Reabsorption

Not to be confused with Resorption.

In renal physiology, reabsorption, more specifically tubular reabsorption, is the process by which the nephron removes water and solutes from the tubular fluid (pre-urine) and returns them to the circulating blood.^[1] It is called reabsorption (and not absorption) because these substances have already been absorbed once (particularly in the intestines) and the body is reclaiming them from a postglomerular fluid stream that is on its way to becoming urine (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 transport from the lumen into the blood by the Na⁺/K⁺ATPase in the basolateral membrane of the epithelial cells. Thus, the glomerular filtrate becomes more concentrated, which is one of the steps in forming urine. Nephrons are divided into five segments, with different segments responsible for reabsorbing different substances.^[2] Reabsorption allows many useful solutes (primarily glucose and amino acids), salts and water that have passed through Bowman's capsule, 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 through cotransport channels driven by the sodium gradient.

Locations of secretion and reabsorption in the nephron

Renin–angiotensin system:

1. The kidneys sense low blood pressure.
2. Release renin into the blood.
3. Renin causes production of angiotensin I.
4. Angiotensin-converting enzyme (ACE) converts angiotensin I to angiotensin II.
5. Angiotensin II stimulates the release of aldosterone, ADH, and thirst.
6. Aldosterone causes kidneys to reabsorb sodium; ADH increases the uptake of water.
7. Water follows sodium.
8. As blood volume increases, pressure also increases.

The bladder is able to separately reabsorb water and solutes such as drugs.^[3] This mechanism is not affected by anticholingeric drugs, unlike renal reabsorption.^[4] This mechanism also does not involve arginine vasopressin.^[5] In fully hydrated frogs, the bladder plays a significant role in reabsorbing water and electrolytes.^[6] The pig urothelium expresses AQP3, AQP9, and AQP11.^[7]

See also

• Transepithelial potential difference driving reabsorption

References

1. Skirving, Mya; Borger, Pam; Chiovtti, Tony; Duncan, Jacinta; Gerdtz, Wayne; Guay, Patrick-Jean; Martin, Genevieve; Walker, Katrina; Woolnough, Jim; Wright, Jane (2020). "Chapter 11: Regulation of Water, Salts and Gases". In Attley, Teresa; Irwin, Kirstie (eds.). Biology WA ATAR Units 3&4 (1st ed.). South Melbourne, Victoria, Australia: Cengage Learning Australia. pp. 374, 402. ISBN 9780170452922.
2. "Tubular reabsorption article (article)". Khan Academy. Retrieved 2022-03-17.
3. Dalton, JT; Weintjes, MG; Au, JL (June 1994). "Effects of bladder resorption on pharmacokinetic data analysis". Journal of pharmacokinetics and biopharmaceutics. 22 (3): 183–205. doi:10.1007/BF02353328. PMID 7884649.
4. Oe, Hideki; Yoshiki, Hatsumi; Zha, Xinmin; Kobayashi, Hisato; Aoki, Yoshitaka; Ito, Hideaki; Yokoyama, Osamu (28 April 2021). "Urinary reabsorption in the rat kidney by anticholinergics". Scientific Reports. 11 (1). doi:10.1038/s41598-021-88738-y.
5. Morizawa, Yosuke; Torimoto, Kazumasa; Miyake, Makito; Hori, Shunta; Gotoh, Daisuke; Tatsumi, Yoshinori; Nakai, Yasushi; Onishi, Sayuri; Tanaka, Nobumichi; Watanabe, Hiroki; Fujimoto, Kiyohide (12 September 2017). "Role of the Urinary Bladder in Water Metabolism—How Does the Bladder Absorb Urine?". ics.org.
6. Sinsch, Ulrich (January 1991). "Reabsorption of water and electrolytes in the urinary bladder of intact frogs (genus Rana)". Comparative Biochemistry and Physiology Part A: Physiology. 99 (4): 559–565. doi:10.1016/0300-9629(91)90131-U.
7. Manso, Marian (September 2019). Fluid reabsorption across pig urinary bladder (PhD (partial) thesis).