3.7 homeostasis and the kidney Flashcards
1
Q
what is homeostasis?
A
- the maintenance/control of the internal environment
- at set points/constant/stable
- despite external changes
- e.g keeping temperature at 37°C, glutoce at 90mg per 100cm^3 blood
2
Q
how does the body accomplish homeostasis?
A
- uses negative feedback
- whereby the body responds in such a way as to reverse the direction of change
3
Q
negative feedback involves:
A
- input - a change away from the set point or norm e.g rise in core body temperature
- receptor - a sensor that detects the change from the set point e.g temperature receptors
- control centre - or coordinator detects signals from receptors and coordinates a response via effectors e.g hypothalamus in the brain
- effector - bring about changes which returns the body to set point e.g glands in skin release sweat
- output - corrective procedure e.g evaporation of sweat cools skin
process repeats
4
Q
an effector is a muscle or a gland
(in negative feedback circuit)
A
5
Q
what is excretion?
A
- the process of removing wastes made by the body
- e.g carbon dioxide and water from respiration and urea from the deamination of excess amino acids
6
Q
- surplus amino acids are deaminated in the ____: the amine group (-NH2) is removed, converted to ___ and then into ___. it is removed from the body by the kidneys. the organic acid that remains can be used in respiration, or converted to lipids or glucose
A
- liver
- ammonia
- urea
7
Q
what are the kidney’s 2 main functions?
A
- excretion - removal of wastes made by the body e.g urea
- osmoregulation - control of the water potential of body fluids by the regulation of the water content of the body
8
Q
- body has 2 kidneys
- each containing around a million nephrons
- each nephron is 30mm long
A
9
Q
- the kidneys are supplied with blood containing oxygen and waste (including urea) from ____
- and filtered blood returns to the general circulation by the _____
- excess water and solutes including urea is called ____
- and it drains into the collecting ducts and pelvis which empties urine into the ____
- each ____ connects to the bladder
A
- renal artery
- renal vein
- urine
- ureter
10
Q
it is important to refer to EXCESS amino acids, as amino acids are used in protein synthesis - only excess ones are deaminated
A
11
Q
watch spelling of ureter and urethra
A
12
Q
does the medulla or cortex contain bowman’s capsules?
A
cortex
13
Q
what is the use of the network of capillaries surrounding the convoluted tubules and loop of Henle?
A
it allows substances to be reabsorbed into the blood
14
Q
what are the capillaries that surround the loop of Henle referred to as?
A
the vasa recta
15
Q
what are the 3 main processes that occur in the nephron?
A
- ultrafiltration
- selective reabsorption
- osmoregulation
16
Q
ultrafiltration summary:
A
- removal of small molecules including water and urea from the blood in the glomerulus of the kidney at high pressure
17
Q
selective reabsorption summary:
A
- useful substances from the glomerular filtrate such as water, glucose and amino acids are reabsorbed into the blood
- but urea is not
- involves membrane transport proteins
18
Q
osmoregulation summary:
A
- the regulation of the water potential of body fluids (e.g blood, tissue fluid, lymph) by the kidney
19
Q
where does ultrafiltration occur?
A
- in the bowman’s capsule
20
Q
where does selective reabsorption occur?
A
- in the proximal convoluted tubule
(some different molecules reabsorbed in the distal convoluted tubule)
21
Q
where does osmoregulation occur?
A
- in the loop of Henle and collecting ducts and distal convoluted tubule
(but ADH can only affect collecting ducts and distal convoluted tubule)
22
Q
how does ultrafiltration work?
