kidney functions urine formation. learning outcomes: describe the function of each section of the...
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KIDNEY FUNCTIONS URINE FORMATION
Learning Outcomes:
• Describe the function of each section of the nephron
• Describe the process of urine formation
• Describe how the kidneys maintain blood pH
Urine Composition
• Water
• Nitrogenous wastes (urea etc.)
• Salts
• May contain H+ and other ions
• May contain drugs, vitamins and other substances
Urine Formation
3 major steps:
1. Glomerular filtration - in the glomerular capsule
2. Tubular reabsorption - in the proximal tubule and loop of Henle
3. Tubular secretion - distal tubule
• Different parts of the nephron have different permeability characteristics
• Substances are filtered out of the blood by the nephron and excreted in urine
• Some substances are reabsorbed back into the blood from the nephron
Pressure Filtration
• Blood enters the glomerulus from the afferent arteriole
• Blood pressure forces small molecules out of the capillaries into the capsule
• Substances that leave the blood:
Water, salts, nutrients (glucose, amino acids), nitrogenous wastes (urea)
• Filtrate resembles plasma
Selective Reabsorption
• At the proximal tubule and loop of Henle
• Some substances must be reabsorbed back into the blood
• Examples:
• Water, nutrients (glucose, amino acids), salts
• Na+ ions, glucose, amino acids - move back into the peritubular capillaries by active transport (why active transport?)
• Cl- ions follow passively
• Water is reabsorbed by osmosis
Absorption of Water at the Loop of Henle
• Function of the loop of Henle is to remove as much water as possible from the filtrate and return it to the blood
• Descending limb is permeable to water but not salt
• Ascending limb is permeable to salt but not water
• Sodium is actively pumped out of the ascending limb
• Medulla is hypertonic to the filtrate (contains high [Na+])
• Water leaves the tubule by osmosis and enters the medulla, then the capillaries
Fig. 16.7
Fig. 16.6
Tubular Secretion
• At the distal tubule• Excess H+ ions, drugs,
and other substances move from the blood into the nephron by active transport
• Ions secreted or kept in the blood as needed, to maintain correct pH
Reabsorption of Water
• More water is reabsorbed into the blood from the collecting duct
• Collecting duct is in the hypertonic medulla so water is drawn out by osmosis
Fig. 16.6
• Urine is now ready to be excreted from the body
• Collects in the renal pelvis before going to the bladder
Path of Blood
1. Renal artery & arterioles
2. Afferent arteriole
3. Glomerulus
4. Efferent arteriole
5. Peritubular capillary network
6. Venules to renal vein
Questions
• How is the composition of blood in the renal artery different from that in the renal vein?
• What is the importance of the hypertonic condition of the renal medulla?
• Why is glucose transported actively instead of passively at the proximal tubule?
Fig. 16.6
Fig. 16.6