Topic 6C - Homeostasis ARN * Flashcards
homeostasis basics control of blood glucose concentration the kidneys controlling blood water potential
what is homeostasis?
the maintenance of a constant internal environment
why is homeostasis important?
keeping your internal environment stable is vital for cells to function normally and to stop them being damaged
what factors does homeostasis maintain?
temperature pH glucose water - hydrolysis ions - nerve transmission
why is it important to control temperature in the body?
too high - enzymes denature, molecules vibrate too much, which breaks the hydrogen bonds that hold them in their 3D shape. active site shape changes, doesn’t work as catalyst.
metabolic reactions are less efficient
too low - enzyme activity reduced, slowing rate of metabolic reactions
highest rate of enzyme activity happens at optimum temperature
why is it important to control pH in the body?
too high or low - enzymes denature
hydrogen bonds that hold them in their 3D shape are broken, so shape of enzymes active site is changed, no longer works as catalyst. metabolic reactions are less efficient
highest rate of enzyme activity at optimum pH
why is it important to control glucose in the body?
too high - water potential of blood reduced, so water moves out of cells by osmosis causing them to shrivel and die
too low - cells unable to carry out normal activities because insufficient glucose for respiration to provide energy
how does homeostasis use negative feedback?
receptors detect when level is too high or low
info communicated to effectors by nervous or hormonal system
effectors respond to counteract the change - bringing it back to normal
why does homeostasis involve multiple feedback mechanisms?
gives more control
actively increase or decrease a level to bring it back to normal
with only 1 you can only turn it on or off, can only actively change a level in 1 direction
only 1 negative feedback mechanism means a slower response and less control
what is positive feedback?
when a change triggers a response which increases the effect of that change
so effectors respond to further increase the level from the normal
when does positive feedback happen?
to rapidly activate something e.g. blood clotting after an injury
when a homeostatic system breaks down e.g hypothermia
is positive feedback involved in homeostasis?
no, it doesn’t keep your internal environment stable
how is blood glucose monitored?
by cells in the pancreas
when does blood glucose concentration change?
it rises after eating food containing carbohydrate
falls after exercise, as more glucose is used in respiration to release energy
how is blood glucose concentration controlled?
using insulin and glucagon
they travel in the blood to their target cells (effectors)
where are insulin and glucagon produced?
clusters of cells in the pancreas called islets of langerhans
β cells - secrete insulin into blood
α cells - secrete glucagon into the blood
describe the role of insulin:
produced when blood glucose is high
binds to specific receptors on cell membrane of liver and muscle cells
increase permeability to glucose by increasing channel proteins - more glucose taken up
activates enzymes that convert glucose into glycogen
glycogen stored in cytoplasm as energy resource
increases rate of respiration of glucose
activates enzymes that catalyse lipogenesis
what is glycogenosis?
the conversion of glucose into glycogen
describe the role of glucagon:
produced when blood glucose is low
binds to specific receptors on cell membranes of liver cells
activates enzymes to break down glycogen into glucose
activates enzymes involved in formation of glucose from glycerol and amino acids
decreases the rate of respiration of glucose in cells
what is glycogenolysis?
the process of breaking down glycogen into glucose
what is gluconeogenesis?
the process of forming glucose from non-carbohydrates e.g. lipids and amino acids
when is insulin released?
when blood glucose concentration is too high
when is glucagon released
when blood glucose concentration is too low
what is GLUT4?
a glucose transporter
it is a channel protein in skeletal and cardiac muscle cells
what happens to GLUT4 when insulin levels are low?
GLUT4 is stored in vesicles in the cytoplasm of cells
what happens to GLUT4 when insulin binds to receptors on the cell surface membrane?
it triggers the movement of GLUT4 to the membrane
glucose can then be transported into the cell by facilitated diffusion through GLUT4
what is adrenaline?
a hormone that’s secreted from your adrenal glands
it increases blood glucose concentration
when is adrenaline secreted?
when there’s a low concentration of glucose in your blood, when you’re stressed and when you’re exercising
what does adrenaline do?
binds to receptors in the cell membrane of liver cells, forming a hormone-receptor complex
causes production of a ‘second messenger’ molecule within the cell
-activates glycogenolysis
-inhibits glycogenesis
activates glucagon secretion and inhibits insulin secretion
why is adrenaline secreted?
it gets the body ready for action by making more glucose available for muscles to respire
how do adrenaline and glucagon activate glycogenolysis inside a cell?
receptors have specific tertiary structures. adrenaline and glucagon bind to their receptors and activate an enzyme called adenylate cyclase
it converts ATP into cyclic AMP
cAMP activates an enzyme called protein kinase A
this acts as a cascade that breaks down glycogen into glucose
what is cyclic AMP?