A
- the afferent arteriole is wider than the efferent arteriole
- which creates a higher blood pressure than normal in the glomerulus (high hydrostatic pressure in glomerulus)
- substances are forced out into the Bowman’s capsule
- this includes glucose, amino acids, salts, water, urea
- this forms the glomerular filtrate
(basement membrane acts as filter - blood cells and most proteins are too big and stay in the blood, although there are some protein exceptions)
23
Q
the movement of filtrate is resisted by:
A
- capillary epithelium which has pores called fenestrae
- basement membrane of bowman’s capsule which acts like a sieve
- wall of the bowman’s capsule is made up of highly specialised squamous epithelial cells called podocytes. filtrate passes between their branches (pedicels)
- hydrostatic pressure in capsule
- low water potential of blood in glomerulus (lowered by loss of water into capsule)
24
Q
what is the filtration rate?
A
- the rate at which fluid passes from the blood in the glomerular capillaries into the bowman’s capsule
25
what determines the filtration rate?
the factors resisting the movement of filtrate
26
the kidneys receive around 1100cm^3 of blood per minute and produce 125cm^3 of glomerular filtrate in the same time
27
around 85% of filtrate is reabsorbed in the proximal convoluted tubule, which includes:
- all glucose
- all amino acids
- most of the water and salts
28
what forms urine?
- urea
- excess water
29
what method of reabsorption do mineral ions and salts use in selective reabsorption?
- facilitated diffusion and active transport into epithelial cells
30
what method of reabsorption does glucose and amino acids use in selective reabsorption?
- co-transport (with sodium ions)
- glucose is co-transported with 2 sodium ions by facilitated diffusion into the cell
- sodium ions and glucose move separately into the capillaries
31
what method of reabsorption does water use in selective reabsorption?
osmosis
32
what is the result of selective reabsorption?
- that the filtrate at the end of the proximal convoluted tubule is isotonic with that of the blood plasma
33
how is the proximal convoluted tubule adapted for selective reabsorption?
- microvilli provide a large surface area for co-transporter proteins
- many mitochondria which produce ATP for active transport of glucose
- close association with capillaries which creates a short diffusion pathway between cells and the peritubular capillaries
- tight junctions are found between adjacent cells which stop reabsorbed materials leaking back into the filtrate
- folded basal membrane provides a large surface area
- peritubular capillaries extend into the medulla enabling reabsorption of materials
34
when explaining adaptations of cells for selective reabsorption you must FULLY explain your answer e.g microvilli provide a large surface area for the absorption of glucose by co-transport with sodium ions
35
what does the glucose threshold refer to?
the maximum mass of glucose that can be reabsorbed in the proximal convoluted tubule
(where blood glucose concentrations are very high e.g in type I and II diabetes, not all of it can be reabsorbed in the tubule, so some remains in the filtrate and therefore in the urine)
36
how much water is reabsorbed in the proximal convoluted tubule?
- most of it
- by osmosis
- but some is reabsorbed in the loop of
Henle
- and the volume of water reabsorbed in the convoluted tubule and collecting duct varies according to the body’s needs
37
why is osmoregulation important in animals?
- prevents cells from bursting or shrinking when water enters or leaves by osmosis
- cellular reactions occur in aqueous solutions so water levels affect concentrations and the rate of reactions in cells
38
what is the loop of Henle responsible for?
- reabsorption of some water from the descending limb
- but its main function is to create an increasing ion concentration within the interstitial region (surrounding loop of Henle) of the medulla, which allows water to be reabsorbed by the collecting duct
39
the volume of water reabsorbed from the distal convoluted tubule and collecting duct, and hence the resulting water potential of the blood, is influenced by what?
- antidiuretic hormone
- which increases the permeability of the tubule and duct walls to water
40
longer loops of Henle are found in mammals that have evolved in ____ habitats?
