5.1.2 Excretion as an example of homeostatic control Flashcards
1
Q
What is excretion?
A
- the removal of metabolic waste from the body
2
Q
What are the main excretory products?
A
- carbon dioxide from respiration
- nitrogen-containing compounds, such as urea
- other compounds, such as bile pigments found in faeces
3
Q
What excretory organs are there?
A
Lungs:
- every living cell produces carbon dioxide as a result of respiration
- it is passed on from cells into the bloodstream, where it is transported to the lungs
- diffuses into alveoli to be breathed out
Liver:
- produces bile for excretion with faeces
- converts excess amino cidsrtourea by the process of deamination (nitrogen-containing part combines with carbon dioxide)
Kidneys:
- urea passed into the bloodstream to the kidneys
- removed from blood then excreted via the urethra
Skin:
- sweat contains salts, urea, water, uric acid and ammonia
4
Q
Why is excretion important?
A
- some metabolic products are toxic
- they interfere with cell processes by altering pH,or act as inhibitors
5
Q
Why is it important to remove CO2 and nitrogenous compounds?
A
CO2:
- most carbon dioxide is transported in the blood as H2CO3 ions, which then dissociates to released hydrogen ions
- occurs inside red blood cells, under influence of enzyme carbonic anhydrase
- hydrogen ions affect pH of the cytoplasm of red blood cells
- also interact with bonds with haemoglobin, changing its 3D shape
- this reduces the affinity of haemoglobin for oxygen
- may combine with haemoglobin to form haemoglobinic acid
- carbon dioxide can also combine with haemoglobin to form carbaminohaemoglobin
- reduces oxygen transport
- reduces in blood pH may cause headaches, drowsiness, tremor etc
Nitrogenous compounds:
- body cannot store excess amino acids
- they are transported to liver and the toxic amino group is removed (deamination) to form ammonia, then converted to urea
- remaining keto acids used in respiration to release energy or to be converted to carbohydrate or fat for storage
6
Q
Why is it essential to ensure that the liver has good blood supply?
A
- hepatocytes carry out many hundred of metabolic processes
- internal structure of the liver ensures that as much blood flows past as many liver cells as possible
- enables liver cells to remove excess or unwanted substances from blood and return substances to the blood to ensure concentrations are maintained
7
Q
What two sources supply the liver with blood and what other vessel is connected to the liver?
A
Hepatic artery:
- oxygenated blood from the heart travels from the aorta via the hepatic artery into the liver
- supplies oxygen for aerobic respiration
Hepatic portal vein:
- deoxygenated blood from the digestive system enters via the hepatic portal vein
- blood is rich in products of digestion
- may also contain toxic compounds
- blood leaves liver via hepatic vein which then rejoins the vena cava and blood returns to normal circulation
Bile duct:
- carries bile from liver to gall bladder, where it is stored
- also contains some excretory products such as bilirubin
8
Q
What is the structure of the liver?
A
- liver is divided into loves which are further divided into lobules, which are cylindrical
- as hepatic artery and hepatic portal vein enter the liver, they split into smaller and smaller vessels
- blood from the two vessels is mixed and pass a chamber called a sinusoid, which is in close contact with hepatocytes
- Kupffer cells move about within the sinusoids, to breakdown and recycle old red blood cells
- bile canaliculi join to form bile duct
9
Q
What is the structure and function of liver cells?
A
- cuboidal shape with many microvilli on their surface
- many metabolic functions such as protein synthesis, transformation and storage of carbohydrates, synthesis of cholesterol and bile salts, detoxification etc
- cytoplasm is dense and specialised in certain organelles
10
Q
What does the liver do?
