Blood Vessels1

Blood is carried through the body by blood vessels. An artery is a blood vessel that carries blood away from the heart, where it branches into ever-smaller vessels. Eventually, the smallest arteries, called arterioles, branch into tiny capillaries, where gases, nutrients, and wastes are exchanged. Capillaries turn into venules, small veins, which carry blood to veins. Veins merge and eventually become the largest veins in the body called the superior and inferior venae cavae that return blood to the heart. As discussed previously in this chapter, arteries and veins transport blood in two distinct circuits: the systemic circuit and the pulmonary circuit. Systemic arteries provide blood rich in oxygen to the body’s tissues. The blood returned to the heart through systemic veins has less oxygen because oxygen carried by the arteries has been delivered to the cells. In contrast, in the pulmonary circuit, arteries carry blood low in oxygen to the lungs for gas exchange. Pulmonary veins then return freshly oxygenated blood from the lungs to the heart to be pumped back out into systemic circulation. See Figure 11.20 for an illustration of blood flow through the systemic and pulmonary circuits in vessels carrying oxygenated blood and deoxygenated blood.

Figure 11.20. Vessels Transporting Oxygenated and Deoxygenated Blood in Pulmonary and Systemic Circulation

A:”2101_Blood_Flow_Through_the_Heart” by OpenStax College is licensed under CC BY 3.0

Shared Features of Arteries and Veins

Although arteries and veins differ structurally and functionally, they share certain features. Arteries and arterioles have thicker walls than veins and venules because they are closer to the heart and receive blood that is surging at a far greater pressure.

Each type of vessel has a lumen—a hollow central passageway through which blood flows. Arteries have smaller lumens than veins, a characteristic that helps to maintain the pressure of blood moving through the system. Together, their thicker walls and smaller diameters give arterial lumens a more rounded appearance in cross section than the lumens of veins.

By the time blood has passed through capillaries and entered venules, the pressure initially generated by heart contraction has diminished. In comparison to arteries, venules and veins have a much lower pressure from the blood that flows through them. Their walls are considerably thinner and their lumens are larger in diameter, allowing more blood to flow with less vessel resistance. In addition, many veins of the body, particularly those of the limbs, contain valves that assist the flow of blood toward the heart. This is critical because blood flow becomes sluggish in the extremities, as a result of the lower pressure and the effects of gravity.

Both arteries and veins have the same three distinct tissue layers, called tunics (from the Latin term tunica) for the garments first worn by ancient Romans. From the inner to outer layers, these tunics are the tunica interna (also known as tunica intima), the tunica media, and the tunica externa.

See Figure 11.21 for an illustration showing these three layers and comparing the differences in the walls of arteries and veins.

Figure 11.21. Comparison of the Walls Arteries and Veins (a) Arteries (b) Veins (c)  Micrograph of Arteries and Veins

A:”2102_Comparison_of_Artery_and_Vein” by OpenStax College is licensed under CC BY 3.0

Tunica Interna

The tunica interna (also called the tunica intima) is composed of smooth epithelial and connective tissue layers. Lining the tunica interna is the specialized simple squamous epithelium called the endothelium, which is continuous throughout the entire vascular system, including the lining of the chambers of the heart. Damage to this endothelial lining and exposure of blood to the deep collagenous fibers are primary causes of clot formation.

Under the microscope, the lumen and the entire tunica interna of a vein will appear smooth, whereas those of an artery will normally appear wavy because of the partial constriction of the smooth muscle in the tunica media.

Tunica Media

The tunica media is the middle layer of the vessel wall. It is generally the thickest layer in arteries. Arteries also have a thicker tunica media compared to veins. The tunica media consists of layers of smooth muscle supported by connective tissue that is primarily made up of elastic fibers, most of which are arranged in circular sheets. Toward the outer portion of the tunic, there are also layers of longitudinal muscle. Contraction and relaxation of the circular muscles decrease and increase the diameter of the vessel lumen, respectively.

Specifically in arteries, vasoconstriction decreases blood flow as the smooth muscle in the walls of the tunica media contracts, making the lumen narrower and increasing blood pressure. Similarly, vasodilation increases blood flow as the smooth muscle relaxes, allowing the lumen to widen and blood pressure to drop.

Tunica Externa

The outer tunic, the tunica externa (also called the tunica adventitia), is a sheath of connective tissue composed primarily of collagenous fibers. Some bands of elastic fibers are found here as well. This is normally the thickest tunic in veins and may be thicker than the tunica media in some larger arteries. The outer layers of the tunica externa are not distinct but rather blend with the surrounding connective tissue outside the vessel, helping to hold the vessel in position. If you are able to palpate some of the superficial veins in your upper limbs and try to move them, you will find that the tunica externa prevents this. If the tunica externa did not hold the vessel in place, any movement would likely result in disruption of blood flow.

Table 11.1 compares and contrasts the tunics of the arteries and veins.

Table 11.1 Comparison of Tunics in Arteries and Veins1

See Figure 10.22 for an illustration comparing the tunics of arteries and veins.

Figure 10.22 Comparison of Arteries and Veins (https://upload.wikimedia.org/wikipedia/commons/6/60/Artery_Vein_Capillary_Comparison.png)

Artery Types and Arterioles

As previously discussed in this chapter, arteries have relatively thick walls that can withstand the high pressure of blood pumped from the heart. Arteries closest to the heart have the thickest walls, containing a high percentage of elastic fibers in all three of their tunics. This type of artery is known as an elastic artery.

Vessels larger than 10 mm in diameter are typically elastic. Their abundant elastic fibers allow them to expand, as blood pumped from the ventricles passes through them, and then to recoil after the surge has passed. If artery walls were rigid and unable to expand and recoil, their resistance to blood flow would greatly increase and blood pressure would be higher which would require the heart to pump harder to maintain adequate pressure and flow. Artery walls would have to become even thicker in response to this increased pressure. An elastic artery is also known as a conducting artery because the large diameter of the lumen enables it to accept a large volume of blood from the heart and conduct it to smaller branches.

Farther from the heart, where the surge of blood has dampened, the percentage of elastic fibers in an artery’s tunica interna decreases, and the amount of smooth muscle in its tunica media increases. The artery at this point is described as a muscular artery. The diameter of muscular arteries typically ranges from 0.1 mm to 10 mm. Their thick tunica media allows muscular arteries to play a leading role in vasoconstriction. In contrast, their decreased quantity of elastic fibers limits their ability to expand. Fortunately, because the blood pressure has eased by the time it reaches these more distant vessels, elasticity is less important.

