All about Homeostasis PDF

All about homeostasis! Selective reabsorption in the proximal convoluted tubule: The proximal convoluted tubule is the longest section of a nephron. In this a large part of the filtrate is reabsorbed. The walls of the tubules are one cell thick and their cells are packed with mitochondria to prod...

All about homeostasis! Selective reabsorption in the proximal convoluted tubule: The proximal convoluted tubule is the longest section of a nephron. In this a large part of the filtrate is reabsorbed. The walls of the tubules are one cell thick and their cells are packed with mitochondria to produce energy for active transport. The cell surface membrane that is in contact with the filtrate has a brush border membrane of microvilli to increase surface area for reabsorption. The mechanism of reabsorption: Active transport of sugar and amino acids across the cell surface membrane by the activity of special carrier protein in a process known as co-transport. In co-transport the carrier protein uses the diffusion of hydrogen ions down their gradient into the cell to drive the uptake of molecules of sugar such as glucose against their concentration gradient. Basically, water is being reabsorbed by very large amounts in the proximal convoluted tubule because selective reabsorption for ions, glucose, amino acids all lower the water potential of the blood so water diffuses into the blood by osmosis. Other essential metabolites are transported similarly like movement of mineral ions by active transport and facilitated diffusion. Diffusion of urea. Movement of proteins by pinocytosis. 1.Na*-K* pumps in the basal membrane of proximal convoluted tubule cells use ATP made by the mitochondria. These pumps decrease the concentration of sodium ions in the cytoplasm. The basal membrane is folded to give a large surface area for many of these carrier proteins, 2.Very close nearby, the blood plasma rapidly removes absorbed Na*, Cl, glucose and amino acids. This helps further uptake from the lumen of the tubule.  3. Microvilli increase surface area, helping bed uptake of solutes. Nat moves passively into the cell down its concentration gradient. It moves in using protein co- transporter molecules in the membrane which bring in glucose and amino acids at the same time. 2. Water conservation in the loop of Henle The structure of the loop of henle is the ascending and descending limb with a parallel blood supply, vasa recta. Vasa recta is a part of the same capillary network that surrounds a nephron The role of the loop of henle: Loops create and maintain an osmotic gradient This gradient allows water to move out from the collecting ducts if required and this is required when water potential decreases How is the gradient created? By something called counter current multiplier and this involves an exchange between fluids slowing in opposite directions in two systems At the limbs: The walls of the ascending limb are impermeable to water and permeable to salts Descending limb is fully permeable to water and some salts 1. Sodium and chloride ions are actively transported out of the ascending limb into the tissue fluid 2. This raises the Na and Cl concentration in the tissue fluid which causes water to pass out into the tissue fluid and Na and Cl to the descending limb 3. As the filtrate flows a high concentration of salts is formed around the hairpin In the medulla, which allows water to be absorbed by nearby collecting ducts. The role of vasa recta: Drive the oxygen to and remove carbon dioxide from the cells of the loop of henle The vasa recta absorb water that passes into the medulla at the collecting duct The blood in the vasa recta becomes saltier as it flows down beside the ascending limb and becomes less salty as it flows back up out of medulla. NEXT: Blood pH and ion concentration regulation in the distal convoluted tubule: Blood proteins prevent any change in blood pH (buffer) If blood deviates from pH 7.4 in the DCT there is controlled secretion of H ions and reabsorption of HCO3 ions The pH of blood remains 7.35-7.45 and urine 4.5-8.2 The concentration of K+ id adjusted by the secretion of any excess present in the plasma into the filtrate The concentration of Na+ is regulated by varying the quantity of sodium ions reabsorbed from the filtrate NEXT: Water reabsorption in the collecting duct Osmoregulation is an example of the control of blood water content Hypothalamus controls many body functions: Composition of blood is monitored here as it circulates through capillary network of the hypothalamus Data is received from sensory receptors Hypothalamus controls the pituitary gland, it is located below the hypothalamus but connected to it In osmoregulation process: The posterior part of the pituitary gland stores and releases ADH ADH is produced in the hypothalamus and stores in vesicles at the end of neurosecretory cells in pituitary gland When nerve impulses from hypothalamus trigger the release of ADH into the capillary network in the posterior pituitary. ADH circulates in the blood stream The targets of this hormones are the walls of the collecting duct. When water levels are too low: The hypothalamus sends signal to pituitary gland to release ADH. So the body reabsorbs more water and less urine is produced. When water levels are too high: The hypothalamus stops sending signals to pituitary gland so not as much ADH is released and the body reabsorbs less water. More urine is produced. How does ADH change the permeability of the walls of the collecting duct? When low