A
- control blood glucose levels, amino acid levels, lipid levels
- synthesis of bile, plasma proteins, cholesterol
- synthesis of red blood cells in the foetus
- storage of vitamins A, D and B12, iron, glycogen
- detoxification of alcohol, drugs
- breakdown of hormones
- destruction of red blood cells
11
Q
Describe the storage of glycogen in the liver
A
- liver stores sugars in the form of glycogen
- able to store around 100-120g of glycogen
- glycogen forms granules in the cytoplasm of hepatocytes
12
Q
Describe detoxification in the liver
A
- toxins can be rendered harmless by oxidation, reduction, methylation etc
- the enzyme catalase: converts hydrogen peroxide to oxygen and water (it has a high turnover number)
- cytochrome P450: a group of enzymes used to breakdown drugs such as cocaine
13
Q
Describe the detoxification of alcohol in the liver
A
- alcohol is broken down in the hepatocytes by the enzyme ethanol dehydrogenase
- this forms ethanal
- which is dehydrogenated further by the enzyme ethanal dehydrogenase
- this produces ethanoate which combines with coenzyme A to form acetyl CoA, used in aerobic respiration
- H atoms released from alcohol are combined with NAD to formed reduced NAD
- if the liver detoxifies too much alcohol and uses up NAD, fatty acids are converted to lipids, causes fatty liver disease
14
Q
Describe the formation of urea in the liver
A
- excess amino acids cannot be stored because amino groups are toxic
- but amino acid molecules contain a lot of energy
- so deamination occurs then the ornithine cycle
15
Q
Describe deamination in how to form urea in the liver
A
- amino group is removed and ammonia is produced
- ammonia is very soluble and highly toxic
- deamination also produces a keto acid, which can enter respiration directly to release energy
16
Q
Describe the ornithine cycle to form urea in the liver
A
- ammonia is combined with carbon dioxide to produce urea
- this then combines with the amino acid ornithine to produce citrulline
- then converted to arginine by addition of further ammonia
- the arginine is then re-converted to ornithine by the removal of urea
- urea is passed back into the blood and transported to the kidneys, where it is then filtered out of blood into urine
17
Q
What is the structure of the kidney?
A
- outer region: the cortex
- inner region: medulla
- centre: pelvis, leads to ureter
18
Q
How does the blood supply work in the kidney?
A
- renal artery splits to form many afferent arterioles, which each lead to a knot of capillaries called the glomerulus
- blood from the glomerulus continues into an efferent arteriole which carries the blood to more capillaries surrounding the rest of the tubule
- these capillaries eventually flow together into the renal vein
- each glomerulus is surrounded by the Bowman’s capsule
- fluid from the blood is pushed into the Bowman’s capsule by ultrafiltration
19
Q
Describe the barrier between the blood in the capillary and the lumen of the Bowman’s capsule
A
Endothelium of the capillary:
- narrow gaps between the cells of the endothelium of the capillary wall
- cells of the endothelium also contain pores, called fenestrations
- these gaps allow blood plasma and the substances dissolved in it to pass out of the capillary
Basement membrane:
- consists of a fine mesh of collagen fibres and glycoproteins
- acts as a filter to prevent the passage of molecule with a relative molecular mass of greater than 69000
- most proteins and red blood cells are held in the capillaries
Epithelial cells of the Bowman’s capsule:
- the cells, called podocytes, have a specialised shape
- they have finger-like projections called major processes
- on each major process are minor processes or foot processes that hold the cells away from the endothelium of the capillary
- ensure that there are gaps between the cells
20
Q
Label a diagram of a nephron
A
21
Q
What is ultrafiltration?
A
- the filtering of blood at a molecular level
- blood flows into the glomerulus through the afferent arteriole, which is wider than the efferent arteriole that carries blood away from the glomerulus
- the difference in diameters ensures that the blood in the capillaries of the glomerulus maintains a pressure higher than the pressure in the Bowman’s capsule
22
Q
What is filtered out the blood during ultrafiltration?
A
blood plasma containing dissolved substances is pushed under pressure from the capillary into the lumen of the Bowman’s capsule
- water
- amino acids
- glucose
- urea
- inorganic mineral ions (sodium, chloride, potassium)
23
Q
What is left in the capillary during ultrafiltration?
