5.2 Excretion as an example of homeostatic control

Question 1

define excretion

Answer 1

the removal of metabollic waste from the body

Question 2

Define metabolic waste

Answer 2

A substance that is produced in excess by the metabolic processes in the cells and it may become toxic

Question 3

What must be excreted

Answer 3

carbon dioxide from respiration
nitrogen containing compounds such as urea
other compounds such as bile

Question 4

what are the excretory organs

Answer 4

the lungs
the liver
the kidneys
the skin

Question 5

what is excreted from the lungs

Answer 5

carbon dioxide

Question 6

What is excreted from the liver

Answer 6

bilirubin - the bile pigment
converts excess amino acids into urea

Question 7

What do the kidneys excrete

Answer 7

urea

Question 8

What does the skin excrete

Answer 8

sweat which contains substances such as salts water uric acid and ammonia

Question 9

Why is excretion important

Answer 9

The build up of some metabolic products could be fatal as they may interfere with cell processes by altering pH etc. other metabolic products may act as inhibitors and reduce enzyme activity

Question 10

Why must carbon dioxide be excreted

Answer 10

It can react with water to form carbonic acid which can dissociate into H+ ions
These ions can affect the pH of red blood cell cytoplasm and the plasma which can alter the structure of proteins and affect their ability to perform their function

Question 11

How is a drop in blood pH detected and counteracted

Answer 11

The extra H+ ions are detected by the respiratory centre in the medulla oblongata and cause an increase in breathing rate to help remove excess CO2

Question 12

What can happen if blood pH drops below 7.35

Answer 12

It may cause headaches, drowsiness, restlessness, tremor and confusion

Question 13

What can cause an increase in blood pH

Answer 13

Diseases or conditions which affect the lungs such as emphysema asthma or bronchitis

Question 14

What is the condition when blood is acidic

Answer 14

respiratory acidosis

Question 15

how do we excrete excess amino acids

Answer 15

The body cannot store excess amino acids so they are transported to the liver where the amino group is removed and initially the soluble and toxic compound ammonia is formed and then converted to the less toxic compound urea which can be transported to the kidneys for excretion

Question 16

Why are excess amino acids not excreted but rather deaminated

Answer 16

They contain a similar amount of energy as carbohydrates so it would be wasteful to excrete them so they are deaminated and the keto acid can be used in respiration

Question 17

Give the equation for the formation of urea from amino acids

Answer 17

amino acid + oxygen → keto acid+ ammonia
ammonia + carbon dioxide → urea + water

Question 18

How does blood flow to the liver

Answer 18

The hepatic artery delivers oxygenated blood to the liver

the hepatic portal vein delivers deoxygenated blood from the digestive system to the liver which is rich in the products of digestion and may also contain toxic compounds which need their concentrations altered before it can continue to circulate around the body

Question 19

How does bood leave the liver

Answer 19

Hepatic vein which then rejoins the vena cava

Question 20

Why are the blood vessels and chambers in the liver arranged in the way that they are

Answer 20

To ensure greatest possible contact between the blood and liver cells

Question 21

What is the liver split into

Answer 21

Lobes which are further split into lobules

Question 22

What do the hepatic artery and hepatic portal vein split into

Answer 22

as they enter the liver they split into smaller and smaller vessels which run between and parallel to the lobules known as interlobular vessels

Question 23

Describe the blood flow from the hepatic artery and hepatic portal vein

Answer 23

They split into smaller vessels running between and parallel the lobules as interlobular vessels.

At intervals they enter the lobule and the blood from the two vessels is mixed and passes along a special chamber called a sinusoid which is lined with liver cells which are able to remove substances from the blood and return other substances to the blood

At the end of the sinusoid the concentrations of many of its components have been altered and at the centre of each lobule is a branch of the hepatic vein known as an intralobular vessel which the sinusoids empty into

The branches of the hepatic vein from the different lobules join together to form the hepatic vein and blood is drained from the liver

Question 24

Describe the contents of the sinusoids

Answer 24

lined with liver cells
contains macrophages called Kupffer cells

Question 25

What is the primary function of the macrophages in the sinusoid

Answer 25

Their primary function appears to be breakdown and recycle of old red blood cells