- dry
- more concentrated urine can be produced because more Na+ and Cl- ions can be actively transported out of the descending limb
- more time for reabsorption of water and salts
41
osmoregulation steps:
1. Na+ and Cl- ions are actively pumped out of the ascending limb
2. this creates an increasing ion concentration in the interstitial region
3. walls of the descending limb are permeable to water so water leaves by osmosis into the interstitial region before entering the capillaries (vasa recta)
4. water is progressively lost down the descending limb (long loops can reach much higher concentrations because more Na+ and Cl- ions can be actively transported out of the ascending limb)
5. the concentration of the filtrate decreases in the lumen of the nephron in the ascending limb, as Na+ and Cl- ions are actively pumped out
6. this creates an increasing ion concentration gradient in the interstitial region towards the base of the loop
7. water passes out of collecting duct by osmosis into the interstitial region before entering the vasa recta
8. as water passes out of filtrate in the collecting duct, the concentration of the filtrate increases, but it is always lower than the fluid in the interstitial region of the medulla, so water will continue to leave by osmosis. the two liquids flow in opposite directions past each other, resulting in a greater exchange of substances between them than if they flowed in the same direction. this is referred to as countercurrent multiplier. this ensures that the concentration of the filtrate is always lower than the interstitial fluid in the medulla
42
in animal cells there is no pressure potential, so the water potential is effectively the solute potential. therefore, water will move from a less concentrated solution to a more concentrated solution by osmosis
e.g water would move from a solute potential of -300 to one of -600kPa
43
what does ADH stand for?
antidiuretic hormone
44
what is ADH produced by?
by what is it secreted?
- produced by the hypothalamus
- secreted by the posterior pituitary
45
what does ADH do?
- increases the permeability of the cells of the distal convoluted tubule and collecting duct walls to water, increasing water reabsorption
46
the water potential of the blood varies when animals become:
- over-hydrated due to excess water intake, or low salt intake
- dehydrated due to low water intake, sweating or high salt intake
47
what is the water potential of the blood controlled by?
- receptors called osmoreceptors in the hypothalamus
- which respond by triggering the release of more or less antidiuretic hormone (ADH) into the blood from the posterior lobe of the pituitary gland
48
what is osmoregulation controlled by?
negative feedback
49
overhydrated:
- osmoreceptors in the hypothalamus
- posterior pituitary gland
- decreased ADH secretion
- decreased permeability of distal convoluted tubule and collecting duct
- less water reabsorbed into blood, dilute urine produced
dehydrated:
- osmoreceptors in the hypothalamus
- posterior pituitary gland
- increased ADH secretion
- increased permeability of distal convoluted tubule and collecting duct
- more water reabsorbed into blood, dilute urine produced
50
how does ADH increase the permeability of the distal convoluted tubule and collecting duct walls?
- ADH binds to membrane receptor proteins found on the surface of cells lining the distal convoluted tubule and collecting duct walls
- the binding of ADH triggers vesicles containing intrinsic membrane proteins called aquaporins containing pores that allow water to move, to fuse with the cell membrane
- the aquaporins allow water to pass through the walls down the water potential gradient
- when ADH concentration falls, aquaporins are removed from the cell membrane
51
what are the main causes of kidney failure?
- diabetes
- high blood pressure
- auto-immune disease
- infection
- crushing injuries
52
how is kidney disease treated?
by balancing body fluids using:
- medication to control blood potassium and calcium levels, which can lead to heart disease and kidney stones if not controlled
- a low protein diet to reduce concentration of excess amino acids, and hence concentration of urea
- drugs to lower blood pressure e.g.
• beta blockers which reduce the effect of adrenaline
• calcium channel blockers which dilate blood vessels lowering blood pressure
• ACE inhibitors which reduce the effect of angiotensin which causes blood vessels to constrict
- dialysis involves using dialysis fluid that contains glucose at the same concentration as the blood, but has no urea and a low ion concentration. the result is that urea, some ions and water diffuse out of the blood, but glucose remains. (there are two types)
- kidney transplant, as a final resort for end-stage renal disease. involves transplanting one kidney from a donor who is closely matched to ensure tissue compatibility. immunosuppressants have to be used afterwards to prevent organ rejection
53
what are the 2 types of dialysis?
