proximal tubule

(noun)

The first and primary sort of water and ion reabsorption in the kidney, where all glucose in the blood is reabsorbed.

Related Terms

Examples of proximal tubule in the following topics:

  • Glucosuria

    • The proximal tubule can only reabsorb a limited amount of glucose.
    • When the blood glucose level exceeds about 160 – 180 mg/dl, the proximal tubule becomes overwhelmed and begins to excrete glucose in the urine.
    • However, in those with renal glycosuria, glucose is abnormally elevated in the urine due to improper functioning of the renal tubules, which are the primary components of the nephrons that act as the filtering units of the kidneys.

  • 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 that actively transports sodium across the basolateral membrane.
    • Approximately 2/3rds of water in the nephron and 100% of the glucose in the nephron are reabsorbed by cotransport in the proximal convoluted tubule.
    • It transfers fluid from the proximal to the distal tubule.
    • Proximal tube, 5.

  • Nephron: The Functional Unit of the Kidney

    • A nephron consists of three parts: a renal corpuscle, a renal tubule, and the associated capillary network, which originates from the cortical radiate arteries.
    • The renal tubule is a long, convoluted structure that emerges from the glomerulus.
    • The first part is called the proximal convoluted tubule (PCT), due to its proximity to the glomerulus.
    • The third part of the renal tubule is called the distal convoluted tubule (DCT); this part is also restricted to the renal cortex.
    • The glomerulus and convoluted tubules of the nephron are located in the cortex of the kidney, while the collecting ducts are located in the pyramids of the kidney's medulla.

  • Tubular Reabsorption

    • The filtrate entering the proximal convoluted tubule is 300 mOsm/L, which is the same osmolarity as normal plasma osmolarity.
    • In the proximal convoluted tubules, all the glucose in the filtrate is reabsorbed, along with an equal concentration of ions and water (through cotransport), so that the filtrate is still 300 mOsm/L as it leaves the tubule.
    • Finally, in the distal convoluted tubule and collecting duct, a variable amount of ions and water are reabsorbed depending on hormonal stimulus.
    • The final osmolarity of urine is therefore dependent on whether or not the final collecting tubules and ducts are permeable to water or not, which is regulated by homeostasis.

  • Chemical Composition of Urine

    • Glycosuria—Glucose in urine, due to excess plasma glucose in diabetes, beyond the amount able to be reabsorbed in the proximal convoluted tubule.

  • Regulation of Urine Concentration and Volume

    • ADH increases the permeability to water of the distal convoluted tubule and collecting duct, which are normally impermeable to water.
    • As the fluid flows along the proximal convoluted tubule useful substances like glucose, water, salts, potassium ions, calcium ions, and amino acids are reabsorbed into the blood capillaries that form a network around the tubules.

  • Internal Anatomy of the Kidneys

    • After entering the capsule, the filtered fluid flows along the proximal convoluted tubule to the loop of Henle and then to the distal convoluted tubule and the collecting ducts, which flow into the ureter.

  • Diuretics

    • Thiazide-type diuretics such as hydrochlorothiazide act on the distal convoluted tubule and inhibit the sodium-chloride symporter leading to a retention of water in the urine, as water normally follows penetrating solutes.
    • Frequent urination is due to the increased loss of water that has not been retained from the body as a result of a concomitant relationship with sodium loss from the convoluted tubule.
    • Carbonic anhydrase inhibitors inhibit the enzyme carbonic anhydrase which is found in the proximal convoluted tubule.
    • Aldosterone normally adds sodium channels in the principal cells of the collecting duct and late distal tubule of the nephron.

  • Peripheral Motor Endings

    • The action potential spreads through the muscle fiber's network of T-tubules, depolarizing the inner portion of the muscle fiber.
    • The depolarization activates L-type, voltage-dependent calcium channels (dihydropyridine receptors) in the T-tubule membrane, which are in close proximity to calcium-release channels (ryanodine receptors) in the adjacent sarcoplasmic reticulum.

  • Malpighian Tubules of Insects

    • Malpighian tubules remove wastes from insects by producing urine and solid nitrogenous waste, which are then excreted from the body.
    • Malpighian tubules line the gut of some species of arthropods, such as bees.
    • The system of malpighian tubules consists of branching tubules, which increase their surface area, near the hemolymph (a mixture of blood and interstitial fluid that is found in insects, other arthropods, and most mollusks) and fat tissues.
    • Malpighian tubules work cooperatively with specialized glands in the wall of the rectum .
    • Explain how insects use malpighian tubules to excrete wastes and maintain osmotic balance