Notice that although the distinctions between elastic and muscular arteries are important, there is no clear boundary where an elastic artery suddenly becomes muscular. Rather, there is a gradual transition as the vascular tree repeatedly branches. In turn, muscular arteries branch to distribute blood to the vast network of arterioles. For this reason, a muscular artery is also known as a distributing artery.

An arteriole is a very small artery that leads to a capillary. Arterioles have the same three tunics as the larger vessels, but the thickness of each is greatly diminished. The critical endothelial lining of the tunica interna is intact. The tunica media is restricted to one or two smooth muscle cell layers in thickness. The tunica externa remains but is very thin. With a lumen averaging 30 micrometers or less in diameter, arterioles are critical in slowing down—or resisting—blood flow and causing a substantial drop in blood pressure. The importance of the arterioles is that they will be the primary site of both resistance and regulation of blood pressure.

See Figure 11.23 for an illustration comparing the walls of an elastic artery, muscular artery, and arteriole.

Figure 11.23. Comparison of Walls of an Elastic Artery, Muscular Artery, and Arteriole

A:”2103_Muscular_and_Elastic_Artery_Arteriole” by OpenStax College is licensed under CC BY 3.0

Capillaries

A capillary is a microscopic vessel that supplies blood to the tissues, a process called perfusion. Exchange of gases, nutrients, wastes, and other substances occurs between the blood in capillaries and the surrounding cells and their tissue fluid (interstitial fluid). The diameter of a capillary lumen ranges from 5–10 micrometers; the smallest capillaries are just barely wide enough for an erythrocyte to squeeze through. Flow through capillaries is often described as microcirculation.

The wall of a capillary consists of the endothelial layer surrounded by a basement membrane with occasional smooth muscle fibers. There is some variation in wall structure. In a large capillary, several endothelial cells bordering each other may line the lumen; in a small capillary, there may be only a single cell layer that wraps around to contact itself.

Veins and Venules

Compared to arteries, veins are thin-walled vessels with large and irregular lumens. Because of the low blood pressure in veins, as well as the pull of gravity, larger veins are commonly equipped with valves that promote the unidirectional flow of blood toward the heart and prevent backflow toward the capillaries.

A venule is an extremely small vein, generally 8–100 micrometers in diameter. Once gas exchange has occurred in the capillaries, blood is ready to start its return to the heart. Venules are the small veins exiting the capillaries. Multiple venules join to form veins. The walls of venules consist of endothelium, a thin tunica media – with a few muscle cells and elastic fibers, plus an outer layer of connective tissue fibers that constitute a very thin tunica externa.

See Figure 11.24 for an illustration comparing large veins, medium veins, and venules.

Figure 11.24. Comparison of Veins and Venules (https://openstax.org/books/anatomy-and-physiology-2e/pages/20-1-structure-and-function-of-blood-vessels)

A:”2106_Large_Medium_Vein_Venule” by OpenStax College is licensed under CC BY 3.0

Table 11.2 compares the features of arteries and veins.

Table 11.2 Comparison of Arteries and Veins1

See Figure 10.25 for an illustration of arteries, arterioles, capillaries, venules, and veins.

Figure 10.25 Arteries, Arterioles, Capillaries, Venules, and Veins (https://upload.wikimedia.org/wikipedia/commons/d/d3/Blood_vessels-en.svg)

Blood Pressure

Blood flow is initiated by the contraction of the ventricles of the heart and refers to the movement of blood through a vessel, tissue, or organ. Ventricular contraction ejects blood into the major arteries, resulting in flow from regions of higher pressure to regions of lower pressure, where blood encounters smaller arteries and arterioles, then capillaries, and then the venules and veins of the venous system.

Hydrostatic pressure is the force exerted by a fluid against the wall of the container in which it is located. One form of hydrostatic pressure is blood pressure, the force exerted by blood on the walls of the blood vessels or the chambers of the heart. Blood pressure typically refers to systemic arterial blood pressure—that is, the pressure of blood flowing in the arteries of the systemic circulation. In clinical practice, this pressure is measured in mmHg (millimeters of mercury) and is typically obtained by applying a sphygmomanometer (blood pressure cuff) on the brachial artery of a patient’s arm.

When systemic arterial blood pressure is measured, it is recorded as a ratio of two numbers expressed as systolic pressure over diastolic pressure. For example, 120/80 mmHg is a normal adult blood pressure. The systolic pressure reflects the arterial pressure resulting from the ejection of blood during ventricular contraction (called systole). The diastolic pressure represents the arterial pressure of blood during ventricular relaxation (called diastole).

Variables Affecting Blood Flow and Blood Pressure

Blood is pumped from the ventricles of the heart into the arteries at high pressure and then blood pressure decreases as blood flows through the arteries, capillaries, and veins. See Figure 11.26 for an illustration of the changes in pressures through the circulatory system from the aorta to the venae cavae back into the heart.

Figure 11.26. Systolic Blood Pressure Changes Through the Circulatory System

A:”2109_Systemic_Blood_Pressure” by OpenStax College is licensed under CC BY 3.0

If the pressure inside the arteries increases and cardiac function does not compensate, blood flow decreases to tissues and organs. In the venous system, the opposite relationship is true. Increased pressure inside the veins (such as through the application of compression socks) increases blood flow back to the heart.

Five variables that influence blood flow and blood pressure are:

• Cardiac output
• Compliance (how stretchy/elastic the walls of a blood vessel are)
• Blood volume
• Blood viscosity
• Blood vessel length and diameter

Cardiac Output

As previously discussed in this chapter, cardiac output is the measurement of blood flow from the heart through the ventricles, and is usually measured in liters per minute. Any factor that causes cardiac output to increase, by elevating heart rate or stroke volume or both, will elevate blood pressure and promote blood flow. For example, sympathetic nervous system stimulation increases the heart rate and cardiac output. Conversely, factors that decrease cardiac output, by decreasing heart rate or stroke volume, will decrease arterial pressure and blood flow. For example, altered levels of potassium or calcium in the blood can cause decreased heart rate and cardiac output.

Compliance

Compliance is the ability to expand or stretch. The greater the compliance of an artery, the more effectively it can expand to accommodate surges in blood flow without increased resistance. (Resistance is anything that slows blood or counteracts blood flow). Veins are more compliant than arteries and can easily expand to hold more blood. However, when vascular disease causes stiffening of the arteries, compliance is reduced and resistance to blood flow is increased, resulting in higher pressure within the vessel and reduced blood flow.

