Renal Physiology: Tubular Reabsorption G28- 2

Water movement through the nephron is regulated by aquaporins and tight junctions. Aquaporins are water channels present in the cell membranes, while tight junctions connect epithelial cells.

In the descending loop of Henle, water permeability is high due to the presence of aquaporins, allowing water to move freely.

The ascending loop of Henle has low water permeability, meaning little water is reabsorbed.

The distal tubule, collecting tubule, and collecting duct have variable water permeability, depending on the presence of ADH (antidiuretic hormone).

Sodium reabsorption from the tubular lumen creates an electrical gradient, causing chloride ions to move passively. Chloride ions are also reabsorbed by secondary active transport.

Urea is passively reabsorbed but to a lesser extent than chloride. The tubule is not easily permeable to urea, so only about half is reabsorbed.

Creatinine is a waste product that is not reabsorbed because the tubule is impermeable to it. Most of it is excreted in urine.

The proximal tubule reabsorbs 65% of filtered sodium and water because of its special cellular characteristics.

The late distal tubule and cortical collecting tubule are impermeable to urea and reabsorb sodium, primarily under aldosterone control. They also secrete potassium. Type A intercalated cells actively secrete hydrogen ions, while type B intercalated cells secrete bicarbonate ions. Water permeability is controlled by vasopressin.

Type A intercalated cells are responsible for secreting hydrogen ions into the tubular lumen, helping to correct acidosis.

These cells are responsible for secreting bicarbonate ions into the tubular lumen, helping to correct alkalosis.

This transporter is located on the apical membrane of intercalated cells and helps move chloride and bicarbonate ions. It becomes more active during chronic metabolic alkalosis.

The sodium-potassium pump is a major player in the transport of these ions in the collecting tubules.

Potassium diffuses down its concentration gradient from the tubular cells into the tubular lumen, primarily in the collecting tubules.

They are the final site for processing urine and play a critical role in determining the final output of water and solids. They reabsorb less than 5% of the filtered water and sodium.

It is controlled by antidiuretic hormone (ADH). High levels of ADH increase water reabsorption, concentrating the urine.

The sodium-potassium ATP pump maintains a low intracellular sodium concentration, which creates a gradient for sodium to move from the tubular fluid into the cell.

The sodium-potassium ATP pump powers the secondary active transport of a cotransporter called the sodium-potassium-chloride transporter.

The thick ascending limb of the loop of Henle is impermeable to water, meaning that water cannot pass through its walls.

Loop diuretics, such as furosemide (Lasix), block the sodium-potassium-chloride transporter, reducing sodium reabsorption and increasing urine output.

The macula densa, a specialized region of the distal tubule, monitors the sodium chloride concentration in the filtrate and communicates with the juxtaglomerular apparatus to regulate blood flow and filtration rate.

The distal tubule is referred to as the diluting segment because it actively reabsorbs sodium and chloride while remaining impermeable to water, leading to dilute urine.

The second half of the distal tubule and the cortical collecting ducts are responsible for fine-tuning the final composition of urine.

Principal cells in the distal tubule and collecting ducts reabsorb sodium and water, and secrete potassium into the tubular lumen, while intercalated cells reabsorb potassium and secrete hydrogen.

The proximal tubule is a part of the nephron responsible for reabsorbing essential nutrients, water, and electrolytes. It's characterized by a large surface area due to its brush border and extensive channels, enabling efficient transport.

Mitochondria are abundant in the proximal tubule to power active transport processes that move essential substances back into the bloodstream.

Co-transport involves moving two substances across a membrane in the same direction. The proximal tubule uses this mechanism to reabsorb glucose, amino acids, and other nutrients along with sodium ions.

The loop of Henle is a U-shaped structure in the nephron responsible for generating a concentration gradient in the kidney, enabling water reabsorption and urine concentration.

The descending limb of the loop of Henle is permeable to water but less permeable to solutes, allowing water to move out and concentrate the urine.

The ascending limb of the loop of Henle, both thin and thick, is impermeable to water, preventing water loss and allowing for the reabsorption of sodium and chloride ions.

The thick ascending limb of the loop of Henle is actively involved in reabsorbing sodium, potassium, and chloride ions, contributing to the concentration gradient and urine formation.

The process where the kidneys adjust the concentration of urine by selectively reabsorbing water and solutes.

Solutes that are not needed by the body, like creatinine and urea, are minimally reabsorbed or even secreted by the kidneys, resulting in higher concentrations in the urine.

Essential nutrients like glucose and amino acids are reabsorbed by the kidneys to a large extent, resulting in low concentrations in the urine.

Inulin, a substance used to measure GFR (glomerular filtration rate), is neither reabsorbed nor secreted by the kidneys. Its concentration changes reflect only water reabsorption.

The ability of the kidneys to independently regulate the reabsorption of different solutes, particularly through hormonal control.

The phenomenon where an increase in the amount of a substance entering the renal tubules results in an increased reabsorption rate of that substance.

The percentage of filtered substances reabsorbed remains relatively constant even as the GFR (glomerular filtration rate) increases.

Multiple nervous, hormonal, and local control mechanisms work together to regulate the precise balance between filtration and reabsorption in the kidneys.