Homeostasis Flashcards
kidney functions
- Osmoregulation. The homeostatic control of the water potential of the blood. Ensures that the blood is iso-osmotic/ isotonic to that of tissue fluid and cell cytoplasm
- Removal/excretion of nitrogenous waste. Cant store protein and these contain nitrogen so need to be converted to nitrogenous waste.
Breakdown of protein
- occurs in the liver
- excess protein is broken down by hydrolysis into amino acids. Deamination then occurs which is when an amine group is removed from an amino acid. This forms the stem of an amino acid and ammonia. Fish excrete their nitrogenous waste in the form of aqueous ammonia
- as ammonia is very toxic it forms urea which is less toxic. Urea is removed in urine
- in birds/ insects urea is broken down into uric acid
- the kidney removes urea from the body after the liver creates it
structure of the mammalian kidney
- two kidneys found at the back of the abdominal cavity, one each side of the spinal chord
- contains fibrous capsule- an outer membrane that protects the kidney
- cortex- a lighter coloured outer region made up of renal (Bowman’s) capsules, convoluted tubules and blood vessels
- medulla- a darker coloured inner region made up of loops of Henle, collecting ducts and blood vessels
- renal pelvis- funnel-shaped cavity that collects urine into the ureter
- ureter- tube that carries urine to the bladder
- renal artery- supplies the kidney with blood from the heart via the aorta
- renal vein- returns blood to the heart via the vena cava
nephron structure
- functional unit of the kidney
- contains Bowman’s (Renal) capsule- closed end at the start of the nephron. It is cup-shaped and surrounds the mass of capillaries known as the glomerulus. Inner layer of the renal capsule is made up of podocytes
- Proximal convoluted tubule- series of loops surrounded by blood capillaries. Its walls are made of epithelial cells which have microvilli
- Loop of Henle- long loop that extends from the cortex into the medulla of the kidney and back again, surrounded by capillaries
- distal convoluted tubule- a series of loops surrounded by blood capillaries. Its walls are made of epithelial cells, but it is surrounded by fewer capillaries than the proximal tubule
- Collecting duct- tube into which a number of distal convoluted tubules from a number of nephrons empty. Lined by epithelial cells and becomes increasingly wide as it empties into the pelvis of the kidney
Blood vessels: - afferent arteriole- tiny vessel that arises from the renal artery and supplies the nephron with blood. The afferent arteriole enters the bowman’s capsule where it forms the glomerulus
-glomerulus- many-branched knot of capillaries from which fluid is forced out of the blood. The glomerular capillaries recombine to form the efferent arteriole - efferent arteriole- 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. Carries blood away from the Bowmans capsule
- blood capillaries- concentrated network of capillaries that surrounds the proximal convoluted tubule, loop of Henle and the distal convoluted tubule and from where they reabsorb mineral salts, glucose and water. These capillaries merge together into venules that in tern merge together to form the renal vein
ultrafiltration
- blood is filtered out from the glomerulus and is collected in the Bowman’s capsule.
- there are more pores in glomerulus capillaries than normal capillaries for quick filtration
- higher hydrostatic blood pressure in capillaries- blood pushed out the pores at a faster rate
- afferent arteriole has larger diameter than the efferent arteriole so it is easier for the blood to enter the glomerulus than leave, this causes a build up of hydrostatic pressure in the glomerulus so water, glucose and mineral ions are squeezed out of the capillary to form the glomerular filtrate. Blood cells and proteins cannot pass across the renal capsule as they are too large
- bottleneck shape of the bowman’s capsule creates pressure
-renal arteries are short and direct branches off the aorta which helps maintain a high blood pressure
-Podocytes keep the capillaries in the glomerulus slightly separated so that the capillaries don’t collapse down and obstruct the formation of filtrate - movement of this filtrate out of the glomerulus is resisted by: the capillary endothelial cells, connective tissue and endothelial cells of the blood capillary, epithelial cells of the renal capsule, the hydrostatic pressure of the fluid in the renal capsule space, the lower water potential of the blood in the glomerulus
- the total resistance would be sufficient enough to prevent filtrate leaving the glomerular capillaries but for some modifications to reduce this barrier to the flow of filtrate:
- inner layer of renal capsule made up of highly specialised podocytes. They have spaces between them to allow filtrate to pass beneath the and through gaps between their branches. Filtrate passes between these cells rather than through them
- endothelium of glomerular capillaries has spaces between its cells, to allow fluid to pass between rather than through the cells
As a result, hydrostatic pressue in the glomerulus is sufficient to overcome the resistance and so filtrate passes from the blood into the renal capsule