- haemodialysis
- continuous ambulatory peritoneal dialysis (APD)
54
haemodialysis:
- takes blood (usually from an artery in the arm) and passes it through a dialyser containing thousands of fibres each consisting of selectively permeable dialysis tubing and dialysis fluid
- to ensure maximum transfer, a countercurrent is used where blood and dialysis fluid move in opposite directions
- heparin is used to prevent the blood from clotting
- dialysis takes hours and is repeated several times each week
55
continuous ambulatory peritoneal dialysis (APD):
- involves passing dialysis fluid into the peritoneum through a catheter
- the peritoneum contains numerous capillaries which exchange materials with the dialysis fluid, which is changed after about 40 minutes, and the process repeated several times a day
- this type has the advantage that the patient is able to move around, but it is less effective than haemodialysis so fluid retention is likely
56
freshwater fish excrete ____
ammonia
57
freshwater fish excrete ammonia:
- ammonia is highly soluble but very toxic so it cannot be stored
- it must be excreted immediately using large volumes of water to dilute it (which is freely available to freshwater fish)
58
mammals excrete ___?
urea
59
mammals excrete urea:
- urea is much less toxic than ammonia and so requires less water to dilute it
- and can be stored for short periods of time
- it requires energy to convert ammonia to urea but is an adaptation to life on land, as it helps prevent dehydration because less water is needed to excrete it
60
birds, reptiles and insects excrete _____?
uric acid
61
birds, reptiles and insects excrete uric acid:
- uric acid is visually non-toxic and therefore requires very little water to dilute it
- the conversion of ammonia to uric acid requires much energy but allows these animals to survive in very arid environments
- a major advantage for birds is that very small volumes of water are needed reducing weight in flight
62
the length of the loop of Henle is an adaptation to where the animal has evolved.
- beavers:
- kangaroo rat:
- beavers, which live in freshwater where water is plentiful, have very short loops of Henle and produce large volumes of dilute urine
- animals that live in arid environments e.g the kangaroo rat, have much longer loops of Henle and produce small volumes of highly concentrated urine
- they have a higher proportion of these nephrons, with the bowman’s capsule being located closer to the medulla, and loops of Henle, which penetrate deep into the medulla
- the longer the loop of Henle, the more concentrated the urine that can be produced (which saves water), because a higher ion concentration in the medulla can be created by the counter current multiplier
- greater time for reabsorption of water and salts
63
what are some adaptations of animals in arid environments?
- longer loop of Henle
- rely more on metabolic water e.g the camel which largely respires fat stored in its hump
- behavioural adaptation such as being nocturnal, coming out at night to forage when it’s cooler
64
why is homeostasis important?
- to ensure optimum conditions for enzymes and cellular processes in the body
65
negative feedback definition
- self-regulatory mechanisms return the internal environment to the optimum when there is a fluctuation
66
positive feedback definition
- a fluctuation which triggers changes that result in an even greater deviation from the normal level
67
describe receptors and effectors:
- receptors = specialised cells located in sense organs that detect a specific stimulus
- effectors = muscles or glands which enables a physical response to a stimulus
68
describe the role of the coordinator
- coordinates information from the receptors and sends instructions to the effectors
69
state the components of a negative feedback system
- input
- receptor
- coordinator
- effector
- output
70
describe how excess amino acids are excreted (in humans)
- amino acids deaminated in the liver (removal of amino group) to form ammonia
- ammonia converted to urea (less toxic)
- urea transported into the blood plasma and eliminated by the kidneys
71
what is the function of the renal artery?
- supplies blood to the kidneys
72
what is the function of the renal vein?
- drains blood from the kidneys
73
what is the function of the ureter?
- takes urine to the bladder from the kidneys
74
what is the function of the urethra?
- releases urine from the bladder, out of the body
75
where are nephrons found within the kidneys?
- part of the nephron is located in the medulla and part in the cortex
76
describe the blood vessels associated with a nephron
- wide afferent arteriole from renal artery enters renal capsule and forms the glomerulus, a branched knot of capillaries which combine to form narrow efferent arteriole
- efferent arteriole branches to form capillary network that surrounds tubules
77
why is there a build up of pressure in the glomerulus?
