6.4 Homeostasis Flashcards
1
Q
What is homeostasis?
A
- The maintenance of a constant internal environment within restricted limits in organisms.
- It involves trying to maintain the chemical make-up, volume and other features of blood and tissue fluid.
- It ensures that cells are in an environment that meets their requirements.
2
Q
Why is it important that core temperature remains stable?
A
- Maintains stable rate of enzyme-controlled reaction and prevent damage to membranes
- temperature too low = enzyme and substrate molecules have insufficient kinetic energy
- temperature too high = enzymes denature
3
Q
Why is it important that blood pH remains stable?
A
- Maintain stable rate of enzyme controlled reaction and optimum conditions for other proteins
- Acidic pH = H+ ions interact with H-bonds and ionic bonds in tertiary structure of enzymes -> shape of active site changes so no ES complexes form
4
Q
Why is it important that blood glucose concentration remains stable?
A
- Maintain constant blood water potential: prevent osmotic lysis/crenation of cells
- Maintain constant concentration of respiratory substrate: organism maintains constant level of activity regardless of environmental conditions
5
Q
Why is homeostasis important for enzymes?
A
- Enzymes that control reactions, and other proteins, are sensitive to pH and temperature changes.
- Changing these reduces the rate of reaction or may prevent them working at all.
- Maintaining a fairly constant internal environment means reactions take place at a suitable rate.
6
Q
Why is homeostasis important for organisms?
A
- Organisms who can maintain a constant internal environment are more independent of changes in the external environment.
- They may have a wider geographical range and therefore greater chance of finding food, shelter etc.
E.g. Mammals are found in most habitats.
7
Q
What are the control mechanisms?
A
- The optimum point, at which the system works best, monitored by a:
- Receptor, which detects deviation from the optimum, and informs the:
- Coordinator, which coordinates information from receptors and sends information to an appropriate:
- Effector, a muscle or gland, which brings about the changes needed to return to the optimum, this return creates a:
- Feedback mechanism, where a receptor responds to a stimulus created by a change to the system brought about by the effector.
8
Q
What is negative feedback?
A
- When the change produced by the control system leads to a change in the stimulus detected by the receptor and turns the system off
- self regulatory mechanisms return internal environment to optimum when there is a fluctuation
9
Q
What is positive feedback?
A
- Occurs when a deviation from an optimum causes changes that result in an even greater deviation from the normal.
E.g. a stimulus leads to a small influx of sodium ions, which increases membrane permeability to sodium ions, more enter, so further increase permeability and more ions.
10
Q
What is the classification of control systems?
A
There are many receptors and effectors, so they have separate mechanisms that each produce a positive movement towards an optimum.
This allows a greater degree of control of the factor being regulated.
Having separate mechanisms that controls departures in different directions from the original state is a general feature of homeostasis.
11
Q
Why is it important information from receptors is analysed by coordinators before action?
A
Temperature receptors in skin may signal that the skin is cold, so to raise the temperature.
But, information from the hypothalamus in the brain may indicate that the blood temperature is above average, e.g. during exercise.
Also, the control centre must coordinate the action of the effectors so they operate harmoniously.
12
Q
Why do coordinators analyse inputs from several receptors before sending an impulse to effectors?
A
- receptors may send conflicting information
- optimum response may require multiple types of effector
13
Q
When does negative feedback occur?
A
when the stimulus causes the corrective measures to be turned off, and returns the system to its optimum level.
14
Q
Why does negative feedback mechanisms control fluctuations in different directions?
A
Provide more control, especially in case of ‘overcorrection’, which would lead to a deviation in the opposite direction from the original one
15
Q
What is negative feedback of glucose?
A
- A fall in glucose concentration is detected by receptors on α-cells.
- These cells secrete glucagon, which causes liver cells to convert glycogen to glucose, which increased blood glucose concentration.
- This blood circulates back to the pancreas, and there is reduced stimulation of α-cells, so they secrete less glucagon.
- So the secretion of glucagon leads to a reduction in its own secretion.
16
Q
What is negative feedback of increased glucose?
A
If blood glucose concentration rises, insulin is produced from the β-cells in the pancreas.
Insulin increases the uptake of glucose by cells and its conversion to glycogen and fat.
The fall in blood glucose concentration that results reduces insulin production.
