reabsorption

Examples of reabsorption in the following topics:

• Tubular Reabsorption

  • Tubular reabsorption is the process by which solutes and water are removed from the tubular fluid and transported into the blood.
  • Reabsorption is a finely tuned process that is altered in maintaining homeostasis of blood volume, blood pressure, plasma osmolarity, and blood pH.
  • Reabsorption in the nephron may be either a passive or active process, and the specific permeability of the each part of the nephron varies considerably in terms of amount and type of substance reabsorbed.
  • The mechanisms of reabsorption into the peri-tubular capillaries include:
  • Cotransport-this process is particularly important for reabsorption of water.

• Bulk Flow: Filtration and Reabsorption

  • When moving from the interstitium into the bloodstream, the process is termed reabsorption and is favored by blood oncotic pressure and interstitial fluid hydrostatic pressure.
  • Tubular reabsorption is the process by which solutes and water are removed from the tubular fluid and transported into the blood.
  • Reabsorption is a two-step process beginning with the active or passive extraction of substances from the tubule fluid into the renal interstitium, and then the transport of these substances from the interstitium into the bloodstream

• Overview of Urine Formation

  • Urine formation occurs during three processes: filtration, reabsorption, and secretion.
  • The next step is reabsorption, during which molecules and ions will be reabsorbed back into the circulatory system.

• Tubular Secretion

  • Tubular secretion is the transfer of materials from peritubular capillaries to renal tubular lumen, and is the opposite process of reabsorption.
  • Urine is the substance leftover in the collecting duct following reabsorption and secretion.
  • The mechanisms by which secretion occurs are similar to those of reabsorption, however these processes occur in the opposite direction.
  • Renal secretion is mainly different from reabsorption because it deals with filtering and cleaning substances from the blood, rather than retaining them.
  • Urine that is formed via the three processes of filtration, reabsorption, and secretion, leaves the kidney through the ureter, and is stored in the bladder before being removed through the urethra.

• Kidney Function and Physiology

  • Most of the solutes are reabsorbed in the PCT by a process called tubular reabsorption.
  • The formation of urine occurs through three steps: glomerular filtration, tubular reabsorption, and tubular secretion.
  • Tubular reabsorption occurs in the PCT part of the renal tubule.
  • Reabsorption of water and key electrolytes are regulated and influenced by hormones.
  • Additional solutes and wastes are secreted into the kidney tubules during tubular secretion, which is the opposite process to tubular reabsorption.

• Renal Clearance

  • Clearance is a function of glomerular filtration, secretion from the peritubular capillaries to the nephron, and reabsorption from the nephron back to the peritubular capillaries.
  • Increased tubular reabsorption will decrease clearance, while increased tubular reabsorption will increase clearance.

• Other Hormonal Controls for Osmoregulation

  • Aldosterone also stimulates potassium secretion concurrently with sodium reabsorption.
  • It acts by inserting aquaporins in the collecting ducts, promoting reabsorption of water.
  • ANP also prevents sodium reabsorption by the renal tubules, decreasing water reabsorption (thus acting as a diuretic) and lowering blood pressure.

• Regulation of Water Output

  • If the body is becoming fluid-deficient, there will be an increase in the secretion of these hormones, causing water to be retained by the kidneys through increased tubular reabsorption and urine output to be reduced.
  • ADH causes the walls of the distal convoluted tubule and collecting duct to become permeable to water, drastically increasing the amount of water that is reabsorbed during tubular reabsorption.
  • This causes greatly increased reabsorption of sodium and water (which follows sodium osmotically by cotransport) while causing the secretion of potassium into urine.
  • Aldosterone therefore increases water reabsorption, however it involves an exchange of sodium and potassium, which ADH reabsoption regulation does not involve.

• Hormonal Regulation of the Excretory System

  • As ADH (which is also known as vasopressin) causes direct water reabsorption from the kidney tubules, salts and wastes are concentrated in what will eventually be excreted as urine.
  • In contrast to ADH, which promotes the reabsorption of water to maintain proper water balance, aldosterone maintains proper water balance by enhancing Na+ reabsorption and K+ secretion from extracellular fluid of the cells in kidney tubules.
  • The reabsorption of Na+ also results in the osmotic reabsorption of water, which alters blood volume and blood pressure.
  • Angiotensin II functions as a hormone, causing the release of the hormone aldosterone by the adrenal cortex, resulting in increased Na+ reabsorption, water retention, and an increase in blood pressure.

• Nephron, Parts, and Histology

  • The proximal tubule is the first site of water reabsorption into the bloodstream, and the site where the majority of water and salt reabsorption takes place.
  • Water reabsorption in the proximal convoluted tubule occurs due to both passive diffusion across the basolateral membrane, and active transport from Na+/K+/ATPase pumps which actively transports sodium across the basolateral membrane.
  • Fluid leaving this tubule generally unchanged due to equivalent water and ion reabsorption, with an osmolarity (ion concentration) of 300 mOSm/L, which is the same osmolarity as normal plasma osmolarity.
  • The distal convoluted tubule and collecting duct is the final site of reabsorption in the nephron.
  • However, anti-diuretic hormone (secreted from pituitary gland as a part of homeostasis) will act on the distal convoluted tubule to increase the permeability of the tubule to water, increasing water reabsorption.