5.3 HOMEOSTASIS

Question 1

What is the coordination of cells?

Answer 1

The coordination of cells:
- evolution of organisms has given rise to multicellular life
- multicellular organisms must coordinate the function of different cells and systems to operate effectively
- very few body cells can work in isolation

Question 2

What is homeostasis?

Answer 2

Homeostasis is the maintenance of a stable equilibrium in the conditions inside the body.

Question 3

What is the role of cell signalling in homeostasis?

Answer 3

Cell signalling is the communication of cells. The nervous and endocrine systems are coordinated by cell signalling. One cell releases a chemical which then has an effect on another cell (target cell).

Question 4

What is signal initiation?

Answer 4

Signal initiation:
- sensory receptors detect changes in the bodies internal and external environments
- information from the receptors is transmitted to the brain via sensory neurones

Question 5

What are the types of signal transfer?

Answer 5

Types of signal transfer:
- locally (e.g neurotransmitters across synapse)
- long distances = using hormones (e.g ADH secretion)

Question 6

What is signal response?

Answer 6

Signal response:
- impulses are sent along motor neurones to effectors which bring about changes to restore equilibrium
- effectors are the muscles or glands that respond to the stimuli

Question 7

How do plants respond to cell signalling?

Answer 7

Plants response:
- don’t have nervous systems but need to respond to internal and external changes
- use plant hormones to achieve a response
- stems grow towards a light source to maximise their rate of photosynthesis (due to auxin)

Question 8

What is negative feedback?

Answer 8

Negative feedback is the reversing of a change in the internal environment.

Question 9

What is positive feedback?

Answer 9

Positive feedback is where a change in the internal environment is detected and then reinforced by effectors to reinforce the change.
- not common
- usually harmful (e.g body temp decreases, enzymes less active, decreased heat)
- blood clotting cascade and role of oxytocin in childbirth

Question 10

What is sensitivity?

Answer 10

Sensitivity is the ability to response to internal and external changes.

Question 11

What is a stimulus?

Answer 11

A stimulus is a detectable change in external or internal environment of an organism.

Question 12

What are receptors?

Answer 12

Receptors are extrinsic glycoproteins that bind to chemical signals, triggering a response by the cell.

Question 13

What are effectors?

Answer 13

Effectors are a muscle or gland which carries out the bodies response to a stimulus

Question 14

What is an example of positive feedback?

Answer 14

An example of positive feedback is labour, where the cervix opens and oxytocin is released. Oxytocin increases contractions which causes the cervix to stretch more.

Question 15

What are ectotherms?

Answer 15

Ectotherms are animals that use their surroundings to warm their bodies, so their core temperature is heavily dependant on the environment (e.g reptiles, fish).

Question 16

What are endotherms?

Answer 16

Endotherms are animals that rely on their metabolic processes to warm their bodies and maintain core temperature (e.g birds, mammals).

Question 17

How is temperature change detected?

Answer 17

Temperature change is detected by:
- peripheral receptors in the skin
- temperature receptors in the hypothalamus detect the temperature of the blood deep in the body
- gives the body greater sensitivity

Question 18

What is the hypothalamus?

Answer 18

The hypothalamus:
- temperature receptors in the hypothalamus act as a thermostat for the body = it controls the responses that a core body temperature of 37 degrees C
- within the hypothalamus there is the heat loss centre and heat gain centre

Question 19

What is vasodilation?

Answer 19

Vasodilation:
- cooling down when core body temp increases
- arterioles near surface of skin dilate when temp increases
- vessels that provide a direct connection between arterioles and venules (arteriovenous shunt vessels) constrict
- forces blood through capillary networks close to the surface of the skin
- skin flushes and cools as a result of increased radiation
- if skin is pressed against cool surfaces then cooling happens from conduction
- as core temp increases, rate of sweating also increases
- sweat evaporates, heat is lost, blood cooled below surface
- in some animals sweat glands are covered by hair, so they pant when they get hot
- erector pilli muscles in the skin collapse, avoids trapping an insulating layer of air

Question 20

What is vasoconstriction?

Answer 20

Vasoconstriction:
- warming up when core body temp decreases
- arterioles near surface of skin constrict when temp decreases
- skin pales, little radiation takes place
- warm blood kept well below surface
- rates of sweating decreases, decreases cooling from skin, evaporation in lungs still occurs
- erector pilli muscles contract, hairs on skin stand up, reduces heat loss, creates insulating layer of air
- shivering occurs

Question 21

What is shivering?

