The structure of the nephron. The mechanism of urine formation

The nephron is a structural unit of the kidney where blood is filtered and urine is produced.
Each kidney contains approximately 1 million nephrons.

Nephron structure

In the cortical layer of the kidney there is a renal capsule (nephron capsule), inside which there is a capillary glomerulus of the convoluted tubule.

In the medulla (pyramidal) layer are convoluted tubules. The tubules form common collecting ducts that drain into the renal pelvis.
From the renal pelvis of each kidney, there is a ureter that connects the kidney to the bladder.

A convoluted tubule of the first order (proximal convoluted tubule) departs from the capsule, which forms a loop in the medulla of the kidney (loop of Henle), then it rises again into the cortical layer, where it passes into a convoluted tubule of the second order (distal convoluted tubule). This tubule flows into the collecting duct of the nephron. All collecting ducts form excretory ducts that open at the tops of the pyramids in the medulla of the kidney.

The afferent renal artery splits into arterioles and then into capillaries, forming the glomerulus of the renal capsule.

The capillaries collect in the efferent arteriole, which again breaks up into a network of capillaries that encircle the convoluted tubules.

The capillaries then form the veins that carry blood to the renal vein.

Urine formation

Urine is formed in the kidneys from blood, which is well supplied to the kidneys. Urine formation takes place in two stages – filtration and reabsorption (reabsorption).

At the first stage, blood plasma is filtered through the capillaries of the Malpighian glomerulus into the cavity of the nephron capsule.

In the capillaries of the glomeruli, the blood pressure is high. Therefore, water and molecules of substances dissolved in plasma are filtered through the thin walls of the capillaries and enter the renal tubule. The resulting filtrate is called primary urine. In composition, it is similar to blood plasma, but does not contain proteins. Primary urine contains both metabolic products (urea and uric acid) and substances necessary for the body (glucose, amino acids, vitamins, etc.).

In the convoluted tubules, primary urine is reabsorbed into the bloodstream and secondary (final) urine is formed. Water, amino acids, carbohydrates, vitamins, and some salts are reabsorbed into the bloodstream.

In secondary urine, the content of urea (65 times) and uric acid (12 times) increases by several tens of times, compared with primary urine. The concentration of potassium ions increases 7 times. The amount of sodium remains practically unchanged.

About 150 liters of primary urine and about 1.5 liters of secondary urine per day are formed per day, which is approximately 1% of the volume of primary urine. Thus, the substances necessary for the body are returned to the blood, and the unnecessary ones are excreted.

Secondary urine enters the renal pelvis from the tubules, and then flows through the ureters into the bladder and is discharged through the urethra.

Regulation of kidney function

Kidney activity is regulated by a neurohumoral mechanism.

Nervous regulation. The blood vessels contain osmo- and chemoreceptors that transmit information about blood pressure and fluid composition to the hypothalamus along the pathways of the autonomic nervous system.

Humoral regulation of kidney activity is carried out by hormones of the pituitary gland, adrenal cortex, parathyroid glands.
A sign of kidney disease is the presence of protein, sugar in the urine, an increase in the number of leukocytes or erythrocytes in the blood.

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