Homeostasis Flashcards
what is meant by homeostasis?
the process of maintaining a constant internal environment, despite external changes
what are 3 examples of homeostasis?
-control of blood pH
-control of core temperature
-control of blood glucose
why does blood pH have to be controlled?
enzymes have an optimum pH and above and below this, the distribution of hydrogen and ionic bonds in the 3D tertiary structure start to change, making the active site no longer complementary to the substrate and decreasing rate of metabolism. enzymes then denature at extremes, so they can no longer function.
how do you calculate pH?
pH=-log10[H+]
why does core temperature have to be controlled?
enzymes have an optimum temperature. as temperature increases, kinetic energy increases, so there are more successful collisions between the active site and the substrate, causing an increased rate of metabolism. however, hydrogen bonds begin to break as temperature increases, so tertiary structure changes. enzymes then denature and stop functioning.
why does control of blood glucose concentration have to be controlled?
if blood glucose concentration falls too low, rate of respiration falls too low and the cell can’t make enough ATP to stay alive
if blood glucose concentration is too high, the water potential of the blood falls so low that water leaves cells by osmosis, down the water potential gradient. if too much water leaves, they shrivel up (crenate) and die.
what is negative feedback?
a mechanism that reverses a change to restore a level back to normal.
what happens when there is a rise in temperature?
-the thermoregulatory centre in the hypothalamus detects the change
-the nervous and hormonal system and carry signals to the skin, liver, and muscles.
-less heat is then generated and more heat is lost
-temperature then falls
what happens when blood glucose concentration becomes higher than normal?
-the pancreas detects the change and secretes insulin
-this travels in the blood and binds to receptors on the liver and muscle cells
-these respond by bringing about changes which increase glucose uptake by the cells so blood glucose concentration falls back to normal
what are the advantages of multiple negative feedback systems and what is an example of this?
-more rapid response to changes
-more control over changes in the internal environment
an example is the body reducing blood glucose by secreting more insulin AND by secreting less glucagon.
what is positive feedback?
a mechanism that amplifies a change away from the normal level
is positive feedback involved in homeostasis?
no
why is positive feedback not involved in homeostasis?
because it doesn’t keep internal environment constant
what is positive feedback useful for?
rapid activation of a process (e.g. in labour)
when does positive feedback also occur (other than for rapid activation of a process)?
when homeostasis systems break down, like in hyperthermia
what cells detect an increase in blood glucose and what do they do?
by beta cells in the pancreas (at islets of langerhans), which secrete insulin to decrease blood glucose
what cells detect an decrease in blood glucose and what do they do?
alpha cells in the pancreas (at islets of langerhans), which secrete glucagon to increase blood glucose
glucose is a _______________ substrate
respiratory
describe the process when blood glucose rises above normal
-insulin is secreted, which binds to insulin receptors on the plasma membrane of liver cells and muscle cells
-insulin decreases blood glucose by:
*causing vesicles storing glucose carrier proteins to fuse with the plasma membrane, so there are more glucose carriers, so increased permeability to glucose, so more glucose enters the cell
*activating enzymes for glycogenesis (converting glucose to glycogen)
*increasing rate of respiration of glucose
why is glycogen a good storage molecule?
it is insoluble
describe the process when blood glucose falls below normal
-glucagon is secreted, which binds to glucagon receptors on the plasma membrane of liver cells
-glucagon increases blood glucose by:
*activating enzymes for gluconeogenesis (conversion of amino acids and glycerol to glucose)
*activating enzymes for glycogenolysis (conversion of glycogen to glucose)
*decreasing rate of respiration of glucose
what is glycogenesis?
condensation of glucose to glycogen
what is gluconeogenesis?
conversion of amino acids and glycerol to glucose
what is glycogenolysis?
hydrolysis of glycogen to glucose
explain two advantages of having two opposing hormones to regulate blood glucose concentration rather than having just one
-more controlled response to blood glucose control being out of the optimum range
-more rapid response, so there is less change from the optimum range
describe and explain 3 ways insulin will result in increased uptake of glucose by binding to insulin receptors on plasma membranes, with reference to glucose carriers and diffusion gradients
-more glucose carriers join the plasma membrane from the fusing vesicles, so the membrane is more permeable to glucose
-insulin activates the enzymes for glycogenesis, turning glucose to glycogen. this decreases the concentration of glucose inside the cell, making a steeper concentration gradient
-insulin increases the rate of respiration of glucose, decreasing the concentration of glucose inside the cell, so facilitated diffusion occurs into the cell
can insulin be made in type 1 diabetes? why?