- afferent arteriole leading into the glomerulus is wider than the efferent arteriole taking blood from the glomerulus
78
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 podocytes
79
how are filtered proteins (and some urea) selectively reabsorbed into the blood?
- via diffusion
80
what is the function of the loop of Henle?
- creates a low water potential in the medulla, enabling the reabsorption of water
81
describe what happens in the loop of Henle:
- active transport of Na+ and Cl- out of ascending limb
- water potential of interstitial fluid decreases
- movement of water out of descending limb via osmosis (ascending limb is impermeable to water)
- water potential of filtrate decreases going down descending limb- lowest in medullary region, highest at top of ascending limb
- hair-pin countercurrent multiplier
82
what is an endocrine gland?
- a gland of the endocrine system that secretes hormones directly into the bloodstream
83
how is the concentration and volume of urine controlled?
by the secretion of anti-diuretic hormone (ADH)
84
explain the role of the hypothalamus in osmoregulation
- osmoreceptors in hypothalamus detect the concentration of the blood plasma
- hypothalamus secretes ADH
85
explain the role of the posterior pituitary gland in osmoregulation
stores and secretes the ADH produced by the hypothalamus
86
describe the effects of kidney failure
- buildup of toxic waste products (e.g urea) causes symptoms such as vomiting
- fluid accumulation leads to swelling
- disruption to electrolyte balance can make bones more brittle
- high concentrations of renin may lead to hypertension
- low concentrations of EPO can lead to anaemia
87
potential treatment for kidney failure summarised:
- low protein diet
- control of blood calcium and potassium levels using medication
- drugs to lower blood pressure
- renal dialysis
- kidney transplant
88
what does dialysis rely on?
- a partially permeable membrane between the patient’s blood and dialysis fluid
89
the ascending limb is (impermeable/permeable) to water?
impermeable
90
the descending limb is (impermeable/permeable) to water?
permeable
91
explain the function of the loop of Henle in osmoregulation [4]
- water leaves descending limb by osmosis/Na+ is retained in descending limb
- at apex Na+ is very concentrated/ the ascending limb receives a filtrate rich in Na+
- Na+ (actively transported/pumped out) of ascending limb
- lowering the water potential in the medulla
- ascending limb (doesn’t allow escape of water/impermeable to water)
92
describe how hormonal control affects the final concentration and volume of urine produced when someone is dehydrated [4]
- (high osmotic pressure / low water potential / low solute potential / high solute concentration) of BLOOD detected by (OSMORECEPTORS/HYPOTHALAMUS)
- secretion of ADH by pituitary
- causes collecting duct (walls) to (become more permeable to water /insertion of aquaporins)
- water moves into medulla by osmosis
- (quickly) removed by the (vasa recta/capillaries/blood)
- LOW VOLUMES of CONCENTRATED urine produced
93
- roughly 60% of the mass of the body is water and despite wide variation in the quantity of water taken in each day, body water content remains incredibly stable
- one hormone responsible for this homeostatic control is anti-diuretic hormone (ADH)
- describe the mechanisms that are triggered in the mammalian body when water intake is reduced [6]
- low water levels in blood/ high osmotic potential
- detected by OSMORECEPTORS
- in HYPOTHALAMUS
- (more) ADH secreted from (posterior lobe of) pituitary
- (into/travels in) blood to
- (collecting duct/distal convoluted tubule) (becomes more permeable/more aquaporins/more water channels in membranes)
- water absorbed
- because of low water potential in medulla
94
explain how the nephron and its blood supply is adapted for ultrafiltration [5]
- mention of glomerulus + bowman’s capsule
- glomerulus has a high hydrostatic/blood pressure
- because afferent arteriole/blood vessel is wider diameter than efferent
- glomerulus CAPILLARIES have many small holes / fenestrations / are leaky
- some correct reference to basement membrane acting as a dialysing membrane
- allowing (small molecules/plasma minus the large proteins) to squeeze through