Question 26

What is the product of haemoglobin breakdown

Answer 26

bilirubin

Question 27

Describe how bile is transported out of the liver

Answer 27

Bile is made in the liver cells and released into the bile canaliculi which join to form the bile duct and is transported to the gall bladder

Question 28

Give the 11 components of the liver

Answer 28

Hepatic artery hepatic portal vein interlobular vessels lobes lobules sinusoids Kupffer cells intralobular vessel hepatic vein bile canaliculi bile duct

Question 29

What are the metabolic functions of the liver

Answer 29

Control of blood glucose levels synthesis of bile, plasma proteins and cholesterol storage of glycogen destruction of red blood cells detoxification of alcohol and drugs hormone breakdown

Question 30

How does the liver detoxify hydrogen peroxide

Answer 30

catalase enzymes convert hydrogen peroxide to hydrogen and water

Question 31

How does the liver detoxify drugs

Answer 31

Cytochrome p450 is a group of enzymes that can breakdown drugs including cocaine. They are also used in electron transport during respiration

Question 32

Why do drugs have side effects

Answer 32

Cytochrome p450s role in metabolising drugs can interfere with other metabolic processes causing the unwanted side effects

Question 33

How does the liver detoxify alcohol

Answer 33

it is broken down in the hepatocytes by ethanol dehydrogenase which produces ethanal This is dehydrogenated further by ethanal dehydrogenase enzyme and the final compound is acetate Acetate can enter respiration by combining with CoA to form acetyl CoA The hydrogen atoms released from the dehydrogenation combine with NAD to form NADH

Question 34

Explain why it would be wastefl to just excrete alcohol

Answer 34

Because it can be dehydrogenated to form acetate which can be used in respiration and it also forms 2NADH

Question 35

Explain why someone that consumes a lot of alcohol may have fatty liver

Answer 35

NAD is also required to oxidise and breakdown fatty acids for use in respiration but if the liver has to detoxify too much alcohol it has insufficient NAD left to deal with the fatty acids so they are converted back to lipids and stored as fats in the hepatocytes

Question 36

What 2 processes do amino acids undergo in the liver

Answer 36

deamination and the ornithine cycle

Question 37

Describe deamination

Answer 37

It removes the amino group and produces ammonia which is very soluble and toxi and keto acid which can enter respiration

Question 38

Describe the ornithine cycle

Answer 38

Ammonia and CO2 combine with the amino acid citrulline and this is converted into arginine by addition of further ammonia The arginine is then re-converted to ornithine by the removal of urea

Question 39

What is the equation for what occurs in the ornithine cycle

Answer 39

2NH3 + CO2 → CO(NH2)2 + H20

Question 40

What type of reactions occur in the ornithine cycle

Answer 40

condensation

Question 41

Define nephron

Answer 41

the functional unit of the kidney

Question 42

Define ultrafiltration

Answer 42

filtration of the blood at molecular level under pressure

Question 43

describe the structure of the kidneys

Answer 43

it has an outer region called the cortex inner region called the medulla the centre is the pelvis which leads to the ureter It is lined with a capsule blood flows through the renal artery and vein Each kidney contains about one million nephrons which starts at the cortex

Question 44

What does the renal artery split into

Answer 44

many afferent arterioles which lead to a knot of capillaries in the glomerulus which then continues to the efferent arterioles

Question 45

How do the afferent and efferent arterioles differ

Answer 45

The afferent arteriole has a large diameter

Question 46

What parts of the nephron are in the cortex?

Answer 46

The glomerulus proximal convoluted tubule distal convoluted tubule 1/2 of collecting duct

Question 47

What parts of the nephron are located in the medulla

Answer 47

The loop of Henle descending limb ascending limb 1/2 of collecting duct

Question 48

what do the capillaries in the kidney go into

Answer 48

the glomerulus

Question 49

What are the filtering layers in ultrafiltration

Answer 49

The endothelium of the capillary the basement membrane the epithelial cells of the Bowmanns capsule

Question 50

Describe the endothelium of the capillary

Answer 50

There are narrow gaps between the cells of the endothelium wall as well as pores called fenestrations the gaps allow blood plasma and substances dissolved in it to pass out of the capillary

