Thursday, August 13, 2009
A. Functions of the kidneys _
The kidneys’ main function is excretion of water and water-soluble substances
(1). This is closely associated with their role in regulating the body’s electrolyte and acid–base balance 2.Both excretion and homeostasis are subject to hormonal control. The kidneys are also involved in synthesizing several hormones (3;. Finally, the kidneys also play a role in the intermediary metabolism (4), particularly in amino acid degradation and gluconeogenesis .The kidneys are extremely well-perfused organs, with about 1500 L of blood flowing through themevery day. Approximately 180 L of primary urine is filtered out of this. Removal of water leads to extreme concentration of the primary urine (to approximately one-hundredth of the initial volume). As a result, only a volume of 0.5–2.0 L of final urine is excreted per day. B. Urine formation _ The functional unit of the kidney is the nephron. It is made up of the Malpighian bodies or renal corpuscles (consisting of Bowman’s capsules and the glomerulus), the proximal tubule, Henle’s loop, and the distal tubule, which passes into a collecting duct. The human kidney contains around one million nephrons. The nephrons form urine in the following three phases.
Ultrafiltration. Ultrafiltration of the blood plasma in the glomerulus gives rise to primary urine, which is isotonic withplasma. The pores in the glomerular basal membrane, which are made up of type IV collagen , have an effectivemean diameter of 2.9 nm. This allows all plasma components with a molecular mass of up to about 15 kDa to pass through unhindered. At increasing masses, molecules are progressively held back; atmasses greater than 65 kDa, they are completely unable to enter the primary urine. This applies to almost all plasma proteins—which in addition, being anions, are repelled by the negative charge in the basalmembrane. Resorption. All low-molecular weight plasma components enter the primary urine via glomerular filtration.
Most of these are transported back into the blood by resorption, to prevent losses of valuable metabolites and electrolytes. In the proximal tubule, organic metabolites (e. g., glucose and other sugars, amino acids, lactate, and ketone bodies) are recovered by secondary active transport . There are several group-specific transport systems for resorbing amino acids, with which hereditary diseases can be associated (e. g., cystinuria, glycinuria, and Hartnup’s disease). HCO3 –, Na+, phophate, and sulfate are also resorbed by ATP-dependent (active) mechanisms in the proximal tubule. The later sections of the nephron mainly serve for additional water recovery and regulated resorption of Na+ and Cl– . These processes are controlled by hormones (aldosterone, vasopressin). Secretion. Some excretable substances are released into the urine by active transport in the renal tubules. These substances include H+ and K+ ions, urea, and creatinine, as well as drugs such as penicillin. Clearance. Renal clearance is used as a quantitative measure of renal function. It is defined as the plasma volume cleared of a given substance per unit of time. Inulin, a fructose polysaccharidewith amass of ca. 6 kDa that is neither actively excreted nor resorbed but is freely filtered, has a clearance of 120mL min–1 in healthy individuals.Further information
Concentrating urine and transporting it through membranes are processes that require
large amounts of energy. The kidneys therefore have very high energy demands. In
the proximal tubule, the ATP needed is obtained from oxidative metabolism of fatty
acids, ketone bodies, and several amino acids. To a lesser extent, lactate, glycerol, and citric acid are also used. In the distal tubule and Henle’s loop, glucose is the main substrate for the energy metabolism. The endothelial cells in the proximal tubule are also capable of gluconeogenesis. The substrates for this are mainly the carbohydrate skeletons of amino acids. Their amino groups are used as ammonia for buffering urine (see p. 311). Enzymes for peptide degradation and the amino acid metabolism occur in the kidneys at high levels of activity (e. g., amino acid oxidases, amine oxidases, glutaminase).