water potential? When osmoreceptors detect low water content in the blood, they send impulses to hypothalamus that sends signals to pituitary gland to release ADH. When ADH is present: ADH causes the protein channels (aquaporins) in the collecting duct cell surface membrane to open As a result water diffuses out of the medulla and is taken up in the body by blood circulation. Only small amount of concentrated urine is formed. The negative feedback for this would be: No or little ADH is secreted and ADH is removed by the liver or kidney When ADH is absent: The aquaporins in the collecting duct are closed and the amount of water that is retained in the medulla is now minimal. The urine formed is now much more and dilute. So basically: Osmoreceptors->impulses->hypothalamus->impulses->pituitary gland->secretion of ADH->target the collecting duct walls->aquaporin channels open->water diffuses into medulla->water is taken up by blood and distributed all over body->normal water content and little concentrated urine. The change in permeability of the collecting duct walls due to the presence of ADH is brought about by increasing the number of aquaporins: 1. ADH binds to receptor which stimulate the production of cAMP 2. This activates enzyme signaling cascade leading to the phosphorylation of the aquaporins 3. This causes vesicles containing aquaporins to move towards cell surface membrane 4. They fuse with the cell surface membrane 5. Water can now move freely through aquaporins down water potential gradient Control of blood glucose concentration: Normal level of blood glucose, each 100cm3 of blood contains between 80-120mg of glucose Low values arise during an extended period without food or after exercise The highest value occurs after a meal rich in carbohydrates All body cells have reservation in the form of Glycogen and this is converted to glucose during exercise In the brain glucose is the only substrate that cells can use and there are no glycogen stores at all Hypoglycemia: Blood glucose falls below 80mg/100cm3. If this is not reversed, we may faint Hyperglycemia: Abnormally high concentration of blood glucose concentration occurs. Regulation of blood glucose level: At pancreas the presence of an excess blood glucose is detected by Islets of Langerhans. These islets are hormone secretion glands and contain alpha cells and beta cells. Beta cells: Detect a raise in blood glucose level and secrete insulin hormone 1. Insulin stimulates the uptake of glucose by cells in the body. In the liver it triggers the conversion of glucose to glycogen (glycogenesis) and glucose to fatty acids. 2. This promotes the distribution of fat around the body and increases rate of respiration. The negative feedback would be: As blood glucose levels return back to normal this is detected in the islets of Langerhans and the beta cells are signaled to stop producing insulin. Alpha cells: Detect low blood glucose levels and release the hormone glucagon Glucagon: 1. Glucagon activates the enzyme that converts glycogen and amino acids to glucose (gluconeogenesis) 2. Activates enzymes in the liver to break down glycogen to glucose (glycogenolysis) 3. This reduces rate of respiration Glucose can only enter cells by facilitated diffusion through transporter proteins called GLUT: 1. Insulin binds to receptor in the cell surface membrane 2. The receptor signals cell so that vesicles with glucose transporter proteins (GLUT) move towards the cell surface membrane and fuse with it 3. Glucose can now diffuse into the cell down its concentration gradient The method of cell signaling in response to glucagon: 1. Glucagon binds to receptor 2. Activating G protein leading to stimulation of enzyme adenylyl cyclase 3. Active adenylyl cyclase produces cAMP from ATP 4. cAMP helps initiate enzyme cascade by activating protein kinase A 5. Enzyme cascade leads to the activation of many molecules of glycogen phosphorylase that break down glycogen. 6. Final product is Glycogen->Glucose Muscles: import glucose but don’t export Liver: Imports glucose when Blood glucose level is high, exports glucose when BGL is low Brain: Imports glucose and no glycogen reserves Small intestines: Glucose absorption Most tissues: Can import or export glucose  Endocrine glands: They are glands of the endocrine system that secrete their hormones directly into the blood rather than through the duct. Measurements of glucose levels in the body: Glucose is completely reabsorbed in the PCT so it is not present in the urine of a healthy person Dipstick: contains two enzymes Glucose oxidase and Peroxidase. Reactions: 1. Glucose + Oxygen -> (enzyme is glucose oxidase) Gluconic acid + hydrogen peroxide 2. Hydrogen peroxide + Colorless chromogen -> (enzyme peroxidase) Oxidized chromogen Brown + Water HOW? (when strip is dipped into urine and it contains urine) The more glucose present the more colored dye is formed and lastly color is compared to the color on printed scale. Glucose biosensor: It is a device that makes use of biological molecules to detect and measure glucose It has immobilized enzyme glucose oxidase 1. The outer membrane is brought in contact with blood that was squeezed from a pinprick at the tip of the finger 2. Once in contact with glucose oxidase, the glucose in the blood plasma is oxidized to gluconic acid and hydrogen peroxide 3. An electrical signal is produced 4. The size of this signal is proportional to the amount of glucose present in patients’ blood 5. A digital read out gives the concentration of glucose present in the persons blood