A
- red blood cells and proteins
- presence of proteins means that there is very low water potential
24
Q
What happens as the fluid from the Bowman’s capsule passes along the nephron tubule?
A
Proximal convoluted tubule:
- fluid is altered by reabsorption of all sugar, most mineral ions and some water
- cells of the tubules have a highly folded surface, increasing its surface area
Descending limb of the loop of Henle:
- water potential of the fluid is decreased by the addition of mineral ions and the removal of water
Ascending limb of the loop of Henle:
- water potential is increased as mineral ions are removed by active transport
Collecting duct:
- the water potential is decreased again by the removal of water
- final product is urine
- this process ensures that urine has a low water potential
25
Q
How are the cells lining the proximal convoluted tubule specialised for selective reabsorption?
A
- involves active transport and cotransport
- cells lining the proximal convoluted tubule are specialised for reabsorption:
- cell surface membrane in contact with tubule fluid is highly folded to formed microvilli, increasing the surface area
- cell surface membrane also contains cotransporter proteins that transport glucose or amino acids from tubule into the cell
- opposite membrane of the cell, close to the tissue fluid and blood capillaries is also folded to increase surface area. it contains sodium/potassium pumps
- cell cytoplasm has many mitochondria to produce ATP
26
Q
Describe the mechanism of selective reabsorption
A
- the movement of sodium ions and glucose into the cell is driven by the concentration gradient created by pumping sodium ions out of the cell
- sodium ions move out of the cell by facilitated diffusion bu they cotransport glucose of amino acids against their concentration gradient
- the movement of these substance reduces water potential of the cells, so water is drawn in from the tubule by osmosis
27
Q
Why does the loop of Henle have a descending limb and an ascending limb?
A
- allows mineral ions (sodium and chloride ions) to be transferred from the ascending limb to the descending limb
- overall effect s to increase the concentration of mineral ions in the tubule fluid, which has a similar effect on the concentration of mineral ions in the tissue fluid
- this gives the tissue fluid in the medulla a very low water potential
28
Q
Describe the reabsorption of water in the loop of Henle
A
- known as the hairpin countercurrent multiplier system
- as mineral ions enter the descending limb, the concentration of the fluid in the descending limb rises
- this decreases its water potential, which then becomes even more negative as it descends into the medulla
- as fluid rises up the ascending limb, mineral ions leave the fluid
- at the bases, this is by diffusion but higher up, active transport is used to move mineral ions out
- this creates a higher water potential in the fluid of the ascending limb and decreases water potential in the tissue fluid of the medulla
- as the fluid passes down the collecting duct, it passes through tissues with decreasing water potential, so there is always a water potential gradient
- water moves out of the collecting duct into the tissue fluid by osmosis
29
Q
Describe the reabsorption of water in the collecting duct
A
- at this stage, the tubule fluid still contains a lot of water and has a high water potential
- the fluid is carried back down through the medulla which as a low water potential
- water moves by osmosis from the tubule fluid into the surrounding tissue, entering the blood capillaries by osmosis
30
Q
Describe and explain a graph of concentration changes in the tubule fluid
A
31
Q
What is osmoregulation?
A
- the control of the water potential in the body
- controls levels of both water and salt in the body
32
Q
Describe the mechanism of osmoregulation
A
- kidneys alter the volume of urine produced by altering the permeability of the collecting ducts:
- if you need to conserve less water, walls of the collecting duct become less permeable, so less water is reabsorbed
- if you need to conserve more water, collecting duct walls are made more permeable
33
Q
How do the cells in the collecting duct alter permeability?
A
- respond to the level of antidiuretic hormone in the blood
- these cells have membrane-bound receptors for ADH
- ADH binds to these receptors and causes a chain of enzyme-controlled reactions inside the cell
- these cause vesicles containing water-permeable channels (aquaporins) to fuse with the cell surface membrane
- this makes the walls more permeable to water
34
Q
What happens when levels of ADH in the blood rises and falls?
