Urine formation is a complex process that involves several steps to filter blood, remove waste products, and maintain the body's fluid and electrolyte balance.
The primary site of urine formation is the nephron, which is the functional unit of the kidney.
There are three main steps in urine formation: glomerular filtration, tubular reabsorption, and tubular secretion.
The physiology of urine formation can be summarized in the following points:
1. Glomerular filtration:
Blood is filtered in the glomerulus, forcing water, electrolytes, and small molecules (e.g., glucose, amino acids, urea) into the Bowman's capsule, forming the glomerular filtrate.
2. Tubular reabsorption:
Essential substances and water are reabsorbed from the filtrate back into the bloodstream as it flows through the renal tubule.
A. Proximal convoluted tubule (PCT): Majority of water, glucose, amino acids, and ions are reabsorbed.
B. Loop of Henle: Water is reabsorbed in the descending limb, while sodium and chloride ions are reabsorbed in the ascending limb.
C. Distal convoluted tubule (DCT): Additional ions and water are reabsorbed; regulated by hormones like aldosterone and ADH.
3. Tubular secretion:
Waste products and ions are actively transported from the bloodstream into the renal tubule, where they become part of the urine.
This occurs primarily in the PCT and DCT.
4. Urine concentration and volume adjustment:
The final concentration and volume of urine are regulated in the collecting ducts under the influence of ADH.
5. Urine transport and elimination:
Urine flows from the collecting ducts to the renal calyces, renal pelvis, and ureter, which transports it to the urinary bladder for storage.
It is then eliminated from the body through the urethra during urination.