16. HOMEOSTASIS Flashcards
Define homeostasis
Homeostasis is the maintenance of a constant internal environment
Recall the four levels in the body that are controlled by homeostasis
Temperature, pH, water potential and blood glucose concentration
Explain why temperature is controlled by homeostasis
To maintain the optimum temperature for enzymes
Explain why pH is controlled by homeostasis
To maintain the optimum pH for enzymes
Explain why water potential is controlled by homeostasis
To prevent the negative effects of osmostic imbalance on cells
Explain why blood glucose concentration is controlled by homeostasis
To prevent the negative effects of osmostic imbalance on cells
What are ectotherms?
Organisms that use the surroundings to warm their bodies
What are endotherms?
Organisms that rely on metabolic processes to warm up and maintain a relatively constant core body temperature, regardless of the temperature of the environment
If an ectotherm’s core body temperature falls below optimum, list the ways they might increase it
Bask in the sun, orientate their bodies towards the sun so the maximum surface area is exposed to the sun, extend their bodies to increase the surface area exposed to the sun, press their bodies against warm ground (conduction)
If an ectotherm’s core body temperature rises above optimum, list the ways they might decrease it
Seek shade, dig burrows, press their bodies against cool earth (conduction), move into water (conduction), minimise movement, orientate their bodies so the minimum surface area is exposed to the sun
Describe the two places body temperature is detected in endotherms
By peripheral temperature sensors in the skin, and by temperature sensors in the hypothalamus, which detect the temperature of the blood
If an endotherm’s core body temperature falls below optimum, list the ways they might increase it
Vasoconstriction, decrease the rate of sweating, shivering, hairs on the skin stand erect
How does vasoconstriction increase core body temperature?
By reducing heat lost through radiation from the blood
How does reducing sweating increase core body temperature?
By reducing heat lost through evaporation
How does shivering increase core body temperature?
By releasing more heat through increased muscle contraction and respiration
How do erect skin hairs increase core body temperature?
By increasing the amount of air trapped on the skin
If an endotherm’s core body temperature increases above optimum, list the ways they might decrease it
Vasodilation, rate of sweating increases, hair on the skin lie flat
How does vasodilation decrease core body temperature?
By increasing the heat lost through radiation from the blood to the environment
How does increasing sweating decrease core body temperature?
By increasing heat loss from the body as sweat evaporates from the skin
How do flat skin hairs decrease core body temperature?
By reducing the amount of air trapped on the skin
In terms of heat exchange, explain the effect of small organisms having a large surface area to volume ratio
A higher rate of heat loss, but also a higher rate of heat absorption
In terms of heat exchange, explain the effect of large organisms having a small surface area to volume ratio
A lower rate of heat loss, but also a lower rate of heat absorption
Define negative feedback
A system in which a change to the normal levels triggers a response which reduces the effect of the change
Give two examples of homeostatic levels that are controlled by negative feedback
The maintenance of blood glucose concentration around an optimum, and the maintenance of temperature around an optimum
Define positive feedback
A system in which a change to the normal levels triggers a response which increases the effect of the change
Which action may increase your blood glucose concentration?
Eating
Which action may decrease your blood glucose concentration?
Exercise
Name the two hormones that are involved in the control of blood glucose concentration
Insulin and Glucagon
Where are insulin and glucagon secreted from?
The pancreas
Insulin and Glucagon are antagonistic - what does this mean?
They work exclusively and oppositely to each other
Define Glycogenesis
The formation of glycogen from glucose
Define Glycogenolysis
The hydrolysis of glycogen to form glucose
Define Gluconeogenesis
The formation of glucose from non-carbohydrate sources (e.g. amino acids, lactate and glycerol)
Which hormone is released from the pancreas when blood glucose concentration rises above normal?
Insulin
How does insulin travel from the pancreas to target cells?
In the bloodstream
Where are the receptors that insulin binds to?
The cell surface membrane on target cells
What does the binding of insulin to receptors cause?
