Homeostasis Flashcards
Define homeostasis
the regulation/maintenance of the internal conditions of a cell/organism to maintain optimum conditions for function, in response to internal and external changes
give examples of physiological factors that are controlled by homeostasis in mammals
core body temperature metabolic waste (e.g. CO2 and urea) blood pH blood glucose concentration blood water potential conc of respiratory gases (CO2 and O2) in blood
what two different coordination systems does homeostasis in mammals rely on?
nervous system - info transmitted as electrical impulses that travel along neurones
endocrine system - info transmitted as hormones that travel in blood
what does homeostasis ensure?
maintenance of optimum conditions for enzyme action and cell function
(that fluid surrounding cells are at optimum conditions)
Why is it important that core temperature
remains stable?
Maintain stable rate of enzyme-controlled reactions & prevent damage to membranes.
Temperature too low = enzyme & substrate molecules have insufficient kinetic energy (i.e. fewer collisions, slower ROR)
Temperature too high = enzymes denature (active site shape changes, no/few E-S complexes form)
Why is is important that blood pH
remains stable?
Maintain stable rate of enzyme-controlled reactions (& optimum conditions for other proteins e.g. channel proteins).
Acidic pH = H+ ions interact with H-bonds & ionic bonds in tertiary structure of enzymes → shape of active site changes so no ES complexes form.
Why is it important that blood glucose concentration remains stable?
● Maintain constant blood water potential: prevent cells bursting or shrivelling (by water entering/leaving by osmosis)
● Maintain constant concentration of respiratory substrate: organism maintains constant level of activity regardless of environmental conditions. (glucose is respiratory substrate, enough available means respiration can occur & ATP can be generated)
Define negative and positive feedback
Negative feedback: self-regulatory mechanisms return 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 (e.g. nervous system/endocrine system) → corrective mechanism by effector (muscle and glands) → receptors detect that conditions have returned to normal
describe the outcome of a negative feedback loop
- factor/stimulus is continuously monitored
- if there’s an increase in the factor, the body responds to make the factor decrease
- if there is a decrease in the factor, the body responds to make the factor increase
give an example of positive feedback
Impulse causes influx of sodium ions which increases permeability of the neuron membrane to sodium ions –> more Na+ ions enter and so on…
give an example of negative feedback
blood glucose regulation, insulin lowers blood glucose when high, glucagon raises blood glucose when levels are low.
what is the optimum point?
Desired level (norm) at which system operates
What is the advantage of having separate negative feedback systems?
- provides greater homeostatic control
- especially in case of ‘overcorrection’, which would lead to a deviation in the opposite direction from the original one.
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 carbohydrate digested from diet (will fall following exercise/not eating)
- rate of glycogenolysis (glycogen to glucose)
- rate of gluconeogenesis (production of glucose)
Define glycogenesis, glycogenolysis and
gluconeogenesis.
Glycogenesis: liver converts glucose into the storage polymer glycogen.
Glycogenolysis: liver hydrolyses glycogen into glucose which can diffuse into blood.
Gluconeogenesis: liver converts glycerol and amino acids into glucose.
Which hormones are produced by the pancreas and are involved in blood glucose concentration?
Insulin and glucagon
Describe what the pancreas consists of?
- composed mostly of cells that produce its digestive enzyme, and scattered within it are islets of Langerhans which have alpha cells and beta cells
What do alpha and beta cells produce?
Alpha - Glucagon
Beta - Insulin
Which cells are bigger, alpha or beta?
Alpha
Where is the liver located & how much does it weigh?
Immediately under the diaphragm
1.5 kg
What are liver cells called?
Hepatocytes
How much glycogen can the liver store + how long can this last?
75-100g which can last for about 12 hours
Why are brain cells the most susceptible to suffer from low levels of glucose?
Glucose is the only substance it can respire
what is adrenaline’s role in blood glucose concentration?
- released by adrenal glands when body anticipates danger
- results in more glucose being released from stores of glycogen in the liver (increases blood glucose conc) by stimulating second messenger model
Which two hormones involved in blood-glucose regulation use the second messenger model?
Adrenaline and glucagon
Describe the steps involved in the second messenger model with adrenaline
1) Adrenaline binds to transmembrane protein receptor within the cell-surface membrane of liver cell
2) Binding of adrenaline causes protein to change shape on inside of membrane
3) Change of protein shape leads to activation of enzyme called adenyl cyclase which then converts ATP to cyclic AMP
4) cAMP acts as a second messenger that binds to protein kinase enzyme, changing its shape and therefore activating it
5) Active protein kinase catalyses conversion of glycogen to glucose which moves out of liver via facilitated diffusion and into the blood through channel proteins
describe the liver’s role in blood glucose regulation
- liver acts as the body’s glucose (or fuel) reservoir
- liver both stores and manufactures glucose depending upon the body’s need
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 and activates enzymes for glycogenolysis and gluconeogenesis. (second messenger model)
- Glucose diffuses from liver into bloodstream
explain how negative feedback works in increasing blood glucose concentration by glucagon
- increases blood glucose concentration back to optimum
- raising of the blood glucose concentration causes alpha cells to reduce secretion of glucagon
Outline the role of adrenaline when blood glucose concentration decreases
- Adrenal glands produce adrenaline. It
binds to surface receptors on liver cells and activates enzymes for glycogenolysis (second messenger model) - 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) change tertiary structure of the channel proteins so more glucose being absorbed by FD
(more protein carriers are incorporated into cell membranes so that more glucose is absorbed from blood into cells)
b) activate enzymes for glycogenesis (liver and muscles)
c) stimulate adipose tissue to synthesise fat
Describe how insulin leads to a decrease in blood glucose concentration
● increasing rate of absorption of glucose in cells (especially muscle cells)
● increasing respiratory rate of cells, which use up more glucose (increasing uptake of glucose from blood)
● increasing rate of conversion of glucose into glycogen (glycogenesis) in cells of liver and muscles
● increasing rate of conversion of glucose to fat
How does insulin increase permeability of cells to glucose?