17
Q
Why is it important there are separate negative feedback mechanisms?
A
- It gives greater homeostatic control, because there are positive actions in both directions.
- E.g. if glucagon raised glucose concentration above the optimum, it would take time for it to decrease again, if it was only by metabolic activity.
- But, by the second hormone insulin, blood sugar concentration is lowered, so is more rapid.
18
Q
What are the characteristics of hormones?
A
- They are produced in glands, which secrete the hormone directly in the blood (endocrine).
- carried in blood plasma to the target cells, which have specific receptors on their cell-surface membrane complementary to the hormone.
- effective in very low concentrations, but have widespread, long-lasting effects.
19
Q
Why is there a time lag between hormone production and response by an effector?
A
It takes time to:
- produce hormone
- transport hormone in the blood
- cause required change to the target protein
20
Q
What is the second messenger model?
A
This mechanism of hormone action is used by adrenaline and glucagon, to regulate blood glucose concentration.
21
Q
What is the process of the mechanism involving adrenaline?
A
- Adrenaline binds to a transmembrane protein receptor in the cell-surface membrane of a liver cell.
- binding causes the protein to change shape on the inside of the membrane.
- activating the enzymes adenylate cyclase, which converts ATP to cyclic AMP (cAMP) (removes 2 phosphates).
- cAMP acts as a second messenger that binds to protein kinase enzymes, changing its shape, activating it.
- The enzyme catalyses the conversion of glycogen to glucose, which moves out the liver by facilitated diffusion and into the blood through channel proteins.
22
Q
What is the pancreas made up of?
A
- Mainly the cells that produce its digestive enzymes.
- Scattered throughout are hormone-producing cells - islets of Langerhans, these include:
α cells produce glucagon.
β cells, which are smaller and produce insulin.
23
Q
What is the liver?
A
- Located above the diaphragm
-weighs up to 1.5kg, and is made up of hepatocytes cells. - involved in regulating blood glucose concentration, by:
Glycogenesis, Glycogenolysis, Gluconeogenesis.
24
Q
Name the factors that effect blood glucose concentration
A
- amount of carbohydrates digested from diet
- rate of glycogenolysis
- rate of gluconeogenesis
25
What is glycogenesis?
- The liver converts glucose into the storage polymer glycogen.
- When blood glucose is higher than normal the liver removes glucose and converts it to glycogen.
- It can store 75-100g of glycogen, sufficient to maintain blood glucose concentration for 12 hours at rest.
26
What is glycogenolysis?
- the liver hydrolyses glycogen into glucose which can diffuse into blood
- When blood glucose is lower than normal, the liver converts stored glycogen back to glucose, which diffuses into the blood to restore the normal blood glucose concentration.
27
What is gluconeogenesis?
- The production of glucose from sources other than carbohydrate.
- When its supply of glycogen is exhausted, the liver can produce glucose from non-carbohydrate sources such as glycerol and amino acids.
- liver converts glycerol and amino acids into glucose
28
Why does blood glucose concentration need regulating?
- If concentration falls too low, cells will be deprived of energy and die - brain cells are especially sensitive because they can only respire glucose.
- If concentration rises too high, it lowers the water potential and creates osmotic problems that can cause dehydration.
29
Where does blood glucose come from?
Normal concentration is 5mmol dm^-3.
- Directly from the diet in the form of glucose absorbed following hydrolysis of carbs - starch, maltose, lactose and sucrose.
- From hydrolysis in the small intestine of glycogen / glycogenolysis store in the liver and muscle cells.
- From gluconeogenesis, production of glucose from other sources.
30
What is insulin?
- The β cells of islets of Langerhans have receptors that detect the stimulus of increased concentration, and respond by secreting insulin into the blood.
- Insulin is a globular protein made up of 51 amino acids.
- All body cells par RBCs have glycoprotein receptors that bind to insulin molecules.
31
What happens when insulin binds to glycoprotein receptors?
- Change in the tertiary structure of the glucose transport carrier proteins, causing them to change shape and open, allowing more glucose in by facilitated diffusion.
- An increase in the number of carrier proteins for glucose.
- Activation of enzymes that convert glucose to glycogen and fat.
32
What happens with insulin and the number of carrier proteins?
- At low insulin concentrations, the protein from which the carrier proteins are made is part of the membrane of vesicles.
