5.2 Excretion as an example of homeostatic control Flashcards
define excretion
the removal of metabollic waste from the body
Define metabolic waste
A substance that is produced in excess by the metabolic processes in the cells and it may become toxic
What must be excreted
carbon dioxide from respiration
nitrogen containing compounds such as urea
other compounds such as bile
what are the excretory organs
the lungs
the liver
the kidneys
the skin
what is excreted from the lungs
carbon dioxide
What is excreted from the liver
bilirubin - the bile pigment
converts excess amino acids into urea
What do the kidneys excrete
urea
What does the skin excrete
sweat which contains substances such as salts water uric acid and ammonia
Why is excretion important
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
Why must carbon dioxide be excreted
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
How is a drop in blood pH detected and counteracted
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
What can happen if blood pH drops below 7.35
It may cause headaches, drowsiness, restlessness, tremor and confusion
What can cause an increase in blood pH
Diseases or conditions which affect the lungs such as emphysema asthma or bronchitis
What is the condition when blood is acidic
respiratory acidosis
how do we excrete excess amino acids
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
Why are excess amino acids not excreted but rather deaminated
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
Give the equation for the formation of urea from amino acids
amino acid + oxygen → keto acid+ ammonia
ammonia + carbon dioxide → urea + water
How does blood flow to the liver
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
How does bood leave the liver
Hepatic vein which then rejoins the vena cava
Why are the blood vessels and chambers in the liver arranged in the way that they are
To ensure greatest possible contact between the blood and liver cells
What is the liver split into
Lobes which are further split into lobules
What do the hepatic artery and hepatic portal vein split into
as they enter the liver they split into smaller and smaller vessels which run between and parallel to the lobules known as interlobular vessels
Describe the blood flow from the hepatic artery and hepatic portal vein
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
Describe the contents of the sinusoids
lined with liver cells
contains macrophages called Kupffer cells
What is the primary function of the macrophages in the sinusoid
Their primary function appears to be breakdown and recycle of old red blood cells
What is the product of haemoglobin breakdown
bilirubin
Describe how bile is transported out of the liver
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
Give the 11 components of the liver
Hepatic artery
hepatic portal vein
interlobular vessels
lobes
lobules
sinusoids
Kupffer cells
intralobular vessel
hepatic vein
bile canaliculi
bile duct
What are the metabolic functions of the liver
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
How does the liver detoxify hydrogen peroxide
catalase enzymes convert hydrogen peroxide to hydrogen and water
How does the liver detoxify drugs
Cytochrome p450 is a group of enzymes that can breakdown drugs including cocaine. They are also used in electron transport during respiration
Why do drugs have side effects
Cytochrome p450s role in metabolising drugs can interfere with other metabolic processes causing the unwanted side effects
How does the liver detoxify alcohol
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
Explain why it would be wastefl to just excrete alcohol
Because it can be dehydrogenated to form acetate which can be used in respiration and it also forms 2NADH
Explain why someone that consumes a lot of alcohol may have fatty liver
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
What 2 processes do amino acids undergo in the liver
deamination and the ornithine cycle
Describe deamination
It removes the amino group and produces ammonia which is very soluble and toxi and keto acid which can enter respiration
Describe the ornithine cycle
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
What is the equation for what occurs in the ornithine cycle
2NH3 + CO2 → CO(NH2)2 + H20
What type of reactions occur in the ornithine cycle
condensation
Define nephron
the functional unit of the kidney
Define ultrafiltration
filtration of the blood at molecular level under pressure
describe the structure of the kidneys
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
What does the renal artery split into
many afferent arterioles which lead to a knot of capillaries in the glomerulus which then continues to the efferent arterioles
How do the afferent and efferent arterioles differ
The afferent arteriole has a large diameter
What parts of the nephron are in the cortex?
The glomerulus
proximal convoluted tubule
distal convoluted tubule
1/2 of collecting duct
What parts of the nephron are located in the medulla
The loop of Henle
descending limb
ascending limb
1/2 of collecting duct
what do the capillaries in the kidney go into
the glomerulus
What are the filtering layers in ultrafiltration
The endothelium of the capillary
the basement membrane
the epithelial cells of the Bowmanns capsule
Describe the endothelium of the capillary
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
Describe the basement membrane
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
Describe the epithelial capillaries of the Bowmann’s capsule
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
Describe how hepatocytes are adapted
They have a very dense nucleus with specialised numbers of certain organelles
Describe how pressure is maintained in the glomerulus
The afferent arteriole is wider than the efferent which tends to push blood into the Bowmann’s capsule that surrounds the glomerulus
What is filtered out of the blood
water
amino acids
glucose
urea
inorganic mineral ions such as sodium
What other pressure is acting on the capillary in the glomerulus
Hydrostatic and osmotic pressure
Why is osmotic pressure beneficial in the glomerulus
This ensures that some of the fluid is retained in the blood due to the low water potential in the capillaries
What is the funtcion of the nephrons
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
Describe how the plasma membrane of the proximal convoluted tubule closest to the tubule fluid is adapted
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
Describe how the plasma membrane of the proximal convoluted tubule closest to the tubule fluid is adapted
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
Describe how the plasma membrane closest to the tissue fluid in the proximal convoluted tubule is adapted
It is also folded to increase its surface area
It contains sodium/potassium pumps
escribe how the cells lining the proximal convoluted tubule are adapted
They contain many mitochondira
Describe the mechanism of reabsorption in the proximal convoluted tubule
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
How are large molecules reabsorbed
endocytosis
Why do mammals need to reabsorb water
The mammal would become dehydrated very quickly otherwise
Why does urea concentration increase in the nephron past the loop of henle
Water is withdrawn from the tubule so the urea concentration increases
Describe how water is reabsorbed in the loop of henle
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
Describe the purpose of the distal convoluted tubule
active transport is used to adjust concentrations of various mineral ions
Why does water potential of the tissue fluid in the medulla get more negative the lower down the medulla
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
What is the effect of alcohol on water absorption
It inhibits the release of ADH which can make you dehydrated
What chain of events occurs when there is a very low water concentration in the blood
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
What chain of vents occurs when water concentration in the blood is too high
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
What happens to excess ADH in the body
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
Why must excess ADH get broken down
So that water potential of the blood doesnt get too high and cells do not undergo lysis
Define hypertonic
High concentration of solutes
Define hypotonic
Low concentration of solute
Define kidney failure
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
How can we assess kidney function
By estimating the glomerular filtration rate and by analysing urine for substances such as proteins
Define glomerular filtration rate
it is a measure of how much fluid passes the nephrons per minute and a normal reading is 90-120 cm/min
What are the causes of kidney failure
diabetes mellitus
heart disease
hypertension and infection
What are the main treatments for kidney failure
renal dialysis and kidney transplant
What are the 2 types of renal dialysis
haemodialysis
peritoneal dialysis
Describe haemodialysis
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
What is added to the blood during haemodialysis
Heparin to prevent clotting and air bubbles are removed
Describe peritoneal dialysis
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
What must be combined with peritoneal dialysis and why
A balanced diet and not to much protein as it turns into urea
What are the advantages of a kidney transplant
Freedom from time-consuming dialysis
feeling physically fitter
better self image
improved quality of life
What are the disadvantages of a kidney transplant
Need to take immunosuppressants and their side effects
need for major surgery
need regular check for rejection
Describe urine analysis
Urine can be tested for molecules with an Mr less than 69000
such as glucose, alcohol, many recreational drugs, anabolic steroids
Describe pregnancy testing
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
Describe testing for anabolic steroids
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
