Normal Renal Function
Functions of the Kidney:
- balances solute and water transport
- excretes metabolic waste products
- conserves nutrient
- regulates acid-base balance
- secretes hormones that help regulate blood pressure, erythrocyte production, and calcium metabolism.
- forms urine
Formation of Urine in the Kidney
A kidney contains around one million nephrons. In general, the kidneys can adequately function with only one third of the normal number of nephrons. Less than that, the body will retain waste products, especially urea and creatinine.
Nephrons process the blood to make urine. A tuft of capillaries called the glomerulus is contained in each nephron. The glomerulus is surrounded by Bowman's capsule. The capillaries are extremely porous, allowing large amounts of solute-rich fluids to pass from the capillaries into the capsule. This fluid is the raw material of urine.
This fluid leaves the capsule and is channeled into the proximal convoluted tubule (PCT) of the nephron. This is where the primary active transport of the sodium ion accounts for about 80% of sodium reabsorption. During the primary active transport of sodium, chloride is simultaneously reabsorbed as are all amino acids and glucose. Primary active transport also occurs to some extent in the other tubules except in the descending loop of Henle.
|The secondary active secretion of H+ during Na+ reabsorption is called countertransport since the ions move in opposite directions. Normally, only 20% of total Na+ reabsorption occurs during active secretion of H+ and K+.|
|When H+ ions are not available for exchange as in alkalemia, K+ ions are secreted. This is why alkalemia may lead to hypokalemia. In H+ and K+ secretion, HCO3- ions are reabsorbed in place of Cl-. When chloride ions are in short supply, there is an increased demand for H+ and K+ secretion to reabsorb sodium.|
|The proximal tubules are responsible for the iso-osmotic reabsorption of water, electrolytes, non-electolytes. As much as 80% of the filtrate is reabsorbed into the capillaries that line the tubules.|
Past the PCT is the loop of Henle which consists of both ascending and descending limbs. The descending limb is freely permeable to water, while the ascending is less permeable. In the Loop of Henle there is continued reabsorption of water, sodium, chloride.
The distal convoluted tubule receives fluid from the loop of Henle. The distal tubules are important in the final regulation of water balance and acid-base balance since hydrogen ion is excreted with ammonia as ammonium and with phosphate buffers. Bicarbonate is regenerated in this process and retained in the body.
The collecting tubule then receives the newly formed urine from the nephrons. In the collecting duct water reabsorption is completed. The final concentration of urine takes place here under the control of anti-diuretic hormone (ADH). In the presence of ADH, more water is reabsorbed.The urine flows through the minor and major calyces of the renal pelvis into the ureter. From the ureter, urine makes its way to bladder.