Structure and Function of the Urinary System

Function of the Kidneys

Kidneys produce urine and play a role in the regulation of water volume, blood pressure, electrolyte concentrations, pH levels, calcium levels, and red blood cell counts.

The kidneys are located near the middle of the back just below the rib cage. Each of two kidneys is about the size of an adult fist. The kidneys are the primary filtration organ in the body. They filter over one quart (approximately one liter) of blood every minute, which adds up to about 45 gallons (170 liters) of blood every day. In addition to their excretory function, the kidneys also play a role in blood pressure control, red blood cell production, calcium regulation, and control of blood acidity. Specialized cells in the filtering tubules of the kidneys sense sodium levels. When blood pressure drops, filtration of sodium also decreases. Holding on to sodium in the blood results in an increase in the amount of water retained in the blood, and this raises blood pressure. The kidneys also regulate blood pressure through the production of the hormone renin. Renin is converted into angiotensin I and then into angiotensin II. Angiotensin II causes blood vessels to constrict in the body, thereby raising blood pressure.

The kidneys play a role in the production of red blood cells in the body. Kidney cells are sensitive to low oxygen levels in the blood. When a low oxygen level is sensed, the kidney cells secrete a hormone called erythropoietin (EPO). Erythropoietin stimulates the formation of new red blood cells in the bone marrow. When calcium levels drop in the blood and the parathyroid hormone responds by releasing parathormone, calcium reabsorption is stimulated from bone and its reabsorption is increased in the intestines and kidney. In order to reabsorb calcium from the intestines, vitamin D is converted into its active form in the proximal convoluted tubule of the kidney. This active vitamin D causes calcium to then be absorbed from the proximal tubule of the kidney, thereby increasing calcium levels in the blood as well. Further, kidneys control pH levels in blood through their ability to absorb carbonic acid in the renal tubule. If blood pH level decreases or becomes too acidic, carbonic acid reabsorption decreases. Conversely, if the blood becomes too alkaline (i.e., the pH increases), reabsorption of carbonic acid increases in the tubule.

Healthy kidneys are working hard 24 hours every day to filter and remove waste from the body, in addition to participating in several other regulatory processes. Unfortunately, when left uncontrolled, diseases such as hypertension (high blood pressure) and diabetes mellitus can lead to kidney damage or even kidney failure over time. This occurs as a result of damage to the fine arteriolar network within the kidneys. The arterioles become narrow and hardened and do not allow for the filtration necessary to keep the blood free of metabolic wastes. Early kidney failure may be asymptomatic, but continued failure can be fatal. Symptoms of kidney failure may include weakness, lethargy, shortness of breath, swelling, and confusion.