Blood Volume

Under normal circumstances, there is little variation in an individual’s blood volume. However, if blood volume increases, blood pressure and blood flow increase. If blood volume decreases, blood pressure and blood flow decrease.

Low blood volume, called hypovolemia, can be caused by bleeding, dehydration, vomiting, severe burns, or side effects of certain medications. Regulatory mechanisms in the body are so effective at maintaining blood pressure that an individual may be asymptomatic until 10–20 percent of the blood volume has been lost. Treatment of hypovolemia includes treating the cause and typically includes intravenous fluid replacement.

Excessive blood volume, called hypervolemia, is caused by excessive retention of water and sodium that can be caused by heart failure, kidney disease, and side effects of steroid medications. Treatment of hypervolemia includes treating the cause and typically includes medications called diuretics that increase the amount of urine produced by the kidneys to excrete excess water and sodium from the body.

Blood Viscosity

Viscosity is a measure of a fluid’s thickness, which directly influences how easily it flows. Clean water, for example, is less viscous than mud. The viscosity of blood is directly proportional to resistance and inversely proportional to flow; therefore, any condition that causes viscosity to increase will also increase resistance and decrease blood flow. For example, imagine sipping milk and then a milkshake through the same sized straw. There is more resistance and, therefore, less flow due to the increased viscosity of the milkshake. Conversely, any condition that causes viscosity to decrease (such as when the milkshake melts) will decrease resistance and increase flow.

Normally, the viscosity of blood does not change over short periods of time. The two primary determinants of blood viscosity are the formed elements and plasma proteins. Any condition causing increased levels of formed elements, such as polycythemia, can increase viscosity. Conversely, because most plasma proteins are produced by the liver, any condition affecting liver function, such as hepatitis or excessive alcohol intake, can decrease the viscosity of the blood.

Vessel Length and Diameter

Blood vessel length is directly proportional to its resistance, meaning the longer the vessel, the greater the resistance and the lower the flow. The length of blood vessels increases throughout childhood, but under normal physiological circumstances does not change in adulthood.. However, the distribution of vessels is not the same in all tissues. Adipose tissue does not have an extensive vascular supply. One pound of adipose tissue contains approximately 200 miles of vessels, whereas skeletal muscle contains more than twice that. An individual weighing 150 pounds has approximately 60,000 miles of vessels in the body. Gaining about 10 pounds adds from 2,000 to 4,000 miles of vessels, depending on the nature of the gained tissue. Blood pressure has to be higher to move blood through the extra miles of vessels. One of the great benefits of weight reduction is the reduced stress to the heart and blood pressure, which does not have to overcome the resistance of as many miles of vessels.

In contrast to length, the diameter of blood vessels varies by vessel type and affects resistance differently. The diameter of any vessel also changes frequently throughout the day in response to neural and chemical signals that trigger contraction and relaxation of smooth muscle, resulting in vasoconstriction or vasodilation of the vessel, respectively. Vessel diameter has an inverse effect on resistance. Vasodilation (widening of vessels) lowers resistance and increases blood flow, whereas vasoconstriction (narrowing of vessels) raises resistance and decreases flow.

See Figure 10.27 comparing blood flow in a normal artery versus a narrowed artery.

Figure 10.27. Blood Flow in a Normal Artery and a Narrowed Artery (https://commons.wikimedia.org/wiki/File:Blausen_0092_BloodPressureFlow.png)

Pulse

After blood is ejected from the heart, elastic fibers in the arteries help maintain a high-pressure gradient as they expand to accommodate the blood and then recoil. This expansion and recoiling effect, known as the pulse, can be palpated manually or measured electronically. Although the effect diminishes farther from the heart, elements of the systolic and diastolic components of the pulse are still evident down to the level of the arterioles.

Pulse is frequently measured clinically to assess a patient’s state of health and is recorded as beats per minute (bpm). The rate and the strength of the pulse are both important clinical indicators of health. For example, a fast or irregular pulse may indicate an underlying heart condition. The pulse strength represents the strength of ventricular contraction and cardiac output. If the pulse is strong, then systolic pressure is high, but if it is weak, medical intervention may be required to improve cardiac output.

Pulse can be palpated manually by placing the tips of the fingers across an artery that runs close to the body surface and pressing lightly. This procedure is typically performed using the radial artery in the wrist or the common carotid artery in the neck, but other common sites include brachial arteries in the upper arms, femoral arteries in the thighs, popliteal arteries behind the knees, posterior tibial arteries near the ankles, and dorsalis pedis arteries in the feet. See Figure 11.28 for an illustration of pulse sites.

Figure 11.28. Pulse Sites (https://openstax.org/books/anatomy-and-physiology-2e/pages/20-2-blood-flow-blood-pressure-and-resistance)

A:”b1d5a333807f4a41c3d26bd19c6264e3ad49b978” by OpenStax College is licensed under CC BY 4.0. Access for free at https://openstax.org/books/anatomy-and-physiology-2e/pages/20-2-blood-flow-blood-pressure-and-resistance

Venous System Pressure

The pumping action of the heart propels the blood into the arteries from an area of higher pressure toward an area of lower pressure. For the blood to flow adequately from the veins back into the heart, the pressure in the veins must be greater than the pressure in the atria of the heart. Two factors help maintain this pressure gradient between the veins and the heart. First, the pressure in the atria during diastole is very low, often approaching zero when the atria are relaxed (atrial diastole). Second, two physiologic “pumps” increase pressure in the venous system, the skeletal muscle pump and the respiratory pump. These “pumps” help return blood to the heart.

Skeletal Muscle Pump

In many body regions, the pressure within the veins can be enhanced by the contraction of the surrounding skeletal muscle. This mechanism, known as the skeletal muscle pump, helps the lower-pressure veins counteract the force of gravity, increasing pressure to return blood to the heart. As leg muscles contract, for example during walking or running, they exert pressure on nearby veins with their numerous one-way valves. This increased pressure causes blood to flow against gravity, opening valves superior to the contracting muscles so blood flows through. Simultaneously, valves inferior to the contracting muscles close, so blood does not flow back down toward the feet. See Figure 11.29 for an illustration of the skeletal muscle pump.

