Topic 14: Homeostasis
Principles of homeostasis
● homeostasis – regulation of int. cond. of a cell/organism → maintain optimum cond. for function in
response to int. & ext. stimuli
● importance in mammals
○ maintain optimal cond. for enzyme action & cell function
○ key physiological factors controlled incl.
■ core body temp.
■ metabolic wastes
■ blood pH
■ blood glucose conc.
■ water pot. of blood
■ respiratory gas conc’s in blood
● negative feedback loops
○ keep physiological factors w/in limits
1. receptor detects int./ext. stimuli involved w/ a physiological factor
2. receptor sends info. thru. coordination systems (nervous & endocrine systems)
3. coordination systems instructs effector (muscles & glands) → carry out corrective actions
4. physiological factor returned to fluctuate closely around set point
Urea production
● excretion – removal of toxic/waste prod. of metabolism from body
● deamination
○ breakdown of excess amino acids in liver
○ removal of amino groups from amino acids + 1 H atom → form NH3
■ H2NCH(R)COOH - 2H + H2O → NH3 + RCOCOOH
○ remainder forms keto acid to harness E. by
■ enter Krebs cycle to be respired
■ converted to glucose
■ converted to glycogen/fat for storage
○ NH3 converted to urea ∵ < soluble & < toxic
■ 2NH3 + CO2 → CO(NH2)2 + H2O
■ NH3 can disrupt blood pH, impact respiration, interferes w/ cell signalling
Urine formation
● ultrafiltration in Bowman’s capsule
1. blood in glomerular capillaries separated from lumen of Bowman’s capsule by 3 layers
a. endothelium of capillary – each cell perforated by many tiny membrane-lined circular
holes
b. basement membrane – network of collagens & glycoproteins
c. epithelium of Bowman’s capsule – podocyte cells w/ many tiny finger-like
projections w/ gaps between them
2. afferent arteriole narrower as branches into glomerular capillaries → ↑ bp
● ∴ diameter of afferent arteriole > efferent arteriole
● although solute conc. of capillary > Bowman’s capsule → blood pressure diff.
outweigh diff. in solute conc. → water pot. of blood > filtrate
, 3. amino acids, water, glucose, urea, inorganic ions in blood forced into Bowman’s capsule
thru. holes in capillary endothelium & gaps between podocytes → form glomerular filtrate
● RBCs, WBCs, platelets = too big to pass thru. holes
● large protein molecules blocked by basement membrane
● selective reabsorption in proximal convoluted tubule
○ process
1. Na-K pumps in basal membranes of p.c.t. epithelial cells pump Na+ from epithelial
cells into adjacent capillaries → [Na+] inside epithelial cells ↓ → Na+ grad. created
2. Na+ in glomerular filtrate diffuse thru. co-transporters in luminal (lumen) membrane
of epithelial cells down conc. grad.
3. Na+ carries solutes thru. co-transporters into epithelial cells
● solutes = glucose, amino acids, vitamins, inorganic ions, urea
4. glucose & amino acids pass thru. transport proteins in basal membranes into blood
by facilitated diffusion
5. water pot. of filtrate ↑, water pot. of blood ↓ → water pot. grad. created → water
moves into blood by osmosis
○ structural adaptations
■ microvilli on luminal membrane → ↑ SA for ↑ co-transporters
■ many co-transporters in luminal membrane → each transports diff. type of solute
■ folded basal membrane → ↑ SA for ↑ Na-K pumps
■ many mito. in epithelial cells → prov. E. for Na-K pumps
■ cells tightly packed → no fluid can leak between cells, all reabsorbed substances
must pass thru. epithelial cells
Osmoregulation
● part of homeostasis → controls water pot. of body fluids
● ↓ water pot. of blood
1. specialised sensory neurones = osmoreceptors in hypothalamus of brain detect stimulus
→ send nerve impulses to posterior pituitary gland
2. p.p.g. stimulated to release antidiuretic hormone (ADH) into blood
3. ADH travels thru. bloodstream to capillaries of collecting duct in kidneys
4. ADH binds to receptor proteins on collecting duct epithelial cells → activates signalling
cascade by converting ATP → cAMP
5. cAMP phosphorylates aquaporins (H2O permeable channels) embedded in membranes of
free-floating vesicles in epithelial cells → energises them
6. entire vesicle w/ aquaporins fuse w/ luminal membrane of collecting duct epithelial cells →
↑ permeability to water flowing down water pot. grad. into capillaries
7. filtrate in collecting duct loses > water → ↓ V. of concentrated urine prod. → ↑ water pot. of
blood
● ↑ water pot. of blood
1. osmoreceptors in hypothalamus not stimulated
2. no nerve impulses are sent to the posterior pituitary gland → no ADH released
3. aquaporins moved out of luminal membranes of collecting duct epithelial cells → ↓
permeability to water
4. filtrate in collecting duct loses < water → ↑ V. of dilute urine prod. → ↓ water pot. of blood
Principles of homeostasis
● homeostasis – regulation of int. cond. of a cell/organism → maintain optimum cond. for function in
response to int. & ext. stimuli
● importance in mammals
○ maintain optimal cond. for enzyme action & cell function
○ key physiological factors controlled incl.