- correct reference to function of podocytes / filtration slits
95
- patients with severe kidney disease may be told to follow-up a protein controlled diet with low sodium chloride (salt) intake
- using using your knowledge of the role of the kidney suggest why this diet is recommended:
- protein controlled diet:
- low sodium chloride (salt) intake:
- high protein means (more) urea which cannot be removed
- excess salt does not need to be removed
- high salt causes high blood pressure /fluid retention
96
give 2 examples of what might cause a rise in blood solute concentration? [1]
- high solute intake or e.g / sweating / low water intake / vomiting / diarrhoea
97
state two variables which would need to be considered when comparing results from a number of people who had drunk similar volumes of water (in an experiment measuring conc of salt in urine) [2]
- age
- sex
- activity levels / exterior temperature
- diet / salt intake
- previous fluid intake
- mass/weight
- external temperature
- normal kidney function
98
describe how ultrafiltration takes place in the kidney [4]
- arterial pressure high/high blood pressure
- afferent vessel wider diameter than efferent
- increase in pressure in glomerulus
- pores between endothelial cells
- pores in basement membrane
- filtration slits podocyte
- e.g of a substance not filtered and one which passes through
99
explain the principle of dialysis as used in kidney machines [4]
- blood is taken from a vein
- dialysate passes in opposite direction in the machine
- dialysis fluid has the same water potential and concentration of ions and glucose as blood of patient
- urea and excess water and salt diffuses
- from blood into dialysate because of concentration gradient
- from the blood into the dialysis fluid
100
the endothelium and basement membrane is found between the glomerulus and bowman’s capsule
101
lungfish survive the drying out of their habitats by burrowing into the mud and decreasing their metabolic rate. explain why, when in water, they release the products of deamination as ammonia but when they are in the dried-out mud, they convert the products to urea [4]
- ammonia is MORE toxic than urea
- so needs to be diluted/washed away constantly
- mud: need to conserve water
- urea needs less water for excretion/if ammonia was used, a lot of water would be lost
- urea can be stored (because it is less toxic)
102
give 4 possible effects of kidney failure [4]
- increased levels of urea in blood
- increased levels of (ions/names ions/salt) in blood
- increased water levels in blood / swelling legs / shortness breath
- increase in water potential of boood
- reduced volume or urine
- blood in urine
- nausea
- itching (caused by high levels phosphate)
- bone damage / slow bone healing
- muscle cramps / abnormal heart rhythm / muscle paralysis
- decreased numbers RBC / tiredness / dizziness
103
suggest why the dialysis fluid moves in the opposite direction to the flow of blood [1]
- countercurrent maximises concentration gradient along length of tube / stop equilibrium
- accept blood and dialysis fluid do not reach same urea concentration
104
describe 2 structural features of glomeruli that allow ultrafiltration to occur [2]
- many (pores/gaps) in the (capillary wall/endothelium/fenestrated wall)
- basement membrane with (pores/molecular sieve) (through which large molecules cannot pass)
- efferent arteriole has (smaller DIAMETER/narrower LUMEN than afferent)
105
the urea concentration of urine is much higher than that of glomerular filtrate.
describe the role of the nephron and collecting duct in achieving this increase in concentration [5]
- water (reabsorbed from filtrate / removed from filtrate)
- less urea reabsorbed / urea not reabsorbed
- (sodium/mineral ions) reabsorbed in proximal convoluted tubule
- (active transport/pumping) of Na+ ions in the ascending limb of loop of Henle
- water reabsorbed from filtrate in the descending limb of loop of Henle/descending limb is permeable to water/ascending limb impermeable
- hypertonic conditions / high solute concentrations in the medulla / lowering water potential of medulla / correct description of concentration gradient towards apex of loop
- therefore water reabsorbed in the collecting duct / distal convoluted tubule
