In mammals the main organs of osmoregulation are the kidneys. They have two linked jobs:• excretion: removing metabolic waste, mainly urea made in the liver, from the blood
• osmoregulation: controlling the water potential of the blood by adjusting how much water and how many ions are kept in the bodyEach kidney contains about a million tiny tubules called nephrons. Blood is filtered at the start of each nephron, making a fluid called filtrate. Useful substances are then taken back into the blood, and the remaining fluid leaves as urine. By changing how much water is reabsorbed, the kidneys change the concentration of the urine:• if the blood water potential is too low (for example after heavy sweating or a salty meal), more water is reabsorbed, so a small volume of concentrated urine is produced
• if the blood water potential is too high (for example after drinking a lot of water), less water is reabsorbed, so a large volume of dilute urine is producedMammals can make urine that is more concentrated than their blood, that is, with a lower water potential. This is an adaptation to life on land, where water is often scarce and is lost continually by evaporation. It depends on the loop of Henle.The kidneys work with the brain. Changes in blood water potential are detected by osmoreceptors in the hypothalamus, which control the release of antidiuretic hormone (ADH) from the posterior pituitary gland. ADH alters how much water the kidneys reabsorb. This control system is covered later in the topic.The kidneys also help to regulate blood pH and blood pressure.
• osmoregulation: controlling the water potential of the blood by adjusting how much water and how many ions are kept in the bodyEach kidney contains about a million tiny tubules called nephrons. Blood is filtered at the start of each nephron, making a fluid called filtrate. Useful substances are then taken back into the blood, and the remaining fluid leaves as urine. By changing how much water is reabsorbed, the kidneys change the concentration of the urine:• if the blood water potential is too low (for example after heavy sweating or a salty meal), more water is reabsorbed, so a small volume of concentrated urine is produced
• if the blood water potential is too high (for example after drinking a lot of water), less water is reabsorbed, so a large volume of dilute urine is producedMammals can make urine that is more concentrated than their blood, that is, with a lower water potential. This is an adaptation to life on land, where water is often scarce and is lost continually by evaporation. It depends on the loop of Henle.The kidneys work with the brain. Changes in blood water potential are detected by osmoreceptors in the hypothalamus, which control the release of antidiuretic hormone (ADH) from the posterior pituitary gland. ADH alters how much water the kidneys reabsorb. This control system is covered later in the topic.The kidneys also help to regulate blood pH and blood pressure.