-** basement membrane** is present on podocytes and it is through the basment membrane that the molecules move out of the blood and into the bowmans capsule
adaptations of the cells of the proximal and distal convoluted tube
- microvilli
- larger amounts of mitochondria for active transport
- water moves down a water potential gradient as lower water potential due to amino acids/glucose
- Na+ moves by active transport
- ammino acids and glucose move by co-transport
- infoldings at their bases to give a larger surface area to transfer reabsorbed substances into blood capillaries
selective reabsorption
selective reabsorption:
- vast majority of the filtrate is reabsorbed back into the blood
- most of what is reabsorbed is glucose and minerals such as Na+ and K+ but urea isn’t reabsorbed. Reabsorption takes place in the proximal convoluted tubule
- concentration of Na+ in the PCT cell is decreased as the Na+ are actively transported out of the PCT cells into the blood in the capillaries
- due to conc gradient Na+ diffuse down the gradient from the lumen of the PCT into the cells lining the PCT. An example of co-transport, as glucose can then diffuse from the PCT epithelial cell in the blood stream. This is how all the glucose is reabsorbed
distal convoluted tubule
- also involved in selective reabsorption
- cells of distal convoluted tubule are same as ones in proximal convoluted tubule eg many mitochondria, microvilli.
-Regulated so under the influence of hormones, which alter the permeability if its walls- can therefore change/tweak amount of reabsorption. - can make small adjustments eg controlling concentration of Na+ in the blood so therefore controlling blood pressure
- also controls blood pH
- has role of active secretion of chemicals body wants to remove. Actively secretes them to be removed in urine
loop of Henle
-ascending limb has thicker walls than descending limb. This makes the walls impermeable to water so water cannot move across by osmosis. Walls of descending limb are very permeable to water
The Loop of Henle acts as a counter-current multiplier.
1. Sodium ions are actively transported out of the ascending limb using ATP into the interstitial space. Chloride ions will also be actively transported into the interstitial space. Some of these sodium ions may also move into the descending limb, futher reducing the water potential in there and allowing for a greater water potential gradient into the I.S.S
2. This creates a lower water potential (high ion concentration) in the interstitial space, than that of the loop of Henle and a higher osmolarity at the equivalent points
3. The filtrate progressively loses water by osmosis as it moves down the descending limb lowering its water potential. Lots of the water that moves into the intersticial space will be reabsorbed. This is because sodium ions are being moved out of the ascending limb into the interstitial space so the water potential of the interstitial space decreases. It reaches its lowest water potential and highest osmolarity at the tip of the hairpin
4. At the base of the ascending limb, sodium ions diffuse out of the filtrate as it moves up the ascending limb these ions are also actively pumped out and therefore the filtrate develops a progressively higher water potential
5. In the interstitial space between the ascending limb and the collecting duct there is a gradient of water potential with the highest water potential in the cortex and an increasingly lower water potential the further into the medulla you go
Regulation of the water potential of the blood
- the water potential of the blood depends on the concentration of solutes like glucose, proteins, sodium chloride, and other mineral ions as well as the volume of water in the body. A rise in solute concentration lowers its water potential. This may be caused by two little water being consumed, much sweating occurring, large amounts of ions eg NaCl being taken in
-osmoreceptors in the hypothalamus of the brain detect the fall in water potential
-it is thought when water potential of the blood is low, water is lost from the osmoreceptor cells by osmosis
- Due to water loss osmoreceptor cells shrink, a change that causes the hypothalamus to produce ADH
- ADH passes to the posterior pituitary gland, from where it is is secreted into the capillaries
- ADH passes in the blood to the kidney, where it increases the permeability to water of the cell-surface membrane of the cells that make up the walls of the distal convoluted tubule and the collecting duct
- specific protein receptors on the cell-surface membrane of these cells bind to ADH molecules, leading to activation of any enzyme called phosphorylase within the cell
- the activation of phosphorylase causes vesicles within the cell to move to, and fuse with, its cell-surface membrane
- these vesicles contain pieces of plasma membrane that have numerous water channel proteins (aquaporins) and so when they fuse with the membrane the number of water channels is considerably increased, making the cell-surface membrane much more permeable to water