no- the body is unable to produce its own insulin, usually due to beta cell damage in the islets of langerhans
can insulin be made in type 2 diabetes? why?
yes- however the body becomes insensitive to this due to loss of functioning insulin receptors on the liver and muscle cells
when does type 1 diabetes occur?
quickly during childhood
when does type 2 diabetes occur?
slowly at 40+ years, or earlier if obese
what are the risk factors for type 1 diabetes?
diseases
genetics
what are the risk factors for type 2 diabetes?
age
obesity
diet high in refined sugars
family history
ethnicity (african/asian)
what is the treatment for type 1 diabetes?
daily insulin injections (a protein, so cannot be taken orally as it is digested in the stomach)
what is the treatment for type 2 diabetes?
diet
exercise
use of drugs which can stimulate insulin production or slow rate of glucose absorption
what is adrenaline?
a hormone secreted by adrenal glands which binds to specific adrenaline receptors on the plasma membrane of liver cells to increase the blood glucose concentration to make more glucose available for muscles to respire to get the body ready for fight or flight
how does adrenaline increase blood glucose concentration? (4 ways)
-activates glucagon secretion
-activates glycogenolysis
-inhibits insulin secretion
-inhibits glycogenesis
explain the process of how adrenaline works
1) adrenaline binds to complementary receptors on the plasma membrane of liver cells.
2) this binding activates adenyl cyclase enzyme.
3) adenyl cyclase converts ATP to cyclic AMP (cAMP), which is the secondary messenger. cAMP then activates protein kinase, an enzyme that activates a cascade of other enzymes needed for glycogenolysis.
what is a primary messenger? give 2 examples
a hormone which exerts its effects by binding to receptors on plasma membranes (does NOT enter the cell)
e.g. adrenaline, glucagon
what is a secondary messenger? give an example
the molecule activated by the primary messenger to bring about a response
e.g. cAMP, used in glycogenolysis
what are the 2 functions of kidneys?
-excretion of urea and other metabolic waste
-osmoregulation (control of water and salt levels)
what is the bowman’s capsule?
the start of the nephron with lots of blood capillaries (the glomerulus), made up of podocytes
what is the proximal convoluted tubule?
a series of loops with blood capillaries, made from epithelial cells lined with microvilli
what is the loop of henle?
a long loop travelling through the cortex to the medulla with capillaries
what is the distal convoluted tubule?
a loop with less capillaries than the proximal tube
what is the collecting duct?
where the nephrons empty
what is ultrafiltration?
the filtration of small molecules and ions out of the blood in the glomerulus into the bowman’s capsule, caused by high hydrostatic pressure.
the glomerular filtrate is forced out the blood in the glomerulus capillaries, collecting in the lumen of the bowman’s capsule. blood cells and large blood proteins stay in the blood as they don’t fit through the “filter”.
why is hydrostatic pressure in the glomerulus capillaries unusually high?
because the efferent arteriole is narrower than the afferent arteriole, therefore maintaining the high pressure
what are the 3 layers that the fluid forced out the blood in the glomerulus capillaries passes through?
-endothelium of capillary walls
-basement membrane
-epithelium of bowman’s capsule (podocytes)
how does the basement membrane act as a filter?
the extracellular protein mesh only allows proteins of 68000 daltons or smaller to pass through
what do the pores in the capillary walls do?
they allow plasma through
what do the podocytes do in the epithelium walls do?
they help with filtration and support the glomerulus
what are the small substances that can pass through the basement membrane?
glucose
ions
amino acids
urea
water
what process occurs in the proximal convoluted tubule?
glucose reabsorption from the filtrate in the nephron
what other substances get reabsorbed slightly during glucose reabsorption?
water and ions
how much fluid do the kidneys filter out the blood each day?
180dm3
how are epithelial cells lining the proximal convoluted tubule specialised for glucose reabsorption? (4 ways)
-sodium potassium pumps actively transport sodium ions out of the cell to get carried away in the blood
-many mitochondria to provide ATP for active transport
-microvilli increase the surface area so there is more space for absorption
-sodium cotransport proteins on the surface of microvilli to reabsorb glucose. there are enough of these to reabsorb ALL the glucose present in normal urine.
describe the process of reabsorption
1) PCT cells have co-transporter proteins on their microvilli which bind to both Na+ and glucose.
2) co-transporter proteins transport Na+ and glucose out of the PCT filtrate and into the PCT cells by *active transport.
3) Na+ ions and glucose cross to the other side of the cell, down a concentration gradient, by *diffusion.
4) sodium potassium pumps on the basement membrane pump Na+ out of PCT cells into surrounding blood capillaries by *diffusion.
