12.2: Main Functions for Homeostasis

Several systems, including the circulatory, respiratory, skeletal, and digestive systems, play major roles in maintaining chemical homeostasis; the urinary system completes this list. Besides regulating numerous chemicals, the urinary system assists other systems in regulating blood pressure.

The urinary system makes seven contributions to homeostasis. Each activity is adjusted to compensate for changing body conditions. Each kidney function is regulated by the nervous system, the endocrine system, or the characteristics of the blood flowing through the kidneys.

Removing Wastes and Toxins

One function of the urinary system is removing wastes and toxins (e.g., heavy metals, dyes) from the blood. Major waste materials removed include urea, uric acid, and ammonia, which result primarily from the metabolism of amino acids and proteins, and creatinine from muscle cells. Although body concentrations of urea and creatinine can become relatively high before causing significant harm, slight elevations in uric acid cause the formation of irritating crystals (e.g., gout), and ammonia is highly toxic at very low concentrations. Many drugs, which can reach toxic levels, are also removed.

Regulating Osmotic Pressure

Osmotic pressure is the total concentration of dissolved materials in a liquid. Since water and many dissolved substances can pass through capillary walls, the osmotic pressures of the blood and the fluid surrounding body cells (interstitial fluid) are equal. The kidneys regulate the osmotic pressure of blood and therefore that of interstitial fluid by adjusting the amounts of water and dissolved materials that leave the body in urine.

If the osmotic pressure of the interstitial fluid is the same as the osmotic pressure inside body cells, osmotic homeostasis exists and the cells remain the same size. However, if the osmotic pressure surrounding the cells rises, water will leave the cells by the process of osmosis, causing them to shrink and their contents to become more concentrated. Conversely, if the osmotic pressure surrounding the cells falls, water will diffuse into the cells, causing them to swell and their contents to become dilute. In either situation the cells malfunction because their structure and chemical concentrations are disturbed. Swelling of brain cells is especially dangerous because the excess pressure that develops inside the skull causes neuron injury and malfunctioning.

Though all substances dissolved in the interstitial fluid contribute to its osmotic pressure, the ratio between water and sodium is the main determinant of its osmotic pressure. Therefore, the kidneys maintain osmotic homeostasis primarily by adjusting the amounts of water and sodium that remain in the blood and the amounts excreted in the urine. The kidneys must frequently alter these amounts to compensate for factors that alter osmotic pressure, including changes in intake (e.g., drinking fluids, eating salty foods) and output (e.g., perspiring, having diarrhea).

Maintaining Individual Concentrations

The urinary system maintains individual homeostatic concentrations of specific minerals such as sodium, potassium, calcium, magnesium, and phosphorus. Each mineral is important for specific cell activities and must be available at the correct concentration for these activities to occur properly. The kidneys adjust the retention and excretion of each substance individually, compensating for changes in input (e.g., eating) and output (e.g., perspiring, bleeding).

Maintaining Acid/Base Balance

Maintaining acid/base balance (pH homeostasis) is important because disturbances disrupt molecular structure and functioning (e.g., enzymes). Many body activities tend to disturb acid/base balance because they produce acids (e.g., carbonic acid, lactic acid, ketoacids). Acid/base balance can also be disturbed by ingesting acidic substances such as vinegar and citrus fruits, ingesting alkaline substances such as sodium bicarbonate and other antacids, or changing CO2 levels through altered respiratory system functioning. The kidneys help compensate for such disturbances and thus help maintain acid/base balance by adjusting acid and buffer materials (e.g., sodium bicarbonate) in the blood.

Regulating Blood Pressure

The kidneys help regulate blood pressure by adjusting the amount of water retained in the blood and thus help determine the volume of blood in the vessels. Low blood pressure can be increased by retaining more water, and high blood pressure can be reduced by allowing more water to leave in the urine.

The kidneys also influence blood pressure by secreting an enzyme (renin) when blood pressure in the kidneys is low. This enzyme causes the formation of another substance in the blood (angiotensin II), which results in increased production of the hormone aldosterone. Angiotensin II and aldosterone increase blood pressure by causing small arteries to constrict and causing the kidneys to retain more water. Conversely, when blood pressure rises, less renin is produced. Then blood pressure can drop back to normal because vessels can dilate and more water can leave in the urine.

Activating Vitamin D

The urinary system helps maintain proper calcium concentrations not only by directly adjusting the retention and excretion of calcium but also by activating vitamin D. Fully activated vitamin D from the kidneys is needed for adequate absorption of calcium by the small intestine and proper calcium retention by the kidneys.

Regulating Oxygen Levels

The urinary system helps regulate oxygen levels. When oxygen levels are low, the kidneys secrete a hormone (erythropoietin) that stimulates red blood cell production in bone marrow. When red blood cells increase, more oxygen enters the blood in the lungs. Conversely, high oxygen levels inhibit erythropoietin production, leading to slower RBC production. As the number of RBCs declines through normal attrition, oxygen levels decrease.