The opening of channel proteins and the activation of carrier proteins, and more carrier and channel proteins to move into the plasma membrane, increasing the permeability of the tissues to glucose
Describe how the effect of insulin on a cell’s permeability to glucose causes the blood glucose concentration to decrease
The increase in permeability to glucose causes more glucose to move from the blood to the cells by facilitated diffusion
Describe three other actions of insulin
It activates enzymes within skeletal muscle and liver cells to catalyse glycogenesis. It stimulates lipid formation from glucose. It raises the rate of respiration.
Which hormone is released from the pancreas when blood glucose concentration falls below normal?
Glucagon
How does glucagon travel from the pancreas to target cells?
In the bloodstream
Where are the receptors that glucagon binds to?
The cell surface membrane on liver and muscle cells
What does the binding of glucagon to receptors cause?
It activates enzymes that catalyse glycogenolysis and gluconeogenesis.
Describe how glucagon increases blood glucose concentration
It causes more glucose to move from the cells to the blood by facilitated diffusion.
Apart from insulin and glucagon, name another hormone that may affect your blood glucose concentration
Adrenaline
Why is adrenaline secreted, and where is it secreted from?
Adrenaline is secreted from your adrenal glands in times of stress and excitement
Name the response adrenaline initiates
The fight or flight response
Why is more glucose required during the fight or flight response?
Because your rate of respiration increases
What is signal transduction?
The transmission of molecular signals from a cell’s exterior to its interior
What does adrenaline require signal transduction to initiate its action?
Because it’s hydrophilic, and therefore can’t pass through the phospholipid bilayer of the cell membrane
Which cells are the target cells of adrenaline, and how does adrenaline bind to them?
Adrenaline binds to receptors on the cell surface membrane of liver and muscle cells
What does the binding of adrenaline to the receptors on target cells initiate?
It activates Adenylate Cyclase
What does activated Adenylate Cyclase produce?
cAMP
What is the purpose of cAMP?
It is a secondary messenger
What does cAMP activate?
The enzyme that catalyses glycogenolysis and gluconeogenesis
What effect does cAMP activating the enzymes that catalyse glycogenolysis and gluconeogenesis have on blood glucose concentration?
This releases more glucose for the increased rate of respiration
Define Diabetes
Diabetes is the inability to control blood glucose concentration
Describe the symptoms of diabetes
More frequent urination, more thirsty than normal, lack of energy
Describe the medical indicator of diabetes
Glucose in the urine
Name the two types of diabetes
Type 1 and Type 2
Describe the cause of type 1 diabetes
The pancreas does not produce insulin
Describe the treatment for type 1 diabetes
Insulin injections and a carbohydrate controlled diet
Name the risk factors for type 2 diabetes
A high carbohydrate diet, a sedentary lifestyle and obesity
Describe the cause of type 2 diabetes
Target cells are less responsive to insulin because there are fewer (normal) receptors in cell surface membranes
Describe the treatments for type 2 diabetes
A carbohydrate controlled diet and exercise
Describe the three ways you can identify a diabetic from a graph of blood glucose concentration over time
A diabetic’s resting blood glucose concentration would be higher, their blood glucose concentration would rise higher after eating, and take longer to return to normal
Explain why a diabetic’s blood glucose concentration returns to normal more slowly after eating
There are fewer normal receptors in the cell surface membrane of target cells, therefore less insulin binds to the receptors, so less channel proteins open and less carrier proteins are activated, so less glucose moves into cells from the blood by facilitated diffusion
Name three methods for testing for glucose
Benedict’s solution, test strips and biosensors
Why can you only use Benedict’s solution to test urine for glucose, and not the blood?
Because the red colour of the blood would mask the colour change of the Benedict’s solution
Describe the disadvantages of using Benedict’s solution to test urine for glucose
It is quantitative, and the colour matching is subjective
Describe the advantage of using test strips to test urine for glucose
They give an immediate result
Describe the disadvantages of using test strips to test urine for glucose
They are qualitative. The colour matching is subjective. The urine glucose concentration is given as a range.
Describe the advantages of using biosensors to test blood for glucose
They are quantitative. They are more accurate. They can monitor blood glucose concentration continuously.
