Module 5.2 - Excretion as an Example of Homeostatic Control

Question 1

(MA) What structures are present in the lobule of the liver?

Answer 1

• hepatic artery
• hepatic portal vein
• bile duct
• sinusoid
• hepatocytes
• Kupffer cells

Question 2

(MA) How do the structures of the liver relate to their function?

Answer 2

• hepatic artery: brings oxygenated blood from heat, supplies hepatocytes w O2 etc for aerobic respiration
• hepatic portal vein: brings deoxygenated blood from digestive system. May contai toxic compounds which need detoxification or products of digestion for storage etc
• bile duct: bile made in hepatocytes (exocrine) + secreted into bile canaliculi which drains into bile duct. Bile stored in gall bladder until release into small intestine. Bile neutralises stomach acid + emulsifies lipids in s intestine-blood from hepatic artery+heptatic portal vein mix + go into a sinusoid surrounded closely by liver cells (hepatocytes)
• blood from sinusoid drains to intra-cellular hepatic vein which return blood to heart
• hepatocytes: remove molecules eg glucose from blood + release others back in eg fibrinogen. Covered in microvilli, large SA for diffusion. Make bile, plasma protein, store vitamins, carry out detoxification. Contain many mitochondria as v metabolically active. High density of organelles eg ribosomes for protein synthesis
• Kupffer cells: macrophages found in sinusoids which break down RBCs. Products of this are released into bile to be sent into digestive system for excretion eg bilirubin from Hb is brown colour in faeces

Question 3

(MA) How is urea made in the liver?

Answer 3

• deamination: amine group removed from amino acids + forms a keto acid (which can be respired) + NH3
• ammonia is highly soluble + highly toxic so must be converted to urea (less toxic/soluble)
• ornithine cycle: ammonia react w CO2 to form urea + H2O. Other products eg ornithing (+ other amino acids) also made
• urea transported in blood for removal by nephrons in kidney into urine

Question 4

(MA) How does detoxification in the liver occur?

Answer 4

-converting toxic molecules to less/non-toxic ones
-needed to remove toxic molecules from diet or from metabolic processes from blood. If weren’t removed could kill us
-detoxification of alcohol
>ethanol dehydrogenase converts ethanol to ethanol
>ethanol converted to ethanoic acid
>both stages: NAD combines w/ H to form reduced NAD which can take them to the inner mitochondrial membrane to make ATP in oxidative phosphorylation
>ethanoic acid converted to acetyl CoA + carries it to the Krebs cycle
-detoxification ot hydrogen peroxide: catalase enzyme in hepatocytes hydrolyses H2O2 (made by WBCs) into H2O + O2

Question 5

(MA) What are the biological problems associated with having too much alcohol?

Answer 5

• NAD needed to break down alcohol
• NAD also needed to break down fatty acids for respiration
• too much alcohol means not enough NAD is present to break down fatty acids
• means fatty acids converted back into lipids + get stored in hepatocytes
• fatty liver can cause cirrhosis of liver

Question 6

(MA) How is the structure of the glomerulus related to its function?

Answer 6

• afferent arteriole wider than efferent arterioles
• causes high HS pressure
• endothelium of capillaries has many small gaps
• allow smaller molecules through for ultrafiltration (by basement membrane)

Question 7

(MA) What are the structures of the nephron?

Answer 7

• Bowman’s capsule
• PCT
• loop of Henle
• DCT
• collecting duct

Question 8

(MA) How is the structure of the Bowman’s capsule related to its function?

Answer 8

• ultrafiltration
• basement membrane stops removal of large molecules e.g. RBCs
• basement membrane mesh like made of glycoproteins
• epithelial cells of Bowman’s capsule are podocytes which have finger like projections
• ensure gaps to allow passage of substances underneath into lumen

Question 9

(MA) How is the structure of the proximal convoluted tubule related to its function?

Answer 9

• epithelial cells covered in microvilli increase SA
• for selective reabsorption
• many co transporter proteins in cell surface membrane of epithelial cells
• to reabsorb all of the glucose + amino acids into blood

Question 10

(MA) How is the structure of the Loop of Henle related to its function?

Answer 10

• hairpin counter current multiplier
• purpose is to cause a decrease in water potential in tissue fluid of medulla
• many carrier proteins to transport ions (out of ascending limb)
• so water can be reabsorbed from the collecting duct

Question 11

(MA) How is the structure of the collecting duct related to its function?

Answer 11

-epithelial cells contain many aquaporins in the cell surface membranes

Question 12

(MA) What happens during ultrafiltration?

