URINARY - REABSORPTION & SECRETION Learning Objectives 1. Explain the importance of the peritubular capillary of the kidney cortex. 2. Describe the cellular mechanism for transport of glucose and bicarbonate by the proximal convoluted tubule (PCT). 3. Contrast the transcellular and paracellular pathways for movement of water across the renal tubular epithelium and explain solvent drag of K+. 4. Define maximal transport rate and transport threshold. 5. Explain secretion of organic compounds in the PCT. 6. Explain the factors that increase K+ secretion in the collecting duct. PROXIMAL CONVOLUTED TUBULE (PCT) FUNCTIONS • Filtration (F) moves solutes and water into the lumen of the tubule. • Reabsorption (R) moves solutes from the lumen of the tubule to the blood. • Secretion (S) moves solutes from the blood to the lumen of the tubule. PORTAL SYSTEM & PRESSURE GRADIENTS REABSORPTION IN PROXIMAL TUBULE (PCT) •2nd active transport uses Na+ for co-transport of many solutes such as glucose and amino acids. • Water follows Na+ to enter the blood. • HCO3 is impermeable. HCO3 combines with H+ to form H2O and CO2 in the lumen in presence of carbonic anhydrase (c.a.). H+ and HCO3 are generated inside the PCT cells by c.a. reaction. HCO3- exits the basal side. H+ exits the luminal side . RATE OF TRANSPORT Why is glucose found in the urine of patients with uncontrolled diabetes mellitus? TERMS TRANSCELLULAR MOVEMENT Solutes cross epithelial cells via secondary active transport coupled to Na+ movement. Others (e.g., urea, Cl-) cross by facilitated diffusion. PARACELLULAR MOVEMENT Water moves between the renal tubular epithelium cells. SOLVENT DRAG K+ moves between the PCT tubular epithelial cells following water. SECRETION IN PROXIMAL CONVOLUTED (PCT) TUBULE • Movement of organic compounds (toxins, metabolic waste products) from blood (peritubular capillary) into the renal tubule occurs by secondary active transport. • Transporters are located in the basal surface of the proximal convoluted tubule (PCT) epithelial cells. • Transport is saturable. SECRETION IN COLLECTING DUCT (CD) • Principal cells reabsorb Na+ and secrete K+. • K+ secretion is increased by increased Na+ delivery to CD and by increased filtrate flow in CD. • K+ secretion can be increased by aldosterone. GENERAL CONCEPTS 1. Reabsorption moves filtered solutes from the renal tubule to the blood. 2. Reabsorption of solutes occurs predominantly within the PCT. 3. Most solutes cross the epithelium in a transcellular manner by secondary active transport. 4. Reabsorption within the PCT occurs isosmotically. Water crosses this epithelium via aquaporin and in a paracellular manner. K+ moves by solvent drag. 5. Secretion is the movement of solutes from the blood to the lumen. This occurs in the PCT by secondary active transport. 6. Secretion of K+ occurs in the CD in response either to increased Na+ delivery or to high filtrate flow. Aldosterone acts here to increase K+ secretion. .