15.2 Functions of the Urinary System
Functions of the Urinary System[1]
The urinary system works with other body systems such as the heart, lungs, and endocrine system to maintain homeostasis. The functions of the urinary organs are vital to well-being and include the following:
- Excrete wastes and produce urine
- Regulate water balance, blood volume, and electrolyte levels
- Regulate blood pH
- Regulate blood pressure
- Produce the hormones erythropoietin and calcitriol
If an individual has decreased kidney functioning, several body functions become compromised with potentially devastating effects on homeostasis. Symptoms may include edema (swelling) and shortness of breath due to excessive fluid retention; weakness, lethargy, and heart arrhythmias from electrolyte imbalances; acidosis (low pH) or alkalosis (elevated pH) from an inability to regulate blood pH; anemia (from decreased erythropoietin levels); and or muscular, neurological, or skeletal effects from decreased amounts of active vitamin D (calcitriol).
The functions of the urinary system are further described in the following subsections.
Excrete Wastes and Produce Urine
The urinary system filters blood to remove wastes and maintain water balance, producing about 1.5 liters (1.6 quarts) of urine per day. The kidneys can adjust the concentration of the urine to adjust to the body’s water needs, conserving water if the body is dehydrated or eliminating excess water when there is too much.
In addition to excreting excess water, the kidneys rid the body of many wastes, including excess salts, ammonia, urea (from the breakdown of amino acids), creatinine (from muscle metabolism), uric acid (from the breakdown of nucleic acids), drugs, and other toxins.
Regulate Blood Volume and Electrolytes
The kidneys play a major role in regulating blood volume. As they filter blood, they control the amount of water and sodium excreted in the urine, thus regulating blood volume. Reabsorbing sodium causes water retention, resulting in increased blood volume. Conversely, sodium excretion causes water excretion, thus decreasing blood volume. The kidneys also reabsorb or excrete other electrolytes such as calcium, potassium, and magnesium.[2]
The renin-angiotensin-aldosterone system (RAAS) influences sodium and water retention by the kidneys and helps regulate blood pressure. When blood volume is low, the kidneys release renin, leading to the release of the hormone aldosterone. Aldosterone increases reabsorption of sodium and water, resulting in decreased urine production, increased blood volume, and increased blood pressure.[3]
Antidiuretic hormone (ADH), also known as vasopressin, also influences blood volume and blood pressure. ADH is produced in the hypothalamus and released by the posterior pituitary gland. ADH’s primary function is to increase water reabsorption by the kidneys, leading to decreased urine production and increased blood volume.[4]
Regulate Blood pH
The pH level of the blood must remain within a narrow range between 7.35 and 7.45 for normal functioning of body systems. A variety of pH regulation systems exist in the body to help maintain the blood pH, including the kidneys, lungs, and buffers in the blood. Kidneys help regulate blood pH by adjusting the amounts of HCO3- and H+ reabsorbed in the blood or excreted into the urine.[5]
Produce Erythropoietin and Calcitriol
The kidneys produce the hormones erythropoietin (EPO) and calcitriol. Erythropoietin stimulates red blood cell production in the bone marrow. Eighty-five percent of the erythropoietin is produced in the kidneys, with the remainder produced in the liver.
The kidneys perform the final step of vitamin D production, converting calcitriol to calcitriol, the active form of vitamin D. Vitamin D helps the body absorb dietary calcium (Ca2+). It also supports the health of the muscles, brain cells, and the immune system. In fact, many scientists suggest that vitamin D be referred to as a hormone because receptors are present in most cells of the body, reflecting the systemic importance of vitamin D.[6][7]
- Betts, J. G., Young, K. A., Wise, J. A., Johnson, E., Poe, B., Kruse, D. H., Korol, O., Johnson, J. E., Womble, M., & DeSaix, P. (2022). Anatomy and physiology 2e. OpenStax. https://openstax.org/books/anatomy-and-physiology-2e/pages/1-introduction ↵
- Sharma, R., & Sharma, S. (2023). Physiology, Blood Volume. StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK526077/ ↵
- Norris, D., & Carr, J. A. (2013). Vertebrate endocrinology (fifth edition). Academic Press. https://www.sciencedirect.com/topics/medicine-and-dentistry/renin-release#:~:text=Release%20of%20renin%20is%20controlled,receptors%20in%20the%20juxtaglomerular%20body ↵
- Gonzalez, A. A., Salinas-Parra, N., Cifuentes-Araneda, F., & Reyes-Martinez, C. (2020). Vasopressin actions in the kidney renin angiotensin system and its role in hypertension and renal disease. Vitamins and hormones, 113, 217–238. https://doi.org/10.1016/bs.vh.2019.09.003 ↵
- Rajkumar, P., & Pluznick, J. L. (2018). Acid-base regulation in the renal proximal tubules: Using novel pH sensors to maintain homeostasis. American Journal of Physiology Renal Physiology, 315(5), F1187–F1190. https://doi.org/10.1152/ajprenal.00185.2018 ↵
- Betts, J. G., Young, K. A., Wise, J. A., Johnson, E., Poe, B., Kruse, D. H., Korol, O., Johnson, J. E., Womble, M., & DeSaix, P. (2022). Anatomy and physiology 2e. OpenStax. https://openstax.org/books/anatomy-and-physiology-2e/pages/1-introduction ↵
- Mayo Clinic. (2025). Vitamin D. https://www.mayoclinic.org/drugs-supplements-vitamin-d/art-20363792#:~:text=Vitamin%20D%20is%20a%20nutrient,muscles%20and%20brain%20cells%20working ↵
Swelling caused by excess fluid trapped in the body's tissues, often occurring in the hands, feet, ankles, or legs.
Low pH.
Elevated pH.
A hormone produced primarily by the kidneys that stimulates the production of red blood cells in the bone marrow.
The active form of vitamin D3 produced in the kidneys under the influence of parathyroid hormone (PTH); it helps the intestines absorb more calcium from food.