Figure 11.29. Skeletal Muscle Pump (https://openstax.org/books/anatomy-and-physiology-2e/pages/20-2-blood-flow-blood-pressure-and-resistance)

A:”2114_Skeletal_Muscle_Vein_Pump” by OpenStax College is licensed under CC BY 3.0

Respiratory Pump

The respiratory pump aids blood flow through the veins of the chest and abdomen. During inhalation, the volume of the chest cavity increases, causing air pressure in the chest cavity to decrease and allowing us to inhale. As air pressure within the chest cavity drops, blood pressure in the thoracic veins also drops, falling below the pressure in the abdominal veins. This causes blood to flow into the thoracic region, where pressure is now lower and promotes the return of blood from the thoracic veins to the atria.

Relationships in the Circulatory System

The circulatory system affects nearly every cell, tissue, organ, and system in the body. This includes the generalized and more specialized functions of transport of materials, capillary exchange, maintaining health by transporting white blood cells and various immunoglobulins (antibodies), hemostasis, regulation of body temperature, and helping to maintain acid-base balance. In addition to these shared functions, many systems enjoy a unique relationship with the circulatory system. Figure 11.30 summarizes these relationships.

Figure 11.30. Interaction of the Circulatory System With Other Body Systems (https://openstax.org/books/anatomy-and-physiology-2e/pages/20-5-circulatory-pathways#fig-ch21_05_01)

A:”2141_CircSyst_vs_OtherSystemsN” by OpenStax College is licensed under CC BY 3.0

As you learn more about specific vessels of the body, notice that many arteries and veins share the same names, parallel one another throughout the body, and are very similar on the right and left sides of the body. For example, there are a pair of femoral arteries and a pair of femoral veins, with one vessel on each side of the body. In contrast, some vessels closer to the midline of the body, such as the aorta, are unique. Some superficial veins, such as the great saphenous vein in the femoral region, have no arterial counterpart.

Names of vessels can change with location. An artery or vein can change names as it passes an anatomical landmark. For example, the left subclavian artery becomes the axillary artery as it passes through the body wall and into the axillary region, and then becomes the brachial artery as it moves from the axillary region into the upper arm (or brachium).

As you read about circulatory pathways, notice that a large artery is referred to as a trunk, indicating that the vessel gives rise to several smaller arteries. For example, the celiac trunk gives rise to the left gastric, common hepatic, and splenic arteries.

As previously discussed in the “Anatomy of the Heart” section, the heart acts as a double pump, simultaneously pumping blood to and from the lungs (pulmonary circulation) and to and from the remainder of the body tissues (systemic circulation). See Figure 10.31 for an illustration of the pulmonary and systemic circulation circuits. The vessels in these two circulation circuits will be further discussed in the following two subsections.

Figure 10.31 Relationship of Pulmonary and Systemic Circulation (https://commons.wikimedia.org/wiki/File:202410_Systemic_and_pulmonary_circulation.svg)

Pulmonary Circulation

Blood returning from the systemic circuit enters the right atrium through the superior and inferior venae cavae as well as the coronary sinus that receives blood from the heart muscle. These vessels will be described more fully later in this section.

This blood is relatively low in oxygen and relatively high in carbon dioxide because much of the oxygen has been dropped off for use by the tissues and carbon dioxide was picked up to be transported to the lungs for exhalation.

From the right atrium, blood moves into the right ventricle, which pumps it to the lungs for gas exchange. This system of vessels is referred to as the pulmonary circuit.

The single vessel exiting the right ventricle is the pulmonary trunk. At the base of the pulmonary trunk is the pulmonary valve, which prevents backflow of blood into the right ventricle during ventricular diastole. As the pulmonary trunk reaches the superior surface of the heart, it bifurcates (divides) into two branches, a left and a right pulmonary artery. The right pulmonary artery carries deoxygenated blood to the right lung, and the left pulmonary artery carries deoxygenated blood to the left lung.

The right and left pulmonary arteries, in turn, branch many times within each lung, forming a series of smaller arteries and arterioles that eventually lead to the pulmonary capillaries. The pulmonary capillaries surround structures known as alveoli, which are the sites of oxygen and carbon dioxide exchange.

Once gas exchange is completed, oxygenated blood flows from the pulmonary capillaries into a series of pulmonary venules that eventually lead to a series of larger pulmonary veins. Four pulmonary veins, two on the left and two on the right, return blood to the left atrium. At this point, the pulmonary circuit is complete. See Figure 11.32 for an illustration of the pulmonary circuit.

Figure 11.32. Pulmonary Circuit

A:”2119_Pulmonary_Circuit” by OpenStax College is licensed under CC BY 3.0

Table 11.3 defines the major arteries and veins of the pulmonary circuit.

Table 11.3 Pulmonary Trunk, Arteries, and Veins1

Systemic Circulation

From the left atrium, blood moves into the left ventricle, which pumps blood into the aorta. The aorta and its branches—the systemic arteries—send blood to virtually every organ of the body. After the systemic capillaries exchange gases, nutrients, and wastes, the systemic veins and superior vena cava and inferior vena cava return the blood back to the right atrium.

Systemic Arteries

A great approach to learning the vessels is to create simple line drawings, labeling each of the major vessels. It is beyond the scope of this text to name every vessel in the body, but this text will cover the major arteries and veins in the body.

See Figure 11.33 for an illustration of the major systemic arteries of the body.

Figure 11.33. Major Systemic Arteries (https://openstax.org/books/anatomy-and-physiology-2e/pages/20-5-circulatory-pathways#fig-ch21_05_03)

A:”2120_Major_Systemic_Artery” by OpenStax College is licensed under CC BY 3.0

Aorta

The aorta is the largest artery in the body. It arises from the left ventricle and eventually descends to the abdominal region, where it bifurcates (divides) at the level of the fourth lumbar vertebra into the two common iliac arteries.

The aorta consists of the ascending aorta (or aortic root), the aortic arch, and the descending aorta, which passes through the diaphragm, a large dome-shaped muscle that divides the thorax from the abdomen. The descending aorta in the chest is called the thoracic aorta. Once the aorta enters the abdominal region, it is called the abdominal aorta. Arteries originating from the aorta ultimately distribute blood to virtually all tissues of the body.

At the base of the aorta is the aortic valve, which prevents backflow of blood into the left ventricle while the heart is relaxing. After exiting the left ventricle, the ascending aorta moves in an upward direction. Following this ascent, it forms a graceful arc to the left, called the aortic arch. The aortic arch descends toward the abdomen and ends at the level of the intervertebral disk between the fourth and fifth thoracic vertebrae. Now called the descending aorta, it passes through an opening in the diaphragm known as the aortic hiatus. Superior to the diaphragm, the aorta is called the thoracic aorta, and inferior to the diaphragm, it is called the abdominal aorta. See Figure 11.34 for an illustration of the ascending aorta, the aortic arch, and the initial segment of the descending aorta plus major branches.

