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6.2.2. Urine formation

Urine formation

The blood carries waste products to the kidney. The urine is a liquid obtained from the blood, composed mainly of water, minerals and excretion products, such as urea and uric acid.

The urine formation process follows the following stages:

Filtration

Blood vessels leading to the nephron form the glomerulus of Malpighi, a microscopic, ball-shaped capillary system surrounded by Bowman's capsule. The blood that reaches the nephrons is under great pressure, and water, glucose, vitamins, amino acids, sodium, potassium, chlorides, urea and other salts leave these capillaries, which pass into Bowman's capsule. 20% of the blood plasma that reaches the nephron is filtered, about 150 liters of primary urine per day. Logically, an organism that loses so much water would dehydrate very quickly, so it cannot afford it.

Reabsorption

In the filtration, waste substances, but also a lot of water and other useful substances, have passed into the Bowman capsule, which are reabsorbed and returned to the blood.

In the proximal convoluted tubule it reabsorbs glucose, amino acids, sodium, chloride, potassium and other substances. Here, approximately 65% ​​of the filtrate is reabsorbed. The remainder is reabsorbed in the loop of Henle and the distal convoluted tubule. The toxic urea cannot escape from the tubules.

With reabsorption, a large part of the water and the filtered useful substances are recovered, leaving only 1.5 liters of urine per day to be reabsorbed, which is directed towards the renal pelvis.

Secretion

It consists of the passage of some substances that have not been filtered, or have been erroneously reabsorbed, from the capillaries that surround the distal convoluted tubule towards its interior. Here some substances such as penicillin, potassium and hydrogen are secreted, which are added to the urine that is being formed.

Thus, this final liquid, urine, will be made up of water, some salts, and urea, and will pass through the collecting tubules to the renal pelvis, and from there, through the ureters, to the urinary bladder.

Answer in your notebook

6.5.- Name all the parts through which a urea molecule passes from when it reaches the kidney until it goes outside.

Answer in your notebook

6.6.- If urine comes from blood, why is it not red or does it have blood cells?

Answer in your notebook

6.7.- How is the blood that goes through the renal artery different from that which goes through the renal vein?