Homeostasis

• Homeostasis:

  the maintenance of a constant internal environment.

  factors Include:

  • temperature
  • blood pH
  • blood glucose
  • if these aren't maintained enzymes will become denatured
• negative feedback mechanisms:

  These are mechanisms which act to reverse a change in the body

  the change is detected by receptors and the reversal is changed by effectors

  negative feedback mechanisms enable the body to stay in certain limits and fluctuates around the normal level
• positive feedback mechanisms:

  amplify a detected change which moves conditions away from a normal level

  used to accelerate a biological pathway

  Not involved in homeostasis but does keep it in a normal range
• control of blood glucose concentration: It is monitored and controlled by the pancreas which releases a hormone called insulin if glucose levels become too high and another hormone called glucagon if levels fall too low. The action of insulin in reducing blood glucose levels and glucagon in increasing blood glucose are examples of negative feedback loops.
• when blood glucose is too high:

  • Cells in the pancreas stimulate beta cells in the islets of Langerhans to secrete insulin.
  • Insulin travels in the blood to liver and muscle cells, it binds to insulin receptors on their cell surface membrane.
  • Insulin increases the permeability of the membrane to glucose, so more glucose is moved from the blood into cells. When insulin binds to its receptor, vesicles with the GLUT4 glucose transporter move and fuse with the plasma membrane.
  • It also stimulates glycogenesis and an increase in the rate of respiration, which helps to lower blood glucose.
• when blood glucose is too low:

  • Cells in the pancreas detect low blood glucose and stimulate alpha cells in the islets of Langerhans to secrete glucagon.
  • Glucagon travels in the bloodstream to liver cells, where it binds to glucagon receptors on their cell surface membrane.
  • Glucagon stimulates the breakdown of glycogenolysis and a decrease in the rate of respiration, which helps to increase blood glucose.
  • It also triggers the production of glucose from non-carbohydrates, such as lipids and amino acids, in a process called gluconeogenesis.
• glycogenolysis: the breakdown of glycogen into glucose
• gluconeogenesis:

  the production of glucose from non-carbohydrate sources
• glycogenesis:

  conversion of glucose into glycogen
• Adrenaline:

  increases blood glucose levels as part of the fight-or-flight response that prepares our body for action. released by the adrenal glands and binds to adrenaline receptors on the membrane of liver cells. It stimulates hydrolysis of glycogen, stimulates the secretion of glucagon and inhibits the secretion of insulin.
• second messengers:

  hormones such as adrenaline, glucagon and insulin can’t get inside target cells to exert their effects directly, they use these to activate reactions inside

  • Adrenaline binds to its receptor on the cell surface membrane of liver cells.
  • This activates the enzyme adenylyl cyclase, which catalyses the production of a second messenger called cyclic AMP
  • cAMP then activates a cascade of intracellular reactions.
  • For example, cAMP activates an enzyme called protein kinase A, which activates a cascade which eventually leads to the breakdown of glycogen
• Type 1 diabetes:

  • onset in childhood
  • immune system destroys the beta cells of the islets of Langerhans so they can no longer produce insulin
  • results In high blood glucose - glucose in urine
  • treated with insulin pumps and injections
• Type 2 diabetes:

  • Onset in adulthood - linked to lifestyle factors
  • body cells stop responding properly to insulin - receptors stop working
  • treated with a managed diet
  • becoming more common
• determining concentration of a glucose solution:

  • make a calibration curve by preparing serial dilutions in five separate test tubes
  • add quantitative Benedict's reagent to each solution
  • use a colorimeter on each solution then plot curve absorbance y and glucose concentration x
  • take a urine sample and add quantitative Benedict’s reagent. Use colorimeter to record absorbance and read off the concentration of the solution using the calibration curve.
• Kidneys structure:

  The inner part of the kidney is the medulla and the outer part is the cortex. Blood is carried to the kidney via the renal artery for the kidney to filter blood and remove waste products. The filtered blood is taken away from kidneys by the renal vein. The individual structures which filter blood are called nephrons. kidneys remove excess water, excess ions and urea from our blood as urine. The removal of these substances from the bloodstream involves ultrafiltration and selective reabsorption.
• Ultrafiltration:

  The Bowman’s capsule surrounds a ball of capillaries called the glomerulus where blood is placed under high pressure. The efferent arteriole is smaller in diameter than the afferent arteriole. so blood inside the glomerulus is under high pressure. Small molecules are pushed out of the bloodstream and into the Bowman’s capsule, while larger molecules stay inside the capillaries. They form a substance called glomerular filtrate, which moves through the nephron
• ultrafiltration small molecules:

  passed between the capillary and kidney they pass through three layers:

  • capillary endothelium
  • basement membrane
  • epithelium of the Bowman's capsule
• selective reabsorption: In the proximal convoluted tubule, loop of Henle and the distal convoluted tubule, useful substances are reabsorbed, passing out of the nephron and back into the capillaries. Glucose is reabsorbed in the PCT by active transport and facilitated diffusion. The PCT epithelium has microvilli for a large surface area for reabsorption. Water is reabsorbed in the loop of Henle, DCT and collecting duct by osmosis. The remaining filtrate is urine. This passes from the collecting duct to the bladder via the ureter.
• Water Reabsorption where:

  Water is reabsorbed along almost the entirety of the nephron, but regulation of water potential mostly takes place in the DCT, loop of Henle and collecting duct with the amounts of water reabsorbed controlled by hormones such as ADH.
• the Kidney where processes take place: The nephron spans two parts of the kidney – the Bowman’s capsule, PCT and DCT are in the cortex of the kidney while the loop of Henle and lower half of the collecting duct are located in the medulla. The loop of Henle is made up of two limbs called the ascending and descending limb, which control the movement of ions and allow water to be reabsorbed
• Water Reabsorption:

  • The ascending limb is permeable to ions but not water. the top of ascending limb, sodium ions are actively pumped into medulla. lowering water potential of the medulla, making water move out of nephron from the descending limb As water moves out of nephron, the filtrate becomes concentrated. This causes sodium ions to leave the nephron at the start of ascending limb, down concentration gradient by facilitated diffusion. lowering water potential of the medulla, causing water to move out of DCT and collecting duct. Water in medulla then moves to capillary
• dry conditions water adaptation:

  • have an extra long loop of Henle meaning more ions can be pumped into the medulla
• ADH:

  • ADH controls the water content of our urine by increasing the reabsorption of water from the collecting duct. It works by increasing the permeability of the collecting duct wall, making it more porous and allowing more water to pass from the kidney into the bloodstream. It is released by the pituitary gland in the brain when water levels in our blood plasma are becoming low. this is a negative feedback system
• ADH Negative feedback system:

  • Osmoreceptors in the hypothalamus detect a drop in blood water potential.
  • The hypothalamus signals to the posterior pituitary gland to secrete ADH.
  • ADH causes the walls of the DCT and collecting duct to become more permeable to water
  • More water moves by osmosis out of the DCT/collecting duct and reabsorbed into the bloodstream, increasing its water potential.
  • A smaller volume of concentrated urine is produced.
• loop of henle - descending limb:

  water leaves by osmosis - passes through aquaporins: the longer the loop the more water flows out and the lower the water potential
• loop of Henle - ascending limb:

  sodium, potassium, and chloride ions are pumped out by active transport so interstitial space has high salt concentration. it is impermeable to water so water remains in the tubule