A
Rises:
- more aquaporins are inserted
- more water is reabsorbed int blood by osmosis
- less urine produced and has a lower water potential
Falls:
- cell surface membrane fold inwards (invaginates) to create new vesicles that remove water-permeable channels from the membrane
- walls less permeable
- less water reabsorbed by osmosis
- greater volume of urine and higher water potential
35
Q
How is the concentration of ADH adjusted in the blood?
A
- osmoreceptors cells in the hypothalamus are sensory receptors that detect the water potential in the blood
- the cells respond to the effects of osmosis
- when water potential of the blood is low, the osmoreceptor cells lose water and shrink
- this stimulates neurosecretory cells in the hypothalamus
- the neurosecretory cells produce and release ADH
- ADH moves down the axon to the terminal bulb in the posterior pituitary gland, where it is stored in the vesicles
- when neurosecretory cells are stimulated, they carry action potential down their axons and cause the release of ADH by exocytosis
- ADH entrees blood capillaries
- transported around the body and acts on the cells of the collecting ducts
36
Q
Draw a negative feedback loop to show the control of water potential of the blood
A
37
Q
How can kidney function be assessed?
A
- by estimating the glomerular filtration rate (GFR): measures how much fluid passes into the nephrons each minute
- normal reading: 90-120cm3min-1
- and by analysing urine for substances such as proteins
38
Q
What is renal dialysis?
A
- waste products, excess fluid and mineral ions are removed from the blood by passing it over a partially permeable dialysis membrane
- dialysis fluid contains correct concentrations of mineral ions, urea, water and other substances
39
Q
Describe haemodialysis
A
- blood from an artery or vein is passed into a machine that contains an artificial dialysis membrane shaped to form many artificial capillaries, increasing surface area for exchange
- heparin is added to avoid clotting
- artificial capillaries surrounded by dialysis fluid, which flows in the opposite direction of blood (countercurrent)
- this improves efficiency of exchange
- any bubbles are removed before blood is return into the body via a vein
- haemos=dialysis is performed at a clinic 2 or 3 times a week
40
Q
Describe peritoneal dialysis
A
- dialysis membrane is the body’s own abdominal membrane (peritoneum)
- a permanent tube is implanted in the abdomen
- dialysis solution is poured through the tube and fills the space between the abdominal wall and organs
- after several hours, solution is drained from the abdomen
- can be done at home or work
41
Q
What are the advantages and disadvantages of kidney transplants?
A
Advantages:
- freedom from time-consuming renal dialysis
- feeling physically fitter
- improved quality of life: able to travel
- improved self image
Disadvantages:
- need to take immunosuppressants
- need for major surgery under general anaesthetic
- regular checks for signs of rejection
- side effects of immunosuppressents
42
Q
What can urine analysis be used to test for?
A
- glucose for diabetes
- alcohol to determine blood alcohol levels
- recreational drugs
- anabolic steroids
- human chorionic gonadotrophin (hCG) for pregnancy
43
Q
What do pregnancy tests test for and how do they work?
A
- detect the hormone human chorionic gonadotrophin (hCG)
- a stick is used with an application area that contains monoclonal antibodies for hCG bound to a blue coloured bead
- monoclonal antibodies are all identical to each other
- when urine is applied to the application area, an hCG will bind to the antibody on the beads
- the urine moves up to the test strip, carrying the beads with it
- the test strip has antibodies to hCG which are immobilised
- if there is hCG present, the test strip turns blue because the immobilised antibody binds to any hCG attached to the blue beads, concentrating the blue beads in that area
- if no hCG is present, the beads will [ass through the test area without binding to anything, so it does not become blue
44
Q
How are anabolic steroids tested?
A
- anabolic steroids have a half-life of about 16 hours and remain in the blood for many days
- they are relatively small molecules and can enter the nephron easily
- a urine sample is analysed using gas chromatography
