Mechanism Of Urine Formation

Glomerular Filtration

Glomerular filtration is the first step in urine formation, where blood plasma is filtered through the kidneys’ glomeruli. The resulting fluid, called glomerular filtrate, enters the capsular space. Typically, about 16–20% of the plasma in the afferent arterioles becomes glomerular filtrate, a value known as the filtration fraction. On average, females produce about 150 liters of filtrate daily, while males produce approximately 180 liters. Remarkably, over 99% of this filtrate is reabsorbed back into the bloodstream, and only 1–2 liters are ultimately excreted as urine.

Filtration Membrane
The filtration process occurs through a specialized structure known as the filtration membrane, composed of glomerular endothelial cells and podocytes. This membrane forms a highly permeable but selective barrier that allows water and small solutes to pass while restricting plasma proteins, blood cells, and platelets. It consists of three main layers: the glomerular endothelial cells, the basal lamina, and the filtration slits between podocyte pedicels.

Glomerular endothelial cells possess large pores (0.07–0.1 µm), which permit solutes to pass but retain blood cells and platelets. Among these cells are mesangial cells, which regulate filtration surface area. The basal lamina, a collagen and proteoglycan-rich layer, lies between the endothelium and podocytes, further restricting large proteins. The podocyte pedicels form thousands of filtration slits covered by slit membranes, allowing only molecules smaller than 0.006–0.007 µm, such as water, glucose, and ions, to pass through. Albumin and similar large proteins are almost entirely retained.

Principle of Filtration
Filtration in the glomerulus relies on pressure to drive fluids and solutes across the membrane, operating more extensively than typical capillary filtration in other tissues. This higher filtration volume results from three factors: a large surface area of glomerular capillaries, a thin and highly permeable filtration membrane (~0.1 µm thick), and elevated blood pressure in the glomerular capillaries, due to a narrower efferent arteriole compared to the afferent arteriole.

Net Filtration Pressure (NFP)
NFP is determined by the balance of pressures across the filtration membrane: the glomerular blood hydrostatic pressure (GBHP), which promotes filtration (around 55 mmHg), and two opposing pressures—capsular hydrostatic pressure (CHP) at ~15 mmHg and blood colloid osmotic pressure (BCOP) at ~30 mmHg. The NFP is calculated as:

NFP = GBHP – (CHP + BCOP) = 55 – (15 + 30) = 10 mmHg,
which drives the filtration process.

Glomerular Filtration Rate (GFR)
The GFR is the rate at which filtrate is formed by all renal corpuscles per minute. It averages about 125 mL/min in males and 105 mL/min in females. Maintaining GFR within a narrow range is essential for the regulation of fluid and electrolyte balance and efficient waste removal.

Regulation of GFR
GFR is regulated by several mechanisms:

Renal Autoregulation maintains consistent renal blood flow and GFR despite fluctuations in systemic blood pressure. It includes:
- Myogenic mechanism: where afferent arteriole smooth muscle contracts in response to stretching.
- Tubuloglomerular feedback: where the macula densa detects Na⁺, Cl⁻, and water levels to adjust arteriole tone.
Neural Regulation involves sympathetic nervous input. Moderate stimulation causes equal vasoconstriction of afferent and efferent arterioles, slightly reducing GFR. In contrast, intense stimulation primarily constricts afferent arterioles, drastically reducing GFR to conserve blood volume during stress.
Hormonal Regulation includes:
- Angiotensin II, which constricts both arterioles and reduces GFR.
- Atrial natriuretic peptide (ANP), which increases GFR by relaxing mesangial cells and expanding the filtration surface.

Reabsorption Routes and Transport Mechanisms
Filtered substances are reabsorbed via two main routes:

Paracellular reabsorption, where fluids pass between tubule cells, accounts for up to 50% of ion and water reabsorption.
Transcellular reabsorption, where substances pass through the tubule cell membranes into interstitial fluid.