Question 51

Describe the basement membrane

Answer 51

It consists of a mesh of collagen fibres and glycoproteins which acts as a filter to prevent the passage of molecules with a molecular mas greater than 69000 to pass through

Question 52

Describe the epithelial capillaries of the Bowmann's capsule

Answer 52

They are cells called podocytes which have a specialised shape They have projections called major processes and on each there are minor processes that hold the cells away from the endothelium of the capillary These projections ensure there are gaps between the cells so that fluid can pass through them into the lumen of the Bowmann's capsule

Question 53

Describe how hepatocytes are adapted

Answer 53

They have a very dense nucleus with specialised numbers of certain organelles

Question 54

Describe how pressure is maintained in the glomerulus

Answer 54

The afferent arteriole is wider than the efferent which tends to push blood into the Bowmann's capsule that surrounds the glomerulus

Question 55

What is filtered out of the blood

Answer 55

water amino acids glucose urea inorganic mineral ions such as sodium

Question 56

What other pressure is acting on the capillary in the glomerulus

Answer 56

Hydrostatic and osmotic pressure

Question 57

Why is osmotic pressure beneficial in the glomerulus

Answer 57

This ensures that some of the fluid is retained in the blood due to the low water potential in the capillaries

Question 58

What is the funtcion of the nephrons

Answer 58

Ensures that urine has a low water potential and a high concentration of solutes that is found in the blood and tissue fluid - metabolic waste

Question 59

Describe how the plasma membrane of the proximal convoluted tubule closest to the tubule fluid is adapted

Answer 59

It is highly folded to form microvilli to increase the surface area for reabsorption It also contains special cotransport proteins that transport glucose or amino acids in association with the sodium ions from the tubule into the cell

Question 59

Describe how the plasma membrane of the proximal convoluted tubule closest to the tubule fluid is adapted

Answer 59

It is highly folded to form microvilli to increase the surface area for reabsorption It also contains special cotransport proteins that transport glucose or amino acids in association with the sodium ions from the tubule into the cell

Question 60

Describe how the plasma membrane closest to the tissue fluid in the proximal convoluted tubule is adapted

Answer 60

It is also folded to increase its surface area It contains sodium/potassium pumps

Question 61

escribe how the cells lining the proximal convoluted tubule are adapted

Answer 61

They contain many mitochondira

Question 62

Describe the mechanism of reabsorption in the proximal convoluted tubule

Answer 62

1- Sodium ions are actively pumped out of the the cells lining the proximal convoluted tubule into the tissue fluid reducing their concentration in the cell 2- Sodium ions diffuse into the cell from the tubule fluid through a cotransport protein carrying a glucose or amino acid - secondary active transport 3-This decreases the water potential in the cell so water moves in by osmosis 4- glucose and amino acids diffuse out of the cell into the tissue fluid repeat

Question 63

How are large molecules reabsorbed

Answer 63

endocytosis

Question 64

Why do mammals need to reabsorb water

Answer 64

The mammal would become dehydrated very quickly otherwise

Question 65

Why does urea concentration increase in the nephron past the loop of henle

Answer 65

Water is withdrawn from the tubule so the urea concentration increases

Question 66

Describe how water is reabsorbed in the loop of henle

Answer 66

1- There is active removal of odium and chlorine ions at the top of the ascending limb it is also impermeable to water 2-As mineral ions diffuse into the descending limb the concentration of the fluid rises so its water potential decreases 3- the fluid becomes increasing more concentrated deeper in the medulla 4- as the fluid rises up the medulla the mineral ions leave the fluid by diffusion 5- the effect of these ionic movements is to create a higher water potential in the fluid of the ascending limb and lower water potential in the tissue fluid of the medulla 6- as water flows down the collecting duct it passes through tissues with an ever-decreasing water potential gradinet so water moves out and into the tissue fluid via osmosis

Question 67

Describe the purpose of the distal convoluted tubule

Answer 67

active transport is used to adjust concentrations of various mineral ions

Question 68

Why does water potential of the tissue fluid in the medulla get more negative the lower down the medulla

Answer 68

This is because the tubule fluid has a very high concentration at the base of the ascending limb and mineral ions diffuse out making the tissue fluid have a very negative water potential compared to higher up