- A rise in insulin concentration results in these vesicles fusing with the cell-surface membrane so increasing the number of glucose transport channels.
33
How is blood glucose concentration lowered by insulin?
- by increasing the rate of absorption of glucose into the cells, especially in muscle cells
- by increasing the respiratory rate of the cells, which therefore use up more glucose, increasing their uptake of glucose from the blood
- by increasing the rate of conversion of glucose into glycogen in the cells of the liver and muscles
- by increasing the rate of conversion of glucose to fat
34
What is the role of glucagon when blood glucose concentration decreases?
1. α cells of the Islets of Langerhans in the pancreas detect decrease and secrete glucagon into bloodstream
2. Glucagon binds to surface receptors on liver cells and activates enzymes for glycogenolysis and gluconeogenesis
3. Glucose diffuses from liver into bloodstream
35
What is the role of adrenaline when blood glucose concentration decreases?
1. adrenal glands produce adrenaline. It binds to surface receptors on liver cells and activates enzymes for glycogenolysis
2. Glucose diffuses from liver into bloodstream
36
What happens when blood glucose concentration increases?
1. β cells in the Islets of Langerhans in pancreas detect increase and secrete insulin into bloodstream
2. Insulin binds to surface receptors on target cells to
a) increase cellular glucose uptake
b) activate enzymes for glycogenesis(liver and muscles)
c) stimulates adipose tissue to synthesis fat
37
What is diabetes?
a metabolic disorder caused by an inability to control blood glucose concentration due to a lack of the hormone insulin or a loss of responsiveness to insulin
38
What is type 1 diabetes?
Insulin dependent
- due to the body being unable to produce insulin
- normally begins in childhood
- may be the result of an autoimmune response whereby the body's immune system attacks its own cells, in this case the β cells of the islets of langerhans
- develops quickly and signs are normally obvious
39
What is type 2 diabetes?
Insulin independent
- normally due to glycoprotein receptors on body cells being lost of losing their responsiveness to insulin
- May also be due to an inadequate supply of insulin from the pancreas
- usually develops in people over the age of 40
- develops slowly, symptoms are normally less severe
- people who are overweight are particularly likely to get it
40
What are the signs of diabetes?
- high blood glucose concentration
- glucose in urine
- need to urinate excessively
- genital itching or regular episodes of thrush
- weight loss
- blurred vision
41
What are the symptoms of diabetes?
- tiredness
- increased thirst and hunger
42
How is type 1 diabetes controlled?
- Controlled by injections of insulin.
- cannot be taken by mouth because being a protein it would be digested in the alimentary canal.
- therefore it's injected typically either two or four times a day.
- The dose of insulin must be matched exactly to the glucose intake. If a person with diabetes takes too much insulin he will experience a low blood glucose concentration that can result in unconsciousness. To ensure the correct dose, blood glucose concentration is monitored using biosensors. By injecting insulin and managing their carbohydrate intake and exercise carefully people with diabetes can lead normal lives.
43
How is type 2 diabetes controlled?
- usually controlled by regulating the intake of carbs in the diet and matching this to the amount of exercise taken
- may be supplemented by injections of insulin or by the use of drugs that stimulate insulin production
- other drugs can slow down the rate at which the body absorbs glucose from the intestine
44
What is osmoregulation?
the homeostatic control of the water potential of the blood
45
Where are the kidneys found?
There are 2 kidneys found at the back of the abdominal cavity, one on each side of the spinal cord
46
What is the structure of a mammalian kidney?
Fibrous Capsule - outermembrane which protects the kidney
Cortex - outer region consists of Bowman's capsules, convoluted tubules and blood vessels
Medulla - inner region consists of collecting ducts and blood vessels, made up of loops of Henle
Renal pelvis - funnel shaped cavity that collects urine into the ureter
Ureter - tube carries urine to bladder
Renal artery - supplies kidney with oxygenated blood
Renal vein - returns deoxygenated blood from kidney to heart
47
What is the structure of a nephron?
Bowman's capsule - at the start of nephron: cup-shaped, surrounds glomerulus, inner layer of podocytes
Proximal convoluted tubule: series of loops surrounded by capillaries, walls made of epithelial cells with microvilli
Loop of Henle: hairpin loop extends from cortex into medulla, surrounded by blood capillaries
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
48
What are the blood vessels associated with a nephron?