Answer 21

Shivering:
- rapid involuntary contracting and relaxing of the large voluntary muscles in the body
- metabolic heat from the exothermic reactions warm up the body instead of moving = effective way of raising the core temperature
- endotherms living in cold climates have adaptations that minimise their SA:V ratio to reduce cooling (e.g small ears)

Question 22

What are the behavioural mechanisms for warming up in ecotherms?

Answer 22

Behavioural mechanisms - warming up:
- basking in the sun (lizards, locusts, butterflies)
- conduction = press bodies against warm group
- ectothermic metabolic reactions = vibration of muscles/wings

Question 23

What are the behavioural mechanisms for cooling down in ecotherms?

Answer 23

Behavioural mechanisms - cooling down:
(important to cool down to stop enzymes denaturing)
- shelter from sun
- press bodies against cool earth or stone
- wallow in water or mud

Question 24

What are physiological responses in ecotherms?

Answer 24

Physiological responses:
- dark colours absorb thermal radiation faster
- alteration of heart rate to increase or decrease metabolic rate

Question 25

What are the advantages of endotherms?

Answer 25

Advantages of endotherms:
- can live in colder habitats
- core body temp kept constant
- can be active at all times of day/year

Question 26

What are the disadvantages of endotherms?

Answer 26

Disadvantages of endotherms:
- huge proportion of energy intake used on temp regulation
- less energy from food is used for growth
- more food must be eaten per day
- may hibernate if food is scarce

Question 27

What are advantages of ectotherms?

Answer 27

Advantages of ecotherms:
- less food is needed per day
- more energy from food is used for growth
- can survive longer without eating
- rate of respiration slower

Question 28

What are disadvantages of ecotherms?

Answer 28

Disadvantages of ectotherms:
- less active in cold temperatures
- must have sufficient energy stores to survive cold weather
- puts them at risk of predation
- may need to be inactive and hibernate in cold weather

Question 29

Introduction to the liver.

Answer 29

Introduction to the liver:
- largest internal organ in a mammals body
-makes up 5% body mass
- involved in many metabolic processes
- receives oxygenated blood from the hepatic artery (off aorta)(25% blood supply)
- receives deoxygenated from the hepatic portal vein (75%)(vein coming from gut)

Question 30

What are hepatocytes?

Answer 30

Hepatocytes (liver cells):
- arranged in single rows along the sinusoids
- as blood flows through each sinusoid to the central branch of the hepatic vein the hepatocytes add materials to the blood- products manufactured by liver e.g plasma proteins
- they also remove materials from blood (oxygen, food substances, toxins e.g alcohol, metabolites) needed by theses cells or produced by other body cells
- levels of cholesterol vitamins A and D and iron are absorbed then stored and released from the liver as necessary
- the cells are identical and contains numerous mitochondria, lysosomes, glycogen granules and fat droplets
- have prominent golgi
- cell membrane facing sinusoids has microvilli

Question 31

What are sinusoids?

Answer 31

Sinusoids:
- open channels through which blood flows in direct contact with hepatocytes
- are different to normal capillaries in that they are wider and the walls are more porous
- blood in sinusoids comes from 2 sources- hepatic portal vein (from small intestine) and hepatic artery
- blood flows to central branch of hepatic vein- blood to ‘hub’
- lined with kupffer cells which move within sinusoid and are involved in defence

Question 32

What are kupffer cells?

Answer 32

Kupffer cells:
- move about the sinusoid and are part of the body’s defence mechanism
- they are phagocytes (macrophages) and ingest matter and are responsible for breakdown of hb in rbc producing bile pigments (billirubin/billiverdin)

Question 33

What are bile canaliculi?

Answer 33

Bile canaliculi:
- also found in lobules
- each canaliculus is a blind-ended tube between two rows of liver cells
- hepatocytes secrete bile into the canaliculi
- canaliculi merge to form bile ducts in which bile is drained from the liver
- movement is in opposite direction to the flow of blood in sinusoids i.e away from centre

Question 34

What are the functions of the liver?

Answer 34

Functions of the liver:
- disposes of worn out rbc (reusing them in masking bile)
- involved in carbohydrate metabolism and regulates blood sugar levels
- removes excess amino acids in the blood, breaking them down into ammonia then urea
- controls the level of fats in the blood by making cholesterol
- helps the body to destroy poisons such as alcohol, and to get rid of other unwanted substances e.g hormones
- makes and regulates many hormones, including sex hormones

Question 35

How does the liver help with the regulation of blood sugar?

