13.14 Homeostasis Flashcards
Define homeostasis
Involves physiological control systems that maintain (keep constant) the internal environment within restricted limits.
What factors affect enzyme activity?
Temperature
pH
Substrate concentration
Enzyme concentration
Inhibitors
Define negative feedback
Where there is an increase/decrease from the set point, the opposite effect is produced that returns value to the norm.
What is the set point?
Optimum/ norm at which the systems operates
Define positive feedback
Occurs when a deviation from an optimum causes even greater deviation from the norm.
Example of positive feedback
A stimulus causes Na+ ions to diffuse through Na+ channel proteins into post SN, once threshold is reached, voltage gated Na+ channels open, so there is rapid diffusion of MORE Na+ ions into neuron.
= SMALL STIMULUS LEADING TO LARGER, RAPID RESPONSE.
What is thermoregulation?
Regulation of body temperature.
Explain what happens to enzymes when body temp is too low .
Enzyme controlled reactions take place too slowly.
Lack of KE
Less successful collisions
Less ESCS form per second
Explain what happens to enzymes when body temp is too high .
Body temp is too high, enzymes could be denatured.
Breaking of hydrogen and ionic bonds which hold specific tertiary structure.
Characteristics of hormones
- Produced by glands which secrete directly into blood
- Carried in blood plasma to target cells
- Receptors are complementary shape to hormone
- Hormones BIND to specific receptors ONLY on target cells which have complementary receptors
Range of blood glucose conc
80-100 mg
100 cm-3
Very low levels of blood glucose
Hypoglycaemia
Very high levels of blood glucose
Hyperglycaemia
Which organ monitors blood glucose conc?
Controlled by pancreas.
Where are alpha and beta cells located and what do they do?
Endocrine cells/islets of langerhans. These secrete hormones.
Alpha cells secrete….
hormone called glucagon.
Beta cells secrete…..
hormone called insulin.
What is the aim of insulin?
To reduce blood glucose levels to normal.
It is synthesised and secreted by beta cells in pancreas.
How does insulin work?
- Insulin binds to complementary receptors on cell surface membrane of target cells.
- Controls uptake of glucose by regulating addition of glucose carrier proteins in surface membranes of target cells.
- Insulin also activates enzymes that stimulate the conversion of glucose to glycogen- GLYCOGENESIS, decreasing blood glucose conc.
What is the aim of glucagon?
To increase blood glucose levels to normal.
It is synthesised and secreted by alpha cells in pancreas.
How does glucagon work?
- Glucagon binds to receptors on cell surface membrane of target cells.
- Activates enzymes involved in hydrolysis of glycogen to glucose- GLYCOGENOLYSIS
- Activates enzymes involved in the conversion of glycerol and amino acids into glucose- GLUCONEOGENESIS
Explain the second messenger model of hormone action.
- Adrenaline/glucagon bind to specific transmembrane protein receptors on surface of target cell membrane- THEY ARE FIRST MESSENGER.
- Hormone-receptor complex is formed.
- Receptor protein changes tertiary structure.
- Hormone-receptor complex activates adenylate cyclase, which is an enzyme which results in the conversion of ATP into cyclic AMP (cAMP)= ACTS AS SECONDARY MESSENGER.
- cAMP is secondary messenger and activates protein kinase enzymes- produces cascade of chain of reactions, catalysing conversion of glycogen to glucose.
What is glucagon?
Hormone causing hydrolysis of glycogen to glucose.
What is glycogen?
Energy storage carbohydrate found in liver and muscles.
What is glycogenolysis?
Glycogen hydrolysed to glucose.
What is glycogenesis?
Glucose to glycogen by condensation.
What is glyconeogenesis?
Glucose from amino acids and glycerol.
Factors affecting blood glucose concentration
Diet
Glycogenolysis
Gluconeogenesis
What is diabetes a disease caused by?
Caused by failure of glucose homeostasis
What is type 1 diabetes?