- ADH increases the permeability of the collecting duct to urea which therefore passes out, further lowering the water potential of the fluid around the duct
- the combined effect is that more water leaves the collecting duct by osmosis, down a water potential gradient and re-enters the blood
- as the reabsorbed water came from the blood in the first place, this will not, in itself increase the water potential of the blood, but merely prevent it getting lower. The osmoreceptors also send nerve impulses to the thirst centre of the brain, to encourage an individual to drink more water
- the osmoreceptors in the hypothalamus detect the rise in water potential and send fewer impulses to the pituitary gland
- the pituitary gland reduces the release of ADH and the permeability of the collecting ducts to water and urea reverts to its former state. This is an example of homeostasis and negative feedback
-a fall in the solute concentration of the blood raises its water potential. This can be caused by large volumes of water being consumed, salts used in metabolism not being replaced in the diet
- this causes the osmoreceptors in the hypothalamus to detect the rise in water potential and increase the frequency of nerve impulses to the pituitary gland to reduce its release of ADH
- less ADH in the blood leads to a decrease in the permeability of the collecting ducts to water and urea
- less water is reabsorbed into the blood from the collecting duct
- more dilute urine is produced and the water potential of the blood falls
- when the water potential of the blood has returned to normal, the osmoreceptors in the hypothalamus cause the pituitary to raise its ADH release back to normal levels
collecting duct
- collecting duct is permeable to water
-as the filtrate exits the descending limb it forms conditions for concentrated urine to be made - the osmolarity as you move down the interstitial space will increase. This allows water to move out of the collecting duct and into the I.S.S via a water potential gradient and osmosis. The collecting duct has an increasing osmolarity as you move down, as water is moving out.
-The water that passes out of the collecting duct is passed into blood vessels that occupy the I.S.S and the water is therefore carried away. - As water passes out of the filtrate its water potential is lowered. However, the water potential of the intersticial space is also lowered as you move down, so water continues to move out by osmosis down the whole length of the collecting duct
- The counter-current multiplier ensures there is always a water potential gradient, moving water out of the tubule
This allows the urine to be hypertonic - body can alter the conditions of urine eg if dehydrated to ensure more water leaves
homeostasis
- maintenance of a constant internal environment
- internal environment is made up of tissue fluids that bathe each cell, supplying nutrients and removing wastes. Maintaining the features of this fluid at the optimum levels protects the cells from changes in the external environment
- also involved of maintaining blood conc/ volume etc
- maintenance of temperature
- maintenance of blood pH
- maintenance of blood pressure
- maintenance of nitrogenous waste
- maintenance of O2 conc
- homeostasis is the ability to return to the optimum point and so maintain organisms in a balanced equilibrium
importance of homeostasis
- Enzymes. that control biochemical reactions within cells and other proteins eg channel proteins are very sensitive to changes in pH and temperature. Any change to these factors reduces the rate of reaction of enzymes or may even denature them. Maintaining a fairly constant internal environment means that reactions take place at a suitable rate
- changes to the water potential of the blood and tissue fluid may cause cells to shrink and expand, resulting in the movement of water by osmosis. In both cases cells cannot operate normally. Maintenance of constant blood glucose concentration is essential in ensuring a constant water potential and availability of the respiratory substrate
- organisms with the ability to maintain a constant internal environment are more independent of changes in the external environment. They may have a wider geographical range and therefore have a greater chance of finding food, shelter etc. eg mammals can be found in most habitats
control mechanisms
- optimum point- point where the system operates best
- receptor- which detects any deviation from the optimum point ie a stimulus and informs the coordinator
- Increases frequency of action potentials along a sensory neurone
- coordinator- coordinates info from receptors and sends instructions to an appropriate effector.
- effector is a muscle or gland that brings about a change to return the system to the optimum point
- this all creates a feedback mechanism by which a receptor responds to a stimulus created by the change brought about by the effector
- may have two antagonistic pairs- one for an increase one for a decrease. This gives a finer level of control, eg insulin and glycogen or having inhibitor and effector neurones
positive feedback
- when a deviation from optimum causes changes that result in an even greater deviation from the normal.
