Physiology of Urine Formation:
Introduction
The main function of the kidneys is to remove waste products from the blood and regulate fluid and electrolyte balance.
This is achieved through the process of urine formation, which occurs in the nephrons, the structural and functional units of the kidneys.
Each kidney contains about one million nephrons, and each nephron performs filtration, reabsorption, and secretion to form urine.
Original Notes:
Steps in Urine Formation
Urine formation takes place in three main stages:
- Glomerular Filtration
- Tubular Reabsorption
- Tubular Secretion
1. Glomerular Filtration
Definition:
It is the process by which blood is filtered in the glomerulus of the nephron to form a fluid called glomerular filtrate.
Process:
- Blood enters the glomerulus through the afferent arteriole and leaves through the efferent arteriole.
- The blood pressure in the glomerulus forces water and small solutes through the thin walls of capillaries into Bowman’s capsule.
- Large molecules like proteins and blood cells are too big to pass through and remain in the blood.
Filtration Rate:
- The total amount of filtrate formed per minute in both kidneys is called Glomerular Filtration Rate (GFR).
- Normal GFR = 125 mL/min or about 180 liters/day.
Filtrate Composition:
- Contains water, glucose, amino acids, salts, urea, and uric acid.
2. Tubular Reabsorption
Definition:
It is the process by which useful substances are reabsorbed from the filtrate back into the blood through renal tubules.
Process:
- As the filtrate passes through the proximal convoluted tubule (PCT), Loop of Henle, distal convoluted tubule (DCT), and collecting duct, essential substances are selectively reabsorbed.
- Water, glucose, amino acids, sodium, and chloride ions are reabsorbed into the peritubular capillaries.
Major Sites of Reabsorption:
- PCT: Reabsorbs about 65% of filtered water and sodium, along with glucose and amino acids.
- Loop of Henle: Reabsorbs water (descending limb) and sodium chloride (ascending limb).
- DCT and Collecting Duct: Fine-tune water and salt reabsorption under hormonal control (ADH and aldosterone).
Hormonal Control:
- Antidiuretic Hormone (ADH): Increases water reabsorption in collecting ducts.
- Aldosterone: Promotes sodium reabsorption and potassium excretion.
3. Tubular Secretion
Definition:
It is the process by which additional unwanted substances are secreted from blood into the tubular fluid.
Process:
- Occurs mainly in the distal convoluted tubule (DCT) and collecting duct.
- Substances such as hydrogen ions (H⁺), potassium ions (K⁺), ammonia, urea, creatinine, and certain drugs are actively secreted from the blood into the filtrate.
Purpose:
- Helps in maintaining acid-base balance (pH).
- Removes toxic substances and drugs from the body.
Final Urine
After filtration, reabsorption, and secretion, the final urine formed is collected in the collecting ducts and passes to the renal pelvis, then to the ureters, and finally stored in the urinary bladder.
Normal Composition of Urine:
- Water: 95%
- Solutes: 5% (urea, uric acid, creatinine, sodium, potassium, chloride, etc.)
- pH: Slightly acidic (around 6)
- Color: Pale yellow (due to urochrome pigment)
Summary of Urine Formation
| Step | Main Site | Function |
|---|---|---|
| Glomerular Filtration | Glomerulus | Filters blood plasma into Bowman’s capsule |
| Tubular Reabsorption | PCT, Loop of Henle, DCT, Collecting Duct | Reabsorbs water and useful solutes |
| Tubular Secretion | DCT and Collecting Duct | Secretes unwanted substances into tubule |
Functions of Urine Formation
- Removes nitrogenous wastes like urea and uric acid.
- Regulates water and electrolyte balance.
- Maintains acid-base balance (pH) of blood.
- Controls blood pressure and volume.
- Helps in the excretion of drugs and toxins.