Answer 35

To help with the regulation of blood sugar, the liver can store 100g of glycogen. Insulin stimulates hepatocytes to convert glucose to glycogen. Glucagon stimulates hepatocytes to convert glycogen to glucose.

Question 36

Why is urea formed?

Answer 36

Urea is formed as the human body is unable to store excess dietary protein and hepatocytes firstly deanimate the amino acids = the amino group is removed and ammonia is formed. Ammonia is then converted to urea as it is toxic and excreted via the kidneys. The rest of the amino acid (keto acids) are either converted to lipids and cholesterol or enter the krebs cycle.

Question 37

What is the ornithine cycle?

Answer 37

Ornithine cycle:
- because ammonia is so toxic and soluble it cannot remain in the body and must be converted to a less toxic substance
- the ornithine cycle is the metabolic pathway which combines ammonia with co2 to form urea (CO(NH2)2)
- uses ATP

Question 38

Where does detoxification occur?

Answer 38

Detoxification occurs in the SER of hepatocytes.

Question 39

What is broken down during detoxification?

Answer 39

Broken down:
hydrogen peroxide - produced by many metabolic reactions, broken down by catalase
alcohol - toxic, small molecule and lipid soluble so can cross phospholipid bilayer easily
hormones - removed so effects do not continue but not toxic e.g thyroxine, oestrogen and testosterone
medicines - e,.g paracetamol and antibiotics

Question 40

What is the process of detoxification of alcohol?

Answer 40

Detoxification of alcohol:
- ethanol converted first to ethanal by ethanol dehydrogenase
- ethanal then converted to ethanoate by ethanal dehydrogenase
- ethanoate combines with COA (to form acetylCOA) and can then enter the krebs cycle in the mitochondria and be used to produce ATP or can be used to build up fatty acids

Question 41

What is a fatty liver?

Answer 41

Fatty liver:
- when a lot of alcohol is drunk then a lot of oxidised NAD is used during the detoxification process
- oxidised NAD is also needed to oxidise fatty acids in the hepatocytes
- the fatty acids are therefore deposited in the liver cells which reduces their ability to function effectively

Question 42

What is cirrhosis?

Answer 42

Cirrhosis:
- alcohol damages the hepatocytes and damaged cells are replaced by fibrous tissue and the structure of the blood supply is lost such that blood does not enter the sinusoids and is not ‘processed’ by liver cells
- the combined effects of fatty liver and damage to hepatocytes results in cirrhosis
- a person with liver cirrhosis cannot convert ammonia to urea as well which can result in damage to the nervous system and occasionally a coma

Question 43

What is the renal artery?

Answer 43

The renal artery brings blood containing urea and other substances in solution to the kidney

Question 44

What is the renal vein?

Answer 44

The renal vein carries blood away from the kidney, after urea and other substances have been removed from the blood by the kidney

Question 45

What is the ureter?

Answer 45

The ureter is a tube through which urine passes from the kidney to the bladder

Question 46

What is the urethra?

Answer 46

The urethra is the tube through which urine passes to the outside of the body

Question 47

What is the role of the kidney?

Answer 47

Role of the kidney:
- two important homeostatic roles in the body
- filtering nitrogenous waste products out of the blood (especially urea)
- maintenance of water balance and pH of the blood

Question 48

Kidney structure?

Answer 48

Kidney structure:
capsule - thin membranus sheath that covers the outer surface of each kidney. protects the vital tissue from injury
pelvis - the central chamber where urine collects before passing out down the ureter
cortex - darker, outer layer where the filtering of the blood takes place. has a very dense network of capillaries carrying blood from the renal artery to the nephrons
medulla - lighter in colour, contains the tubules of the nephrons that form the pyramids of the kidney and collecting ducts

Question 49

What is the nephron?

Answer 49

The nephron:
- nephrons are the functional units of the kidneys
- each nephron is 3cm long
- 1.5 million nephrons in each kidney
- blood is filtered in the nephrons and the majority of filtered material is returned to the blood removing nitrogenous waste and balancing mineral ions and water

Question 50

What is the function of the nephron?

Answer 50

Function of the nephron:
- ultrafiltration
- absorption
- osmoregulation
- excretion

Question 51

What is ultrafiltration?

