Kidney functions are monitored and regulated by the hormonal feedback mechanisms of the hypothalamus, juxtaglomerular apparatus or JGA and heart.
Our body has several osmoreceptors that are activated by changes in the volume of body fluids, volume of blood and ionic concentration. An osmoreceptor is a sensory receptor that detects changes in osmotic pressure.
For example, a decrease in the level of body fluids activates the osmoreceptors, which stimulate the hypothalamus, which, in turn, neurally stimulates the neurohypophysis of the pituitary to release the antidiuretic hormone or ADH or vasopressin.
ADH prompts the distal convoluted tubules or DCTs and the collecting ducts to reabsorb more water, thereby preventing diuresis. On the other hand, if the fluid volume in the body increases, the osmoreceptors suppress the release of ADH, leading to increased excretion of water in urine.
The juxtaglomerular cells detect a fall in the glomerular blood pressure or GFR and release a peptide hormone called renin. Renin converts angiotensinogen in the blood into angiotensin one, which is further converted into angiotensin two. Angiotensin two, a powerful vasoconstrictor, constricts the blood vessels thereby increasing blood pressure, which stimulates the cortex of the adrenal gland to secrete aldosterone.
Aldosterone increases the reabsorption of sodium and water from the distal parts of the renal tubule, resulting in an increase in blood volume, which increases blood pressure and GFR. This complex mechanism is commonly known as the renin-angiotensin mechanism.
The urine passes into the urinary bladder, where it is stored until a signal is received by the central nervous system or CNS. The CNS, in turn, sends motor messages that make the smooth bladder muscles contract and the urethral sphincter relax, resulting in the release of urine.
This process of disposing urine is called micturition and the neural mechanism responsible for the process is known as the micturition reflex. On average, an adult human releases about one to one point five litres of urine every day.
The characteristics of urine can change according to different body conditions. In fact, a urine test helps to diagnose many metabolic disorders in the body and any malfunction in the kidneys.
For instance, the presence of glucose in urine, which is called glycosuria, is indicative of diabetes mellitus. Moreover, the urine of diabetic patients sometimes shows ketone bodies, which is called ketonuria.
The malfunctioning of the kidneys can also cause uremia, a condition where large amounts of urea accumulate in the blood. Uremia can even lead to kidney failure. A life-saving process for uremic patients is hemodialysis, wherein excess urea in the blood is removed.
However, in cases of acute renal or kidney failure, the only option is kidney transplantation. In this process, a functioning kidney from a donor is transplanted in the patient. Usually, a close relative of the patient is the preferred donor to minimise the chances of the patient’s immune system rejecting the kidney.
Other disorders due to the malfunctioning of the kidneys include the formation of stones or insoluble masses of crystallised salts within the kidney, a condition known as renal calculi. These stones are usually salts of calcium, mainly calcium oxalate or calcium phosphate. Such stones are also formed in the ureter and the urinary bladder.