Name: Excretion in Human Beings
Uploaded: 2019-07-19
Duration: 2 min 52 s
Description: Kidneys eliminate the toxic nitrogenous wastes generated in the body as a result of metabolism. The liver, too, plays a crucial role in this. Take a sneak peek.

Question

Briefly state the mechanism of urine formation in human kidney.

Accepted Answer

There are three stages involved in the process of urine formation. They are glomerular filtration or ultra-filtration, selective reabsorption, and tubular secretion.

1. Glomerular filtration: This occurs through the semipermeable walls of the glomerular capillaries and Bowman’s capsule.

The afferent arterioles supplying blood to the glomerular capsule carry a mixture of useful and harmful substances.

The useful substances include glucose, amino acids, and the harmful substances are metabolic wastes like urea, etc.

The diameter of efferent arterioles is narrower than afferent arterioles. Due to the difference in diameter of arteries, blood leaving the glomerulus creates hydrostatic pressure.

Water and a few small molecules pass through the filtration slits but blood cells, plasma proteins, and other large molecules are too large to be filtered and remain in the capillaries.

The filtrate containing water, glucose, amino acids, uric acid, urea, electrolytes, etc in the glomerular capsule is known as the glomerular filtrate.

The volume of filtrate formed by both the kidneys each minute is called the glomerular filtration rate.

2. Selective reabsorption: As the filtrate passes to the renal tubules, substances like water, electrolytes, and organic nutrients such as glucose, amino acids, etc are selectively reabsorbed from the filtrate back into the blood in the PCT, DCT, loop of Henle, and collecting duct.

Reabsorption of some molecules and ions are passive (negatively charged ions), while some are actively transported (glucose, amino acids, positively charged ions). Most water gets reabsorbed by osmosis and small proteins get reabsorbed by pinocytosis.

Only 60–70% of the filtrate enters the loop of Henle. Water, sodium, and chloride, are reabsorbed in the loop so that only 15–20% of the initial filtrate enters the DCT. Most electrolytes get reabsorbed here, especially Na⁺. So the filtrate entering the collecting ducts is dilute.

3. Tubular secretion: This occurs from the blood in the peritubular capillaries to the filtrate in the renal tubules (PCT and DCT). This ensures that wastes such as creatinine or excess H⁺ or K⁺ ions are actively secreted into the filtrate and excreted.

Excess K⁺ ions are secreted in the tubules while Na⁺ ions get reabsorbed.

Tubular secretion of hydrogen ions (H⁺) is a significant step in maintaining normal blood pH.

The tubular filtrate is finally known as urine. Human urine is usually hypertonic.

Briefly state the mechanism of urine formation in human kidney.

Important Questions on Excretory Products and Their Elimination

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