Answer 51

Ultrafiltration is the process of filtering blood plasma through the Bowmans capsule under pressure

Question 52

How does ultrafiltration work?

Answer 52

Ultrafiltration works:
- the difference in diameter between the afferent and efferent arterioles ensures high pressure in the glomerulus so substances are pushed out of the plasma into the nephron
- water, amino acids, glucose, urea and inorganic ions form the glomerular filtrate
- anything with a molecular mass >69000 is not able to filter out (e.g proteins and blood cells)- this allows blood to maintain a lower water potential for the process of water reabsorption that happens later

Question 53

Rate of filtration.

Answer 53

Rate of filtration:
- water potential affects the rate of filtration, but it is affected by highs and solute concentration
- overall, the effect of pressure outweighs the effect of solute conc, so the movement is from the glomerulus into the Bowman’s capsule

Question 54

What is the structure of the Bowman’s capsule?

Answer 54

Structure of Bowman’s capsule:
capillary endothelium
- very thin
- perforated with thousands of pores about 10nm in diameter
- provides a barrier to cells but not plasma membranes
basement membrane
- meshwork of collagen and glycoprotein fibres
- water and small molecules can pass through
- proteins are too large and are repelled by negative charges on fibres
podocytes
- epithelial cells that make up the lining of the Bowmans capsule
- also have extensions called pedicels that wrap around the capillaries, forming slits that ensure that any larger substances such as blood cells, platelets or proteins that may have made it through in the capsule lumen

Question 55

What is the false colour SEM micrograph of the nephron?

Answer 55

False colour SEM micrograph:
- podocytes are the blue cells
- capillaries of the glomerulus are purple

Question 56

What are convoluted tubules?

Answer 56

Convoluted tubules:
proximal convoluted tubule
- lots of infoldings
- lots of microvilli
- lots of mitochondria
- active reabsorption of solutes
distal convoluted tubule
- columnar epithelium
- no infoldings
- few mitochondria
- some ion transport and water reabsorption

Question 57

What is the process of reabsorption?

Answer 57

Reabsorption happens in the proximated convoluted tubule, reabsorption of ions, water and valuable nutrients such as glucose and amino acids
1. Na+ actively transported out into the blood by Na+/K+ pump , this requires ATP
2. Na+ levels inside the epithelial cell falls
3. Na+ is then able to diffuse into the cell from the lumen of the proximal convoluted tubule through a co-transport protein bringing glucose and amino acids with it
4. glucose, amino acids and Na+ diffuse into the blood from cells
5. water moves down the wp gradients and Cl- ions move down their electrochemical gradient into the blood
6. blood plasma rapidly removes glucose, amino acids, water, Na and Cl ions along to allow further uptake

Question 58

What is the success of reabsorption?

Answer 58

Success of reabsorption:
- 80% of glomerular filtrate is reabsorbed at this stage
- most urea is also reabsorbed here as it is small and has moved down its conc gradient
- by the end of the proximal tubule, glomerular filtrate is isotonic to tissue fluid and blood

Question 59

What does reabsorption have to do with diabetes and glucose?

Answer 59

Diabetes and glucose:
- normally, all glucose is reabsorbed in selective reabsorption
- however, if there is too much glucose then there will not be enough co-transporter proteins
- if there is still glucose left in urine - suggestive of diabates

Question 60

Water reabsorption and the loop of Henle.

Answer 60

Water reabsorption - loop of Henle:
- the loop of Henle is able to create a very high concentration or salts in the tissue fluid, so water can be reabsorbed from the loop and collecting duct. this allows urine to be more concentrated than blood
- descending loop → permeable to water and ions
- ascending loop → permeable to sodium ions

Question 61

What is the descending loop of Henle?

Answer 61

descending loop:
- water enters the descending limb from the proximal convoluted tubules
- upper part or descending loop is impermeable to water
- lower part or descending loop is permeable to water
- not permeable to Na or Cl ions
- water moves out of limb by osmosis down water potential gradient into tissue fluid (due to an increasingly concentrated medulla tissue fluid) then into blood capillaries

Question 62

What is the ascending loop of Henle?

Answer 62

ascending loop of Henle:
- impermeable to water
- first (thin) section allows Na and Cl ions to diffuse out
- second (thick) section actively pumps out ions
- this increases the number of ions in the tissue fluid. therefore, lowering the water potential

Question 63

What is the result of water reabsorption?

