5.1.2 - Excretion as an example of homeostatic control

Question 1

Outline the 3 main waste products in mammals?

Answer 1

• carbon dioxide - from cellular respiration - excreted by lungs
• bile pigments - formed from breakdown of haemoglobin - excreted in the bile from liver into the small intestine via gall bladder and bile duct
• nitrogenous waste products (urea) - due to breakdown of excess amino acids by liver - fish produce ammonia - excreted by kidneys in urine

Question 2

Briefly outline the liver as a major organ?

Answer 2

• major metabolic importance (500 different metabolic pathways) fast growing and able to regenerate when damaged
• oxygenated blood supplied to liver by hepatic artery and returned to heart in hepatic vein
• hepatic portal vein also supplies liver with blood loaded with products of digestion from the intestines (main blood flow route)

Question 3

Outline the structure of the liver?

Answer 3

Liver cells are hepatocytes (large nuclei, prominent Golgi and lots of mitochondria) - metabolically active cells (Can divide/replicate)

• blood from hepatic artery and hepatic portal vein mixed in a spaces (sinusoids) surrounded by hepatocytes (provides them enough oxygen)
• sinusoids contain kupffer cells which act as macrophages ingesting foreign particles

Question 4

Outline the role of hepatocytes in the liver?

Answer 4

Liver cells (large nuclei, prominent Golgi and lots of mitochondria) - metabolically active cells (Can divide/replicate)

• surround sinusoids which provides them with enough oxygen to function
• secrete bile from the breakdown of the blood in spaces called canaliculi - bile then drains into bile ductucles where its taken to the gall bladder

Question 5

Outline the role of the liver regarding carbohydrate metabolism?

Answer 5

When blood glucose levels rise - insulin levels rise and stimulate hepatocytes to convert glucose to storage carbohydrate glycogen

• when blood sugar levels fall - hepatocytes convert glycogen back to glucose under influence of glucagon (hormone)

Question 6

Explain the process of transamination?

Answer 6

The conversion of one amino acid into another - important because diets does not always contain the required balance of amino acids

Question 7

Explain the process of deamination in the liver?

Answer 7

Removal of an amine group from a molecule (body can not store proteins or amino acids)

• amino acid deaminated - removes amino group - converts it into ammonia (highly toxic and highly soluble) then urea (toxic in high concentrations) - urea excreted by kidneys
• remainder of the amino acids can be fed into cellular respiration or converted into lipids for storage

ornithine cycle

Question 8

Explain what happens to the ammonia from deamination - why is this necessary?

Answer 8

Ammonia converted into urea - through enzyme-controlled reactions (ornithine cycle)

ammonia is highly toxic and highly soluble

Question 9

Explain the process of detoxification in the liver - give 2 examples?

Answer 9

Breakdown of hydrogen peroxide (by-product of various metabolic pathways) hepatocytes contain enzyme catalase which splits hydrogen peroxide into oxygen and water

• another example is the detoxification of ethanol - hepatocytes contains enzyme alcohol dehydrogenase breaks ethanol to ethanal which is then converted to ethanoate (used to build up fatty acids for respiration)

Question 10

Give the overall reaction for ammonia conversion to urea?

Answer 10

Ammonia + carbon dioxide - urea + water

overall output of cycle is urea and water

Question 11

Outline the ornithine cycle - what substances are involved?

Answer 11

Ammonia + carbon dioxide (Inputs) - citrulline - arginine - urea + water (outputs) - ornithine

Question 12

Outline the overall structure of the kidneys - give 3 main areas?

Answer 12

• made up of millions of structures called nephrons (act as filtering units)
• cortex - filtering of blood happens here (dense capillary network carrying blood from renal artery to nephrons)
• medulla - contains tubules of nephrons that form pyramids of kidneys and collecting ducts
• pelvis - central chamber - urine collects here

Question 13

Outline the structure of the nephron - focus on the role and function of the bowman’s capsule?

Answer 13

Cup-shaped structure which contains glomerulus (tangle of capillaries)

• more blood goes into glomerulus then leaves it due to ultrafiltration taking place

walls of bowman’s capsule contains podocytes (act as an additional filter - have expansion which wrap around capillaries forming slits which prevents contents getting through to tubule

Question 14

Outline the structure of the nephron - focus on the role and function of the proximal convoluted tubule?

