Homeostasis

• What is homeostasis?

  Maintenance of a stable internal environment. Involves control systems that keep internal environment roughly constant (within certain limits). Internal environment kept in state of dynamic equilibrium.
• Why is it important to maintain right core body temperature, blood pH & blood glucose concentration?

  Temperature & pH affect enzyme activity, & enzymes control rate of metabolic reactions. Cells need glucose for energy. Blood glucose concentration affects water potential of blood.
• How does body temperature affect enzyme activity?

  Rate of metabolic reactions increases when temperature increases. If temperature too high, reaction stops-rise makes enzyme's molecules vibrate more. Vibration breaks some of hydrogen bonds that hold enzyme in its 3D shape-active site changes so enzyme denatures. Temperature too low, enzyme activity reduced, slowing rate of metabolic reactions.
• How does pH affect enzyme activity?

  Blood pH too high or too low, enzymes denatured. Ionic & hydrogen bonds holding them in 3D shape are broken, so active site shape changes & no longer works as catalyst.
• Blood glucose concentration & cells:

  Too high, water potential of blood reduced to point where water molecules diffuse out of cells into blood by osmosis-can cause cells to shrivel up & die. Too low, cells unable to carry out normal activities as there isn't glucose for respiration to provide energy.
• What are negative & positive feedback mechanisms?

  Homeostatic systems involve receptors, communication system (nervous or hormonal) & effectors.
• What is negative feedback?

  Restores level to normal (only works within certain limits). Multiple negative feedback mechanisms for each thing controlled- more control & faster.
• What is positive feedback?

  Amplifies change. Effectors further increase level away from normal level. Rapidly activates processes in body. Can also happen when a homeostatic system breaks down.
• Examples of positive feedback:

  1.)During blood clot formation after injury, platelets activated & release a chemical-triggers more platelets to activate-platelets quickly form clot. Ends with negative feedback, when body detects clot formed. 2.) Homeostatic system breaks down. E.g, hypothermia (low body temperature). Heat lost quicker than it's produced. Brain doesn't work properly & shivering stops- temperature falls even more. Positive feedback takes temperature further away & decreases until action taken.
• What is insulin?

  Lowers blood glucose concentration when it's high-binds to specific receptors on cell membranes of muscle & liver cells. Increases permeability of muscle cell-membranes to glucose, so cells take up more glucose (increases number of channel proteins in membrane). Activates enzymes in muscle & liver cells that convert glucose into glycogen (glycogenesis). Cells able to store glycogen in their cytoplasm as energy source. Increases rate of respiration of glucose, especially in muscle cells. Increases rate of conversion of glucose to fat.
• Where is insulin secreted from?


  Secreted by beta cells in islets of Langerhans in pancreas.
• What is glucagon?

  Hormone raises blood glucose concentration when it's low. Binds to specific receptors on cell membranes of liver cells & activates enzymes that break glycogen into glucose (glycogenolysis). Also activates enzymes involved in formation of glucose from glycerol & amino acids (gluconeogenesis). Decreases rate of respiration of glucose in cells. Secreted by alpha cells in islets of Langerhans in pancreas.
• What happens when blood glucose concentration is too high?

  Pancreas detects this-beta cells secrete insulin & alpha cells stop secreting glucagon. Insulin binds to receptors on liver & muscle cells (effectors)-cells respond to decrease blood glucose concentration. E.g, glycogenesis activated.
• What are glucose transporters?

  Channel proteins allowing glucose transport across cell membrane. Skeletal & cardiac muscle cells contain transporter GLUT4. Insulin levels low- GLUT4 stored in vesicles in cell cytoplasm. When insulin binds to receptors on cell-surface membrane (high insulin levels) it triggers movement of GLUT4 to membrane-glucose transported into cell through GLUT4 protein by facilitated diffusion.
• What is the '2nd messenger model'?

  Binding of hormone to cell receptors activates enzyme on inside of cell membrane, which then produces chemical (2nd messenger). Chemical activates other enzymes in cell to bring about response.
• What is adrenaline?