Figure 11.34. Regions of the Aorta

A:”2121_Aorta” by OpenStax College is licensed under CC BY 3.0

Table 11.4 summarizes the structures of the aorta.

Table 11.4 Structures of the Aorta1

Coronary Arteries

The first vessels that branch off of the ascending aorta are the paired coronary arteries. Refer back to Figure 11.34 to view the coronary arteries emerging from the aorta. The coronary arteries encircle the heart, forming a ring-like structure that divides into the next level of branches that supply blood to the myocardium of the heart.

Aortic Arch Branches

There are three major branches from the aortic arch: the brachiocephalic artery, the left common carotid artery, and the left subclavian (literally “under the clavicle”) artery:

• The brachiocephalic artery is located only on the right side of the body; there is no corresponding artery on the left. The brachiocephalic artery branches into the right subclavian artery and the right common carotid artery.
• The left common carotid artery branches directly from the aortic arch.
• The left subclavian artery branches directly from the aortic arch.

Refer back to Figure 11.34 to view these three major aortic arch branches emerging from the aortic arch.

The right and left subclavian arteries supply blood to the arms, chest, shoulders, back, and central nervous system. The right and left common carotid arteries divide into internal and external carotid arteries.

The external carotid artery supplies blood to numerous structures within the face, lower jaw, neck, esophagus, and larynx. These branches include the lingual, facial, occipital, maxillary, and superficial temporal arteries. The internal carotid arteries, along with the vertebral arteries, are the two primary suppliers of blood to the human brain. Given the central role and vital importance of the brain to life, it is critical that blood supply to this organ remains uninterrupted. Both the carotid and vertebral arteries branch once they enter the cranial cavity, and some of these branches form a structure known as the arterial circle (or circle of Willis), an anastomosis that is remarkably like a traffic circle that sends off branches (in this case, arterial branches to the brain). As a rule, branches to the anterior portion of the cerebrum are normally fed by the internal carotid arteries; the remainder of the brain receives blood flow from branches associated with the vertebral arteries. See Figure 11.35 for an illustration of arteries supplying the head and neck and Figure 11.36 for an illustration of arteries supplying the brain.

Figure 11.35. Arteries Supplying the Head and Neck (https://openstax.org/books/anatomy-and-physiology-2e/pages/20-5-circulatory-pathways)

A:”2122_Common_Carotid_Artery” by OpenStax College is licensed under CC BY 3.0

Figure 11.36. Arteries Supplying the Brain (https://openstax.org/books/anatomy-and-physiology-2e/pages/20-5-circulatory-pathways)

A:”2123_Arteries_of_the_Brain” by OpenStax College is licensed under CC BY 3.0

Table 11.5 summarizes the aortic arch branches, including the major branches supplying the brain.

Table 11.5 Major Aortic Arch Branches and Arteries Supplying the Brain1

Major Branches of the Thoracic Aorta

The thoracic aorta begins at the level of vertebra T5 and continues to the diaphragm. As it passes through the thoracic region, the thoracic aorta gives rise to several branches, which are collectively referred to as visceral branches and parietal branches. These branches supply various organs and muscles in the thoracic region. See Figure 11.37 for an illustration of major branches of the thoracic aorta.

Figure 11.37. Major Branches of the Thoracic Aorta (https://openstax.org/books/anatomy-and-physiology-2e/pages/20-5-circulatory-pathways#fig-ch21_05_07)

A:”2124_Thoracic_Abdominal_Arteries” by OpenStax College is licensed under CC BY 3.0

Abdominal Aorta and Major Branches

After passing through the diaphragm, the thoracic aorta is called the abdominal aorta. It formally ends at approximately the level of vertebra L4, where it divides to form the common iliac arteries. The abdominal aorta gives rise to several important branches:

A single celiac trunk emerges and divides into the left gastric artery to supply blood to the stomach and esophagus, the splenic artery supplies blood to the spleen, and the common hepatic artery, which, in turn, gives rise to the hepatic artery proper to supply blood to the liver.

Two additional single vessels arise from the abdominal aorta. These are the superior and inferior mesenteric arteries. The superior mesenteric artery arises approximately 2.5 cm inferior to the celiac trunk and branches into several major vessels that supply blood to the small intestine (duodenum, jejunum, and ileum), the pancreas, and a majority of the large intestine. The inferior mesenteric artery supplies blood to the distal segment of the large intestine, including the rectum. It arises approximately 5 cm superior to the common iliac arteries.

In addition to these single branches, the abdominal aorta gives rise to several significant paired arteries along the way. These include the inferior phrenic arteries, the adrenal arteries, the renal arteries, the gonadal arteries, and the lumbar arteries.

Each inferior phrenic artery is a counterpart of a superior phrenic artery and supplies blood to the inferior surface of the diaphragm. The adrenal artery supplies blood to the adrenal glands and arises near the superior mesenteric artery. Each renal artery branches approximately 2.5 cm inferior to the superior mesenteric arteries and supplies a kidney. The right renal artery is longer than the left because the aorta lies to the left of the vertebral column and the vessel must travel a greater distance to reach its target. Renal arteries branch repeatedly to supply blood to the kidneys.

Each gonadal artery supplies blood to the gonads, or reproductive organs, and is also described as either an ovarian artery or a testicular artery, depending upon the sex of the individual. An ovarian artery supplies blood to an ovary, Fallopian tube, and the uterus and is located within the suspensory ligament of the uterus. It is considerably shorter than a testicular artery, which ultimately travels outside the body cavity to the testes, forming one component of the spermatic cord. The gonadal arteries arise inferior to the renal arteries and are generally retroperitoneal. The ovarian artery continues to the uterus where it forms an anastomosis with the uterine artery that supplies blood to the uterus. Both the uterine arteries and vaginal arteries, which distribute blood to the vagina, are branches of the internal iliac artery.

The aorta divides at approximately the level of vertebra L4 into a left and a right common iliac artery but continues as a small vessel, the median sacral artery, into the sacrum.

The common iliac arteries provide blood to the pelvic region and, ultimately, to the lower limbs. They split into external and internal iliac arteries approximately at the level of the lumbar-sacral articulation. Each internal iliac artery sends branches to the urinary bladder, the walls of the pelvis, the external genitalia, and the medial portion of the femoral region. In females, they also provide blood to the uterus and vagina. The much larger external iliac artery supplies blood to each of the lower limbs. Figure 11.38 shows the distribution of the major branches of the aorta into the thoracic and abdominal regions.