-one example occurs in neurones where a stimulus leads to a small influx of sodium ions and this influx increases the permeability of the neurone membrane to sodium ions, more ions enter, causing a further increase in permeability and even more rapid entry of ions. In this way, a small stimulus can bring about a large and rapid response - control systems have many receptors and effectors. This allows them to have separate mechanisms that each produce a positive movement towards the optimum, allowing for a greater degree of control of the particular factor being regulated.
negative feedback
- occurs when the stimulus causes the corrective measures to be turned off. In doing so this tends to return the system to its original (optimum) level (and prevents any overshoot)
- there are separate negative feedback mechanisms to regulate departures from the norm in each direction. This allows for a greater degree of homeostatic control. This is because there are positive actions in both directions
- eg control of blood glucose
characteristics of hormones
Hormones differ from one another chemically but all have certain characteristics
- produced in glands, which secrete the hormone directly into the blood (endocrine glands)
- carried in the blood plasma to the cells on which they act- known as target cells- which have specific receptors on their cell-surface membranes that are complementary to a specific hormone
- are effective in very low concentrations but often have widespread and long-lasting effects
the pancreas
- pancreas is situated in upper abdomen behind the stomach. Produces enzymes (protease, amylase and lipase) for digestion and hormones (insulin and glucagon) for regulating blood glucose concentration
- contains groups of hormone producing cells known as islets of Langerhans. Include alpha cells, which are larger and produce glucagon and beta cells which are smaller and produce insulin
-glucose is important as a substrate for respiration. Also in controlling water potential of the blood so cannot be too low or too high - blood glucose comes from: the diet- hydrolysis of carbohydrates such as starch, maltose, lactose and sucrose, from hydrolyses in the small intestine of glycogen
- from gluceogenesis
rise in blood glucose conc
- normal conc of blood glucose is 5mmol/ dm3
- this can rise due to eating something containing a high level of carbohydrate
- beta cells of the islets of Langerhans in the pancreas have receptors that will detect the stimulus of a rise in blood glucose
- this will cause an increased amount of insulin to be produced by the beta cells and the insulin will be secreted directly into the blood plasma
- the cells primarily in the liver and muscles will absorb the glucose and convert it into glycogen, using insulin
decrease in blood glucose concentration
- can be brought about by prolonged exercise and a lack of digestion
- causes glucose levels to decrease in the blood
- alpha cells in the islets of Langerhans detect the drop in blood glucose
- the alpha cells then secrete glucagon directly into the blood plasma
- glucagon hydrolyses glycogen into glucose
- this is negative feedback as when blood glucose conc raises again, back to optimum, the alpha cells will reduce the secretion of glucagon
glycogenesis
- condensation reaction
- conversion of glucose to glycogen
- controlled by insulin
- takes place in liver
glycogenolysis
-breakdown of glycogen to form glucose.
- takes place in the liver
-Hydrolysis reaction
- controlled by glucagon
glucagon’s actions
- attach to specific protein receptors on the cell-surface membrane of liver cells
- activating enzymes that convert glycogen to glucose
- activating enzymes involved in conversion of amino acids and glycerol into glucose= gluconeogenesis
Overall effect is to increase conc of glucose in the blood and return it to its optimum concentration - Action of glucagon can be used to demonstrate the principles of cell signalling
- alpha and beta cells act as the receptors and alpha cells release glucagon. The liver cells act as the effectors in response to glucagon
- the beta cells respond by stopping the secretion of insulin and a decrease in the use of glucose by liver and muscle cells
gluconeogenesis
- production of glucose from other sources than carbohydrates
- when its supply of glycogen is exhausted, the liver can produce glucose from other non-carbohydrate sources like glycerol, fatty acids and amino acids
- condensation reaction
- triggered by glucagon that activates enzymes in the liver
hormone interaction in regulating blood glucose
when too high less glucagon and more insulin. This causes the negative feedback loop. These too hormones are said to act antagonistically. System is self-regulating through the negative feedback in that it is the concentration of glucose in the blood that determines the quantity of insulin and glucagon produced
- the interaction of these two hormones allows for a highly sensitive control of the blood glucose concentration
- concentration of glucose isn’t constant, but fluctuates around an optimum point
- only when the blood glucose concentration falls below the set point is insulin secretion reduced, leading to a rise in blood glucose concentration. In the same way when the concentration exceeds the set point, glucagon secretion is reduced, causing a fall in the blood glucose concentration
effects of insulin
- produced by beta cells in the islets of Langerhans- interact with receptors. Almost all body cells (other than red blood cells) have glycoprotein receptors on their cell-surface membranes that bind specifically with insulin molecules
- causes an increasing rate of respiration (activation of enzymes)
- increases the ability of facilitated diffusion molecules to absorb glucose (change tertiary shape and allow them to open)
- activate enzymes associated with making glycogen and fat
- incorporate more facilitated diffusion molecules into the cell membrane. Insulin does this as when insulin binds to the insulin receptor, it stimulates a vesicle containing facilitated diffusion molecules to fuse to the cell membrane and therefore increases the number of facilitated diffusion molecules in the overall membrane
- as a result the blood glucose concentration is lowered in one or more of the following ways:
- by increasing the rate of absorption into the cells, especially muscle cells
- increasing respiration rate, which therefore uses up more glucose and increases the uptake of glucose from the blood
- by increasing the rate of conversion of glucose into glycogen in the cells of the liver and muscles
- increasing the rate of conversion of glucose to fat
When blood concentration is lowered the beta cells reduce their secretion of insulin = negative feedback
role of adrenaline in regulating the blood glucose level
- initiates fight or flight response
- adrenaline is produced by the adrenal glands that lie above the kidneys
raises the blood glucose concentration by: - attaching to protein receptors on the cell-surface membrane of target cells
- activating enzymes that causes the breakdowns of glycogen to glucose in the liver. The glucose produced remains in the muscle cells for respiration
- involved in the second messenger model
diabetes
- diabetes is a metabolic disorder causes by an inability to control blood glucose concentration due to a lack of insulin or a loss of responsiveness to insulin.