  Hormone secreted from adrenal glands when low glucose concentration in blood, when stressed or exercising. Binds to receptors in cell membrane of liver cells. Activates glycogenolysis & inhibits glycogenesis to increase glucose concentration. Activates glucagon secretion & inhibits insulin secretion. Body ready for action by making for glucose available for muscles to respire.
• What are the symptoms of diabetes?

  Tiredness & increased thirst & hunger.
• What are the signs of diabetes?

  High blood glucose concentration, presence of glucose in urine, weight loss, need to urinate excessively, blurred vision & genital itching or regular episodes of thrush.
• What is glomerular filtrate?

  Substances that enter Bowman's capsule (tubular fluid).
• Where does blood water potential regulation mainly take place?

  Loop of Henle, DCT & collecting duct.
• Loop of Henle location & function:

  Located in medulla (inner layer of kidneys). Made of 2 'limbs'-descending limb & ascending limb. Limbs control movement of sodium ions so water can be reabsorbed by blood.
• 2nd messenger model example:

  Adrenaline & glucagon can both activate glycogenolysis inside cell. Receptors have specific tertiary structures making them complementary in shape to the hormones. Bind to receptors & activate enzyme adenylate cyclase-converts ATP into chemical called cyclic AMP (CAMP) which is 2nd messenger. CAMP activates enzyme protein kinase A - activates cascade that breaks glycogen into glucose (glycogenolysis).
• What is type 1 diabetes?

  Immune system attacks beta cells so they can't produce insulin. After eating, blood glucose level rises & stays high (hyperglycaemia)-can result in death if left untreated. Kidneys can't reabsorb all this glucose, so some excreted in urine.
• How is type 1 diabetes treated?

  Insulin therapy (carefully controlled as too much can produce dangerous drop in glucose levels. (hypoglycaemia)). Most need regular insulin injections throughout day, but some use insulin pump to deliver insulin continuously instead. Eating regularly & controlling simple carb intake helps avoid sudden rise in glucose.
• What is type 2 diabetes?


  Usually acquired later in life than type 1. Often linked with obesity & is more likely in people with family history of condition. Other risk factors= lack of exercise, age or poor diet. Beta cells don't produce enough insulin or body's cells don't respond properly to insulin. (insulin receptors on cell membranes don't work, so cells don't take up enough glucose).
• How is type 2 diabetes treated?


  Insulin injections may be needed eventually, glucose-lowering medication can be taken if diet & exercise can't control it & eating healthy, balanced diet, losing weight (if necessary) & regular exercise.
• What are nephrons?

  Long tubules along with bundles of capillaries where blood is filtered. Around 1 million nephrons in each kidney.
• Why is the efferent arteriole smaller in diameter compared to afferent?

  Afferent= takes blood into each glomerulus arteriole. Efferent= takes filtered blood away glomerulus. Efferent smaller in diameter than afferent, so blood in glomerulus is under high pressure. Forces liquid & small molecules in blood out of capillary & into Bowman's capsule.
• What happens when body dehydrated?

  Water potential drops as blood water content too low-detected by osmoreceptors in hypothalamus. Posterior pituitary gland stimulated to release more ADH into blood. More ADH= DCT & collecting duct more permeable, so more water reabsorbed into blood via osmosis. Small amount of concentrated urine produced & less water lost.
• What does urine contain?

  Water & dissolved salts, urea & other substances, e.g, hormones & excess vitamins. No proteins or blood cells usually-too big to be filtered out blood. Glucose actively reabsorbed back into blood, so not usually found either. Urea=waste product produced from breakdown of amino acids in liver.
• Features of PCT that allow efficient reabsorption:

  Length of tubule & presence of microvilli- large S.A for diffusion & facilitated diffusion. In-foldings of membranes next to subcellular spaces give large S.A-accommodate carriers for active transport. Mitochondria for active transport. Continuous flow of filtrate & blood maintains concentration gradient between them. Thin squamous endothelial cells of capillary create short diffusion distance into blood.
• Excretion of waste products:


  Blood enter kidney through renal artery & passes through capillaries in cortex (outer layer) of kidneys. Substances filtered out blood & into long tubules that surround the capillaries (ultrafiltration). Useful substances & right amount of water reabsorbed into blood (selective reabsorption). Remaining unwanted substances pass along to bladder & excreted as urine.