Insulin-dependent diabetes
Can’t produce insulin
Severe insulin deficiency due to autoimmune killing of beta cells or faulty gene (may be due to a virus)
What is type 2 diabetes?
Non-insulin dependent diabetes
Body is insulin resistant
Insulin is produced but insulin receptors in target cells are unresponsive, so insulin has no effect.
Lack or sensitivity to insulin
Symptoms of diabetes
High thirst- osmosis of water from cells to blood= low water potential
Large volumes of urine production- excess water in blood
Poor vision- osmotic loss of water from eye lens
Tiredness- loss of glucose inn urine and poor glucose uptake by liver and muscle cells.
Muscle wasting- gluconeogenesis caused by increased glucagon
How can diabetes be treated?
Injections with insulin
Careful diet
How do insulin injections and careful diet work in type 2 diabetes?
So any receptors that are not faulty receive insulin so sugar can be stored in cells.
Diet must also be monitored to ensure blood glucose is kept within manageable levels.
Structure of kidney
Medulla
Cortex
Fibrous capsule
Renal pelvis
Ureter
Renal Artery
Renal Vein
Outline process of osmoregulation
- Osmoreceptors in hypothalamus detect fall in water potential.
- ADH secreted into capillaries by posterior pituitary gland.
- ADH travels to kidneys in blood and increases permeability of cells of distal convulated tubule and collecting duct to water.
- ADH works by causing vesicles bound with aquaporin proteins to fuse with cell surface membranes of cells of distal convoluted tubule and collecting duct.
Nephron structure
Cortex:
- Glomerulus and bowmans capsule
- Proximal convoluted tubule
- Distal convulated tubule
Medulla:
- Collecting duct
- Loop of Henle- ascending and descending limb
How does ultrafiltration produce glomerular filtrate?
- Blood enters kidney to Bowmans capsule through renal artery.
- High hydrostatic pressure of blood in capillaries.
- High hydrostatic pressure forces water, glucose, urea, and mineral ions out of basement membrane.
- Proteins are too large to pass through fenetrations in capillary endothelium, so stay behind in blood.
- Podocytes present and act as a barrier, preventing large molecules from entering filtrate.
Two processes of urine production
Ultrafiltration
Selective reabsorption
Where does ultrafiltration occur?
Bowmans capsule and glomerulus
How is urea removed from blood?
High hydrostatic pressure
Causes ultrafiltration at Bowmans capsule
Through basement membrane
Enabled by small size of urea molecule
Explain how urea is concentrated in the filtrate
- Reabsorption of water by osmosis;
- At the PCT and descending LoH;
- At the DCT and CD;
- Active transport of ions/glucose creates gradient
Where does reabsorption of water and glucose occur?
Proximal convoluted tubule to loop of henle
Ascending limb
Impermeable to water
More wide
Descending limb
Highly permeable to water
More narrow
Role of distal convoluted tubule
Makes final adjustments to the water and salts that are reabsorbed
To achieve this, the permeability of its walls becomes altered under the influence of various hormones = osmoregulation
Role of collecting duct
Highest WP in cortex lowest WP in medulla
Collecting duct is permeable to water and so water continues to move out by osmosis …
(as the countercurrent multiplier ensures there is always a water potential gradient along the entire collecting duct)
Water is reabsorbed into blood and everything left in collecting duct becomes urine.
Explain why a thicker medulla leads to more concentrated urine.
- Thicker medulla means a longer loop of Henle;
- (The longer the loop of Henle means) increase in sodium ion concentration (in medulla) so sodium ion gradient maintained for longer (in medulla);
- (Therefore) water potential gradient maintained (for longer), so more water (re)absorbed (from loop and collecting duct) by osmosis;
Explain process of reabsorption of water in the loop of henle (3)
Concentration increases in descending limb because water moves out by osmosis and Na+ enter by facilitated diffusion
Concentration decreases in ascending limb because Na+ leave by active transport using ATP
Water remains because walls of ascending limb are impermeable