Answer 63

Water reabsorption results:
- having two limbs working in the opposite direction, enables a maximum concentration to build up outside the loop-this is called the counter current multiplier

Question 64

What is the distal convoluted tubule?

Answer 64

distal convoluted tubule:
- permeability or the walls varies depending on the levels of ADH
- if the body lacks salt, Na+ actively pumped out or tubule and Cl- follows
- water can also leave depending on ADH levels

Question 65

What is the collecting duct?

Answer 65

Collecting duct:
- when water reaches the cd, water also leaves due to the high solute conc in the medulla
- water moves from the cd by osmosis until the water potential of the urine is equal to the water potential, of the tissue fluid
- ADH controls the permeability of the collecting duct walls

Question 66

What is osmoregulation?

Answer 66

Osmoregulation is the balancing and control of the water potential in the blood
- kidneys are the main organ responsible for osmoregulation

Question 67

What is ADH?

Answer 67

ADH- Anti Diuretic Hormone:
- a homone that increases the permeability of the distal convouted tubule and collecting duct to water
-a protein hormone made of 9 amino acids.

Question 68

ADH release.

Answer 68

ADH release:
- osmoreceptor cells in the hypothalamus monitor the water potential in the blood
- when water potential falls, osmoreceptor cells trigger neurosecretory cells in hypothalamus
- ADH released from nerve endings in the prosteror pituitary gland
- ADH released into the blood and travels to the kidneys

Question 69

ADH action.

Answer 69

ADH action:
-low water potential= more ADH released, high water potential= ADH release inhibited
1. ADH acts on cells in the collecting duct and distal convoluted tubule
2. ADH binds to membrane-bound receptors on the plasma membrane - starting a cascade or events
3. binding triggers formation or cyclic AMP (secondary messenger)
4. phosphorylase enzyme is activated
5. vesicles (containing aquaporins in their membrane) fuse with cell surface membrane on the side of the cell in contact with medulla tissue fluid
6. aquaporins make cell membrane more permeable to water - therefore more ADH means more water channels
7. water moves into the medulla through aquaporins and is reabsorbed into capillaries by osmosis.

Question 70

What are the effects of ADH?

Answer 70

Affects of ADH :
- more ADH
• more reabsorption or water into blood
• decrease volume of water in urine
• more concentrated urine.
- less ADH
• less reabsorption or water into blood
• increased volume or water in urine
• more dilute urine

Question 71

What things are currently tested for using urine?

Answer 71

Currently tested for via urine:
- diabetes (glucose or ketones will be present)
- nephritis (proteins and bloud cells will be present, also more solutes than normal)
- bacterial infection (leukocytes will be present)
- muscle dumage (creatine will be present)
- pregnancy (HCG hormone
- drug taking (eg steroids, cannabis)

Question 72

Pregnancy testing.

Answer 72

Pregnancy testing:
- modern pregnancy tests test for HCG (human chorionic gonadotrophin hormone) in the urine
- they rely on the use of monoclonal antibodies (an antibody produced by a single clone of cells or cell line and consisting or identical antibody molecules)

Question 73

How are monoclonal antibodies produced?

Answer 73

Production of monoclonal antibodies:
1. a mouse is injected with HCG so it produces the appropriate antibody
2. B-cells that make the antibody are removed from the spleen
3. the antibodies are fused with a mycloma - a type of cancer cell which divides very rapidly
4. new fused cell is known as a hybridoma
s. each hybidama reproduces rapidly
6. monoclonal antibodies are collected - complementary to HCG

Question 74

In a pregnancy test, what happens if pregnant?

Answer 74

If pregnant:
• monoclonal antibodies with small coloured beads attached
• immobilised monoclonal antibodies bind to form HCG-antibody
complex so coloured line shows
- binds to just antibody (doesn’t require HCG) so gets a coloured line

Question 75

In a pregnancy test, what happens if not pregnant?

Answer 75

If not pregnant:
• no hcG to bind
• no hcG so cannot bind
• binds to just antibody (doesn’t require HCG) so gets a
coloured line

Question 76

Urine testing and anabolic steroids.

Answer 76

Urine testing and anabolic steroids:
•anabolic steroids are drugs that mimic the action of testosterone and stimulate the growth of muscles - they are extracted in the urine
• urine can be tested using gas chromatography and mass spectrometry
• urine sample is vapourised with a known solvent and passed along a tube. the lining of the tube then absorbs the gases and the resulting chromatogram can be analysed

Question 77

What are the causes of kidney failure?

