The formation of urine takes place in the nephron, which is the structural and fundamental unit of the kidney. It occurs through three steps: glomerular filtration, tubular reabsorption, and tubular secretion.

The first step of the formation of the urine is glomerular filtration in the glomerules. The process of glomerular filtration filters out most of the solutes, particularly large solutes like proteins, due to the high blood pressure and specialized membranes in the afferent arteriole. The “leaky” connections between the endothelial cells of the glomerular capillary network allow solutes to pass through easily. All solutes in the glomerular capillaries, including sodium ions and negatively and positively charged ions, pass through by passive diffusion; the only exception is macromolecules such as proteins. There is no energy requirement at this stage of the filtration process. Glomerular filtration rate (GFR) is the volume of glomerular filtrate formed per minute by the kidneys.

The second step is the tubular reabsorption. Tubular reabsorption occurs in the PCT (proximal convoluted tubule) part of the renal tubule. Almost all nutrients are reabsorbed; this happens either by passive or active transport. Reabsorption of water and key electrolytes are regulated and influenced by hormones. Sodium (Na⁺) is the most abundant ion; most of it is reabsorbed by active transport and then transported to the peritubular capillaries. Because Na⁺ is actively transported out of the tubule, water follows to even out the osmotic pressure. Water is also independently reabsorbed into the peritubular capillaries due to the presence of aquaporins, or water channels, in the PCT. This occurs due to the low blood pressure and high osmotic pressure in the peritubular capillaries.

In the loop of Henle, the permeability of the membrane changes. The descending limb is permeable to water, not solutes; the opposite is true for the ascending limb. Additionally, the loop of Henle invades the renal medulla, which is naturally high in salt concentration. It tends to absorb water from the renal tubule and concentrate the filtrate. The osmotic gradient increases as it moves deeper into the medulla. Because two sides of the loop of Henle perform opposing functions, it acts as a countercurrent multiplier. The vasa recta around the loop of Henle acts as the countercurrent exchanger. The descending limb is water permeable. Water flows from the filtrate to the interstitial fluid, so osmolality inside the limb increases as it descends into the renal medulla. At the bottom, the osmolality is higher inside the loop than in the interstitial fluid. Thus, as filtrate enters the ascending limb, Na+ and Cl- ions exit through ion channels present in the cell membrane. Further up, Na+ is actively transported out of the filtrate and Cl- follows. Osmolarity is given in units of milliosmoles per liter (mOsm/L).

Additional solutes and wastes are secreted into the kidney tubules during tubular secretion (the last step), which is the opposite process of tubular reabsorption. The collecting ducts collect filtrate coming from the nephrons and fuse in the medullary papillae. Aquaporins on the apical side of epithelial cells in the collecting ducts filter water out and the urine is concentrated here before it enters the ureters through the renal pelvis to go to the bladder. Aquaporin expression at the collecting duct membrane can be mediated by vasopressin.

1. The glomerulus forces solutes out of the blood by pressure.
2. The proximal convoluted tubule reabsorbs ions, water, and nutrients from the filtrate into the interstitial fluid, and actively transports toxins and drugs from the interstitial fluid into the filtrate. The proximal convoluted tubule also adjusts blood pH by selectively secreting ammonia (NH₃) into the filtrate, where it reacts with H⁺ to form NH₄⁺. The more acidic the filtrate, the more ammonia is secreted.
3. The descending loop of Henle is lined with cells containing aquaporins that allow water to pass from the filtrate into the interstitial fluid.
4. In the thin part of the ascending loop of Henle, Na⁺ and Cl⁻ ions diffuse into the interstitial fluid. In the thick part, these same ions are actively transported into the interstitial fluid. Because salt but not water is lost, the filtrate becomes more dilute as it travels up the limb.
5. In the distal convoluted tubule, K⁺ and H⁺ ions are selectively secreted into the filtrate, while Na⁺, Cl⁻, and HCO₃⁻ ions are reabsorbed to maintain pH and electrolyte balance in the blood.
6. The collecting duct reabsorbs solutes and water from the filtrate, forming concentrated urine.

Practice Questions

 

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Key Points

• Glomerular filtration, tubular reabsorption, and tubular secretion are the three primary steps in which kidneys filter blood and maintain proper electrolyte balance.

• Glomerular filtration removes solutes from the blood; it is the first step of urine formation.

• In tubular reabsorption, the second step of urine formation, almost all nutrients are reabsorbed in the renal tubule by active or passive transport.

• Tubular secretion is the last step of urine formation, where solutes and waste are secreted into the collecting ducts, ultimately flowing to the bladder in the form of concentrated urine.

Key Terms

Arteriole: one of the small branches of an artery, especially one that connects with capillaries

Countercurrent: a current that flows against the prevailing one

Electrolyte: any of the various ions (such as sodium or chloride) that regulate the electric charge on cells and the flow of water across their membranes

Diffusion: the movement of substances from an area of high concentration to low concentration

Glomerulus: a small intertwined group of capillaries within a kidney’s nephron that filters the blood to make urine.

Loop of Henle: a structure in a kidney’s nephron that connects the proximal convoluted tubule to the distal convoluted tubule.

Proximal convoluted tubule: involved in the resorption of sugar, sodium and chloride ions, and water from the glomerular filtrate

Collecting duct: the final site of reabsorption in the nephron

Nephron: structural and functional unit of the kidney

Interstitial fluid: a thin layer of fluid which surrounds the body’s cells.