The kidneys are responsible for keeping the homeostatically constant, which is achieved by regulating the volume and concentration of body fluids by selectively filtering and reabsorbing materials from the blood.
To achieve this, the kidneys are controlled by various factors and the neural and endocrine systems act from outside the excretory system to help achieve this balance. Apart from that the kidneys also have their own auto-regulation mechanism.
The volume of fluid in various fluid compartments in the body depends on the balance of fluid intake and fluid output. Fluid intake in response to thirst is a source of fluid, and receptor cells found in the hypothalamus activate thirst when exposed to hypertonic conditions - such as when water loss has been excessive or salt intake high.
These receptor cells will only cease to give a thirst message when the hypertonic status has been corrected.
A large intake of fluid will also be corrected by the specialized receptors in the nervous system, since they monitor the changes in blood volume. Should you for instance take in a liter of fluid, the kidneys will excrete the excess, simply by increasing its urine production eight fold within thirty minutes.
When blood volume increases the pressure is increased within the atria of the heart which then activates stretch receptors, which in turn send a signal for reduction of ADH release in the posterior pituitary, leading to less fluid to be reabsorbed by the kidneys. It secondly reduces the vasomotor tone of the blood vessels, which leads to dilation of the blood vessels, causing an increase of the glomerular blood pressure and increased filtration is achieved, with less water being reabsorbed by osmosis.
The reserve happens when blood volume decreases and less urine is formed with more fluid being retained and so correcting blood volume.
The primary effect of ADH is to limit the amount of water being lost in urine, by increasing the amount of water being reabsorbed into the blood. The ADH targets the cells of the tubules and collecting ducts, which causes an increase of permeability of the cell surfaces, where the water then leaves the renal tubules by means of osmosis.
With more fluid being reabsorbed, the blood volume increases while the solutes concentration becomes more diluted.
As soon as the osmolarity of the blood and body fluids is reduced, with more fluid being reabsorbed by the tubules in the kidneys, the receptors in the hypothalamus are no longer stimulated and the level of ADH stimulation is reduced, which then in turn signals to the kidneys to start excreting more water in the urine production until the blood osmolarity increases enough for the cycle to be started again.
ADH is not the only hormone that helps with the regulation of kidney function - aldosterone (from the adrenal cortex) as well as parathyroid hormone (from the parathyroid glands) affects the balance and regulation of electrolyte content of the blood and body fluids.
When aldosterone is present in the blood, the distal renal tubules increase their re-absorption of sodium and the secretion of potassium. With this action, more water is retained in the body and a person with high aldosterone content can have “puffy” features from the increased water volume.
Aldosterone is secreted by the adrenal glands when the level of the potassium in the blood is increased, as well as the self-regulatory action of the kidneys by means of the renin-angiotensin system.
When blood pressure increases so does the glomerular filtration increase, but when blood pressure falls, the filtration level drops and the glomerular filtration rate then needs another system to increase the filtration rate.
This is made possible with tubuloglomerular feedback where a specialized region of the nephron - the juxtaglomerular complex - will detect the decreased fluid flow within the nephron tubules and will increase the glomerular filtration.
The renin-angiotensin system will not only assist to increase filtration of the glomerular, but can also affect the adrenal secretion of aldosterone, which will help to bring the low systemic blood pressure up to normal.
Parathyroid hormone (PTH) is responsible for the endocrine regulation of calcium and phosphate. When blood levels of calcium decrease it stimulates the production of PTH, which has three physiological effects, one having a direct bearing on the kidneys.
In the kidneys the parathyroid hormone increases calcium re-absorption in the renal tubules, while phosphates are not really affected.
The kidneys are primarily involved in removing nitrogen-containing wastes to prevent toxic build-up.
The metabolic wastes occur during breakdown of nitrogen-containing proteins and purine nitrogenous bases. This metabolism also involves the removal of nitrogen from amino acids and amino nitrogen is often removed as ammonia, which is extremely toxic to cells and needs to be removed from the blood and body fluids. The liver cells can also combine amino nitrogen with carbon dioxide, which then produces urea.
Urea is excreted by the kidneys, and is less toxic than ammonia and can be transported in the blood from the liver where it is formed from ammonia. This molecule is very small, diffuses easily across cell membranes and requires no specialized transport system.
It is osmotically active and can function in regulating osmotic pressure of blood and other body fluids.
Uric acid is less toxic than urea and is also excreted by the kidneys, and while it is a bigger molecule than urea and less soluble, the uric acid that appears in urine is mostly secreted by the tubule cells in the kidneys.
Uric acid is formed from the metabolism of nucleic acids. Nucleotides adenosine and guanosine are purine nucleotides as they contain purine nitrogenous bases, which are double-ring organic compounds, which also contain nitrogen in their ring structure. Nucleotide metabolism metabolizes purine nitrogenous bases to uric acid by the liver cells.