Answer 77

Causes of kidney failure:
- kidney infections - the structure of podocytes and tubules themselves may be damaged or destroyed
- raised blood pressure - can damage the structure or the epithelial cells and basement membrane of the Bowman’s capsule
- genetic conditions- such as polycystic kidney disease where the healthy kidney tissue is replaced by fluid-filled cysts or damaged by pressure from cysts

Question 78

What are the effects of kidney failure?

Answer 78

Effects of kidney failure:
- if kidneys are infected or affected by high blood pressure
• protein in the urine - if the basement membrane or podocytes of the Bowman’s capsule are damaged, they no longer act as filters and large plasma proteins can pass into the filtrate and are passed out in the urine
• blood in the urine - another symptom the filtering process is no longer working
- if kidneys fail completely, the concentrations or urea and
mineral ions build up in the body
• loss of electrolyte balance - if the kidneys fail, the body cannot excrete excess sodium, potassium and chloride ions. this causes osmotic imbalances in the tissues and eventual death
• build up of toxic urea in the blood- if the kidneys fail, the body cannot get rid of urea and it can poison the cells
• high blood pressure-the kidneys play an important role in controlling the blood pressure by maintaining the water balance of the blood. if the kidneys fail, the blood pressure increases and this can cause a range of health issues including heart problems and strokes
• weakened bones -the calcium/phosphorus balance in the blood is lost
• pain and stiffness in joints - abnormal proteins build up in blood
• anaemia - the kidneys are involved in the production ot a hormone called erythropoietin that stimulates the formation of red blood cells causing tiredness and lethargy

Question 79

Measuring glomerular filtration rate.

Answer 79

Measuring glomerular filtration rate:
• kidney problems almost always affect the rate at which blood is filtered in the Bowman’s capsule or the nephrons
• glomerular filtration rate (GFR) is widely used as a measure to indicate kidney disease
• rate or filtration is not measured directly - a simple blood test measures the levels of creatine in the blood
• creatine is a breakdown product of muscles and is usel to give an estimate GR (eGER) units in cm3/min
• if the levels of creatine in the blood go up, it is a signal that the kidneys are not working properly
• certain factors need to be taken into account to in the calculations to work out the GER
e.g GER decreases steadily with age even it you are healthy, and men usually have more muscle mass and therefore more creatine than women

Question 80

Treating kidney failure with dialysis.

Answer 80

Treating kidney failure with dialysis:
• normal GERs do not fall below 70 even in very old people
• a GER below 60 for more than a 3 months is taken to indicate moderate to severe chronic kidney disease- and if it falls below 15, that is kidney fullure, as the kidneys are filtering so little blood they are basically ineffective
- two main ways kidney failure is treated
• renal dialysis where the function of the kidnays is carried out artificially
• transplant where a new, healthy kidney is put into the body to replace the failed kidneys
- two main types of dialysis- haemodialysis and perinatal dialysis

Question 81

What is haemodialysis?

Answer 81

Haemodialysis:
• involves the use of a dialysis machine so usually carried out in hospital
• blood leaves the patients body from an artery and flows into the dialysis machine, where it flows between partially permeabie dialysis membranes
• these membranes mimic the basement of the Bowman’s capsule
-on the other side of the membranes is the dialysis fluid
•during dialysis it is vital that patients lose the excess urea and mineral ions that have built up in the blood
• it is equally important they do not lose substances such as glucose and mineral ions
•the loss of these substances is prevented by careful control or the dialysis fluid
• it contains normal plasma levels of glucose to ensure there is no net movement of glucose out of the blood
• the dialysis fluid also contains normal plasma levels or mineral ions, so any excess mineral ion in the blood move out by diffusion down a concentration gradient into fluid, thus restoring the correct electrolyte balance of the blood.
• dialysis fluid contains no urea so steep conc gradient between fluid and blood and urea leaves blood
• blood and dialysis fluid flow in opposite directions to maintain a countercurrent exchange system and maximise exchange
• takes about 8 hours and has to be repeated regularly - several times a
week
• diets managed carefully - reactively little protein and salt and monitoring fluid intake
at beginning or dialysis process can eat what they like.

Question 82

What is perineal dialysis?