● Increases number of glucose carrier
proteins.
● Triggers conformational change which
opens glucose carrier proteins.
How does insulin increase the glucose concentration gradient?
● Activates enzymes for glycogenesis in
liver & muscles.
● Stimulates fat synthesis in adipose
tissue.
Explain how insulin and glucagon act
- Insulin and glucagon act antagonistically
- conc of glucose in blood determines quantity of glucagon and insulin produced, concentration of glucose fluctuates around the optimum (isn’t constant).
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.
what is diabetes?
- metabolic disorder caused by an inability to control blood glucose concentration
- due to a lack of insulin or loss of responsiveness to insulin
Signs of diabetes
● high blood glucose concentration ● presence of glucose in urine ● need to urinate excessively ● genital itching or regular episodes of ● thrush ● weight loss ● blurred vision
Symptoms of diabetes
● tiredness
● increased thirst and hunger
state the difference between causes of type I and type II diabetes
type I caused by inability to produce insulin
type II caused by receptors on body cells losing their responsiveness to insulin
state one difference between main ways of controlling type I and type II diabetes
type I controlled by injection of insulin
type II controlled by regulating intake of carbohydrate in diet and matching this to amount of exercise taken
suggest an explanation for why tiredness is a symptom of diabetes
- In diabetes insulin not produced by pancreas
- leads to fluctuations in blood glucose levels, if level below normal there might be insufficient glucose for release of energy by cells during respiration
muscle and brain cells may therefore be less active, leading to tiredness
suggest what lifestyle advice you might give someone in order to help them avoid developing type II diabetes
- match your carbohydrate intake to the amount of exercise you take
- avoid becoming overweight by not consuming excessive quantities of carbohydrate and by taking regular exercise
what does a colorimeter do?
can measure the absorbance of light waves
Outline the procedure of this practical
- Make a serial dilution of glucose, ranging from 0 to 10 mmol dm-3.
- Place 2 cm3 of each of the unknown samples in separate boiling tubes.
- Do Benedict’s test on all solutions (i.e. add benedict’s reagent, heat in water bath) (filter out precipitates and put filtrate into curvette)
- Zero the colorimeter using a cuvette with distilled water and set to red filter.
- Place known samples into a cuvette and measure the absorbance of each using the colorimeter.
- Plot calibration curve: absorbance (y-axis), glucose concentration (x-axis).
- Measure the absorbance of the unknown samples using the colorimeter. 8. Use the calibration curve to determine glucose concentrations
What is a serial dilution?
A dilution where successive concentrations increase/decrease in a logarithmic fashion
What would a high glucose concentration in urine suggest?
- may suggest diabetes
- lack of insulin (and sensitivity to insulin of liver cells) leads to high blood glucose concentration, hence high concentration in the glomerular filtrate (blood is filtered in the glomerulus), so not all glucose can be reabsorbed in the proximal convoluted tubule (and remains in filtrate and therefore in urine)
How can you increase the accuracy of the estimate of the unknown glucose solution?
- increase the number of concentrations (at smaller intervals) for the calibration curve within the range of concentrations that the unknown solution belongs in
How can Benedict’s solution be used to measure the concentration of glucose in a solution?
- use colorimeter to measure the absorbance of a series of solutions of known concentrations to create a calibration curve
- compare absorbance of an unknown sample to the calibration curve
Define osmoregulation
Control of blood water potential via
homeostatic mechanisms
Describe the structure of a nephron
● Bowman’s capsule at start of nephron: cup-shaped, surrounds glomerulus, 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 into medulla.
● Distal convoluted tubule : similar to PCT but fewer capillaries.
● Collecting duct: 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
explain why urine doesn’t contain glucose and proteins and blood cells
Proteins and blood cells - are too large to be filtered out
Glucose - all glucose is absorbed at the selective reabsorption in the PCT
what is ultrafiltration?
The filtering of substances out of the blood
What is selective reabsorption?
The reabsoprtion of useful substances and the right volume of water into the blood
function of nephron
- filter the blood to remove waste and selectively reabsorb useful substance back into the blood
What happens during ultrafiltration?