Symptoms: - thirst- insulin cant remove glucose from the blood so water potential in the blood is too low. Low water potential detected by osmoreceptors.
- hypoglycaemic shock- too low blood glucose
- urinating more. When the kidney undergoes ultrafiltration some glucose remains and is therefore found in the urine. This lowers the water potential of the urine, causing some water to move in and therefore leads to a higher volume of urine
- blurred vision- too much glucose in fluid part of the eye, causes water to move in down water potential gradient which damages/ stretches the eye
- tiredness. Insulin is needed to allow glucose to be absorbed so less respiration will occur and you will be more tired
- weight loss. Loosing calories in body through urine as glucose in urine
type 1 diabetes
- body unable to produce insulin. May be result of an autoimmune disease whereby the body’s immune system attacks its own cells, in this case the beta cells of the islets of Langerhans.
- begins in childhood normally
- controlled by injections of insulin. Need to also manage carbohydrate intake and exercise carefully
- insulin is injected as it is a protein. If it was taken orally it would be digested by the enzyme protease in the gut before entering the blood stream
type two diabetes
- glycoprotein receptors on body cells being lost or losing their responsiveness to insulin. May also be due to an inadequate supply of insulin from the pancreas.
- controlled by regulating the intake of carbohydrate in the diet and matching this to the amount of exercise taken. Sometimes can be treated with insulin injections
second messenger signalling pathways
- controlled by two hormones involved in regulation of blood glucose concentration- adrenalin and glucagon
1. Adrenalin (or glucagon) binds to a transmembrane protein receptor within the cell-surface membrane of a liver cell, beta receptor. Adrenalin is the first messenger
2. The binding of adrenalin causes a G. protein (on the inside of the membrane) to be activated, due to the shape of the protein changing
3. This then leads to the activation of an enzyme called adenyl(ate) cyclase, also on the inside of the membrane, as it changes shape. The adenyl cyclase(ate) converts ATP into cyclic AMP (cAMP). Cyclic adenine monophosphate occurs when two phosphate molecules have been removed from ATP. Then forms a cyclic structure as the two end phosphate groups bind back towards each other. This cAMP is the second messenger.
4. The cAMP then binds to protein kinase enzyme, changing its shape and therefore activating it
5. The active protein kinase enzyme then activates phosphorylase kinase by adding a phosphate group to it
6. Activated phosphorylase kinase molecules activate glycogen phosphorylase
7. Activated glycogen phosphorylase then breaks down glycogen into glucose, which moves out of the liver cell by facilitated diffusion and into the blood through channel proteins. This is glycogenolysis
When a high enough blood glucose concentration is reached, phosphodiesterase enzymes turn cyclic AMP into AMP. AMP is inactive so stops the system and therefore will stop the production of glucose.
- Has multiple enzymes/ steps to result in a cascade reaction. This is to allow for amplification of the amount of glucose that is produced. When enzymes break substrates int products, multiple products are often formed eg one adrenalin will stimulate the formation of lots of different glucose molecules
- same second messenger signalling pathway can happen with both adrenalin and glucagon as the first messengers
Why glucose in urine of someone with diabetes
- high glucose concentration in the filtrate as not all glucose is reabsorbed in the proximal convoluted tubule. This is because the carrier proteins moving glucose into the blood are fully saturated
