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Regulation of Water and Electrolytes

Regulation of Urine Production

Imbalance in water and electrolytes in the body can result in a change in volume or concentration of urine to keep the body at homeostasis.

The production of urine is regulated by hormonal and neural mechanisms. Sensory receptors in the brain, heart, and blood vessels detect changes in solute concentration and blood pressure (related to blood volume) and set into motion a negative feedback response to correct any deviations in blood concentration and volume. The kidneys adjust the volume and concentration of urine to offset any changes in the blood volume or composition. Blood pressure changes are detected by baroreceptors in the aorta and carotid arteries. A drop in blood pressure, which could indicate a drop in blood volume, will result in a retention of fluids and a decrease in urine output. An increase in blood pressure will result in an increase in fluids being filtered through the kidneys and increased urine output. An imbalance in any of the electrolytes (charged chemicals) in the blood is detected by osmoreceptors in the hypothalamus (region of the brain that regulates homeostatic processes), which stimulates the release of hormones that will cause an increase in reabsorption or secretion of the solute in order to restore normal levels.

Hormones involved in the production of urine include aldosterone, natriuretic peptides, and antidiuretic hormone (ADH). Aldosterone and antidiuretic hormone are released when blood volume or pressure drops. They act directly on the nephron to regulate the volume and concentration of urine. Aldosterone is released when blood osmolarity decreases. This hormone stimulates an increase in sodium and water reabsorption and a decrease in urine production, resulting in an increase in blood pressure and circulating volume. The release of antidiuretic hormones is stimulated by the hypothalamus sensing low blood volume and increased blood osmolarity. Antidiuretic hormone acts on the kidneys to inhibit urine production and retain water, increasing circulating volume and blood pressure. Natriuretic peptides are secreted in response to increased blood pressure or an increase in circulating volume, and their presence stimulates urine production and helps rid the body of excess fluid by producing a dilute urine.

The DCT and the collecting duct contain the target cells (cells with the receptors for a particular chemical messenger) for the hormones that regulate urine production. Aldosterone acts on the sodium and potassium active transporters in the DCT and the collecting duct, increasing the rate of sodium reabsorption and potassium secretion. Because water follows sodium and sodium content ultimately determines extracellular fluid volume, water reabsorption increases as well. When aldosterone is present, less urine is produced, and it is more concentrated. Natriuretic peptides inhibit sodium transporters, preventing sodium ions from being reabsorbed into the blood in the DCT. This action increases the elimination of sodium in the urine. The presence of ADH at the distal convoluted tubule and the collecting duct results in increased numbers of aquaporins (water channels) to be inserted into the cell membranes in the DCT and collecting duct. This allows for the reabsorption of water as the collecting duct descends to the highly concentrated regions of the renal medulla, resulting in the production of a highly concentrated urine.
Low fluid volume and increased osmotic pressure stimulate hormonal responses, which act on the kidneys to decrease the rate of urine production, resulting in an increase in circulating blood volume and decrease in osmotic pressure.