Kidneys

• In humans, as in all mammals, the kidneys are a pair of dark reddish brown organs attached to the back of the abdominal cavity.
• The kidneys are surrounded by a thick layer of fat, which helps to protect them from mechanical damage.
• The colour of urine varies from almost colourless to deep yellow according to its concentration.
• The colour of urine depends on the colours of food.
• Glucose and protein should never appear in urine, if they do, it indicates the presence of problems elsewhere in the body or the kidneys are not working properly.
• Each kidney is made up of about 1.5 million microscopic tubules called nephrons, which are 2-4 cm long.
• There are two main types of nephron: cortical nephrons, found mainly in the renal cortex, and juxtamedullary nephrons, which have long loops of Henle that penetrate right through the medulla.
• Cortical nephrons are particularly efficient at producing concentrated urine.
• The balance of these different types of nephrons varies in different organisms.
• The mammalian kidney has two main roles in the body: excretion, the removal of urea from the body, and osmoregulation, the control of water potential of the blood that passes through them.
• In humans, around 120 cm3 of blood per minute passes through the kidneys, a rate that means all of the blood in your body travels through the kidneys and is filtered and balanced approximately once every hour.
• The kidney carries out three main functions in its osmoregulatory role: ultrafiltration, selective reabsorption and tubular secretion.
• Ultrafiltration in the kidney tubules occurs due to a combination of very high blood pressure in the glomerular capillaries, and the structure of the Bowman's capsule and glomerulus.
• High blood pressure develops in the glomerular capillaries because the diameter of the blood vessel coming into the glomerulus is greater than that of the blood vessel leaving, which squeezes the blood out through the pores in the capillary wall.
• The size of the pores means that almost all the contents of the plasma can pass out of the capillary — only the blood cells and the largest plasma proteins cannot get through the pores.
• The cells of the Bowman's capsule next to the capillaries act as an additional filter.
• The wall of the capsule is made up of special cells called podocytes.
• Podocytes have extensions called pedicels that wrap around the capillaries, forming slits that ensure any cells, platelets or large plasma proteins that have left the capillary do not get through into the tubule itself.
• The glomerulus and the Bowman's capsule together make up the Malpighian body.
• The filtrate that enters the capsule contains glucose, salt, urea and many other substances in the same concentrations as they are in the blood plasma.
• If all of the filtrate produced in the Malpighian bodies over a 24 hour period was then passed out of the body, we would produce around 200 dm3 of urine a day and would have to drink continually to replace it!
• The average daily urine production is 1-2 dm3 because most of the filtrate is later reabsorbed into the blood.
• Ultrafiltration is passive and it removes urea from the blood, but it also removes a lot of water along with glucose, salt and other substances that are present in the plasma.
• Glucose is needed for cellular respiration and is never, under normal circumstances, excreted.
• Most of the water, salt and other inorganic ions passed into the tubule during ultrafiltration are also needed by the body.
• The main function of the kidney tubule after the ultra-filtrate has entered the nephron is to return most of what has been removed from the blood back to its rightful place.
• The cells lining the proximal convoluted tubule are covered with microvilli, which greatly increase the surface area through which substances can be absorbed.
• The cells also have large numbers of mitochondria, indicating that they are involved in active processes.
• Over 80% of the glomerular filtrate is reabsorbed back into the blood in the first or proximal convoluted tubule.
• All of the glucose, amino acids, vitamins and most hormones are moved back into the blood by active transport.
• About 85% of the sodium chloride and water is reabsorbed as well.
• The sodium ions are actively transported, and the chloride ions and water follow passively down concentration gradients.
• Once the substances are removed from the tubule cells into the intracellular spaces, they then pass by diffusion into the extensive capillary network that surrounds the tubules.
• The blood is constantly moving through the capillaries, maintaining a concentration gradient for diffusion.
• By the time the filtrate reaches the loop of Henle it is isotonic with the tissue fluid that surrounds the tubule.
• The amount of reabsorption that occurs in the proximal tubule is always the same — the fine tuning of the water balance takes place further along the nephron in the loop of Henle.
• The loops of Henle are found in the medulla of the kidney in close contact with the network of capillaries known as the vasa recta.
• The loop of henle has two limbs: Descending limb and Ascending limb.
• The loops of henle create a water potential gradient between the filtrate and the medullary tissue fluid that enables water to be reabsorbed from the distal convoluted tubule and collecting duct.
• The creation of the high concentration of sodium and chloride ions in the tissue fluid of the medulla is due to the flow of fluid in opposite directions in the adjacent limbs of the loop of Henle, combined with the different permeability of the different sections to water and a region of active transport.