Homeostasis - maintenance of stable environment Flashcards
Topic 6.4
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 repiratory substrate: organism maintains constant level of activity regardless of environmental conditions
Define negative and positive feedback
Negative: self-regulatory mechanisms return internal environment to optimum when there is a fluctuation
Positive: fluctuation triggers changes that result in even greater deviation from normal level
General staged involved in negative feedback
Receptors detect deviation -> coordinator -> corrective mechanism by effector -> receptors detect that conditions have returned to normal
Factors that affect blood glucose concentration
- Amount of carbohydrate digested from diet
- Rate of glycogenolysis
- Rate of gluconeogenesis
Define glycogenesis, glycogenolysis & gluconeogenesis
Glycogenesis: liver converts glucose into storage polymer glycogen
Glycogenolysis: liver hydrolyses glycogen into glucose which can diffuse into blood
Gluconeogenesis: liver converts glycerol & amino acids into glucose
Role of glucagon when blood glucose conc. decreases
- alpha cells in Islets of Langerhans in pancreas detect decrease & secrete glucagon into bloodstream.
- Glucgon binds to surface receptros on liver cells & activates enzymes for glycogenolysis & gluconeogenesis
- Glucose diffuses from liver into bloodstream.
Role of adrenaline when blood glucose conc. decreases
- Adrenal glands produce adrenaline. It binds to surface receptors on liver cells & activates enzymes for glycogenolysis.
- Glucose diffuses from liver into bloodstream
What happens when blood glucose conc. increases
- Beta 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 (liver & muscles)
c) stimulate adipose tissue to synthesise fat
How insulin leads to decrease in blood glucose concentration
- Increases permeability of cells to glucose
- Increases glucose concentration gradient.
- Triggers inhibition of enzymes for glycogenolysis
How insulin increases permeability of cells to glucose?
- Increases number of glucose carrier proteins.
- Triggers conformational change which opens glucose carrier proteins
How insulin increase glucose concentration gradient?
- Activates enzymes for glycogenesis in liver & muscles
- Stimulates fat synthesis in adipose tissue
Use secondary messenger model to explain how glucagon & adrenaline work
- Hormone-receptor complex forms
- Conformational change to receptor activates G-protein
- Activates adenylate cyclase, which converts ATP to cAMP (cyclic AMP)
- cAMP activates protein kinase A pathway
- Results in glycogenolysis
Cause of Type 1 diabetes & how it can be controlled
Body cannot produce insulin (e.g. due to autoimmnue response which attacks Beta cells of Islets of Langerhans
Treat by injecting insulin
Cause of Type 2 diabetes & 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
Signs & symptoms of diabetes
- High blood glucose concentration
- Glucose in urine
- Polyuria
- Polyphagia
- Polydipsia
- Blurred vision
- Sudden weight loss
Describe gross structure of 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, loop of Henle, blood vessels
Renal pelvis: cavity collects urine into ureter
Ureter: tube carries urine to bladder
Renal artery: supplied kidney with oxygenated blood
Renal vein: returns deoxygenated blood from kidney to heart
Structure of Nephron
Bowman’s capsule at start of nephron: cup-shaped, surrounds golemrulus, inner layer of podocytes.
Proximal convoluted tubule (PCT): series of loops surrounded by capillaries, walls made of epithelial cells with microvilli
Loop of Henle: hairpin loop extends from cortex from medulla
Distal convoluted tubule (DCT): similar to PCT but fewer capillaries.
Collecting duct: DCT from several nephrons empty into collecting duct, which leads into pelvis of kidney.
Describe blood vessels associated with a nephron
Wide afferent arteriole from renal artery enters renal capsule & forms golmerulus: branched knot of capillaries which combine to form narrow efferent arteriole.
Efferent arteriole branches to form capillary network that surrounds tubules.
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 Bowman’s capsule adapated for ultrafiltration?
- Fenestration between epithelial cells of capillaries.
- Fluid can pass between & under folded membrane of podocytes.
What happens during selective reabsorption & where it occurs
Useful molecules from glomerular filtrate (e.g. glucose) are reabsorbed into the blood
Occurs in proximal convoluted tubule
How cells in proximal convoluted tubule are 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 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 (ascending limb is impermeable to water)
- Water potential of filtrate decreases going down, descending limb: lowest in medullary region, highest at top of ascending limb
Role of distal convoluted tubule (DCT)
Reabsorption:
a) of water via osmosis
b) of ions via active transport
permeability of walls is determined by action of hormones
Role of collecting duct
Reabsorption of water from filtrate into interstitial fluid via osmosis through aquaporins
Why is it important to maintain Na+ gradient?
Countercurrent multiplier: filtrate in collecting ducts is always beside an area of interstitial fluid that has a lower water potential
Maintains water potential gradient for maximum reabsorption of water.
What could cause blood water potential to change?
- level of water intake
- level of ion intake in diet
- level of ions used in metabolic processes or excreted
- sweating
Role of hypothalamus in osmoregulation
- Osmosis of water out of osmoreceptors in hypothalamus causes them to shrink.
- This triggers hypothalamus to produce more antidiuretic hormone (ADH)
Role of pituitary gland in osmoregulation
Stores & secretes ADH produced by hypothalamus
Role of ADH in osmoregulation
- Makes cells lining collecting duct more permeable to water:
- Binds to receptor -> activates phosporylase -> vesicles with aquaporins on membrane fuse with cell-surface membrane - Makes cells lining collecting duct more permeable to urea:
- water potential in interstitial fluid decreases
- more water reabsorbed = more concentrated urine