5) glucose then moves out of the PCT cells into surrounding blood capillaries by *co-transport.
6) this removal of Na+ and glucose from the filtrate increases water potential of filtrate and decreases the water potential of the PCT cells, so water enters these through osmosis.
7)blood in the efferent arteriole has a higher concentration of blood proteins, so blood in surrounding capillaries has a very low water potential, so water leaves the PCT cells by osmosis and enters this blood down the water potential gradient.
8) by reabsorbing water, the PCT reduces filtrate volume by 60%.
when is water reabsorbed in the proximal convoluted tubule?
always
when is water reabsorbed in the descending limb of the loop of Henle?
always
when is water reabsorbed in the distal convoluted tube?
when ADH is present
when is water reabsorbed in the collecting duct?
when ADH is present
water is absorbed from the filtrate (_______ water potential) by cells lining the _________ (____ water potential) by ____________ down the water potential gradient, and enters the blood in surrounding ______________.
high
nephron
low
osmosis
capillaries
why does water not leave the filtrate in the ascending limb of the loop of Henle? what does it do instead?
it is impermeable to water
instead, it pumps Na+ and Cl- ions into the surrounding medulla tissue, creating the low water potential needed to reabsorb water from the descending limb and collecting duct
what is the role of the loop of Henle?
to create a very low water potential in the tissue of the surrounding medulla, so that water can be reabsorbed from the filtrate in the descending limb and collecting duct
what happens in the descending limb of the loop of Henle?
-the descending limb is permeable to water, so water leaves the filtrate by osmosis, because surrounding medulla tissue has a lower water potential due to high salt conc.
-water reabsorbed enters the surrounding blood capillaries.
-the filtrate water potential decreases as it flows down the descending limb due to loss of water and because Na+ and Cl- ions diffuse into the descending limb from the medulla tissue.
-however, filtrate water potential in the descending limb is never as low as in the surrounding medulla tissue, so water continues to be absorbed along the ENTIRE limb.
what happens in the ascending limb of the loop of Henle?
-the ascending limb is impermeable to water, so cells lining this pump Na+ and Cl- ions out of the fluid and into the surrounding medulla tissue, creating a high salt concentration in the medulla, which causes water to leave fluid in the descending limb by osmosis.
-filtrate water potential increases as it flows up the ascending limb due to loss of salt WITHOUT loss of water (impermeability).
what is the countercurrent multiplier principle?
the filtrate flows in opposite directions in the 2 limbs of the loop of Henle, helping to produce a very high concentration of NaCl in the surrounding medulla tissue
what is the role of the distal convoluted tubule and collecting duct?
to reabsorb water from the filtrate but ONLY when the body needs it
what does the permeability to water of the cells lining the DCT and CD depend on?
the levels of ADH in the blood
what does ADH do?
increases the amount of aquaporins in the cell surface membranes of cells in the DCT and CD, so increases the rate at which cells in this region of the nephron absorb water out of the filtrate by osmosis
what are aquaporins?
water-permeable channels
how does ADH result in a smaller volume of more concentrated urine?
-ADH in the blood (only affects collecting duct)
-vesicles in the blood containing aquaporins move and fuse with membranes lining the collecting duct, so more water is reabsorbed
-this causes a smaller volume of more concentrated urine
what does ADH do to urine?
produces a low volume with a high urea concentration, due to stimulating water reabsorption
what is osmoregulation?
the control of water and salt levels in the blood and tissue fluids
what could happen if water potential was too high?
cells would gain too much water by osmosis and burst (lysis)
what could happen if water potential was too low?
cells would lose too much water by osmosis and shrink (crenate)
what is the role of the hypothalamus?
it contains osmoreceptors to detect osmotic pressure, so stimulates ADH production when low water potential and inhibits ADH production when high water potential
what is the role of the pituitary gland?
the posterior pituitary synthesises ADH and secretes it into the blood
describe what happens when you do not get enough fluids
1) receptors in the hypothalamus detect the low blood volume and high blood concentration.
2) the pituitary gland releases more ADH.
3) more ADH increases permeability of collecting duct to water, so there is increased water reabsorption into the vasa recta.
4) in the bladder, there is a decreased volume of more concentrated urine, returning water levels to normal.
describe what happens when you drink lots of fluids
1) receptors in the hypothalamus detect the high blood volume and low blood concentration.
2) the pituitary gland releases less ADH.
3) less ADH decreases permeability of collecting duct to water, so there is decreased water reabsorption into the vasa recta.
4) in the bladder, there is an increased volume of more dilute urine, returning water levels to normal.
