homeostasis Flashcards
What is homeostasis?
Maintenance of a constant internal environment via physiological control systems.
What does negative feedback involve?
Restorative systems are put in place to return deviations from normal values back to original levels.
What are the Islets of Langerhans?
Regions in the pancreas that detect changes in blood glucose levels
- contain endocrine cells (alpha and beta) which release hormones to restore blood glucose levels
What do alpha cells in the Islets of Langerhans do?
Release glucagon when blood glucose concentration is too low.
What do beta cells in the Islets of Langerhans do?
Release insulin when blood glucose concentration is too high.
What factors affect blood glucose concentration?
Eating carbohydrates increases glucose in the blood; exercise increases respiration using glucose.
What is the action of insulin?
- Binds to specific receptors on membranes of liver cells
- increases permeability of cell membrane (GLUT-4 channels fuse with membrane)
- glucose can enter from blood by facilitated diffusion
- activation of enzymes in liver for glycogenesis
- rate of respiration increases
What is the action of glucagon?
- Binds to specific receptors on membranes of liver cells
- activates enzymes for glycogenolysis
- activates enzymes for gluconeogenesis
- rate of respiration decreases
- blood glucose concentration increases
What is the role of adrenaline in blood glucose regulation?
- Secreted by adrenal glands above the kidney when glucose concentration is too low (exercising)
- activates secretion of glucagon
- glycogenolysis and gluconeogenesis
- works via secondary messenger model
What is gluconeogenesis?
- Creating glucose from non- carbohydrate stores in liver e.g. amino acids -> glucose
- occurs when all glycogen has been hydrolysed and body requires more glucose
- initiate by glucagon when blood glucose concentrations are low
What is glycogenolysis?
Hydrolysis of glycogen back into glucose
occurs due to the action of glucagon to increase blood glucose concentration
What is glycogenesis?
Conversion of glucose to glycogen when blood glucose is higher than normal, caused by insulin.
What is a second messenger model?
- Stimulation of a molecule (usually an enzyme) which can then stimulate more molecules to bring about desired response
- adrenaline and glucagon demonstrate this because they cause glycogenolysis to occur inside the cell when binding to receptors on the outside
What is the process of the second messenger model?
- Adrenaline/glucagon bind to specific complementary receptors on the cell membrane
- activate adenylate cyclase
- converts ATP to cyclic AMP (secondary messenger)
- cAMP activates protein kinase A (enzyme)
- protein kinase A activates a cascade to break down glycogen to glucose (glycogenolysis)
What is diabetes?
A disease when blood glucose concentration cannot be controlled naturally.
What is Type 1 diabetes?
An autoimmune disease where the body cannot produce insulin, starting in childhood.
What is Type 2 diabetes?
Occurs when target cell receptors lose responsiveness to insulin, usually due to obesity and poor diet.
What is osmoregulation?
Process of controlling the water potential of the blood, regulated by hormones.
e.g., antidiuretic hormone (affects distal convoluted tubule and collecting duct)
What is the nephron?
The structure in the kidney where blood is filtered and useful substances are reabsorbed.
How is glomerular filtrate formed?
- Diameter of efferent arteriole is smaller than afferent arteriole
- build-up of hydrostatic pressure
- water/glucose / ions squeezed out capillary into Bowman’s capsule through pores in capillary endothelium, basement membrane and podocytes
- large proteins too large to pass
Reabsorbtion of glucose by PCT
- Co-transport mechanism
- walls made of microvilli epithelial cells to provide large surface area for diffusion of glucose into cells from PCT
- sodium actively transported out cells into intercellular space to create a concentration gradient
- glucose can diffuse into the blood again
What is the counter current multiplier mechanism?
- Describes how to maintain a gradient of Na+ in medulla by the loop of Henle.
- Na+ actively transported out ascending limb to medulla to lower water potential
- water moves out descending limb + DCT + collecting duct by osmosis due to this water potential gradient
How is water reabsorbed by the DCT and collecting duct?
- Water moves out of DCT and collecting duct by osmosis down a water potential gradient
- controlled by ADH which changes the permeability of membranes to water
What is the role of the hypothalamus in osmoregulation?
- Contains osmoreceptors which detect changes in water potential
- produces ADH
- when blood has low water potential, osmoreceptors shrink and stimulate more ADH to be made so more released from the pituitary gland
