The main function of the kidney in a human body is to filter the blood in order to remove cellular waste products from the body.
At any given time, 20 percent of the body’s blood is in the kidneys.
The main metabolic wastes are urea, uric acid, and creatinine, all of which have nitrogen as a major component.
Urea is produced in the liver from the breakdown of excess amino acids that are the building blocks of proteins.
Although the amine group can combine with a hydrogen ion to form toxic ammonia, the ammonia is transformed through a complex series of metabolic reactions in the liver into the less toxic urea before being released into the bloodstream.
To form urea and water the liver combines ammonia with carbon dioxide in the urea cycle.
The urea cycle
Uric acid is the end product of purine metabolism.
Creatinine is a breakdown product of creatine phosphate in muscle.
Phosphocreatine, also known as creatine phosphate (CP) or PCr (Pcr), is a phosphorylated creatine molecule that serves as a rapidly mobilizable reserve of high-energy phosphates in skeletal muscle and the brain.
Phosphocreatine is synthesized in the liver and transported to the muscle cells, via the bloodstream, for storage.
Phosphocreatine can anaerobically donate a phosphate group to ADP to form ATP during the first 2 to 7 seconds following an intense muscular or neuronal effort.
The kidneys are more than excretory organs; they are one of the major homeostatic organs of the body.
In addition to filtering the blood to remove wastes they also control:
- the water balance,
- levels of sodium (Na), potassium (K), bicarbonate (HCO3−), and calcium (Ca) ions.
The kidneys also secrete a hormone (erythropoietin, EPO) that stimulates red blood cell production, and they activate vitamin D production in the skin.
Erythropoietin is produced in response to hypoxia (a low level of oxygen in the blood). EPO stimulates the cells of red bone marrow to increase their output of red blood cells. Oxygen levels in the blood increase as more red blood cells mature and enter the bloodstream.
Calcitriol is the active form of vitamin D in the body.
Calcitriol is produced from inactive vitamin D molecules and travels from the kidneys through the bloodstream to the intestines, where it increases the absorption of calcium from food in the intestinal lumen.
Each kidney is composed of three sections — the outer cortex, the medulla, and the hollow inner pelvis where urine accumulates before it travels down the ureters.
Within the cortex and medulla of each kidney are about one million tiny filters called nephrons.
Each nephron consists of five parts:
- the Bowman’s capsule,
- the proximal tubule,
- the loop of Henle,
- the distal tubule,
- the collecting duct.
The upper portions of the nephron are found in the renal cortex, while the loop of Henle is located in the renal medulla.
The tubes of the nephron are surrounded by cells, and a network of blood vessels spreads throughout the tissue. Any material that leaves the nephron enters the surrounding cells and eventually returns to the bloodstream through the network of blood vessels.
Blood enters the cavity of the ball-shaped Bowman’s capsule through a tiny artery that branches to form a network of porous, thin-walled capillaries called the glomerulus.
Under the influence of blood pressure, some blood plasma and small particles are forced out of the capillaries and into the surrounding capsule.
Larger blood components, such as blood cells and proteins, remain in the capillaries.
The fluid in the Bowman’s capsule is called nephric filtrate, and it is pushed out of the capsule into the proximal tubule.
About 20 percent of the blood plasma that enters the kidney becomes nephric filtrate.
When the nephric filtrate enters the proximal tubule, reabsorption begins.
Osmosis, diffusion, and active transport draw water, glucose, amino acids, and ions from the filtrate into the surrounding cells. From here the materials return to the bloodstream. This process is aided by active transport of glucose and amino acids out of the filtrate.
When the filtrate reaches the end of the proximal tubule, the fluid is isotonic with the surrounding cells, and the glucose and amino acids have been removed from the filtrate.
A fluid is isotonic when it has the same concentration of water and solutes as that in the cells surrounding it.
From the proximal tubule, the filtrate moves to the loop of Henle.
The primary function of the loop of Henle, which first descends into the inner renal medulla and then turns to ascend back towards the cortex, is reabsorption of water from the filtrate by the process of osmosis.
The cells of the medulla have an increased concentration of sodium ions (Na+). These ions increase in a gradient starting from the area closest to the cortex and moving toward the inner pelvis of the kidney. This increasing gradient acts to draw water from the filtrate in the loop of Henle. This process continues down the length of the descending loop due to the increasing level of Na+ in the surrounding tissue. The high levels of Na+ in the surrounding medulla tissue are the result of active transport of Na+ out of the ascending loop of Henle.
The amount of water removed from the filtrate by the time it reaches the bottom of the loop of Henle results in an increased concentration of all of the materials dissolved in the remaining filtrate, including Na+.
Thus, as the filtrate moves up the ascending loop of Henle, Na+ is actively pulled from the filtrate into the surrounding tissue. At the same time, the water that left the descending loop cannot re-enter the ascending loop because this loop is impermeable to water.
Chloride ions tend to follow the sodium ions because of the electrical attraction between the negative chloride ions and the positive sodium ions. In addition, as the water concentration in the filtrate decreases, the chloride ion concentration in the filtrate increases, resulting in still more chloride diffusion out of the ascending loop.
The distal tubule is responsible for a process called tubular secretion.
Tubular secretion involves active transport to pull substances such as hydrogen ions, creatinine, and drugs such as penicillin out of the blood and into the filtrate.
The fluid from a number of nephrons moves from the distal tubules into a common collecting duct, which carries what can now be called urine into the renal pelvis.
At that point, 99 percent of the water that entered the proximal tubule as nephric filtrate has been returned to the body. In addition, nutrients such as glucose and amino acids have been reclaimed.
The permeability of the distal tubule and collecting duct is controlled by a hormone called anti-diuretic hormone (ADH).
ADH is secreted by a gland attached to the hypothalamus called the pituitary gland. Anti-diuretic hormone increases the permeability of the distal tubule and collecting duct, thus allowing more water to be removed from the nephric filtrate when the body has a need to conserve water.
The pituitary gland is controlled by the hypothalamus.
The hypothalamus acts to regulate the body’s feedback systems. When the body needs to eliminate excess water, anti-diuretic hormone is inhibited and more water is excreted in the urine. Drugs such as alcohol and caffeine block the release of ADH and increase the volume of urine.
The kidneys regulate the acid-base balance of the blood.
To remain healthy, our blood pH should stay around 7.4, which is slightly basic.
One way in which blood pH is controlled at this level is by regulation of the active transport of hydrogen ions (H+) into the nephric filtrate.
The respiratory system works with the kidneys to help maintain the pH of the blood at 7.4. The two systems depend on chemicals called buffers to control pH.
|Regulation of blood pH|
The main buffer in the blood is carbonic acid (H2CO3), a weak acid that reacts to release H+ and the bicarbonate ion (HCO3−).
Levels of H2CO3 are linked to levels of CO2 , which are regulated by breathing.
Another example of buffering is the combining of hydrogen ions in the blood with ammonia from the cells that line the nephron.