- wide afferent arteriole from renal artery enters renal capsule and forms glomerulus: branched knot of capillaries which combine to form narrow efferent arteriole
- efferent arteriole branches to form capillary network that surrounds tubules
49
What are the afferent arterioles?
a tiny vessel that arises from the renal artery and supplies the nephron with blood.
- the afferent arteriole enters the renal capsule of the nephron where it forms the glomerulus
50
What is the golmerulus?
a many-branched knot of capillaries from which fluid is forced out of the blood.
the glomerular capillaries recombine to form the efferent arteriole
51
What is the efferent arteriole?
- a tiny vessel that leaves the renal capsule
- has a smaller diameter than the afferent arteriole and so causes an increase in blood pressure within the glomerulus
- the efferent arteriole carries blood away from the renal capsule and later branches to form the blood capillaries
52
What are blood capillaries?
- a concentrated network of capillaries that surrounds the proximal convoluted tubule, the loop of Henle and the distal convoluted tubule and from where the reabsorb mineral salts, glucose and water
- these capillaries merge together into venules that in turn merge together to form the renal vein
53
How does the nephron cause osmoregulation?
- the formation of glomerular filtrate by ultrafiltration
- reabsorption of glucose and water by the proximal convoluted tubule
- maintenance of a gradient of sodium ions in the medulla by the loop of Henle
- reabsorption of water by the distal convoluted tubule and collecting ducts
54
How is glomerular filtrate 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 and large molecules e.g. proteins remain in capillary
55
How are cells of the Bowman's capsule adapted for ultrafiltration?
- fenestrations between epithelial cells of capillaries
- fluid can pass between and under folded membrane of pogocytes
56
What is the movement of filtrate out of the glomerulus resisted by?
- capillary epithelial cells
- connective tissue and epithelial cells of the blood capillary
- epithelial cells of the renal capsule
- the hydrostatic pressure of the fluid in the renal capsule space
- the low water potential of the blood in the glomerulus
57
What are the modifications to reduce the barrier to the flow of filtrate through the glomerular capillaries?
- podocytes have spaces between them allowing filtrate to pass beneath them and through the gaps between their branches
- filtrate passes between these cells rather than through them
- the endothelium of the glomerular capillaries has spaces up to 100nm wide between its cells
- fluid can therefore pass between them, rather than through these cells
- so hydrostatic pressure of the blood in the glomerulus is sufficient to overcome the resistance and so filtrate passes from the blood into the renal capsule
- the filtrate doesn't contain cells or plasma proteins which are too large to pass across the connective tissue
58
What is the reabsorption of glucose by the PCT?
- In the PCT nearly 85% of the filtrate id reabsorbed back into the blood
- Ultrafiltration operates on the basis of size of molecule - small ones are removed
- Some such as urea are wastes but most are useful and so are absorbed
59
How are the epithelial cells the proximal convoluted tubules have adapted to reabsorb substances into the blood?
- Microvilli: provide large surface area for cotransporter proteins
- infoldings at their bases to give a large surface area to transfer reabsorbed substances into blood capillaries
- high density of mitochondria: to provide ATP for active transport of glucose into intercellular spaces
60
What is the process of reabsorption of glucose and water by the PCT?
- sodium ions(Na) are actively transported out of the cells lining the PCT into blood capillaries which carry them away. The sodium ion conc. of these cells is therefore lowered
- Na now diffuse down the conc. gradient from the lumen of the PCT into the epithelial lining cells but only through special carrier proteins by facilitated diffusion
- these carrier proteins are of specific types each of which carries another molecule along with Na, known as co-transport
- the molecules which have been co-transported into the cells of the PCT then diffuse into the blood. As a result all the glucose and most of the other valuable molecules are reabsorbed as well as water
61
What is the function of the loop of Henle?
Responsible for water being reabsorbed from the collecting duct, thereby concentrating the urine so that it has a lower water potential than the blood.
62
What are the regions of the loop of Henle?
- the descending limb which is narrow, with thin walls that are highly permeable to water
- the ascending limb, which is wider, with thick ass walls that are impermeable to water
63
How is the loop of Henle a counter-current multiplier?
