1. Active and Passive Tubular Reabsorption
The kidney receives roughly 20 percent of the cardiac output, and 99 percent of this blood flow goes to the renal cortex and one percent to the renal medulla (Stewart, 1998). Each kidney contains roughly one million nephrons, the filtration units of the kidney responsible for reabsorption and secretion. The function of the renal tubules is to reabsorb roughly 99 percent of the glomerular filtrate, and the proximal tubule reabsorbs 60 percent of all the solutes in the glomerular filtrate, including 100 percent of the glucose and amino acids, 90 percent of the bicarbonate and 80 percent to 90 percent of the inorganic phosphate and water (Stewart, 1998). The reabsorption is either active or passive. Most of the solute reabsorption is active, that is, it requires energy, and consumes oxygen. Active transport requires carrier molecules, and so is limited to their availability (Applegate, 2000, 382). Negative ions often follow passively the active transport of positive ions to preserve electrical neutrality.
When active reabsorption of the solute from the tubule occurs, there is osmotic activity within the tubule, causing water to be reabsorbed by passive osmosis (Stewart, 1998). Urine is concentrated in the Loop of Henle by water reabsorption. This occurs because of the high concentration of the solute in the interstitium in the renal medulla. The Loop of henle is a counter current multiplier, allowing the high concentrations of solute to remain in the medulla for water reabsorption. The vasa recta is the counter current exchanger.
The descending limb of the Loop of Henle is relatively impermeable to solute but very permeable to water, allowing water to move out by osmosis (Stewart, 1998). The thin section of the ascending Loop of Henle is impermeable to water but permeable to solute, especially sodium and chloride ions. As the ions move out, the contents of the tubule ...