Figure 11.38. Major Branches of the Aorta (https://openstax.org/books/anatomy-and-physiology-2e/pages/20-5-circulatory-pathways#fig-ch21_05_08)

A:”2125_Thoracic_Abdominal_Arteries_Chart” by OpenStax College is licensed under CC BY 3.0

Figure 11.39 shows the distribution of the major branches of the common iliac arteries.

Figure 11.39. Major Branches of the Iliac Arteries (https://openstax.org/books/anatomy-and-physiology-2e/pages/20-5-circulatory-pathways#fig-ch21_05_09)

A:”2126_Iliac_Artery_Branches_Chart” by OpenStax College is licensed under CC BY 3.0

Table 11.6 summarizes the major branches of the abdominal aorta.

Table 11.6 Vessels of the Abdominal Aorta1

Arteries Serving the Upper Limbs

As the subclavian artery exits the thorax into the axillary region, it is renamed the axillary artery. Although it does branch and supply blood to the region near the head of the humerus, the majority of the vessel continues into the upper arm, or brachium, and becomes the brachial artery.

The brachial artery supplies blood to much of the brachial region and divides at the elbow into several smaller branches, including the deep brachial arteries, which provide blood to the posterior surface of the arm, and the ulnar collateral arteries, which supply blood to the region of the elbow. As the brachial artery approaches the coronoid fossa, it bifurcates (divides) into the radial and ulnar arteries, which continue into the forearm, or antebrachium.

The radial artery and ulnar artery parallel their namesake bones, giving off smaller branches until they reach the wrist, or carpal region. At this level, they fuse to form the superficial and deep palmar arches that supply blood to the hand, as well as the digital arteries that supply blood to the digits.

See Figure 11.40 for an illustration of the major arteries of the upper limbs.

Figure 11.40. Major Arteries of the Upper Limb (https://openstax.org/books/anatomy-and-physiology-2e/pages/20-5-circulatory-pathways#fig-ch21_05_10)

A:”2127_Thoracic_Upper_Limb_Arteries” by OpenStax College is licensed under CC BY 3.0

Figure 11.41 shows the distribution of systemic arteries from the heart into the upper limb.

Figure 11.41. Major Arteries of the Upper Limb (https://openstax.org/books/anatomy-and-physiology-2e/pages/20-5-circulatory-pathways#fig-ch21_05_11)

A:”2128_Thoracic_Upper_Limb_Arteries_Chart” by OpenStax College is licensed under CC BY 3.0

Table 11.7 summarizes the arteries serving the upper limbs.

Table 11.7 Arteries Serving the Upper Limbs1

Arteries Serving the Lower Limbs

The external iliac artery exits the body cavity and enters the femoral region of the lower leg. When it enters the thigh, it is renamed the femoral artery. It gives off several smaller branches, as well as the lateral deep femoral artery that, in turn, gives rise to a lateral circumflex artery. These arteries supply blood to the deep muscles of the thigh, as well as ventral and lateral regions of the integument. The femoral artery also gives rise to the genicular artery, which provides blood to the region of the knee. As the femoral artery passes posterior to the knee near the popliteal fossa, it is called the popliteal artery. The popliteal artery branches into the anterior and posterior tibial arteries.

The anterior tibial artery is located between the tibia and fibula and supplies blood to the muscles and skin of the anterior tibial region. Upon reaching the tarsal region, it becomes the dorsalis pedis artery, which branches repeatedly and provides blood to the tarsal and dorsal regions of the foot.

The posterior tibial artery provides blood to the muscles and skin on the posterior surface of the tibial region. The fibular or peroneal artery branches from the posterior tibial artery. It bifurcates (divides) and becomes the medial plantar artery and lateral plantar artery, providing blood to the plantar surfaces.

There is an anastomosis with the dorsalis pedis artery and the medial and lateral plantar arteries that form two arches called the dorsal arch (also called the arcuate arch) and the plantar arch, which provide blood to the remainder of the foot and toes.

See Figure 11.42 for an illustration of the major arteries in the leg.

Figure 11.42. Major Arteries Serving the Lower Limb (https://openstax.org/books/anatomy-and-physiology-2e/pages/20-5-circulatory-pathways#fig-ch21_05_12)

A:”2129ab_Lower_Limb_Arteries_Anterior_Posterior” by OpenStax College is licensed under CC BY 3.0

Figure 11.43 shows the distribution of the major systemic arteries in the lower limb.

Figure 11.43. Systemic Arteries of the Lower Limb (https://openstax.org/books/anatomy-and-physiology-2e/pages/20-5-circulatory-pathways#fig-ch21_05_13)

A:”2130_Lower_Limb_Arteries_Chart” by OpenStax College is licensed under CC BY 3.0

Table 11.8 summarizes the major arteries serving the lower limbs.

Table 11.8 Arteries Serving the Lower Limbs1

Systemic Veins

Systemic veins return blood to the right atrium. Because the blood has already passed through the systemic capillaries, it will be relatively low in oxygen concentration. In many cases, there will be veins draining organs and regions of the body with the same name as the arteries that supplied these regions, and the two often parallel one another. This is often described as a “complementary” pattern. However, there is a great deal more variability in the venous circulation than normally occurs in the arteries.

In both the neck and limb regions, there are often both superficial and deeper veins. The deeper veins generally correspond to their complementary arteries. The superficial veins do not normally have direct arterial counterparts, but in addition to returning blood, they also contribute to maintaining body temperature. When the ambient temperature is warm, more blood is diverted to the superficial veins where heat can be more easily dissipated to the environment. In colder weather, there is more constriction of the superficial veins, and blood is diverted deeper where the body can retain more of the heat.

See Figure 11.44 for an illustration of the major systemic veins from an anterior view.

Figure 11.44. Major Systemic Veins of the Body (https://openstax.org/books/anatomy-and-physiology-2e/pages/20-5-circulatory-pathways#fig-ch21_05_14)

A:”2131_Major_Systematic_Veins” by OpenStax College is licensed under CC BY 3.0

The right atrium receives all the systemic venous return. Most of the blood flows into either the superior vena cava or inferior vena cava. The superior vena cava receives blood from the head, neck, chest, shoulders, and upper limbs. The exception to this is that most venous blood flow from the coronary veins flows directly into the coronary sinus and from there directly into the right atrium. Inferior to the diaphragm, systemic venous blood from the abdominal and pelvic regions and the lower limbs enters the inferior vena cava to be returned to the right atrium.