Tight junctions between tubular cells help regulate the movement of substances. Solutes are transported directionally by different proteins in apical and basolateral membranes. Sodium reabsorption is vital and driven by sodium-potassium ATPase pumps on basolateral membranes. These pumps consume approximately 6% of the body’s ATP at rest.

Reabsorption occurs through:

Primary active transport, using ATP directly (e.g., Na⁺/K⁺ pump),
Secondary active transport, utilizing ion gradients:
- Symporters move substances in the same direction.
- Antiporters move them in opposite directions.

Water Reabsorption is osmotic, driven by solute movement:

Obligatory reabsorption (90%) occurs in the proximal convoluted tubule (PCT) and descending loop of Henle.
Facultative reabsorption (10%) occurs in collecting ducts and is regulated by ADH.

Reabsorption and Secretion in the Proximal Convoluted Tubule (PCT)
The PCT reabsorbs:

65% of water, Na⁺, and K⁺,
100% of glucose and amino acids,
50% of Cl⁻,
80–90% of HCO₃⁻,
50% of urea, plus variable amounts of Ca²⁺, Mg²⁺, and HPO₄²⁻.

The PCT also secretes H⁺, NH₄⁺, and urea. Na⁺ is reabsorbed via symporters and antiporters, including the Na⁺-glucose symporter. Na⁺/H⁺ antiporters play a key role in acid-base balance, as intracellular CO₂ reacts with water to form H⁺ and HCO₃⁻ (via carbonic anhydrase). HCO₃⁻ is reabsorbed to maintain buffering capacity.

Solute reabsorption increases osmolarity, drawing water into peritubular capillaries through aquaporin-1. Electrochemical gradients aid in passive reabsorption of ions like Cl⁻, K⁺, Ca²⁺, and urea.

Ammonia and Urea Handling
Ammonia, produced by amino acid metabolism in the liver, is converted to urea and secreted by the PCT. Additional NH₃ is formed from glutamine, producing NH₄⁺ and HCO₃⁻, aiding acid-base balance.

Reabsorption in the Loop of Henle
As fluid enters the loop at 40–45 mL/min, it lacks glucose and amino acids. Reabsorption rates include 15% of water, 20–30% of Na⁺ and K⁺, 35% of Cl⁻, and 10–20% of HCO₃⁻.

The thick ascending limb actively reabsorbs ions via Na⁺–K⁺–2Cl⁻ symporters but is impermeable to water. K⁺ leaks back into tubular fluid, creating a gradient that facilitates reabsorption of cations via the paracellular route.

The descending limb is water-permeable, while the ascending limb is not, allowing osmolarity regulation independent of solute concentration.

Hormonal Regulation of Tubular Reabsorption and Secretion
Several hormones tightly regulate renal function:

Angiotensin II: Reduces GFR and enhances Na⁺, Cl⁻, and water reabsorption.
Aldosterone: Promotes Na⁺ and Cl⁻ reabsorption and K⁺ secretion, increasing blood volume.
ADH: Increases water permeability in the distal tubules and collecting ducts, reducing urine output.
ANP: Inhibits Na⁺ and water reabsorption, increases urine output, and reduces blood volume.
PTH: Stimulates Ca²⁺ reabsorption and inhibits phosphate reabsorption.

Production of Dilute and Concentrated Urine
The kidneys adjust urine concentration based on fluid intake and ADH levels.

In dilute urine formation, filtrate begins isotonic, but as it descends the loop of Henle, water is reabsorbed. In the thick ascending limb, solutes are reabsorbed without water, diluting the fluid. Without ADH, collecting ducts remain impermeable to water, and urine can become as dilute as 65–70 mOsm/L.

In concentrated urine formation, high ADH levels increase water reabsorption. An osmotic gradient, maintained by Na⁺, Cl⁻, and urea, spans from 300 mOsm/L in the cortex to 1200 mOsm/L in the medulla. Countercurrent mechanisms, including multiplication in the loop of Henle and exchange in the vasa recta, help maintain this gradient and concentrate the urine efficiently.