Most of the water and electrolytes such as sodium, potassium, and chloride are reabsorbed

A small portion of urea is reabsorbed

Creatinine is not reabsorbed at all

The proximal convoluted tubule has the strongest reabsorption capacity (brush border) and reabsorbs a large amount and variety of substances.

The medullary loops mainly reabsorb water and NaCl

The distal convoluted tubule and collecting duct also have the function of reabsorbing Na⁺ and water (the amount of absorption is less than that of the proximal convoluted tubule and is regulated by vasopressin and aldosterone)

Renal tubules and collecting ducts have certain limits on the reabsorption of different substances.

mechanism

thin segment of descending branch of medullary loop

Thin segment of ascending branch of medullary loop

Thick segment of ascending branch of medullary loop

distal convoluted tubule

Posterior distal tubule and collecting duct

(Na⁺-H⁺ exchanger, Na⁺-K⁺ exchanger) (Na⁺ reabsorption is mainly regulated by aldosterone; water reabsorption depends on the water content in the body, mainly through transcellular pathways, and is regulated by vasopressin )

80%-90% is reabsorbed in the proximal tubule (Na⁺-H⁺)

10% is reabsorbed in the thick ascending branch of the medullary loop

A small amount is reabsorbed in the distal tubule and collecting duct

On the luminal membrane

basolateral membrane

It enters the cells as CO₂ and enters the intercellular fluid through the transporter on the basolateral membrane along the electrochemical concentration.

65%-70% is reabsorbed in the proximal tubule

25%-30% is reabsorbed in the thick ascending branch of the medullary loop

K⁺ in final urine is mainly secreted by the distal convoluted tubule and collecting duct

Active transport, transport against electrochemical gradient (the mechanism is not yet clear)

parts

mechanism

Same as glucose, but with multiple amino acid transporters

The small amount of protein that normally enters the ultrafiltrate is reabsorbed through the phagocytosis function of the proximal tubule epithelial cells.

Rhythm cells secrete H⁺. There is a proton pump (H⁺-ATPase) on the luminal membrane of the distal convoluted tubule and collecting duct, which can pump intracellular H⁺ into the tubule fluid.

Distal tubule and collecting duct

The secretion of K⁺ by the second half of the distal tubule and collecting duct chief cells (accounting for 90% of epithelial cells) is closely related to the active reabsorption of Na⁺. Only with the reabsorption of Na⁺ can there be secretion of K⁺ (Na⁺- K⁺ exchange)

The Na pump on the basement membrane side pumps Na⁺ out of the cell and at the same time pumps K⁺ in the interstitial fluid into the cell, making the K⁺ concentration in the epithelial cells higher than the K⁺ ion concentration in the tubular fluid.

The concentration difference causes K⁺ to enter the small tube liquid along the concentration gradient.

Active reabsorption of Na⁺ brings the tubular fluid to a negative potential (K⁺ enters the tubular fluid along the electrochemical gradient)

There is H⁺-Na⁺ and K⁺-Na⁺ exchange on the renal tubular epithelial cells, and the two are in a competitive relationship

Proximal tubule, thick ascending branch of medullary loop, distal tubule and collecting duct

proximal tubule

collecting tube

2/3 of free uric acid is excreted by the kidneys and 1/3 by the intestines

Mode Glomerular filtration and tubular secretion

Improve the water permeability of distal convoluted tubule and collecting duct epithelial cells, promote water reabsorption, concentrate urine, and reduce urine output.

Plasma crystalloid osmolality (the most important factor) "water diuresis"

Changes in circulating blood volume

"Bao Na⁺ row K⁺"

RAAS

Concentrations of Na⁺ and K⁺ in blood →Zona glomerulosa of adrenal cortex

effect