Filtration, Reabsorption, and Secretion - Anatomy and Physiology
- What are the three basic processes of urine formation?
- Glomerular filtration, tubular reabsorption, and tubular secretion.
- In a healthy person, how much filtered glucose is reabsorbed, and where?
- Essentially all of it, in the proximal convoluted tubule.
- Urine creatinine is 120 mg/dL, urine flow is 1 mL/min, and plasma creatinine is 1 mg/dL. What is creatinine clearance?
- 120 mL/min. Clearance = (urine concentration x urine flow) / plasma concentration = (120 x 1) / 1.
- Glomerular hydrostatic pressure is 55 mmHg, capsular hydrostatic pressure is 15 mmHg, and glomerular oncotic pressure is 30 mmHg. What is net filtration pressure?
- 10 mmHg. Net filtration pressure = 55 - (15 + 30) = 10 mmHg.
- What is the normal glomerular filtration rate in a healthy adult?
- About 120 to 125 mL/min, which is about 180 L of filtrate per day.
- Which plasma components are normally kept out of the glomerular filtrate?
- Blood cells and plasma proteins such as albumin, which are blocked by size and negative charge.
- Which nephron segment reabsorbs the largest share of filtered water and Na+?
- The proximal convoluted tubule, which reabsorbs about 65 percent of both.
- What three layers make up the glomerular filtration membrane?
- Fenestrated capillary endothelium, the glomerular basement membrane, and the filtration slits between podocyte foot processes.
- How does constriction of the afferent arteriole affect GFR?
- It lowers GFR, because less blood enters the glomerulus and glomerular hydrostatic pressure falls.
- Why does glucose appear in the urine when plasma glucose rises to 300 mg/dL?
- Filtered glucose exceeds the transport maximum of the tubular carriers, which are saturated, so the excess is excreted.
- What is the main purpose of tubular secretion?
- To move substances from peritubular blood into the tubule, removing drugs, excess K+ and H+, and other wastes, and helping control blood pH.
- Why does the clearance of inulin equal the GFR?
- Inulin is freely filtered and is neither reabsorbed nor secreted, so the plasma volume cleared equals the volume filtered.