Homeostasis Flashcards
What is homeostasis?
Internal environment is maintained within set limits around an optimum.
Why is it important that blood glucose concentration remains stable?
- maintain constant blood water potential: prevent osmotic lysis/crenation of cells
- maintain constant concentration of respiratory substrate: organism maintains constant level of activity regardless of environmental conditions.
Define negative and positive feedback.
Negative feedback: self-regulatory mechanisms return the internal environment to optimum when there is a fluctuation
Positive feedback: a fluctuation triggers changes that result in an even greater deviation from the normal level.
Outline the general stages involved in negative feedback.
Receptors detect deviation —> coordinator —> corrective mechanism by effector —> receptors detect that conditions have returned to normal
Suggest why separate negative feedback mechanisms control fluctuations in different directions.
Provides more control, especially in case of ‘overcorrection’, which would lead to a deviation in the opposite direction from the original.
Suggest why coordinators analyse inputs from several receptors before sending an impulse to effectors.
- receptors may send conflicting information
- optimum response may require multiple types of effector
Why is there a time lag between hormone production and response by an effector?
It takes time to:
- produce hormone
- transport hormone in the blood
- cause required change to the target protein
Name the factors that affect blood glucose concentration.
- amount of carbohydrates digested from diet
- rate of glycogenolysis
- rate of gluconeogenesis
Define glycogenesis, glycogenolysis, and gluconeogenesis.
Glycogenesis: liver converts glucose into the storage polymer glycogen
Glycogenolysis: liver hydrolyses glycogen into glucose which can diffuse into the blood
Gluconeogenesis: liver converts glycerol & amino acids into glucose
Outline the role of glucagon when blood glucose concentration decreases.
- α cells in islets of Langerhans in pancreas detect decrease & secrete glucagon into bloodstream
- Glucagon binds to surface receptors on liver cells & activates enzymes for glycogenolysis & gluconeogenesis
- Glucose diffuses from liver into bloodstream
Outline the role of adrenaline when blood glucose concentration decreases.
- Adrenal glands produce adrenaline. It binds to surface receptors on liver cells & activates enzymes for glycogenolysis.
- Glucose diffuses from liver into bloodstream
Outline what happens when blood glucose concentration increases.
- β cells in islets of Langerhans in pancreas detect increase & secrete insulin into bloodstream
- Insulin binds to surface receptors on target cells to:
a) increase cellular glucose uptake
b) activate enzymes for glycogenesis
c) stimulate adipose tissue to synthesise fat
Describe how insulin leads to a decrease in blood glucose concentration.
- increases permeability of cells to glucose
- increases glucose concentration gradient
- triggers inhibition of enzymes for glycogenolysis
How does insulin increase permeability of cells to glucose?
- increase no. glucose carrier proteins
- triggers conformational change which open glucose carrier proteins
How does insulin increase the glucose concentration gradient?
- activates enzymes for glycogenesis in liver & muscles
- stimulates fat synthesis in adipose tissue
Use the secondary messenger model to explain how glucagon and adrenaline work.
- Hormone-receptor complex forms
- Conformational change to receptor activates G-protein
- Activates adeylate cyclase, which converts ATP to cyclic AMP (cAMP)
- cAMP activates protein kinase A pathway
- Results in glycogenolysis
Explain the causes of Type 1 diabetes and how it can be controlled.
Body cannot produce insulin e.g. due to autoimmune response which attacks β cells of islets of Langerhans.
Treat by injecting insulin.
Explain the causes of Type 2 diabetes and how it can be controlled.
Glycoprotein receptors are damaged or become less responsive to insulin.
Strong positive correlation with poor diet/obesity.
Treat by controlling diet and exercise regime.
Name some signs and symptoms of diabetes.
- high blood glucose conc
- glucose in urine
- polyuria
- polyphagia
- polydipsia
- blurred vision
- sudden weight loss
- blurred vision
Suggest how a student could produce a desired conc of glucose solution from a stock solution.