1. Active transport of Na+ and Cl- out of ascending limb using ATP
2. Creating a low water potential in the medulla/ water potential of interstitial fluid decreases
3. Osmosis of water out of descending limb (ascending limb is impermeable to water)
4. Water potential of filtrate decreases going down descending limb: lowest in medullary region, highest at top of ascending limb
5. ions are reabsorbed by the blood so the concentration falls again
6. The longer the loop of Henle the more water can be reabsorbed so camels and gerbils would have really long loops of Hnele
64
What is the role of the DCT?
- Role is to make fine adjustments to the water and salt concentration of the filtrate/urine and control the pH of the blood by selecting which ions should be reabsorbed
- the cells have many microvilli and mitochondria
- the reabsorption of ions and water is under control of a hormone called ADH
65
What is the role of the collecting duct?
Reabsorption of water from filtrate into interstitial fluid via osmosis through aquaporins
66
Why is it 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
67
What may cause blood water potential to change/cause a rise in solute concentration?
- too little water being consumed
- sweating
- large amount of ions
- level of ion intake in the diet
- level of ions used in metabolic processes or excreted
68
What is the role of the hypothalamus in osmoregulation?
1. osmoreceptors detect the fall in water potential
2. osmosis of water out of osmoreceptors in hypothalamus causes them to shrink
3. this triggers hypothalamus to produce more antidiuretic hormone (ADH)
69
What is the role of the posterior pituitary gland in osmoregulation?
Stores and secretes the ADH produced by the hypothalamus
70
What is the role of ADH in osmoregulation?
1. Makes cells lining collecting duct more permeable to water:
- Binds to receptor, activates phosphorylase causing vesicles with aquaporins on membrane to fuse with cell-surface membrane
2. Makes cells lining collecting duct more permeable to urea:
- water potential in interstitial fluid decreases
- more water reabsorbed = more concentrated urine
71
How does a fall in solute concentration occur?
- large volumes of water being consumed
- salts used in metabolism or excreted not being replaced in the diet
72
How does the body respond to the rise in water potential?
- osmoreceptors detect a rise in water potential and increase the frequency of nerve impulses to the pituitary gland to reduce its release of ADH
- Less ADH, via blood, leads to decrease in the permeability of the collecting ducts to water and urea
- Less water reabsorbed into the blood from the collecting ducts
- more dilute urine is produced and water potential of blood falls
- when water potential of blood has returned to normal, the osmoreceptors in the hypothalamus cause the pituitary to raise its ADH release back to normal levels
73
What is the function of the Bowman's capsule?
Ultrafiltration, i.e. filtering small molecules and ions out of the blood but keeping large molecules and cells in the blood
74
What is the function of the Proximal convoluted tubule?
selective reabsorption (glucose, amino acids, and most Na+ ions, Cl- ions, and water molecules are reabsorbed into the blood)
75
What is the function of the Loop of Henle?
Establishing a water potential gradient, i.e. the water potential in the nephron filtrate becomes higher than it is in the medulla; additional water can then be reabsorbed from the collecting duct
76
What is the function of the Distal convoluted tubule?
additional ion reabsorption
77
What is the function of the collecting duct?
determination of urine concentration and volume, i.e. additional reabsorption of water into blood
78
How are tissues adapted in the Bowman's capsule?
Three filtering systems help to prevent cells and large molecules from leaving the blood:
1. narrow gaps (fenestrations) between endothelium cells in capillaries
2. Podocytes (epithelial cells of the capsule that have finger like projections, which form filtration slits)
3. Basement membrane (a mesh of collagen and glycoprotein around the glomerulus)
79
How are tissues adapted in the PCT?
PCT cells are adapted for reabsorption in the following ways:
- Microvilli to increase surface area for reabsorption
- Plasma membranes have many pumps and transporter proteins for active transport and facilitated diffusion
- many mitochondria to produce ATP for active transport
80
How are tissues adapted in the loop of Henle?
- the descending limb is permeable to water
- However, the ascending limb is impermeable to water, but Na+ and Cl- ions are actively transported from the filtrate into the medulla, which raises the water potential of the filtrate
81
How are tissues adapted in the DCT?
Plasma membranes of DCT cells have many pumps and transporter proteins for active transport and facilitated diffusion
82
How are tissues adapted in the collecting duct?
the cells of the collecting duct wall contain aquaporin proteins, which can be placed in the plasma membranes of these cells to enable additional water reabsorption
