renal physiology Renal physiology (Latin language, Latin ''renes'', "kidneys") is the study of the physiology of the kidney. This encompasses all functions of the kidney, including maintenance of acid-base balance; regulation of fluid balance; regulation of sodiu ...

, reabsorption, more specifically tubular reabsorption, is the process by which the

nephron The nephron is the minute or microscopic structural and functional unit of the kidney. It is composed of a renal corpuscle and a renal tubule. The renal corpuscle consists of a tuft of capillaries called a glomerulus and a cup-shaped structu ...

removes water and solutes from the tubular fluid (pre-urine) and returns them to the circulating blood. It is called ''reabsorption'' (and not ''absorption'') because these substances have already been absorbed once (particularly in the

intestine The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The tract is the largest of the body's systems, after the cardiovascular system. T ...

s) and the body is reclaiming them from a post glomerular fluid stream that is on its way to becoming

urine Urine is a liquid by-product of metabolism in humans and many other animals. In placental mammals, urine flows from the Kidney (vertebrates), kidneys through the ureters to the urinary bladder and exits the urethra through the penile meatus (mal ...

(that is, they will soon be lost to the urine unless they are reabsorbed from the tubule into the peritubular capillaries). This happens as a result of

sodium Sodium is a chemical element; it has Symbol (chemistry), symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the peri ...

transport from the lumen into the

blood Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood is com ...

by the Na⁺/K⁺ATPase in the basolateral membrane of the

epithelial cell Epithelium or epithelial tissue is a thin, continuous, protective layer of Cell (biology), cells with little extracellular matrix. An example is the epidermis, the outermost layer of the skin. Epithelial (Mesothelium, mesothelial) tissues line ...

s. Thus, the glomerular filtrate becomes more concentrated, which is one of the steps in forming

. Nephrons are divided into five segments, with different segments responsible for reabsorbing different substances. Reabsorption allows many useful solutes (primarily

glucose Glucose is a sugar with the Chemical formula#Molecular formula, molecular formula , which is often abbreviated as Glc. It is overall the most abundant monosaccharide, a subcategory of carbohydrates. It is mainly made by plants and most algae d ...

and

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...

s),

salt In common usage, salt is a mineral composed primarily of sodium chloride (NaCl). When used in food, especially in granulated form, it is more formally called table salt. In the form of a natural crystalline mineral, salt is also known as r ...

s and

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

that have passed through

Bowman's capsule Bowman's capsule (or the Bowman capsule, capsula glomeruli, or glomerular capsule) is a cup-like sac at the beginning of the Renal tubule, tubular component of a nephron in the mammalian kidney that performs the first step in the filtration of b ...

, to return to the circulation. These solutes are reabsorbed isotonically, in that the osmotic potential of the fluid leaving the proximal convoluted tubule is the same as that of the initial glomerular filtrate. However, glucose, amino acids, inorganic phosphate, and some other solutes are reabsorbed via secondary

active transport In cellular biology, active transport is the movement of molecules or ions across a cell membrane from a region of lower concentration to a region of higher concentration—against the concentration gradient. Active transport requires cellula ...

through cotransport channels driven by the sodium gradient.

Renin–angiotensin system The renin–angiotensin system (RAS), or renin–angiotensin–aldosterone system (RAAS), is a hormone system that regulates blood pressure, fluid, and electrolyte balance, and systemic vascular resistance. When renal blood flow is reduced, ...

: # The kidneys sense low blood pressure. # Release renin into the blood. # Renin causes production of angiotensin I. #

Angiotensin-converting enzyme Angiotensin-converting enzyme (), or ACE, is a central component of the renin–angiotensin system (RAS), which controls blood pressure by regulating the volume of fluids in the body. It converts the hormone angiotensin I to the active vasocon ...

(ACE) converts angiotensin I to

angiotensin II Angiotensin is a peptide hormone that causes vasoconstriction and an increase in blood pressure. It is part of the renin–angiotensin system, which regulates blood pressure. Angiotensin also stimulates the release of aldosterone from the ...

. # Angiotensin II stimulates the release of

aldosterone Aldosterone is the main mineralocorticoid steroid hormone produced by the zona glomerulosa of the adrenal cortex in the adrenal gland. It is essential for sodium conservation in the kidney, salivary glands, sweat glands, and colon. It plays ...

, ADH, and thirst. # Aldosterone causes kidneys to reabsorb sodium; ADH increases the uptake of water. # Water follows sodium. # As blood volume increases, pressure also increases. The bladder is able to separately reabsorb water and solutes such as drugs. This mechanism is not affected by anticholingeric drugs, unlike renal reabsorption. This mechanism also does not involve ADH. In fully hydrated frogs, the bladder plays a significant role in reabsorbing water and electrolytes. The pig urothelium expresses AQP3, AQP9, and AQP11.

References

{{Reflist Renal physiology

See also

References