Volume of stock solution = required conc x final vol needed/ conc of stock solution
Volume of distilled water = final vol later - vol of stock solution
Define osmoregulation.
Control of blood water potential via homeostatic mechanisms.
Describe the gross structure of a mammalian kidney.
Fibrous capsule: protects kidney
Cortex: outer region consists of Bowman’s capsules, convoluted tubules, blood vessels
Medulla: inner region consists of collecting ducts, loops of Henle, blood vessels
Renal pelvis: cavity collects urine into ureter
Ureter: tube carries urine to bladder
Renal artery: supplies kidney with oxygenated blood
Renal vein: returns deoxygenated blood from kidney to heart
Describe the structure of a nephron.
Bowman’s capsule: at start of the nephron — cup-shaped, surrounds glomerulus, inner layer of podocytes
Proximal convoluted tubule: series of loops surrounded by capillaries, walls made of epithelial cells with microvilli
Loop of Henle: hairpin loop extends from cortex into medulla
Distal convoluted tubule: similar to PCT but fewer capillaries
Collecting ducts: DCT from several nephrons empty into collecting duct, which leads into pelvis of kidney
Describe the blood vessels associated with a nephron.
Wide afferent arteriole from renal artery enters renal capsule & forms glomerulus: branched knot of capillaries which combine to form narrow efferent arteriole.
Efferent arteriole branches to form capillary network that surrounds tubules.
Explain how glomerular filtrate is formed.
Ultrafiltration in Bowman’s capsule.
High hydrostatic pressure in glomerulus forces small molecules (urea, water, glucose, mineral ions) out of capillary fenestrations AGAINST osmotic gradient.
Basement membrane acts as filter. Blood cells & large molecules, e.g. proteins remain in capillary
How are cells of the Bowman’s capsules adapted for ultrafiltration?
- Fenestrations between epithelial cells of capillaries
- Fluid can pass between & under folded membrane of podocytes
State what happens during selective reabsorption and where it occurs.
Useful molecules from glomerular filtrate e.g. glucose are reabsorbed into the blood.
Occurs in proximal convoluted tubule.
How are cells in the proximal convoluted tubule adapted for selective reabsorption?
- microvilli: large surface area for co-transporter proteins
- many mitochondria: ATP for active transport of glucose into intercellular spaces
- folded basal membrane: large surface area
What happens in the loop of Henle?
- Active transport of Na+ & Cl- out of ascending limb
- Water potential of interstitial fluid decreases
- Osmosis of water out of descending limb
- Water potential of filtrate decreases going down descending limb: lowest in medullary region, highest at top of ascending limb
Explain the role of the distal convoluted tubule.
Reabsorption:
a) of water via osmosis
b) of ions via active transport
permeability of walls is determined by action of hormones
Explain the role of the collecting duct.
Reabsorption of water from filtrate into interstitial fluid via osmosis through aquaporins
Explain why it is important to maintain an Na+ gradient.
Countercurrent multiplier: filtrate in collecting ducts is always beside an area of interstitial fluid that has a lower water potential.
Maintain water potential gradient for max reabsorption of water.
What might cause blood water potential to water?
- level of water intake
- level of ion intake in diet
- level of ions used in metabolic processes or excreted
- sweated
Explain the role of the hypothalamus in osmoregulation.
- osmosis of water out of osmoreceptors in hypothalamus causes them in shrink.
- This triggers hypothalamus to produce more ADH
Explain the role of the posterior pituitary gland in osmoregulation.
Stores and secrete the ADH produced by the hypothalamus.
Explain the role of ADH in osmoregulation.
- makes cells lining collecting ducts more permeable to water:
- binds to receptor —> activates phosphorylase —> vesicles with aquaporins on membrane fuse with cell-surface membrane - Makes cells lining collecting dicts more permeable to urea:
- water potential in interstitial fluid decreases
- more water reabsorbed = more concentrated urine.
