Homeostasis Flashcards
What is homeostasis?
Maintenance of a constant internal environment while adjusting to external conditions.
What is the importance of homeostasis?
- Temperature: prevent denaturation of enzymes, maintain optimum temperatures for enzyme catalysed reactions
- pH: prevent denaturation of enzymes, maintain optimum pH for enzyme catalysed reactions
- Blood glucose: affects water potential of blood, respiratory substrate
What is negative feedback?
Negative feedback loops reduce the effect of the original stimulus to keep a factor within a normal range.
Outline the process of ultrafiltration.
- High hydrostatic pressure
- Allows ions, water, glucose and urea to pass out
- Through pores in capillary endothelium
- And through the basement membrane
Describe and explain the adaptations of the proximal convoluted tubule.
Microvilli: increases SA for reabsorption
Co-transport Proteins: transport of specific solute across luminal membrane
High number of mitochondria: releases energy for Na+/K+ pump
Tightly packed cells: no fluid can pass between cells
Outline the role of the proximal convoluted tubule in selective reabsorption.
- Na+ actively transported from lining of tubule to blood
- Co-transport of glucose and amino acids from lumen to the lining with Na+ down its concentration gradient (Na+ now low in lining)
- Facilitated diffusion of glucose and amino acids into blood
- Increased water potential of lumen so water is absorbed into blood by osmosis
Outline the role of the loop of Henle in selective reabsorption.
Descending limb: narrow, thin wall, highly permeable to water, lots of aquaporins
- osmolarity in tissue fluid higher than in limb so water moves OUT of limb by osmosis → counter current multiplier
Ascending limb: wide, thick wall, impermeable to water, no aquaporins
- Na+ ions actively transported out
- re-establishes initial osmolarity
Outline the role of the distal convoluted tubule in selective reabsorption.
Final adjustments to water, pH and salt concentrations (under hormonal control).
How does ADH control water absorption in the collecting duct.
- Osmoregulators in the hypothalamus detect the water potential of the blood
- If the osmoreceptors detect a decrease in the water potential of the blood, nerve impulses are to the posterior pituitary gland
- Release ADH
- Increases permeability of the luminal membrane of the collecting duct
- Increase number of aquaporins
- Water molecules are absorbed from the collecting duct (high water potential), into the tissue fluid and blood plasma in the medulla (low water potential) by osmosis
- A small volume of concentrated urine is produced
What is osmoregulation?
The control of the water potential of body fluids.
What cells secrete insulin?
β cells
What cells secrete glucagon?
α cells
Outline the action of insulin.
- Insulin binds to specific receptors on cell-surface membrane.
- Chemical signal causes vesicles with glucose transporters on membrane to fuse with cell-surface membrane.
- Exocytosis of vesicles = more glucose transporters on cell membrane.
- Increased transport of glucose into cells, Increased respiration.
- Stimulates glycogenesis (glucose → glycogen).
Outline the action of glucagon.
- Glucagon (the first messenger) binds to receptors in the cell surface membranes of liver cells.
- This binding causes a conformational change in the receptor protein that activates a G protein.
- This activated G protein activates the enzyme adenylyl cyclase.
- Active adenylyl cyclase catalyses the conversion of ATP to the second messenger, cyclic AMP (cAMP).
- cAMP binds to protein kinase A enzymes, activating them.
- Active protein kinase A enzymes activate phosphorylase kinase enzymes by adding phosphate groups to them.
- Active phosphorylase kinase enzymes activate glycogen phosphorylase enzymes.
- Active glycogen phosphorylase enzymes catalyse the breakdown of glycogen to glucose (glycogenolysis).
What is glycogenesis?
- Glycgogenesis is the synthesis of glycogen from glucose molecules
- Insulin triggers this process after it detects an increased blood glucose concentration
