 In this video, I will describe how each of the following works to regulate reabsorption and secretion to affect urine volume and composition. The renin angiotensin aldosterone system, antidiuretic hormone, natriuretic peptides, and parathyroid hormone. Aldosterone is a steroid hormone that is secreted by cells in the zona glomerulosa of the adrenal cortex. Aldosterone is also known as mineral corticoid, and it binds to an intracellular receptor known as the mineral corticoid receptor. The stimulus for the production of aldosterone is the renin angiotensin mechanism leading to the production of angiotensin II. Angiotensin II binds to receptors on the surface of cells in the adrenal cortex in the zona glomerulosa and then stimulates the production of aldosterone. However, angiotensin II is not the only stimulus for the production of aldosterone. An elevated plasma potassium concentration, also known as hyperkalemia, is a stimulus for aldosterone production. And another hormone, adrenocorticotropic hormone, commonly abbreviated ACTH, is secreted by the anterior pituitary gland in response to stress. And ACTH can also stimulate the production of aldosterone. Here we see a summary of the renin angiotensin aldosterone system, starting with the kidneys releasing renin into the blood. This would occur in response to a low sodium concentration of the filtrate. Renin will convert angiotensinogen to angiotensin I, then angiotensin I is converted to angiotensin II by the angiotensin converting enzyme in the lungs. Angiotensin II binds to receptors on the surface of cells in the zona glomerulosa of the adrenal cortex to stimulate the production of aldosterone. Then, aldosterone binds to the mineral corticoid receptors in the cells of the distalconvoluted tubule and collecting ducts of the nephron, leading to increased sodium and water reabsorption. Aldosterone works by binding to an intracellular receptor, known as the mineral corticoid receptor, and stimulating increased transcription of genes for the epithelial sodium channel and sodium and water reabsorption. The epithelial sodium channel enables the facilitated diffusion of sodium from the filtrate into the epithelial cells of the distalconvoluted tubule or collecting duct. The sodium potassium pump enables the facilitated diffusion of sodium from the filtrate into the epithelial cells of the distalconvoluted tubule or collecting duct. And then the sodium potassium pump enables a primary active transport mechanism by increasing the expression of the genes for these proteins. An increased number of sodium channels and sodium potassium pumps leads to an increased rate of sodium reabsorption. And at the same time, this will lead to an increased rate of potassium secretion. So the stimuli for aldosterone production are increased blood potassium or low blood sodium levels. And as aldosterone has its effects in the kidney at the distalconvoluted tubule and collecting duct, it leads to potassium secretion and sodium reabsorption, helping to restore the homeostatic concentrations of sodium and potassium. Aldosterone also works together with another hormone known as antidiuretic hormone. The secretion of antidiuretic hormone is regulated by osmoreceptors of the hypothalamus, which are neurons sensitive to the concentration of solutes. In response to a high solute concentration and a high osmolarity that is detected by the osmoreceptors of the hypothalamus, antidiuretic hormone is released from the posterior pituitary gland. And then travels throughout the body in the blood. But when antidiuretic hormone binds to receptors on the surface of cells in the distalconvoluted tubule and collecting duct, it will stimulate watery absorption. So by stimulating water reabsorption, antidiuretic hormone can work to increase the blood volume and decrease the concentration of solutes in the blood, helping to maintain a homeostatic osmolarity of blood plasma. So while aldosterone will stimulate sodium reabsorption, antidiuretic hormone stimulates water reabsorption. But reabsorption of sodium will create a driving force to help fuel the reabsorption of water as increasing sodium reabsorption creates an osmotic driving force that will fuel further reabsorption of water. So both aldosterone and antidiuretic hormone can work to increase water reabsorption to increase or help maintain blood volume. In response to dehydration, we typically produce a small volume of relatively concentrated urine, whereas if we're very well hydrated, we'll produce a larger volume of more dilute urine. And this works to help maintain water balance in the body. If the blood osmolarity becomes too high, this will be detected by osmoreceptors in the hypothalamus that stimulate the secretion of antidiuretic hormone. This will also stimulate thirst, leading us to go find a drink in order to increase the amount of water intake to help restore water balance of the body. At the same time as antidiuretic hormone will have its actions in the kidney to help produce a smaller volume of more concentrated urine. A decrease in blood volume can also lead to decreasing blood pressure. And this is a stimulus that will then lead to the activation of the renin angiotensin aldosterone mechanism. Angiotensin 2 will have a widespread effect through the body of vasoconstriction. Similarly, antidiuretic hormone causes vasoconstriction as a mechanism to help increase blood pressure, maintaining the circulation despite a decreasing blood volume in the state of dehydration. Osmoreceptors found in the hypothalamus are neurons that produce antidiuretic hormone and have their axons extending down into the posterior pituitary gland. In response to a high osmolarity during dehydration, osmoreceptors will secrete antidiuretic hormone into the blood. Here we can see the major effects of antidiuretic hormone. Antidiuretic hormone is produced by osmoreceptors of the hypothalamus and released from the posterior pituitary gland. Then, antidiuretic hormone binds to receptors on vascular smooth muscle cells, causing vasoconstriction to help increase total peripheral resistance and increase blood pressure. Then, antidiuretic hormone also has its effect in the kidneys to stimulate the reabsorption of water, producing a small volume of concentrated urine. A synonym for antidiuretic hormone is vasopressin, and this synonym relates to the fact that antidiuretic hormone has an action of stimulating vasoconstriction. Aquaporin channels are proteins that enable facilitated diffusion of water. In response to antidiuretic hormone, the epithelial cells of the collecting duct and distal convoluted tubule will increase the number of aquaporin channels. This increased expression of aquaporins is responsible for increasing water reabsorption, leading to the small volume of concentrated urine in response to antidiuretic hormone. The atrial natiuretic peptide will also have an influence on the reabsorption of sodium and water. If we are very well hydrated and have a very large plasma volume, this can lead to stretching of the heart, stretching of the atrial chambers of the heart will lead to increased secretion of atrial natiuretic peptide. Then, ANP will inhibit the renin angiotensin aldosterone system, also inhibit the secretion of antidiuretic hormone, and have the indirect effects of decreasing the reabsorption of sodium and water as a result of inhibiting the signaling of aldosterone and antidiuretic hormone. This will lead to an increase in the urine volume and a decrease in the concentration of that urine. Parathyroid hormone stimulates the reabsorption of calcium in the distal convoluted tubule. Parathyroid hormone is secreted by the parathyroid gland in response to low blood calcium concentrations, also known as hypocalcemia. Parathyroid hormone will bind to receptors on the surface of cells in the distal convoluted tubule, stimulating calcium reabsorption, and also stimulating the activation of vitamin D to the calcitriol form. Calcitriol will then function as a hormone that stimulates calcium absorption in the intestines.