a chemical signal called a second messenger
what is Type I diabetes?
insulin dependent - when the immune system attacks β cells so they can’t produce insulin
what do scientists think causes Type I diabetes?
some people have a genetic predisposition
may be triggered by a viral infection
what happens in Type I diabetes?
after eating, blood glucose levels rise and stay high - hyperglycaemia
it can result in death if left untreated
kidneys can’t reabsorb all this glucose, so some is excreted in urine
how is Type I diabetes treated?
insulin therapy - regular insulin injections throughout the day or insulin pump for continuous insulin
must be carefully controlled because too much insulin can produce a dangerous drop in blood glucose levels
eating regularly and controlling simple carbohydrate intake helps to avoid a sudden rise in glucose
when is Type II diabetes acquired?
usually in later life than Type I
what causes Type II diabetes?
linked with obesity more likely in people with a family history of the condition lack of exercise age poor diet
what is Type II diabetes?
insulin independent - occurs when the body’s cells don’t respond to insulin.
insulin receptors on their membranes don’t work properly, so cells don’t take up enough glucose.
so blood glucose concentration is higher than normal
how can Type II diabetes be treated?
eating a healthy, balanced diet, losing weight and regular exercise
glucose-lowering medication can be taken if diet and exercise can’t control it.
eventually, insulin injections may be needed
what do health advisors recommend to reduce the risk of developing Type II diabetes?
eat a diet that’s low in fat, sugar and salt, with plenty of whole grains, fruit and vegetables
take regular exercise
lose weight if necessary
what are food companies doing to reduce risk of Type II diabetes?
using sugar alternatives to sweeten food/drinks
reducing the sugar, fat and salt content of products
how to determine the concentration of glucose in a urine sample using colorimetry?
quantitative benedict’s solution loses its blue colour without producing a red precipitate when heated with glucose
a colorimeter can be used to measure the light absorbance of the solution
the higher the concentration of glucose, the more blue colour lost, decreasing the absorbance of the solution
what are some of the functions of the kidneys?
to excrete waste products, such as urea
to regulate water potential of the blood
what happens in ultrafiltration?
blood passes through capillaries in the cortex of the kidneys
substances are filtered out of the blood and into long tubules that surround the capillaries
what happens in the kidneys?
ultrafiltration selective reabsorption Water absorption at the Loop of Henle Water absorption at the collecting duct remaining unwanted substances pass along to the bladder and are excreted as urine
what happens in selective reabsorption?
useful substances, such as glucose and the right amount of water, are reabsorbed back into the blood
what are nephrons?
the long tubules along with the bundle of capillaries where the blood is filtered in the kidneys
what happens when blood enters the kidneys?
blood from renal artery enters smaller arterioles in the cortex
each arteriole splits into a glomerulus
this is where ultrafiltration takes place
what is a glomerulus?
a bundle of capillaries looped inside a hollow ball called a Bowman’s capsule
what are the arterioles attached to the glomerulus called?
afferent arteriole - takes blood in
efferent arteriole - takes filtered blood away
how are the arterioles around the glomerulus structured?
efferent is smaller in diameter so blood in the glomerulus is under high pressure
why is it important that blood in the glomerulus is under high pressure?
it forces liquid and small molecules in the blood out of the capillary and into the Bowman’s capsule
what happens in ultrafiltration, for substances to enter the Bowman’s capsule?
the liquid and small molecules pass through 3 layers to enter the Bowman’s capsule and enter the nephron tubules:
the capillary wall
the basement membrane
the epithelium of the Bowman’s capsule
what can and can’t go through ultrafiltration?
larger molecules like proteins and blood cells can’t pass through, so stay in the blood
substances that enter the Bowman’s capsule are known as the glomerular filtrate
what happens to the glomerular filtrate?
it passes along the rest of the nephron and useful substances are reabsorbed along the way
the filtrate flows through the collecting duct and passes out of the kidneys along the ureter
where does selective reabsorption occur?
as the glomerular filtrate flows along the proximal convoluted tubule, the loop of Henle and the distal convoluted tubule
how is the epithelium of the wall of the proximal convoluted tubules adapted?
it has microvilli to provide a large surface area for the reabsorption of useful materials from the glomerular filtrate into the blood
what happens in the proximal convoluted tubule?
useful solutes, like glucose, are reabsorbed along the PCT by active transport and facilitated diffusion
Glucose via co-transport with Na+
Mitochondria provide ATP for active transport
where is water reabsorbed along the nephron tubules?
water enters the blood by osmosis because the water potential of the blood is lower than that of the filtrate
happens in the PCT, loop of Henle, DCT and collecting duct
the filtrate that remains is urine, which passes along the ureter to the bladder
what is urine usually made up of?
water and dissolved salts
urea
other substances such as hormones and excess vitamins
what doesn’t urine usually contain?