Answer 12

• afferent arteriole wider than efferent arteriole
• causes high HS pressure
• endothelium of capillaries has many small gaps to allow smaller molecules through
• basement membrane stops removal of large molecules e.g. RBCs
• epithelial cells of Bowman’s capsule are podocytes (have finger like projections)
• ensure gaps to allow passage of substances underneath into lumen

Question 13

(MA) What happens during selective reabsorption?

Answer 13

• occurs in PCT
• Na pumped into blood + K pumped into epithelial cells from blood
• reabsorption of all glucose + amino acids into blood
• through co transporter proteins by facilitated diff (carry glucose/aa and Na+)
• water follows by osmosis

Question 14

(MA) What happens in the Loop of Henle?

Answer 14

• purpose: decrease water potential of tissue fluid of medulla
• ions actively transported out of ascending limb into tissue fluid of medulla
• water potential of tissue fluid surrounding LoH + collecting duct is lower than inside them
• descending limb + collecting duct are permeable to water
• water is absorbed from descending limb + collecting duct of nephron into tissue fluid + then into blood

Question 15

(MA) Describe the release of ADH.

Answer 15

• osmoreceptors in hypothalamus monitor water potential of blood
• neurosecretory cell bodies in hypothalamus produce ADH
• ADH travels down axon of neurosecretory cells to terminal bulb in posterior pituitary gland to be stored
• if osmoreceptors sense water potential of blood is too low, they lose water by osmosis
• triggers an action potential down axon of neurosecretory cell
• stimulates the release of ADH from the terminal bulb in posterior pituitary gland

Question 16

(MA) Describe the role of ADH in osmoregulation on hot days.

Answer 16

• posterior pituitary gland secretes lots of ADH
• ADH travels in blood
• target cells for ADH are cells lining collecting duct in nephrons of kidneys
• ADH binds to receptors of the cell surface membranes of target cells, triggers a series of enzyme controlled reactions
• causes vesicles containing aquaporins to be made + fuse w the membranes, increasing membrane’s permeability to H2O
• more ADH = more aquaporins in cell surface membrane = collecting duct more permeable to water = more reabsorption of water from collecting duct via osmosis
• produces a smaller volume of more conc urine

Question 17

(MA) Describe the role of ADH in osmoregulation on cold days.

Answer 17

• posterior pituitary gland secretes less ADH
• less ADH travels in blood
• target cells for ADH are cells lining collecting duct in nephrons of kidneys
• less ADH binds to receptors on cell surface membranes of target cells
• fewer vesicles containing aquaporins made + fuse w membranes decreasing membrane’s permeability to water
• less ADH = less aquaporins in cell surface membrane = collecting duct less permeable to water = less reabsorption of water from collecting duct via osmosis
• produce a larger volume of less conc urine

Question 18

(MA) How does haemodialysis work?

Answer 18

• blood from artery removed
• blood pumps keep blood moving
• heparin added to prevent clotting (anticoagulant) during dialysis
• blood passes into a machine w partially permeable artificial dialysis membrane
• on other side of membrane is dialysis fluid which has correct conc of glucose, ions, urea
• blood + dialysis fluid flow in opposite directions to one another to maintain conc grad for diff
• urea diffuses from blood to dialysis fluid, excess water moves into it by osmosis
• air trap + air detector needed to remove bubbles before blood returned to a vein
• 3-4 hours, 2-4 times per week

Question 19

(MA) What is a kidney transplant?

Answer 19

A new kidney is given to a patient surgically under general anaesthetic

Question 20

(MA) What are the advantages of getting a kidney transplant?

Answer 20

• no need for time consuming dialysis
• diet less limited
• better qol than on dialysis ie can travel

Question 21

(MA) What are the disadvantages of getting a kidney transplant?

Answer 21

• have to take immunosuppressants for lifetime of organ to avoid rejection; more likely to get infections
• risk of major surgery ie infections
• risk of rejecting organ

Question 22

(MA) Why do kidney transplants need to be a match?

Answer 22

• donated kidneys will be recognised as foreign
• as antigens on kidney will be different to own ones
• causes rejection by immune system
• use immunosuppressant drugs to decrease chance of rejection
• also needs to be a match in size ie wouldn’t use child’s organ in an adult

Question 23

(MA) Ethically, should kidney transplant be used?

Answer 23

• people should have a right to choose what to do with their kidneys
• donor could be paid + benefit from money by selling kidney
• people can donate a kidney to a family member
• people can donate w/out payment
• poorer people could be exploited into donating by offering them a lot of money
• whether you get a kidney or not shouldn’t depend on if you can pay for it

Question 24

(MA) How do pregnancy tests work?

