Overview
This lecture explains the processes of tubular reabsorption and secretion in the nephron, which are essential for urine formation and maintaining the body's fluid and electrolyte balance.
Structure and Function of the Nephron
- The nephron is the kidney's filtration unit, each kidney contains about 1 million nephrons.
- Blood enters the nephron via the afferent arteriole and is filtered at the glomerulus.
- About 180 liters of filtrate are produced daily, but only 1% (1.8 liters) becomes urine.
Tubular Reabsorption
- Reabsorption returns filtered substances from the nephron back into the bloodstream.
- Proximal convoluted tubule reabsorbs ~65% of filtrate, including sodium, potassium, chloride, calcium, magnesium, bicarbonate, water, amino acids, urea, and all glucose.
- The thin descending limb of the loop of Henle reabsorbs only water, aiding urine concentration.
- The thick ascending limb reabsorbs ions (sodium, potassium, chloride, calcium, magnesium) and bicarbonate.
- The distal convoluted tubule reabsorbs ions, bicarbonate, and water.
- The collecting duct reabsorbs sodium, chloride, bicarbonate, water, and urea.
Tubular Secretion
- Secretion moves substances from blood into the nephron for excretion.
- Proximal convoluted tubule secretes urea, uric acid, creatinine, hydrogen ions, drugs, and ammonia.
- The loop of Henle secretes urea.
- Distal convoluted tubule and collecting duct secrete hydrogen ions, some drugs, and ammonia.
Importance and Consequences
- Proper reabsorption is vital; even a 10% reduction drastically increases urine output and is life-threatening.
- Reabsorption of all glucose occurs in the proximal tubule; excess glucose results in glucose in urine, often seen in diabetes.
Key Terms & Definitions
- Nephron — Functional unit of the kidney responsible for urine formation.
- Glomerulus — Capillary network that filters blood in the nephron.
- Reabsorption — Movement of substances from nephron back into the bloodstream.
- Secretion — Transfer of substances from blood into nephron tubules.
- Filtrate — Fluid filtered from blood into the nephron.
- Peritubular Capillaries — Blood vessels surrounding nephron tubules aiding reabsorption and secretion.
Action Items / Next Steps
- Review the steps and locations of reabsorption and secretion in the nephron.
- Remember the key substances reabsorbed and secreted at each nephron segment.
- Understand the consequences of impaired reabsorption, especially regarding glucose and water balance.