Blood enters from the renal artery into smaller arterioles
- The afferent arteriole takes blood to the glomerulus
- High hydrostatic pressure forces out liquid and small molecules into the Bowman’s capsule (forming glomerular filtrate)
- The efferent arteriole transports the blood away which now contains only large proteins/blood cells
Why is there a high pressure in the glomerulus?
The efferent arteriole is smaller in diameter than the afferent arteriole
What is the glomerular filtrate?
- substances from the blood that enter the Bowman’s capsule
What cells makes up the epithelium of the Bowman’s capsule?
Podocytes
why can’t large molecules form glomerluar filtrate and enter bowman’s capsule in relation to structure of bowman’s capsule?
- bowman’s capsule has cells called podocytes
- they have extensions called pedicels wrapped around the capillaries meaning any cells, large proteins or platelets which leave the capillary walls don’t enter the tubule.
what does glomerular filtrate contain?
- water, amino acids, glucose, ions and importantly: urea and other nitrogenous waste products
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
Describe the process of selective absorption
- Concentration of Na+ ions in the PCT cells decreases as Na+ ions actively transported out PCT cells into blood in capillaries
- So Na+ ions diffuse down conc gradient from lumen of PCT into cells lining PCT - this is co-transport as proteins that transport Na+ ions in carry glucose with it.
- Glucose can then diffuse down its concentration gradient from PCT epithelial cell to blood stream - all glucose REabsorbed
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+
and 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
what do cells in ascending limb of loop of Henle have lots of and why?
- mitochondria
- for active transport of Na+ and Cl- ions into interstitial space (to reduce WP)
Why is it important that the ascending limb is impermeable to water?
- so that water doesn’t move out of the ascending limb and into the medulla and increase the water potential
What is the whole point of the loop of Henle?
- To make the water potential of the medulla very low
- So water moves out of the DESCENDING LIMB
How are the distal convoluted tubule, medulla, loop of Henle and collecting duct linked?
- loop of henle lowers the water potential of the medulla
- water moves out of the DCT and collecting duct into the medulla
How is the water that enters the medulla reabsorbed into the blood?
- through the capillary network
Why is the countercurrent of the loop of Henle important?
- maintains a concentration gradient in the medulla across the whole length of the loop
- more water can move into the medulla
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. - Maintains water potential gradient for maximum reabsorption of water.
Describe the reabsorption of water at the DCT and collecting duct
● Collecting duct permeable to water
● So filtrate moves down it and water passes out of it by osmosis into blood vessels that occupy this space and carried away
● water potential is lowered in interstitial space (by active transport of Na+ ions in loop of Henle) so water continues to move out by osmosis along whole length of collecting duct.
● Counter-current multiplier ensures that there’s always a water potential gradient drawing water out of tubule
Suggest if length of loop of Henle is longer or shorter and explain in a desert animal and how it helps animals in dry areas to survive.
● Longer loop of henle
● to give a longer countercurrent multiplier
● so more absorption of water by collecting duct
- more Na+ ions actively transported out and therefore an even more negative water potential is created.
- this results in more water being reabsorbed into the blood and very concentrated urine.
(Prevents dehydration)
What might cause blood water potential to change?
● level of water intake ● level of ion intake in diet (e.g. NaCl..salt) ● level of ions used in metabolic processes or excreted ● sweating
Explain the role of the hypothalamus in
osmoregulation
- Osmosis of water out of osmoreceptors
in hypothalamus causes them to shrink. - This triggers hypothalamus to produce
more antidiuretic hormone (ADH).
Explain the role of the posterior pituitary
gland in osmoregulation
Stores and secretes the ADH produced
by the hypothalamus
Explain the role of ADH in osmoregulation
- Makes cells lining collecting duct more permeable to water:
● Binds to receptor → activates phosphorylase → 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.
What happens when the water potential of the blood is too low?
- Osmoreceptors in the hypothalamus detect the decrease in water potential
- Posterior pituitary gland releases ADH
- ADH binds to receptors on cell membranes of DCT and collecting duct cells
- Aquaporins move to and fuse with cell membrane
- More water can pass out and into the medulla so more water is reabsorbed
What happens to the urine when the water potential of the blood is too low?
Very concentrated and little volume
as more water has been reabsorbed
What happens when the water potential of the blood is too high?
- Osmoreceptors in hypothalamus detect increase in water potential
- Posterior pituitary gland releases less ADH
- DCT and collecting tube are less permeable to water
- Less water moves out and into the medulla so less is reabsorbed
What happens to urine when the water potential of the blood is too high?
Large volume and very dilute
as Less water reabsorbed
What is ADH?
Anti-diuretic hormone
what do osmoreceptors also do?
osmoreceptors also send nerve impulses to the thirst centre of brain, encouraging individuals to drink water
What cells makes up the epithelium of the Bowman’s capsule?
Podocytes
Role of podocytes
- have slits within them which allow the filtrate to pass though (prevent large molecules entering glomerular filtrate)
- they have foot like processes which increase their surface area
When a person is dehydrated, the cell volume of an osmoreceptor decreases. Explain why.
- water potential of blood decreases
- water moves from osmoreceptors into blood by osmosis