■ core body temp.
■ metabolic wastes
■ blood pH
■ blood glucose conc.
■ water pot. of blood
■ respiratory gas conc’s in blood
● negative feedback loops
○ keep physiological factors w/in limits
1. receptor detects int./ext. stimuli involved w/ a physiological factor
2. receptor sends info. thru. coordination systems (nervous & endocrine systems)
3. coordination systems instructs effector (muscles & glands) → carry out corrective actions
4. physiological factor returned to fluctuate closely around set point
Urea production
● excretion – removal of toxic/waste prod. of metabolism from body
● deamination
○ breakdown of excess amino acids in liver
○ removal of amino groups from amino acids + 1 H atom → form NH3
■ H2NCH(R)COOH - 2H + H2O → NH3 + RCOCOOH
○ remainder forms keto acid to harness E. by
■ enter Krebs cycle to be respired
■ converted to glucose
■ converted to glycogen/fat for storage
○ NH3 converted to urea ∵ < soluble & < toxic
■ 2NH3 + CO2 → CO(NH2)2 + H2O
■ NH3 can disrupt blood pH, impact respiration, interferes w/ cell signalling
Urine formation
● ultrafiltration in Bowman’s capsule
1. blood in glomerular capillaries separated from lumen of Bowman’s capsule by 3 layers
a. endothelium of capillary – each cell perforated by many tiny membrane-lined circular
holes
b. basement membrane – network of collagens & glycoproteins
c. epithelium of Bowman’s capsule – podocyte cells w/ many tiny finger-like
projections w/ gaps between them
2. afferent arteriole narrower as branches into glomerular capillaries → ↑ bp
● ∴ diameter of afferent arteriole > efferent arteriole
● although solute conc. of capillary > Bowman’s capsule → blood pressure diff.
outweigh diff. in solute conc. → water pot. of blood > filtrate
, 3. amino acids, water, glucose, urea, inorganic ions in blood forced into Bowman’s capsule
thru. holes in capillary endothelium & gaps between podocytes → form glomerular filtrate
● RBCs, WBCs, platelets = too big to pass thru. holes
● large protein molecules blocked by basement membrane
● selective reabsorption in proximal convoluted tubule
○ process
1. Na-K pumps in basal membranes of p.c.t. epithelial cells pump Na+ from epithelial
cells into adjacent capillaries → [Na+] inside epithelial cells ↓ → Na+ grad. created
2. Na+ in glomerular filtrate diffuse thru. co-transporters in luminal (lumen) membrane
of epithelial cells down conc. grad.
3. Na+ carries solutes thru. co-transporters into epithelial cells
● solutes = glucose, amino acids, vitamins, inorganic ions, urea
4. glucose & amino acids pass thru. transport proteins in basal membranes into blood
by facilitated diffusion
5. water pot. of filtrate ↑, water pot. of blood ↓ → water pot. grad. created → water
moves into blood by osmosis
○ structural adaptations
■ microvilli on luminal membrane → ↑ SA for ↑ co-transporters
■ many co-transporters in luminal membrane → each transports diff. type of solute
■ folded basal membrane → ↑ SA for ↑ Na-K pumps
■ many mito. in epithelial cells → prov. E. for Na-K pumps
■ cells tightly packed → no fluid can leak between cells, all reabsorbed substances
must pass thru. epithelial cells
Osmoregulation
● part of homeostasis → controls water pot. of body fluids
● ↓ water pot. of blood
1. specialised sensory neurones = osmoreceptors in hypothalamus of brain detect stimulus
→ send nerve impulses to posterior pituitary gland
2. p.p.g. stimulated to release antidiuretic hormone (ADH) into blood
3. ADH travels thru. bloodstream to capillaries of collecting duct in kidneys
4. ADH binds to receptor proteins on collecting duct epithelial cells → activates signalling
cascade by converting ATP → cAMP
5. cAMP phosphorylates aquaporins (H2O permeable channels) embedded in membranes of
free-floating vesicles in epithelial cells → energises them
6. entire vesicle w/ aquaporins fuse w/ luminal membrane of collecting duct epithelial cells →
↑ permeability to water flowing down water pot. grad. into capillaries
7. filtrate in collecting duct loses > water → ↓ V. of concentrated urine prod. → ↑ water pot. of
blood
● ↑ water pot. of blood
1. osmoreceptors in hypothalamus not stimulated
2. no nerve impulses are sent to the posterior pituitary gland → no ADH released
3. aquaporins moved out of luminal membranes of collecting duct epithelial cells → ↓
permeability to water
4. filtrate in collecting duct loses < water → ↑ V. of dilute urine prod. → ↓ water pot. of blood