Answer 24

• embryos secrete human chorionic gonadotrophin, released into urine during pregnancy
• hCG acts as antigen
• has complementary shape to free monoclonal antibodies (anti-hCG)
• binds to free antibodies (w coloured beads on them)
• hCG-antibody complex moves along the test strip w urine
• hCG-antibody complex binds w immobilised antibodies specific to the complex
• binding of antibodies produces coloured line (due to coloured beads) = pregnant
• control antibodies bind w any urine + bind to immobilised antibodies on control line to form a coloured strip to indicate if test is working
• 2 lines = pregnant

Question 25

(MA) How does anabolic steroid testing work?

Answer 25

• gas chromatography used to test urine for presence of anabolic steroids
• sample is vaporised in gaseous solvent
• moves down tubes lined w absorption agent
• different substances will be at different but unique times (retention value)
• this is analysed on a chromatogram

Question 26

(MA) What are the side effects of anabolic steroids?

Answer 26

• increased risks of heart attacks
• liver damage
• infertility
• facial hair growth, breasts get smaller in women
• testes + breasts develop in men

Question 27

(MA) Why are anabolic steroids banned in sport?

Answer 27

• gives people who take them an unfair advantage
• many side effects so it’s dangerous e.g. liver damage
• doesn’t reflect athlete’s natural ability

Question 28

What are the main excretory products from the body?

Answer 28

• CO2 from respiration
• nitrogen containing compounds eg urea (nitrogenous waste)
• other compounds eg bile pigments found in faeces

Question 29

How are the lungs used as an excretory organ?

Answer 29

CO2 passed from cells of respiring tissues to bloodstream where transported to lungs. Here, CO2 diffuses into alveoli to be excreted as breathed out

Question 30

How is the liver used as an excretory organ?

Answer 30

Not directly involved in excretion. Has many metabolic processes + some substances produced passes to bile for excretion eg pigment bilirubin. Involved in converting excess amino acids to urea. Amino acids broken down by deamination. Nitrogen containing part of molecules combined w CO2 to make urea

Question 31

How are the kidneys used as an excretory organ?

Answer 31

• urea passes to bloodstream to be transported in solution - dissolved in the plasma
• in kidneys: urea removed from blood to become part of urine
• urine stored in bladder before excreted from body via urethra

Question 32

How is the skin used as an excretory organ?

Answer 32

• sweat contains salts, urea, water, uric acid + ammonia
• urea, uric acid + NH3 all excretory products
• loss of water/salts may be important part of homeostasis (maintaining body temp + water potential of blood)

Question 33

Why is excretion important?

Answer 33

Build up of metabolic waste could be fatal esp if toxic ie NH3, CO2 as interfere w cell processes by altering pH

Question 34

What causes build up of H+ ions in blood plasma and how is this prevented?

Answer 34

• dissociation of carbonic acid for transportation of CO2 around body
• change in pH detected by respiratory centre in medulla oblongata of brain causing increased breathing rate to help remove excess CO2

Question 35

What happens if blood pH gets too low from too many H+ ions?

Answer 35

• below pH 7.35
• may cause headaches, drowsiness, restlessness, tremor, confusion
• may be a rapid heart rate + changes in blood pressure (respiratory acidosis). Can be caused by diseases or conditions that affect lungs (eg emphysema, chronic bronchitis, asthma, severe pneumonia).
• Blockage of airway due to swelling, foreign object or vomit can also induce acute respiratory acidosis

Question 36

Why does deamination occur?

Answer 36

• Body can’t store excess amino acids but they contain almost as much energy as carbs so would be wasteful to excrete
• potentially toxic amino group removed (deamination)
• initially forms NH3 (v soluble/toxic) so converted to less soluble/toxic urea which can be transported to kidneys for excretion
• remaining keto acid used directly in respiration to release its energy or converted to a carb/fat for storage

Question 37

What is the word equation for deamination?

Answer 37

amino acid + oxygen –> keto acid + ammonia

Question 38

What is the word equation for the formation of urea?

Answer 38

ammonia + carbon dioxide –> urea + water

Question 39

What is the formula for urea?

Answer 39

(NH2)2CO

Question 40

What are liver cells called?

Answer 40

hepatocytes

Question 41

What is the name of the blood vessel that supplies the liver with oxygen and what is the blood used for?

Answer 41

• hepatic artery
• oxygenated blood from heart travels from aorta via hepatic artery to liver
• supplies O2 for aerobic respiration
• liver cells v metabolically active, many of these processes require ATP, so important liver has a good supply of O2 for aerobic respiration

Question 42

What is the name of the blood vessel that supplies the liver with the products of digestion and why does it do this?

