Question

Explain the three processes that are necessary for Urine formation, in which part of the nephron are these functions performed?

Answer 1

Kidneys are the major organs of the urinary system. The filtration of blood takes place in the kidneys through the nephrons. Two ureters carry the urine from the kidneys to the urinary bladder. From the bladder, urine is excreted out through the urethra.

The three basic steps in the formation of urine include the following:

1. Glomerular filtration

2. Tubular reabsorption

3. Tubular secretion.

Filtration: The movement of water and solutes from the plasma into the renal tubules is known as filtration. The major driving force of the filtration is hydrostatic pressure in the glomerulus capillaries.

Blood passes through the capillaries in the glomerulus, due to the hydrostatic pressure in the Bowman’s capsule, plasma is filtered into the Bowman's capsule except for blood proteins (this is called ultra-filtration). This filtrate is called glomerular filtrate. Hydrostatic pressure is the pressure exerted by the filtrate in Bowman's capsule (15 mm Hg).

Reabsorption:

It is the movement of water and solutes from the renal tubule into the plasma. The process of absorption mostly occurs in the proximal tubules (approximately 70% of the filtrate is reabsorbed from the proximal tubules). The process of reabsorption is driven by hydrostatic and oncotic pressure, which is strictly under hormonal regulation. Oncotic pressure is the pressure exerted by the blood proteins, which draws water into the circulation.

Secretion:

Tubular reabsorption of water and other substances back to the blood occur while the glomerular filtrate passes through the tubular portion (from the proximal tubule to the collecting duct) of the nephron. About 99% of water and electrolytes are reabsorbed during this process by both active and passive reabsorption process. During tubular secretion, substances pass from the blood into renal tubules.