Major Veins That Flow Into the Superior Vena Cava

The superior vena cava drains most of the body superior to the diaphragm. On both the left and right sides, the subclavian vein forms when the axillary vein passes through the body wall from the axillary region. It fuses with the external and internal jugular veins from the head and neck to form the brachiocephalic vein. The superior vena cava then receives blood from the brachiocephalic veins. See Figure 11.45 for an illustration of the veins that flow into the superior vena cava.

Figure 11.45. Veins That Flow Into the Superior Vena Cava (https://openstax.org/books/anatomy-and-physiology-2e/pages/20-5-circulatory-pathways#fig-ch21_05_15)

A:”2132_Thoracic_Abdominal_Veins” by OpenStax College is licensed under CC BY 3.0

Table 11.9 summarizes the veins of the thoracic region that flow into the superior vena cava.

Table 11.9 Major Veins of the Thoracic Region1

Major Veins of the Head and Neck

Blood from the brain and the superficial facial vein flow into each internal jugular vein. Blood from the more superficial portions of the head, scalp, and cranial regions, including the temporal vein and maxillary vein, flow into each external jugular vein. Although the external and internal jugular veins are separate vessels, there are anastomoses between them close to the thoracic region. Blood from the external jugular vein empties into the subclavian vein. See Figure 11.46 for an illustration of the internal and external jugular veins.

Figure 11.46. Veins of the Head and Neck (https://openstax.org/books/anatomy-and-physiology-2e/pages/20-5-circulatory-pathways#fig-ch21_05_16)

A:”2133_Head_and_Neck_Veins” by OpenStax College is licensed under CC BY 3.0

Table 11.10 summarizes the major veins of the head and neck.

Table 11.10 Major Veins of the Head and Neck1

Major Veins Draining the Upper Limbs

The digital veins in the fingers come together in the hand to form the palmar venous arches. From here, the veins come together to form the radial vein, the ulnar vein, and the median antebrachial vein. The radial vein and the ulnar vein parallel the bones of the forearm and join together at the antebrachium to form the brachial vein, a deep vein that flows into the axillary vein in the brachium.

The median antebrachial vein parallels the ulnar vein, is more medial in location, and joins the basilic vein in the forearm. As the basilic vein reaches the antecubital region, it gives off a branch called the median cubital vein that crosses at an angle to join the cephalic vein. The median cubital vein is the most common site for drawing venous blood in humans. The basilic vein continues through the arm medially and superficially to the axillary vein.

The cephalic vein begins in the antebrachium (between the elbow and wrist) and drains blood from the superficial surface of the arm into the axillary vein. It is extremely superficial and easily seen along the surface of the biceps brachii muscle in individuals with good muscle tone and in those without excessive adipose tissue in the arms.

The subscapular vein drains blood from the subscapular region and joins the cephalic vein to form the axillary vein. As it passes through the body wall and enters the thorax, the axillary vein becomes the subclavian vein.

See Figure 11.47 for an illustration of the veins in the upper limb.

Figure 11.47. Veins of the Upper Limb (https://openstax.org/books/anatomy-and-physiology-2e/pages/20-5-circulatory-pathways)

A:”2134_Thoracic_Upper_Limb_Veins” by OpenStax College is licensed under CC BY 3.0

Many of the larger veins of the thoracic and abdominal region and upper limb are further represented in the flow chart in Figure 11.48.

Figure 11.48. Veins Flowing Into the Superior Vena Cava (https://openstax.org/books/anatomy-and-physiology-2e/pages/20-5-circulatory-pathways#fig-ch21_05_18)

A:”2135_Veins_Draining_into_Superior_Vena_Cava_Chart” by OpenStax College is licensed under CC BY 3.0

Table 11.11 summarizes the veins of the upper limbs.

Table 11.11 Major Veins of the Upper Limbs1

Major Veins That Flow Into the Inferior Vena Cava

Most of the blood inferior to the diaphragm drains into the inferior vena cava before it is returned to the heart. The inferior vena cava parallels the abdominal aorta, where it can receive blood from abdominal veins.

Blood supply from the kidneys flows into each renal vein, the largest veins entering the inferior vena cava. A number of other smaller veins empty into the left renal vein. Each adrenal vein drains the adrenal or suprarenal glands located immediately superior to the kidneys. The right adrenal vein enters the inferior vena cava directly, whereas the left adrenal vein enters the left renal vein.

From the male reproductive organs, each testicular vein flows from the scrotum, forming a portion of the spermatic cord. Each ovarian vein drains an ovary in females. Each of these veins is generically called a gonadal vein. The right gonadal vein empties directly into the inferior vena cava, and the left gonadal vein empties into the left renal vein.

Blood supply from the liver drains into each hepatic vein and directly into the inferior vena cava. Because the inferior vena cava lies primarily to the right of the vertebral column and aorta, the left renal vein is longer, as are the left adrenal and gonadal veins. The longer length of the left renal vein makes the left kidney the primary target of surgeons removing this organ for donation.

See Figure 11.49 for an illustration of the veins that flow into the inferior vena cava.

Figure 11.49. Veins That Flow Into the Inferior Vena Cava

A:”2132_Thoracic_Abdominal_Veins” by OpenStax College is licensed under CC BY 3.0

Figure 11.50 provides a flow chart of the veins flowing into the inferior vena cava.

Figure 11.50. Venous Flow Into Inferior Vena Cava (https://openstax.org/books/anatomy-and-physiology-2e/pages/20-5-circulatory-pathways#fig-ch21_05_19)

A:”2140_FlowChart_Veins_into_VenaCava” by OpenStax College is licensed under CC BY 3.0

Table 11.12 summarizes the major veins of the abdominal region.

Table 11.12 Major Veins of the Abdominal Region1

Major Veins Draining the Lower Limbs

The superior surface of the foot drains into the digital veins, and the inferior surface drains into the plantar veins, which flow into a complex series of anastomoses in the feet and ankles, including the dorsal venous arch and the plantar venous arch.

From the dorsal venous arch, blood supply drains into the anterior and posterior tibial veins. The anterior tibial vein drains the area near the tibialis anterior muscle and combines with the posterior tibial vein and the fibular vein to form the popliteal vein. The posterior tibial vein drains the posterior surface of the tibia and joins the popliteal vein. The fibular vein drains the muscles and skin in proximity to the fibula and also joins the popliteal vein. The small saphenous vein located on the lateral surface of the leg drains blood from the superficial regions of the lower leg and foot and flows into the popliteal vein. As the popliteal vein passes behind the knee in the popliteal region, it becomes the femoral vein. It is palpable in patients without excessive adipose tissue.