Which method would doctors use to diagnose diabetes, and why?
Test strips, as the medical indicator for this is presence of glucose in the urine (not concentration)
Which method would diabetics use to monitor their blood glucose concentration, and why?
Biosensors, because they have to calculate the concentration of insulin to inject, so the result must be quantitative
What are the two main functions of the kidneys?
Excretion and osmoregulation
What is osmoregulation?
The control of the water potential of the blood
Describe what would happen if the blood was hypertonic compared to red blood cells
Too much water moves from cells to the blood by osmosis, and cells can crenate and die.
Describe what would happen if the blood was hypotonic compared to red blood cells
Too much water moves from the blood to cells by osmosis. Cells can lyse and die.
What is the function of the renal artery?
It transports blood from the heart to the kidney
What is the function of the renal vein?
It transports blood from the kidney to the heart
What is the function of the ureter?
It transports filtrate (urine) from kidney to the bladder for excretion
Name the two main structural parts of the kidney
Cortex and medulla
What is the structural unit of the kidney called?
Nephron
What is a renal pyramid?
A section of the medulla and cortex containing many nephrons
Name all the blood vessels of the kidney, in the order that blood is transported through them
Renal artery, afferent arteriole, glomerulus, efferent arteriole, capillaries, renal vein
Name all the tubules of the kidney that carry filtrate, in the order that filtrate is transported through them
Bowman’s capsule, proximal convoluted tubule, descending loop of henle, ascending loop of henle, distal convoluted tubule, collecting duct
Which structures of the nephron are in the cortex?
Bowman’s Capsule,Glomerulus, ProximalConvoluted Tubule,Distal ConvolutedTubule, top of theCollecting Duct.
Which structures of the nephron are in the medulla?
Loop of Henle, bottom of the Collecting Duct
In a healthy person, what substances should be present in their urine?
(Excess) Water, (Excess) Dissolved ions, Urea, Other small substances, e.g. hormones and excess vitamins
In a healthy person, what substances shouldn’t be present in the urine?
Protein, blood cells and glucose
Why shouldn’t proteins and blood cells be present in the urine of a healthy person?
Because they are too large to be filtered out the blood
Name the three stages involved in the production of urine
Ultrafiltration, selective reabsorption and concentration
In which parts of the nephron does ultrafiltration occur?
The glomerulus and bowman’s capsule
What is ultrafiltration?
Ultrafiltration is where all small molecules are filtered from the blood (glomerulus) into the bowman’s capsule (over the basement membrane)
What is the basement membrane?
The combination of the epithelial layers of the capillaries (glomerulus) and the bowman’s capsule
Describe how the small molecules are filtered from the blood into the bowman’s capsule
Blood enters the afferent arteriole, which splits into smaller capillaries which make up the glomerulus. Because the diameter of the blood vessel decreases, the hydrostatic pressure increases.
Describe the journey of the small molecules as they are forced out of the blood and into the bowman’s capsule
They are forced through the pores between their epithelial cells and through the basement membrane, into the bowman’s capsule
What liquid does ultrafiltration form in the bowman’s capsule?
Filtrate
Describe the contents of the filtrate in the bowman’s capsule
Excess water, excess ions, glucose, hormones and vitamins
In which part of the nephron does selective reabsorption occur?
The proximal convoluted tubule
What is selective reabsorption?
The selective reabsorption of ions, water and glucose in the proximal convoluted tubule (85% of the filtrate gets reabsorbed)
How much urea is reabsorbed from the filtrate back into the blood?
None
How much water is reabsorbed from the filtrate back into the blood?
Some
By which type of molecular transport is water reabsorbed from the filtrate back into the blood?
By osmosis down a water potential gradient
How much Na+ is reabsorbed from the filtrate back into the blood?
Some are actively transported back into the blood
How much glucose is reabsorbed from the filtrate back into the blood?
All of the glucose is reabsorbed
By which type of molecular transport is glucose reabsorbed from the filtrate back into the blood?
