The kidney regulates fluid balance through several mechanisms:
- Filtration: The kidney filters blood, removing waste products and excess fluid in the form of urine. This process is controlled by the glomerulus, which acts as a sieve to selectively filter substances based on their size and charge.
- Reabsorption: After filtration, the filtrate travels through the renal tubules where various substances, including water and electrolytes, are selectively reabsorbed back into the bloodstream. The amount of water that is reabsorbed depends on the body’s hydration status and the levels of certain hormones such as antidiuretic hormone (ADH) and aldosterone.
- Secretion: The kidney also secretes certain solutes, such as hydrogen ions and potassium ions, into the urine. This helps to maintain electrolyte balance in the body.
- Regulation of blood pressure: The kidney plays a crucial role in regulating blood pressure. When blood pressure drops, special cells in the kidney sense the low pressure and release the enzyme renin. Renin stimulates a series of reactions that ultimately lead to the production of angiotensin II, a hormone that constricts blood vessels and helps to raise blood pressure. In addition, the kidney also produces a peptide hormone called atrial natriuretic peptide (ANP), which promotes relaxation of blood vessels and excretion of sodium and water, thus lowering blood pressure.
Overall, the kidney continuously monitors and adjusts the filtration, reabsorption, and secretion of fluid and electrolytes to maintain fluid balance and blood pressure within a narrow range.
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