Answer 82

Perineal dialysis :
• done inside the body -makes use of natural dialysis membranes formed by the lining or the abdomen (Perionum).
• usually done at home, patient can carry on with normal life
• dialysis fluid introduced to abdomen using a catheter
• left for several hours for dialysis to take place and across perineal membranes, so urea and excess mineral ions pass out of blood capillaries into tissue fluid and out across mumbrane into dialylis fluid
• fluid then drained and discarded

Question 83

Kidney transplant.

Answer 83

Transplant:
- long term dialysis can have side effects, best solution is transplant
• single healthy kidney from donor placed within body, blood vessels are joined and the ureter of new kidney is inserted in bladder.
• if sucessful, new kidney will function for many years
•main problem is risk of rejection
- antigens on donated organ differ from antigens on cells of recipient and immune system is likely to recognise
- antigen matching (close as possible) - tissue type similar (people from same blood group)
- drugs to suppress immune response for rest of life, however prevent patients from responding to infectious disease
• transplant organs dont last forever kidney usually last 9-10 years however some have lasted for around 50.

Question 84

Structure of the nephron?

Answer 84

Structure of nephron:
. bowman’s capsule - cup shaped structure that contains the glomerulus (a tangle of capillaries). more blood goes into the glomerulus than leaves it due to the ultrafiltration processes that take place
. proximal convoluted tubule - the first, coiled region of the tubule after the bowman’s capsule, found in the cortex of the kidney. this is where many of the substances needed by the body are reabsorbed into the blood
. loop of henle - a long loop of tubule that creates a region with a very high solute concentration in the tissue fluid deep in the kidney medulla. the descending loop runs down from the cortex through the medulla to a hairpin bend at the bottom of the loop. the ascending limb travels back up through the medulla to the cortex
. distal convoluted tubule - a second twisted tubule where the fine tuning of water balance of the body takes place. the permeability of the walls to water varies in response to the levels of ADH in the blood. further regulation of the ion balance and pH of the blood also takes place in this tubule
. collecting duct - the urine passes down the collecting duct through the medulla to the pelvis more fine tuning of the water balance takes place, the walls of this part of the tubule are also sensitive to ADH
- the nephron has a network of capillaries around it which finally lead to a venule and then to the renal vein. the blood that leaves the kidney has greatly reduced levels of urea, but the levels of glucose and other substances such as amino acids are needed by the body are almost the same as when the blood entered the kidneys (may be slightly less as some glucose will have been used for selective reabsorption). the mineral ion concentration in the blood has also been restored to ideal levels

Question 85

What is the process of ultrafiltration in the glomerulus?

Answer 85

Process of ultrafiltration:
1. high hydrostatic pressure in capillaries of glomerulus forces small molecules such as urea, glucose, amino acids, ions and water into the bowman’s capsule
2. small molecules forced through pores/gaps/fenestrations in the endothelium of the capillaries in the glomerulus
3. small molecules forced through the basement membrane
4. basement membrane acts as a secondary sieve, blood cells and many proteins are too big to leave so are retained in the capillary due to size
5. podocytes in walls of bowman’s capsule act as an additional filter to wrap around capillaries and form slits to ensure any cells, platelets or large plasma proteins do not enter the tubule itself
6. the filtrate that enters the capsule contains glucose, salt, urea and many other substances in the same concentrations of blood plasma

Question 86

What is the mechanism of ADH action?

Answer 86

Mechanism of ADH action:
1. ADH binds to receptors on the cells membrane of tubule cells and triggers the formation of cAMP, which causes a cascade of events
2. vesicles in the cells lining of the collecting duct fuse with the cell surface membranes on the side of the cell in contact with the tissue fluid of the medulla
3. the membranes of these vesicles contain protein based water channels (aquaporins) and when they are inserted into the cell surface membrane, they make it permeable to water
4. this provides a route for water to move out of the tubule cells into the tissue fluids of the medulla and the blood capillaries by osmosis
5. the more ADH released the more water channels are inserted into the membranes of the tubule cells. this makes it easy for more water to leave the tubules by diffusion, resulting in the formation of a small amount of very concentrated urine.
6. water is returned to the capillaries, maintaining the water potential of the blood and therefore the tissue fluid of the body
7. when ADH levels fall, the reverse happens. levels of cAMP falls then the water tubules are removed from the tubule cell membranes and enclosed in vesicles again. the collecting duct duct becomes impermeable to water once more, so no water can leave. this results in the production of large amounts of very dilute urine, and maintains the water potential of the blood and the tissue fluid.