Answer 14

First coiled region of tubule after bowman’s capsule (found in cortex of kidney) - many substances needed by body re-absorbed here

glucose reabsorbed into blood here
most water reabsorbed into blood here

Question 15

Outline the structure of the nephron - focus on the role and function of the loop of henle (LOH)?

Answer 15

Long loop of tubule which creates a region with very high solute concentration in the tissue fluid deep in the kidney medulla

• descending loop runs down from cortex through the medulla and ascending limb travels back up through medulla to cortex

acts as a countercurrent multiplier - uses energy to produce conc gradients results in movement of substances

Question 16

Outline the structure of the nephron - focus on the role and function of the distal convoluted tubule?

Answer 16

Second twisted tubule where fine-tuning of water balance takes place - permeability of walls to water varies in response to ADH

• ion balance and pH regulation also takes place

Question 17

Outline the structure of the nephron - focus on the role and function of the collecting duct?

Answer 17

Urine passed down collection duct through medulla to pelvis (more fine tuning of water balance takes place - walls sensitive to ADH)

• involves secondary messenger cAMP and aquaporins in vesicles - provide channel for water through tubule cells

Question 18

Give the name of the artery and vein which supply the kidneys?

Answer 18

• renal vein
• renal artery

Question 19

Explain the process of ultrafiltration in the kidneys - focus on the formation of tissue fluid?

Answer 19

Glomerulus supplied with blood by afferent arteriole (from renal artery) blood leaves through efferent arteriole (narrower - results in significant pressure in glomerulus)

• pressure forces blood out through capillary wall - fluid passes through basement membrane (made of network of collagen fibres and proteins) most plasma contents can pass through, large proteins remain

Question 20

Explain the role of podocytes in ultrafiltration?

Answer 20

• cells located in the wall of the bowman’s capsule
• have extensions (pedicels) which wrap around capillaries forming slits - make sure any cells or large plasma proteins that managed to get through epithelia cells and basement membrane do not get through into tubule itself

Question 21

What is the products of ultrafiltration?

Answer 21

Filtrate which enters the capsule contains glucose, salt, urea and other substances (same conc as blood)

• up to 20% of the water and solutes are removed from the plasma as it passes though glomerulus

Question 22

Explain the need for reabsorption - how is it achieved?

Answer 22

Ultrafiltration removes urea and other products from the blood (many substances which are still needed - eg glucose)

• main function of the nephron after bowman’s capsule is to return filtered substances back into blood

Question 23

Outline the role of proximal convoluted tubule in depth - what and how are substances re-absorbed?

Answer 23

• all the glucose, amino acids, vitamins and hormones are moved from filtrate back into blood by active transport
• majority of sodium chloride and water reabsorbed as well - sodium ions moved by AT (chloride ions and water follow passively down conc gradient)

Question 24

Explain 2 key adaptions of the cells lining to proximal convoluted turbinal?

Answer 24

• covered with microvilli - greatly increases SA for reabsorption of substances
• lots of mitochondria - provide ATP for AT

Question 25

Explain what happens to filtrate of the proximal convoluted tubule?

Answer 25

Filtrate reaching the loop of henle (at end of PCT) is isotonic (same conc) with the tissue fluid surrounding the tubule and the blood

• at this stage over 80% of the glomerular filtrate has been absorbed back into the blood
• substances diffuse into extensive capillary network which surrounds tubules down steep conc gradient (maintained by constant flow of blood)

Question 26

What is the job of the kidneys?

Answer 26

2 important homeostatic roles - involved in excretion and Osmoregulation (filtration of nitrogenous waste products)and maintenance of water balance and pH of blood

Question 27

Outline what happens in the descending limb of the loop of henle?

Answer 27

• leads from the PCT - is a region where water moves out of the filtrate down a conc gradient
• upper part impermeable to water but lower is permeable (runs down into medulla)
• filtrate entering is isotonic with blood - water moves out via osmosis (not permeable to sodium and chloride ions so no AT)

Fluid which reaches bend very concentrated and hypotonic

Question 28

Outline what happens in the ascending limb of the loop of henle?

Answer 28

• first section permeable to sodium and chloride ions (move out of concentrated solution by conc gradient)
• second section - sodium and chloride ions are actively pumped into the medulla tissue fluid against a conc gradient (high sodium and chloride conc in medulla tissue)
• walls are impermeable to water - water can not follow - leaving fluid to become dilute

Question 29

Explain the importance of the walls of the ascending limb of the LOH being impermeable?

