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
20
Q
Label a diagram of a nephron
A
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
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
30
Q
Describe and explain a graph of concentration changes in the tubule fluid
A
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
36
Q
Draw a negative feedback loop to show the control of water potential of the blood
A
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
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
