The kidney participates in whole-body homeostasis, regulating acid-base balance, electrolyte concentrations, extracellular fluid volume, and regulation of blood pressure.
The kidney accomplishes these homeostatic functions both independently
and in concert with other organs, particularly those of the endocrine system. Various endocrine hormones coordinate these endocrine functions; these include renin, angiotensin II, aldosterone, antidiuretic hormone, and atrial natriuretic peptide, among others.
Many of the kidney's functions are accomplished by relatively simple
mechanisms of filtration, reabsorption, and secretion, which take place
in the Nephron. Filtration, which takes place at the renal corpuscle,
is the process by which cells and large proteins are filtered from the
blood to make an ultrafiltrate that eventually becomes urine. The kidney
generates 180 liters of filtrate a day, while reabsorbing a large
percentage, allowing for the generation of only approximately 2 liters
of urine. Reabsorption is the transport of molecules from this
ultrafiltrate and into the blood. Secretion is the reverse process, in
which molecules are transported in the opposite direction
BASICAL MUST KNOW FUNCTION OF THE KIDNEY
The kidneys excrete a variety of waste products produced by metabolism. These include the nitrogenous wastes called "urea", from protein catabolism, as well as uric acid, from nucleic acid
metabolism. Formation of urine is also the function of the kidney. The
concentration of nitrogenous wastes in the urine of mammals and some
birds is dependent on an elaborate countercurrent multiplication
system. This requires several independent nephron characteristics to
operate: a tight hair pin configuration of the tubules, water and ion
permeability in the descending limb of the loop, water impermeability in
the ascending loop and active ion transport out of most of the
ascending loop. In addition, countercurrent exchange by the vessels carrying the blood supply to the nephron is essential for enabling this function.
Two organ systems, the kidneys and lungs, maintain acid-base homeostasis, which is the maintenance of pH around a relatively stable value. The lungs contribute to acid-base homeostasis by regulating carbonic gas (CO2)
concentration. The kidneys have two very important roles in maintaining
the acid-base balance: to reabsorb bicarbonate from urine, and to
excrete hydrogen ions into urine
Any significant rise in plasma osmolality is detected by the hypothalamus, which communicates directly with the posterior pituitary gland. An increase in osmolality causes the gland to secrete antidiuretic hormone
(ADH), resulting in water reabsorption by the kidney and an increase in
urine concentration. The two factors work together to return the plasma
osmolality to its normal levels.
ADH binds to principal cells in the collecting duct that translocate
aquaporins to the membrane, allowing water to leave the normally
impermeable membrane and be reabsorbed into the body by the vasa recta,
thus increasing the plasma volume of the body.
There are two systems that create a hyperosmotic medulla and thus
increase the body plasma volume: Urea recycling and the 'single effect.'
Urea is usually excreted as a waste product from the kidneys.
However, when plasma blood volume is low and ADH is released the
aquaporins that are opened are also permeable to urea. This allows urea
to leave the collecting duct into the medulla creating a hyperosmotic
solution that 'attracts' water. Urea can then re-enter the nephron and
be excreted or recycled again depending on whether ADH is still present
or not.
The 'Single effect' describes the fact that the ascending thick limb of the loop of Henle is not permeable to water but is permeable to NaCl. This allows for a countercurrent exchange
system whereby the medulla becomes increasingly concentrated, but at
the same time setting up an osmotic gradient for water to follow should
the aquaporins of the collecting duct be opened by ADH.
Blood pressure regulation
Long-term regulation of blood pressure predominantly depends upon the kidney. This primarily occurs through maintenance of the extracellular fluid compartment, the size of which depends on the plasma sodium
concentration. Although the kidney cannot directly sense blood. Renin
is the first in a series of important chemical messengers that comprise
the renin-angiotensin system. Changes in renin ultimately alter the output of this system, principally the hormones angiotensin II and aldosterone.
Each hormone acts via multiple mechanisms, but both increase the
kidney's absorption of sodium chloride, thereby expanding the
extracellular fluid compartment and raising blood pressure. When renin
levels are elevated, the concentrations of angiotensin II and
aldosterone increase, leading to increased sodium chloride reabsorption,
expansion of the extracellular fluid compartment, and an increase in
blood pressure. Conversely, when renin levels are low, angiotensin II
and aldosterone levels decrease, contracting the extracellular fluid
compartment, and decreasing blood pressure.
The kidneys secrete a variety of hormones, including erythropoietin, and the enzyme renin. Erythropoietin is released in response to hypoxia (low levels of oxygen at tissue level) in the renal circulation. It stimulates erythropoiesis (production of red blood cells) in the bone marrow. Calcitriol, the activated form of vitamin D, promotes intestinal absorption of calcium and the renal reabsorption of phosphate. Part of the renin-angiotensin-aldosterone system, renin is an enzyme involved in the regulation of aldosterone levels..
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Regulates Salt Level in Human Also
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