Formation of Urine
Introduction:
The formation of urine is one of the most vital functions of the kidneys, carried out by nephrons and collecting ducts. It involves three primary physiological processes:
- Glomerular filtration
- Tubular reabsorption
- Tubular secretion
Three Basic Processes of Urine Formation:
1. Glomerular Filtration
In the first step of urine production, water and most solutes in the blood plasma move across the wall of glomerular capillaries. They are filtered into the glomerular capsule (Bowman’s capsule) and then enter the renal tubule.
2. Tubular Reabsorption
As the filtered fluid flows through the renal tubules and collecting ducts, about 99% of the filtered water and many useful solutes are reabsorbed by tubular cells. These substances return to the bloodstream through the peritubular capillaries and vasa recta.
Note: Reabsorption refers to returning substances to the bloodstream, whereas absorption refers to entry of new substances into the body (e.g., from the digestive tract).
3. Tubular Secretion
As the filtrate flows through the renal tubules and collecting ducts, the renal tubule cells actively secrete additional materials like wastes, drugs, and excess ions into the fluid. Tubular secretion thus helps remove substances from the blood.
Detailed Physiology of Urine Formation
The nephrons of the kidneys perform all three processes in sequence. Let’s discuss each in detail.
(a) Glomerular Filtration
This occurs in the glomerular capsule. The barrier between the blood in the glomerular capillaries and the inner portion of Bowman’s capsule acts as an ultrafiltration membrane. This membrane consists of:
- Endothelial cells of the capillaries
- Basement membrane
- Podocytes (specialized epithelial cells)
The ultrafiltration membrane allows free passage of water and small solutes but prevents the passage of blood cells and plasma proteins. Hence, the glomerular filtrate has the same composition as plasma except for proteins and cells.
Pressures Affecting Glomerular Filtration
Three pressures act across the ultrafiltration barrier:
- Glomerular capillary blood pressure (Hydrostatic pressure): About 60 mm Hg — drives filtration.
- Plasma oncotic pressure: About 27 mm Hg — opposes filtration due to unfiltered plasma proteins.
- Capsular hydrostatic pressure: About 15 mm Hg — opposes filtration by the fluid already in the capsule.
Net Filtration Pressure (NFP) = 60 − (27 + 15) = 18 mm Hg
The total glomerular filtration rate (GFR) in normal adults is about 120 mL/min or 180 liters/day. However, only about 1.5 liters/day of urine is excreted, meaning roughly 179 liters are reabsorbed back into the body.
Glomerular Filtration Rate (GFR)
The GFR represents the amount of filtrate formed per minute by both kidneys. It depends on the Net Filtration Pressure (NFP), which in turn depends on capillary hydrostatic pressure. This pressure is influenced by the diameter of the afferent and efferent arterioles and systemic blood pressure.
Despite variations in blood pressure, the kidneys maintain a relatively constant GFR through a mechanism called renal autoregulation.
Renal Autoregulation
This mechanism ensures that filtration continues even during changes in systemic blood pressure. It is controlled by the juxtaglomerular apparatus (JGA), which releases the enzyme renin. Renin, along with other substances like angiotensin II, helps regulate the tone of arterioles and stabilize GFR.
(b) Tubular Reabsorption
During tubular reabsorption, the composition and volume of the glomerular filtrate are modified as it passes through the nephron. This process helps conserve essential substances and maintain fluid and electrolyte balance.
- High-threshold substances: Substances like glucose, amino acids, fatty acids, and vitamins are completely reabsorbed into the blood.
- Low-threshold substances: Waste products like urea and uric acid are reabsorbed only slightly.
- Non-reabsorbed substances: Some compounds, such as creatinine sulfate, are not reabsorbed at all.
Reabsorption occurs by both active transport and passive diffusion. Active reabsorption involves carrier proteins in tubular cells and has a limit, known as the transport maximum (Tm). When plasma concentration exceeds this limit, the excess substance appears in the urine (e.g., glucose in diabetes mellitus).
(c) Tubular Secretion
Tubular secretion helps remove additional waste materials that were not filtered at the glomerulus. Substances like hydrogen ions (H⁺), potassium ions (K⁺), drugs (e.g., penicillin, aspirin), and ammonium ions are secreted from the peritubular capillaries into the tubular fluid.
This process is essential for maintaining acid-base balance (blood pH) and ensuring the elimination of toxins from the body.
Detailed Notes:
For full-color illustrated notes and diagrams, open the detailed study material below.