Answer 29

Water can not follow the chloride and sodium ions down the concentration gradient

• tissue fluid of medulla has high conc of ions - this is needed to produce urine more concentrated than blood - part of countercurrent multiplier system

by time dilute fluid reaches top of ascending limb it is hypotonic to blood

Question 30

Explain what happens in the distal convoluted tubule?

Answer 30

• permeability of walls of tubule vary with the levels of ADH (balances water needs of body)
• cells lining the DCT have lots of mitochondria for AT
• if body lacks salt, sodium ions can actively be pumped out of the DCT with chloride ions flowing down an electrochemical gradient - water can also leave to concentrate the urine

Question 31

Explain what happens in the collecting duct?

Answer 31

Passes down through concentrated tissue fluid of medulla

• water moves out of collecting duct by diffusion down conc gradient as it passes through renal medulla (urine becomes more concentrated)
• permeability of collecting duct to water controlled by ADH (water can be removed from duct all along its length - producing very hypotonic urine when needed)

Question 32

What is Osmoregulation?

Answer 32

the active regulation of the osmotic pressure of an organism’s body fluids, detected by osmoreceptors, to maintain the homeostasis of the organism’s water content

• amount of water lost controlled by ADH in a negative feedback system

Question 33

Explain the role and function of ADH - where is it stored and released?

Answer 33

• ADH produced in hypothalamus - secreted into posterior pituitary gland (where its stored)
• increases permeability of distal convoluted tubule and the collecting duct to water
• osmoreceptors in hypothalamus are sensitive to concentration of inorganic ions in blood and cause ADH to be released by pituitary gland and carried in blood to cells of collecting duct - binds to receptors on cell membrane

Question 34

Explain how ADH changes permeability of the walls of the collecting duct?

Answer 34

• binds to receptors on cell membrane - triggers formation of secondary messenger - cyclic AMP (cAMP)
• cAMP causes vesicles (in the cells lining the collecting duct) to fuse with the cell surface membranes of the cells in contact with tissue fluid of the medulla - which provides a route for water through aquaporins

Question 35

Explain in depth how cAMP provides a route for water through membranes of tubule cells?

Answer 35

• membrane of these vesicles contain protein based water channels (aquaporins) which when inserted into the cell surface membrane make it permeable to water
• provides route for water to move out of tubule cells into tissue fluid of medulla and blood capillaries by osmosis

more ADH more water channels = concentrated urine

Question 36

Explain what happens when ADH levels fall?

Answer 36

Levels of cAMP fall - water channels are removed from tubule cell membranes (enclosed within vesicles again)

collecting duct becomes impermeable = large quantities of dilute urine

Question 37

Outline the role of the loop of henle (LOH)?

Answer 37

Main job is to facilitate the production of urine which is more concentrated than blood

• acts as a counter-current multiplier to ensure tissue fluid in the medulla is increasing concentrated (Lower WP) compared to filtrate in nephron
• purpose of this ‘salt-bath’ is to facilitate reabsorption of water from collecting duct

Question 38

Outline fully the process of selective reabsorption in the proximal convoluted tubule?

Answer 38

1) Na+ actively transported out of cells of the PCT though outer membranes (requires ATP)
- K+ (potassium ions) diffuse into cell to maintain electrical balance

2) Na+ conc in cells reduced - creates conc gradient from filtrate in PCT down to cytoplasm of cells lining the PCT
- Na+ diffuses from lumen of the PCT into cells of PCT via co-transporter proteins (glucose + amino acids transported simultaneously)

3) influx of solutes into cells decreases WP - water moves down gradient
- urea reabsorbed by diffusion

Question 39

Why might the kidneys fail?

Answer 39

• kidney infection
• structure of the podocytes and tubules are damaged or destroyed
• raised blood pressure (can damaged epithelial cells and basement membrane of BC)
• genetic conditions

Question 40

Explain the effects of kidney damage from high blood pressure?

Answer 40

• protein in urine - damage to basement membrane or podocytes of bowman’s prevent ability to filter
• blood in urine - sign filtering process is not working

Question 41

Explain 4 of the effects of complete kidney failure?