Answer 42

• hepatic portal vein
• deoxygenated blood from digestive system enter liver
• rich in products of digestion. Concs of various substances is uncontrolled as just entered body from products of digestion in intestines
• blood may also contain toxic compounds absorbed from intestine
• important such substances don’t continue to circulate body before concs have been adjusted

Question 43

How does blood enter/leave the liver?

Answer 43

hepatic vein - rejoins vena cava + blood returns to body’s normal circulation

Question 44

What is bile?

Answer 44

-secretion from liver which has functions in digestion/excretion

Question 45

What does the bile duct do?

Answer 45

Carries bile from liver to gall bladder where it’s stored until required to aid digestion of fats in small intestine. Also contains some excretory products eg bile pigments eg bilirubin, which will leave body in faeces

Question 46

What are inter-lobular vessels?

Answer 46

Run between+parallel to lobules. The splitting of hepatic artery and portal vein

Question 47

What happens in the sinusoid?

Answer 47

(Is lined with liver cells) Blood from hepatic artery and hepatic portal vein mix. As blood flows it’s in close contact w liver cells. Able to remove substances from blood + return other substances

Question 48

What is the function of Kupffer cells?

Answer 48

• move within sinusoid
• breakdown + recycle old RBCs
• one product of Hb breakdown is bilirubin, which is one of the bile pigments excreted as part of bile

Question 49

How does bile travel from where it’s produced to where it’s stored?

Answer 49

• made in liver cells
• released into bile canaliculi
• they join to form a bile duct which transports bile to gall bladder

Question 50

What is at the centre of each lobule?

Answer 50

branch of the hepatic vein known as the intra-lobular vessel

• sinusoids empty into this vessel
• branches of hepatic vein from different lobules join to form hepatic vein, which drains blood from liver

Question 51

What is the structure of liver cells?

Answer 51

• relatively unspecialised

- simple cuboidal shape w many microvilli on surface

Question 52

What are the metabolic functions of hepatocytes?

Answer 52

• protein synthesis
• transformation and storage of carbs
• synthesis of cholesterol + bile salts
• detoxification
• means their cytoplasm must be v dense + is specialised in numbers of certain organelles it contains

Question 53

What are the metabolic functions of the liver?

Answer 53

• control blood glucose/amino acid/lipid levels
• synthesis of bile/plasma proteins/cholesterol
• synthesis of RBCs in fetus
• store vitamins A, D, B12, iron, glycogen
• detoxification of alcohol, drugs
• breakdown of hormones
• destruction of RBCs

Question 54

How is glycogen stored in the liver?

Answer 54

• store 100-120g of glycogen (8% of fresh weight of liver)
• glycogen forms granules in cytoplasm of hepatocytes
• glycogen can be broken down to release glucose into blood as required

Question 55

How is hydrogen peroxide detoxified in the liver?

Answer 55

• catalase
• converts H2O2 to O2 + H2O
• particularly high turnover no. (no of molecules one molecule of catalase can render harmless/sec) of 5 million

Question 56

How are drugs such as cocaine detoxified in the liver?

Answer 56

• cytochrome P450 (group of enzymes)
• these enzyme’s role in metabolising drugs can interfere w their other metabolic processes eg electron transport in respiration + can cause unwanted side effects in some medicinal drugs

Question 57

What is formed throughout the breakdown of ethanol?

Answer 57

ethanol –> ethanol –> ethanoic acid –> acetyl coenzyme A

Question 58

What happens if the liver has to detoxify too much alcohol?

Answer 58

Stores of NAD used up + insufficient left to deal w fatty acids so fatty acids converted back to lipis + stored as fat in hepatocytes so liver becomes enlarged (fatty liver, which can lead to alcohol related hepatitis or cirrhosis)

Question 59

What is the ornithine cycle?

Answer 59

A series of biochemical reactions that convert ammonia to urea

Question 60

What is the formula of keto acid?

Answer 60

RC=OCOOH

Question 61

Why must ammonia quickly be converted to urea?

Answer 61

Highly soluble + toxic

Question 62

In the ornithine cycle, what is formed when ammonia and carbon dioxide combine?

Answer 62

citrulline

Question 63

How is citrulline converted to arginine?

Answer 63

Further addition of ammonia

Question 64

How is arginine re converted to ornithine?

Answer 64

Removal of urea

Question 65

What equation can be used to summarise the ornithine cycle?

Answer 65

2NH3 + CO2 –> CO(NH2)2 + H2O

ammonia + carbon dioxide –> urea + water