Functions of Blood^[1]

Blood is a connective tissue. Like all connective tissues, it is made up of cellular elements and an extracellular matrix. The cellular elements – referred to as the formed elements – include erythrocytes, also called red blood cells (RBCs); leukocytes, also called white blood cells (WBCs), and thrombocytes, also called platelets. The extracellular matrix, called plasma, makes blood unique among connective tissues because it is a fluid. This fluid, which is mostly water, suspends the formed elements and enables them to circulate throughout the body within the cardiovascular system.

The primary function of blood is transportation. Other functions of blood also include regulation of homeostasis and protection.

Transportation

Blood transports nutrients, oxygen, carbon dioxide, hormones, and wastes. Nutrients from the food you eat are absorbed in the digestive tract and travel through the bloodstream to the liver, where they are processed and released back into the bloodstream for delivery to body cells. Oxygen diffuses into the blood in the lungs and is carried to tissues and cells throughout the body. Carbon dioxide is picked up from cells and transported to the lungs, where it is exhaled. Hormones are released by endocrine glands and carried to target cells in the bloodstream. Blood also picks up cell wastes and by-products from cells and transports them to various organs for removal.

Regulation of Homeostasis

Blood helps regulate homeostasis in many ways. Proteins and other molecules in the blood act as buffers that help to regulate the pH of body tissues. Blood also helps to maintain the chemical balance of the body and regulate the water content of body cells. Additionally, blood helps regulate body temperature through a classic negative feedback loop. For example, if you exercise outside on a warm day, your rising body temperature triggers several homeostatic mechanisms, including increased transportation of blood from your core to the skin, which is typically cooler. As blood passes through the vessels of the skin, heat dissipates to the environment, resulting in the blood returning to your core being cooler. In contrast, on a cold day, blood is diverted away from the skin to prevent heat dissipation, which maintains a warmer core body temperature.

Protection

Many types of white blood cells (WBCs) protect the body from external threats, such as disease-causing bacteria that enter the bloodstream. Other types of WBCs seek out and destroy internal threats, such as cells with mutated DNA that could multiply and become cancerous or cells that are infected with viruses.

Platelets help protect the body from excessive bleeding. If a blood vessel is damaged from an injury, platelets stimulate blood clot formation to prevent further blood loss.