All of the glucose is co-transported (using Na+) back into the blood
Describe the only active part of the co-transport of glucose from the PCT into the blood
The active transport of Na+ from the PCCT epithelial cells into the tissue fluid
Why are Na+ actively transported from the PCT epithelial cells into the tissue fluid
To reduce the concentration of Na+ in the PCT epithelial cells compared to the glomerular filtrate in the lumen
Describe how Na+ co-transports glucose from the PCT lumen into the PCT epithelial cells
Na+ moves from the PCT lumen to the PCT epithelial cell by facilitateddiffusion down a concentration gradient, transporting glucose with it.
How does Na+ and glucose then move into the blood
By diffusion
Describe the two adaptations of the PCT cells to maximise the rate of reabsorption
PCT cells have microvilli on the side facing the lumen, and the epithelial cells have lots of mitochondria
Why do PCT epithelial cells have microvilli on the side facing the lumen?
To provide a large surface area for reabsorption
Why do PCT epithelial cells have lots of mitochondria?
To provide more energy for active transport
In which parts of the nephron does concentration occur?
The Loop of Henle and the DCT
What is the function of the Loop of Henle?
To maintain a sodium ion concentration gradient
Why does the descending limb of the loop of henle have thinner walls?
So it is permeable to water, but impermeable to ions
Why does the ascending limb of the loop of henle have thicker walls?
So it is impermeable to water, but permeable to ions
Describe the role of the descending loop of henle in the concentration of filtrate / urine
Na+ are actively transported out of the filtrate in the ascending limb into the interstitial space. This lowers the water potential of the interstitial space.
Describe the role of the descending loop of henle in the concentration of filtrate / urine
More water to move out of the descending limb into the interstitial space by osmosis, where it is reabsorbed into the blood
Why does the filtrate become more concentrated as it moved down the descending limb?
Because it loses more water by osmosis
Describe how the concentration gradient is maintained between the filtrate in the descending limb and the interstitial space
More Na+ are actively transported out on the bottom of the ascending limb into the interstitial space
Why does the concentration of the filtrate decrease as it moves up the ascending limb?
Because more Na+ are actively transported out, but no more water is lost
Describe how the filtrate is concentrated as it moved through the DCT
The medulla surrounding the DCT is more concentrated than the filtrate, so water moved from the DCT into the interstitial space by osmosis
What happens to the filtrate that remains in the collecting duct?
It is transported through the ureter to the bladder to be stored as urine, and eventually excreted
Name the hormone that controls osmoregulation
ADH (Anti-Diuretic Hormone)
What is a diuretic?
Something that increases removal of water from the body
What is an anti-diuretic?
Something that increases reabsorption of water into the body
Which receptors detect the water potential of the blood?
Osmoreceptors in the hypothalamus
Describe the effect on the osmoreceptors if the water potential of the blood is too low
Water movesoutof theosmoreceptors, causing them toshrivel upand the hypothalamus toproduce ADH.
Describe the effect on the osmoreceptors if the water potential of the blood is too high
Water moves into the osmoreceptors, causing the hypothalamus to reduce the production of ADH.
Where is ADH produced and secreted from?
Produced from the hypothalamus, and secreted from the posterior pituitary
How does ADH reach its target cells?
ADH is transported in the bloodstream until it reaches the kidney cells
How does ADH increase reabsorption of water into the blood?
It increases the permeability to water of the epithelial cells of the DCT and the collecting duct, which means more water leaves the DCT and collecting duct, and is reabsorbed into the blood
How does ADH increase the permeability of the epithelial cells of the DCT and collecting duct to water?
It activates an enzyme called phosphorylase in the cells, which causes more aquaporins to be inserted into the cell membranes of epithelial cells of the DCT and collecting duct
How would an increase in the production of ADH affect the concentration and volume of urine?
It would increase the concentration and decrease the volume of urine, because more water would have been reabsorbed into the blood
How would a decrease in the production of ADH affect the concentration and volume of urine?
It would decrease the concentration and increase the volume of urine, because less water would have been reabsorbed into the blood