proteins and blood cells - too big to be filtered out of the blood
glucose - actively reabsorbed back into blood
what is osmoregulation?
the kidneys regulate the water potential of the blood so the body has just the right amount of water
what happens when the water potential of the blood is too low?
more water is reabsorbed by osmosis into the blood from the tubules of the nephrons.
urine is more concentrated, so less water lost during excretion
where does regulation of water potential take place?
reabsorption occurs along almost all of nephron
regulation mainly takes place in the loop of Henle, DCT and collecting duct.
volume of water reabsorbed by DCT and collecting duct is controlled by hormones
what is the Loop of Henle made up of?
the descending and the ascending limb
what happens in the ascending limb?
near the top, Na+ ions are pumped into medulla by active transport.
ascending limb is impermeable to water, so water stays inside.
this creates a low water potential in the medulla, due to high conc. of ions
what happens in the descending limb?
low water potential in medulla
water moves out of descending limb into medulla by osmosis
making filtrate more concentrated
water in medulla is reabsorbed into the blood through the capillary network
what happens as you move down the ascending limb?
more Na+ ions diffuse out into medulla than further up, further lowering water potential in medulla.
what happens in the distal convoluted tubule?
water moves out by osmosis and is reabsorbed into the blood
Ions actively transported out
To make slight adjustments to blood pH
what is the role of the loop of Henle?
it massively increases the ion concentration in the medulla, which lowers the water potential. this causes water to move out of the collecting duct by osmosis.
the water in medulla is reabsorbed into blood through the capillary network
how is the volume of water reabsorbed controlled?
by changing the permeability of the DCT and the collecting duct
how is the water potential of the blood monitored?
by cells called osmoreceptors in the hypothalamus of the brain
how do osmoreceptors detect a change in blood water potential?
when water potential of the blood decreases, water will move out of the osmoreceptor cells by osmosis
cells decrease in volume
this sends signal to hypothalamus, which sends signal to the posterior pituitary gland, causing it to release ADH into the blood
what is ADH?
antidiuretic hormone
what does ADH do?
makes walls of DCT and collecting duct more permeable to water
so more water reabsorbed into medulla and blood
what does blood ADH do when you’re dehydrated?
blood ADH levels rise
what does ADH do when you’re hydrated?
water content of blood rises, so does water potential
detected by osmoreceptors
posterior pituitary gland releases less ADH into the blood
less ADH means DCT and collecting duct become less permeable, so less water is reabsorbed into blood by osmosis
a large amount of dilute urine urine is produced and more water is lost
what is negative feedback?
when a change triggers a response which decreases the effect of the change
what is the liver?
a glandular organ containing hepatocytes
what are hormones?
chemical messengers secreted by glands
specific - bind to complementary receptors on target cells
effective in very low concentrations
long lasting widespread effects
what is lipogenesis?
when glucose is converted to fats
what is water potential?
it is determined by the number of ions relative to water
osmole = unit used to describe the number of ions that affect water potential
what does the nephron do?
form glomerular filtrate via ultracentrifugation at the Bowman’s capsule
reabsorb glucose and water at the PCT
maintain a Na+ gradient and reabsorb water in the medulla via the loop of Henle
reabsorb water at the PCT and CD
what is the first layer of ultrafiltration?
small substances can pass through pores between endothelial cells of capillaries
what is the 2nd layer of ultrafiltration?
small substances can pass through gaps in connective tissue of capillaries and Bowman’s capsule
what is the third layer of ultrafiltration?
small substances can pass through gaps between foot - like processes of podocyte cells that make up the inner membrane of the Bowman’s capsule
What is the exam technique for ultrafiltration in the kidneys?
Blood pressure/ hydrostatic pressure
Small molecules
Pass through basement membrane/ basement membrane acts as a filter
Protein too large to go through
Presence of pores in capillaries/ podocytes
How to ensure water is reabsorbed along the whole length of the nephron?
The counter-current movement of liquids up the ascending limb and collecting duct ensures a constant gradient is maintained between collecting duct and interstitial space
what can blood water potential be affected by?
an increase or decrease in blood solutes
an increase or decrease in water intake
exam technique for when osmoreceptors detect low water potential of blood:
ADH produced in hypothalamus, secreted into posterior pituitary gland and then released into capillaries
presence of ADH detected by receptors in membrane of collecting duct/ DCT
phosphorylase enzyme is activated
vesicles containing aquaporins move to and fuse with membrane of the cells lining collecting duct/DCT
water moves down water potential gradient out of collecting duct/ DCT and into blood
how does osmoregulation cause the increase of blood water potential?
the reabsorption of water will not increase the water potential of the blood - it will just slow down the decrease.
osmoreceptors send an impulse to the thirst centre of the brain