Answer 41

Concentrations of urea and mineral ions build up leads to:

• loss of electrolyte balance - causes osmotic imbalances in tissues and death
• build up of toxic urea - can poison cells
• high bp - can lead to heart problems (strokes)
• weakened bones - calcium/phosphorus balance in blood lost

Question 42

Explain the use of the glomerular filtration rate (GFR)?

Answer 42

Used as measure to indicate kidney disease (kidney problems always affect rate blood if filtered) - done through blood test to measure levels of creatinine (breakdown product of muscles - gives estimate for eGFR)

• other factors involved in calculation of GFR

Question 43

What are 2 ways kidney failure is treated?

Answer 43

• dialysis (hemodialysis or peritoneal)
• transplant

Question 44

What is kidney dialysis?

Answer 44

Renal dialysis is where the function of the kidneys are carried out artificially - two ways haemodialyssis or peritoneal

Question 45

Explain what haemodialysis is?

Answer 45

• uses dialysis machine - blood leaves patients body through artery to machine and flows between partially permeable membranes (mimic basement membrane) dialysis fluid on other side
• urea and excess mineral ions move out of the blood but there is no movement of sugars and other substances

takes 8 hours and relies on diffusion - also requires strict diet and frequent sessions a week

Question 46

Outline and explain how haemodialysis works - with reference to exchanges that take place?

Answer 46

• no net movement of glucose out of blood (dialysis fluid on other side of membrane contains normal plasma levels of mineral ions and sugar - mimics the blood)
• any excess mineral ions/salts move out blood by diffusion down conc gradient
• dialysis fluid contains no urea - steep conc gradient so most urea leaves the body

blood and dialysis fluid flow in opposite directions - maintains counter current exchange system

Question 47

Explain what peritoneal dialysis is?

Answer 47

Done inside body - using natural dialysis membranes of the abdomens lining (peritoneum)

• patient can carry on with normal actives (often at home)

Question 48

Explain how peritoneal dialysis works?

Answer 48

Dialysis fluid injected into abdomen truth catheter (left for few hours)

• urea and excess mineral ions pass out capillaries into tissue fluid (and out across peritoneal membranes into dialysis fluid)
• fluid than drained and discarded

initially fast due to steep conc gradient but eventually reaches equilibrium when conc gradient is 0

Question 49

Explain how kidney failure is treated by transplant?

Answer 49

Best option (long term dialysis as serious side effects)

Healthy kidney from donor placed within body and joined by vessels and ureter is inserted into the bladder (can last for many years)

main problem is risk of rejection if antigens on donor organ differ from antigens of recipients cells - leads to rejection and destruction of kidney

Question 50

Explain how rejection of donor kidney can be avoided?

Answer 50

• using a close match between antigens of donor and recipient
• drugs to suppress immune response (immunosuppressants) - need to be taken for rest of life

Question 51

Explain 3 problems with kidney transplants?

Answer 51

• problem with taking immunosuppressants is they prevent patients from responding effectively to infectious diseases - more vulnerable when ill
• transplanted organs wont last forever - around 10 years
• short supply - not a lot of people register to be organ donors (lived donors is possible though)

Question 52

Explain the hormone hCG?

Answer 52

Human chorionic gonadotropin - produce by the site of the developing placenta

• pregnancy tests will test for hCG in urine using monoclonal antibodies

Question 53

Explain how monoclonal antibodies are made?

Answer 53

Mouse is injected with hCG - so it makes appropriate antibody

• B-cells which make antibody are removed and fused with a cancer cell (divides rapidly) forming a hybridoma - reproduce rapidly forming millions of clones
• monoclonal antibodies collected and purified

Question 54

Outline in full the sages of a pregnancy test?

Answer 54

• wick soaked in urine (usually first morning urine - highest hCG)
• test has monoclonal antibodies with attached coloured beads - bind to hCG forming hCG/antibody complex
• urine carries on until it reaches a window - here immobilised monoclonal antibodies arranged in pattern (eg +) which only bind to hCG/antibody complex
• urine continues up through test to second window - line of immobilised monoclonal antibodies bind only to mobile antibodies (wether there bound to hCG or not) coloured line forms (acts as control to indicate test works)

Question 55

Explain how anabolic steroids and drugs can be tested for in urine?

Answer 55

Testing urine using gas chromatography and mass spectrometry can show presence of drugs (urine sample vaporised by solvent and passed along tube which absorbs the gases)

• drugs like alcohol, cocaine and cannabis can be tested in a similar way