The great saphenous vein, the deep femoral vein, and the femoral circumflex vein drain into the femoral vein. The great saphenous vein is a prominent surface vessel located on the medial surface of the leg and thigh and collects blood from the superficial portions of these areas. It runs from the ankle to the groin and is the longest vein in the body. The deep femoral vein, as the name suggests, drains blood from the deeper portions of the thigh. The femoral circumflex vein forms a loop around the femur just inferior to the trochanters and drains blood from the areas in proximity to the head and neck of the femur.

As the femoral vein penetrates the body wall from the femoral portion of the leg, it becomes the external iliac vein, a large vein that drains blood from the leg to the common iliac vein. The pelvic organs and skin drain into the internal iliac vein, which forms from several smaller veins in the region. The external and internal iliac veins combine near the inferior portion of the sacroiliac joint to form the common iliac vein. In addition to blood supply from the external and internal iliac veins, the middle sacral vein drains the sacral region into the common iliac vein. Similar to the common iliac arteries, the common iliac veins come together at the level of L5 to form the inferior vena cava.

See Figure 11.51 for an illustration of the major veins flowing in the lower limbs.

Figure 11.51. Major Veins Serving the Lower Limbs (https://openstax.org/books/anatomy-and-physiology-2e/pages/20-5-circulatory-pathways#fig-ch21_05_20)

A:”2136ab_Lower_Limb_Veins_Anterior_Posterior” by OpenStax College is licensed under CC BY 3.0

See Figure 11.52 for a flow chart of veins flowing into the lower limb.

Figure 11.52. Major Veins of the Lower Limb (https://openstax.org/books/anatomy-and-physiology-2e/pages/20-5-circulatory-pathways#fig-ch21_05_21)

A:”2137_Lower_Limb_Veins_Chart” by OpenStax College is licensed under CC BY 3.0

Table 11.13 summarizes the major veins of the lower limbs.

Table 11.13 Major Veins of the Lower Limbs1

Hepatic Portal System

The liver is a complex biochemical processing plant. It processes nutrients absorbed by the digestive system; produces plasma proteins, clotting factors, and bile; and disposes of worn-out cell components and waste products. Instead of entering the circulation directly, absorbed nutrients and certain wastes (for example, materials produced by the spleen) travel to the liver for processing via the hepatic portal system. See Figure 11.53 for an illustration of the hepatic portal system.

Figure 11.53. Hepatic Portal System (https://openstax.org/books/anatomy-and-physiology-2e/pages/20-5-circulatory-pathways#fig-ch21_05_22)

A:”2138_Hepatic_Portal_Vein_System” by OpenStax College is licensed under CC BY 3.0

Portal systems begin and end in capillaries. In this case, the initial capillaries from the stomach, small intestine, large intestine, and spleen lead to the hepatic portal vein and end in specialized capillaries within the liver, the hepatic sinusoids. The hepatic portal vein delivers materials from these digestive and circulatory organs directly to the liver for processing.

Because of the hepatic portal system, the liver receives its blood supply from two different sources: from normal systemic circulation via the hepatic artery and from the hepatic portal vein. The liver processes the blood from the portal system to remove certain wastes and excess nutrients, which are stored for later use. This processed blood, as well as the systemic blood that came from the hepatic artery, exits the liver via the right, left, and middle hepatic veins, and flows into the inferior vena cava.

Fetal Circulation

In a developing embryo, the heart develops enough by post-fertilization Day 21 to begin beating. Circulation patterns are clearly established by the fourth week of embryonic life. It is critical to the survival of the developing human that the circulatory system forms early to supply the growing tissue with nutrients and gases and to remove waste products. As the embryo grows within the mother’s uterus, its requirements for nutrients and gas exchange also grow. The placenta (a circulatory organ unique to pregnancy) develops jointly from the embryo and uterine wall structures to fill this need. Emerging from the placenta is the umbilical vein that carries oxygen-rich blood from the mother to the fetus.

There are three major shunts (alternate paths for blood flow) found in the fetal circulatory system. The first two shunts are critical during fetal life when gas exchange is provided by the placenta, not the fetal lungs that are compressed and filled with amniotic fluid. These shunts close shortly after birth when the newborn takes its first breaths. The third shunt persists a bit longer but becomes nonfunctional once the umbilical cord is severed. The three shunts are as follows:

• The ductus venosus is a temporary blood vessel that branches from the umbilical vein, allowing much of the freshly oxygenated blood from the mother and the placenta to bypass the immature fetal liver to flow directly to the fetal inferior vena cava and into the heart, where it is pumped into fetal circulation. The ductus venosus closes slowly during the first weeks of infancy and degenerates to become the ligamentum venosum.
• The foramen ovale is an opening in the interatrial septum that allows blood to flow from the right atrium to the left atrium and bypass the lungs because they are not required for oxygenation. A valve associated with this opening prevents backflow of blood during the fetal period. After childbirth when the newborn begins to breathe, blood pressure in the atria increases, and this shunt closes. The fossa ovalis remains in the interatrial septum after birth, marking the location of the former foramen ovale.
• The ductus arteriosus is a short, muscular vessel that connects the pulmonary trunk to the aorta, diverting most of the blood pumped from the right ventricle into the aorta. A small amount of blood reaches the fetal lungs to supply the developing lung tissue. When a newborn takes their first breath, the lungs expand and the pressure within the lungs drops dramatically, causing the pulmonary vessels to expand. As the amount of oxygen increases, the smooth muscles in the wall of the ductus arteriosus constrict, sealing off the passage. Eventually, the muscular and endothelial components of the ductus arteriosus degenerate, leaving only the connective tissue component of the ligamentum arteriosum.

Two umbilical arteries carry oxygen-depleted fetal blood, including wastes and carbon dioxide, to the placenta. Remnants of the umbilical arteries remain in the adult.

See Figure 11.54 for an illustration of fetal circulation with oxygen-rich blood received from the placenta.

Figure 11.54. Fetal Circulation (https://openstax.org/books/anatomy-and-physiology-2e/pages/20-6-development-of-blood-vessels-and-fetal-circulation#fig-ch21_06_01)

A:”2139_Fetal_Circulation” by OpenStax College is licensed under CC BY 3.0