Regulation of Kidney Function by Feedback Circuits
- Control by Antidiuretic Hormome (ADH)
- Control by Juxtaglomerular Apparatus (JGA)
Still another hormone, a peptide called Atrial Natriuretic Factor (ANF), opposes the regulation by RAAS. The walls of the atria of the heart release ANF in response to an increase in blood volume and pressure. ANF inhibits the release of renin from the JGA, and thereby, inhibits NaCl reabsorption by the collecting duct and reduces aldosterone release from adrenal gland. Thus, ADH, the RAAS and ANF provide an elaborate system of checks and balance that regulate the kidney functioning, to control body fluid osmolarity, salt concentrations, blood pressure and blood volume.
Micturition and Constituents of Urine
Micturition is the act of voiding the urine. The release of urine is accomplished by the simultaneous contraction of the smooth muscle of the urinary bladder wall and the relaxation of the skeletal muscle sphincter around the opening of the bladder. As the bladder wall is stretched by gradual filling of the bladder, stretch receptors in the wall of the bladder generate nerve impulses that are carried by sensory neurons to the spinal cord and brain, producing the sensation of 'fullness' (around 500 ml). The sphincter can then be relaxed by inhibition of motor impulses allowing the smooth muscle of the bladder wall to contract under autonomic control and empty the contents.
Urine is a pale-yellow colored, slightly acidic (average pH 6.0; range 4.5-8.2) watery fluid (specific gravity 1.015-1.025) with a characteristic smell. Human adults produce around 1.5 L of urine per day. Composition, colour and volume, however, may change with the nature of dietary intake.
Abnormal Urine: Various metabolic errors of kidney malfunctioning is reflected in the changes of the composition of urine. Occurrence of ketone bodies, glucose, albumin, blood cells, excess pigments, pus cells, calculi or casts (kidney stones) are some of the major abnormal constituents of urine. Notable abnormal conditions are as follows:-
Proteinuria - excess protein level in urine.
Albuminuria - presence of albumin in urine; usually occurs in nephritis (inflammation of glomeruli), when the size of the filtering slits enlarges.
Glycosuria - Presence of glucose in urine, as in the case of diabetes mellitus.
Ketonuria - Presence of abnormally high ketone bodies in urine.
Hematuria - Presence of blood or blood cells in urine.
Hemoglobinuria - Presence of hemoglobin in urine.
Uremia - Presence of excess urea in urine.
Haemodialysis and Kidney Transplantation
If the kidneys become impaired by disease or injury, the blood must be filtered by an artificial device. Such filtering of the blood is called haemodialysis. Dialysis means using a semi-permeable membrane to separate large non-diffusible particles form smaller diffusible one. One of the best known devices for haemodialysis is the kidney machine. A tube connects the machine with the artery through the tubes to one side of a semi-permeable dialyzing membrane made of cellophene sheets. The other side of the membrane is continually washed with an artificial solution called the dialyzing solution. The blood that passes through the artificial kidney is treated with an anticoagulant. Only about 500 ml of the patients blood is designed to be in the machine at any given time.
All substances in the blood except the protein molecules and blood cells can diffuse back and forth across the semipermeable membrane. The electrolyte level of the plasma is controlled by keeping the electrolytes of the dialyzing solution at the same concentration as found in normal plasma. Any excess plasma electrolytes move down the concentration gradient and into the dialyzing solution. If the plasma electrolyte level is normal, it is in equilibrium with the dialyzing solution and no wastes, substances such as urea move down the concentration gradient and into the dialyzing solution. Thus wastes are removed and normal electrolyte balance is maintained.
There are some drawbacks in this process. Anticoagulants must be added to the blood during dialysis. A large amount of the patients blood must flow throughout the machine to make the treatment effective. The slow rate at which the blood can be processed makes the treatment time consuming. Recently a method has been found out which makes the process of haemodialysis easier and less time consuming. It is called continuous ambulatory peritoneal dialysis (CAPD). CAPD uses the peritoneum instead of cellophane sheets as the dialyzing membrane. Since the peritoneum is a semipermeable membrane it allows rapid bidirectional transfer of substances. A catheter is placed in the patients peritoneal cavity and connected to a supply of dialyzing fluid. Gravity feeds the solution into the abdominal cavity from its plastic container. When the process is complete, the dialyzing fluid is returned from the abdominal cavity to the plastic container and then discarded. No artificial kidney has been implanted permanently, so far. However, diseased kidneys may be replaced with healthy ones by kidney transplantation. For the normal life of human beings one healthy kidney is more than enough. Therefore a healthy person can donate one of his kidneys to patients with both impaired kidneys. Many lives are saved every day by the technique of kidney transplantation all over the world.