11.2 Urine formation and concentration mechanisms

Urine formation is a complex process involving filtration, reabsorption, and secretion in the nephron. These mechanisms work together to regulate fluid balance, remove waste, and maintain homeostasis. Hormones like ADH and aldosterone play crucial roles in fine-tuning urine composition.

The kidney's ability to concentrate urine relies on the countercurrent multiplication system in the loop of Henle. This creates a high osmotic gradient in the medulla, allowing for water reabsorption and urine concentration. Urea recycling further enhances this process.

Urine Formation

Glomerular Filtration and Tubular Processes

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• Glomerular filtration occurs in the Bowman's capsule where blood is filtered under pressure, allowing small molecules and water to pass through while retaining larger molecules (proteins, blood cells)
  • Filtration is driven by Starling forces, which include hydrostatic and oncotic pressures
  • The filtrate entering the tubular system is similar in composition to blood plasma without the proteins
• Tubular reabsorption involves the selective reuptake of essential nutrients, ions, and water from the filtrate back into the bloodstream
  • Reabsorption occurs throughout the nephron segments (proximal convoluted tubule, loop of Henle, distal convoluted tubule, collecting duct)
  • Transport mechanisms include passive diffusion, facilitated diffusion, and active transport
• Tubular secretion is the process of removing specific substances from the blood and adding them to the filtrate
  • Secretion helps eliminate waste products (hydrogen ions, potassium, drugs) and regulates blood pH
  • Occurs primarily in the proximal and distal convoluted tubules

Hormonal Regulation of Urine Formation

• Antidiuretic hormone (ADH), also known as vasopressin, is released by the posterior pituitary gland in response to increased blood osmolarity or decreased blood volume
  • ADH increases water reabsorption in the collecting ducts by inserting aquaporin channels into the luminal membrane
  • This leads to the production of more concentrated urine and helps maintain fluid balance
• Aldosterone, a mineralocorticoid hormone secreted by the adrenal cortex, regulates sodium and potassium balance
  • Aldosterone promotes sodium reabsorption and potassium secretion in the distal convoluted tubule and collecting duct
  • Increased sodium reabsorption leads to increased water retention, helping to maintain blood volume and pressure

Urine Concentration

Countercurrent Multiplication and Osmotic Gradient

• Countercurrent multiplication is a process that establishes a high osmotic gradient in the medulla of the kidney
  • The loop of Henle has a descending limb permeable to water and an ascending limb impermeable to water but actively pumps out sodium chloride (NaCl)
  • As the filtrate flows down the descending limb, water is drawn out by the high osmolarity of the interstitium, concentrating the filtrate
  • In the ascending limb, NaCl is actively transported out, diluting the filtrate and contributing to the high osmolarity of the interstitium
• The osmotic gradient in the medulla increases from the cortex to the papilla
  • This gradient is maintained by the countercurrent exchange of NaCl and urea between the loops of Henle and the collecting ducts
  • The high osmolarity in the medulla allows for the reabsorption of water from the collecting ducts, concentrating the urine

Urea Recycling and Concentrating Ability

• Urea recycling contributes to the high osmolarity in the medulla
  • Urea is passively reabsorbed from the inner medullary collecting ducts into the interstitium
  • Some of the reabsorbed urea diffuses back into the loop of Henle, reinforcing the osmotic gradient
• The concentrating ability of the kidney depends on the presence of a hypertonic medulla
  • The maximum concentrating ability is determined by the length of the loop of Henle and the efficiency of countercurrent multiplication
  • In humans, the kidney can produce urine up to 4 times more concentrated than blood plasma (1200 mOsm/L)
  • Desert animals (kangaroo rats) have longer loops of Henle and can produce highly concentrated urine (up to 25 times more than plasma) to conserve water