Question 69

What is the effect of alcohol on water absorption

Answer 69

It inhibits the release of ADH which can make you dehydrated

Question 70

What chain of events occurs when there is a very low water concentration in the blood

Answer 70

Osmoreceptors in the hypothalamus lose water by osmosis and shrink stimulating neurosecretory cells in the hypothalamus This causes ADH to move down the axon body to the terminal bulb in the posterior pituitary gland where it is stored in vesicles Action potentials cause the release of more ADH The ADH is detected by cell surface receptors and a chain of enzyme controlled reactions occurs Vesicles containing aquaporins move and fuse to the plasma membrane and this makes the plasma membrane more permeable to water

Question 71

What chain of vents occurs when water concentration in the blood is too high

Answer 71

Osmoreceptors in the hypothalamus swells and stimulates neurosecretory cells in the hypothalamus ADH moves down the axon body to the terminal bulb in the posterior pituitary gland where it is stored in vesicles Action potentials stimulate the release of less ADH The ADH is detected by cell surface receptors and a chain of enzyme controlled reactions occurs The plasma membrane invaginates (folds inwards) to create new vesicles that remove aquaporins making the walls less permeable to water

Question 72

What happens to excess ADH in the body

Answer 72

Once the water potential of the blood rises again less ADH is released Remaining ADH is broken down as it has a half life of 20 minutes

Question 73

Why must excess ADH get broken down

Answer 73

So that water potential of the blood doesnt get too high and cells do not undergo lysis

Question 74

Define hypertonic

Answer 74

High concentration of solutes

Question 75

Define hypotonic

Answer 75

Low concentration of solute

Question 76

Define kidney failure

Answer 76

If the kidneys fail completely they are unable to regulate the levels of water and electrolytes in the body or to remove waste products such as urea from the blood

Question 77

How can we assess kidney function

Answer 77

By estimating the glomerular filtration rate and by analysing urine for substances such as proteins

Question 78

Define glomerular filtration rate

Answer 78

it is a measure of how much fluid passes the nephrons per minute and a normal reading is 90-120 cm/min

Question 79

What are the causes of kidney failure

Answer 79

diabetes mellitus heart disease hypertension and infection

Question 80

What are the main treatments for kidney failure

Answer 80

renal dialysis and kidney transplant

Question 81

What are the 2 types of renal dialysis

Answer 81

haemodialysis peritoneal dialysis

Question 82

Describe haemodialysis

Answer 82

Blood from an artery is passed into a machine that contains an artificial dialysis membrane shaped to form many artificial capillaries and increase surface area the dialysis fluid contains the correct concentrations of mineral ions glucose urea and water and flows in a counter current to the blood It is performed at a clinic 2-3 times a week for several hours a session

Question 83

What is added to the blood during haemodialysis

Answer 83

Heparin to prevent clotting and air bubbles are removed

Question 84

Describe peritoneal dialysis

Answer 84

The dialysis membrane is the body's own abdominal membrane First a surgeon implants a permanent tube in the abdomen. Dialysis solution is poured through the tube and fills the space between the abdominal wall and the organs and after several hours the solution is drained This can be carried out at home or work

Question 85

What must be combined with peritoneal dialysis and why

Answer 85

A balanced diet and not to much protein as it turns into urea

Question 86

What are the advantages of a kidney transplant

Answer 86

Freedom from time-consuming dialysis feeling physically fitter better self image improved quality of life

Question 87

What are the disadvantages of a kidney transplant

Answer 87

Need to take immunosuppressants and their side effects need for major surgery need regular check for rejection

Question 88

Describe urine analysis

Answer 88

Urine can be tested for molecules with an Mr less than 69000 such as glucose, alcohol, many recreational drugs, anabolic steroids

Question 89

Describe pregnancy testing

Answer 89

Urine is poured onto a test stick and hCG binds to the mobile antibodies attached to a blue bead the mobile antibodies move down the test stick If hCG is present it binds to fixed antibodies holding a bead in place and a blue line forms Mobile antibodies with no hCG attached bind to another fixed site to show the test is working

Question 90

Describe testing for anabolic steroids

Answer 90

Anabolic steroids increase protein synthesis within cells which results in the build-up of cell tissue especially in the muscles. anabolic steroids have a half life of 16 hours so the urine sample is analysed using gas chromatography