Excretion Flashcards
What is excretion
The removal of waste products
Examples of waste products
Nitrogenous waste
Carbon dioxide
Bile pigments
What happens if waste products aren’t removed
They can become toxic
Describe carbon dioxide as a waste product
Carbon dioxide is a waste product of respiration
It is excreted by the lungs
Describe urea as a waste product
Excess amino acids are broken down in the liver to ammonia and then urea (because ammonia is toxic).
Urea is excreted through the kidneys
Describe bile pigments as a waste product
Formed by the breakdown of haemoglobin from old red blood cells in the liver
Excreted in the bile from the liver into the small intestine via the gall bladder
What is the function of the liver
Formation of urea
Stores glycogen
Detoxification
How is the liver adapted to its function
Contains a range of different enzymes
Very large organ
Good blood supply (blood carried in by both the hepatic artery and hepatic portal vein, carried out by hepatic vein)
Structure of the liver
Cells called hepatocytes
Many mitochondria, large nuclei and prominent golgi apparatus which enable a high metabolic rate
The liver is made of hexagonal structures called lobules
Hepatic artery and portal vein are on the outside, arterial has a thicker wall.
There are kupffer cells within sinusoids which allow protection against disease by engulfing any foreign particles
Hepatocytes produce bile which is secreted into the canaliculi where it then runs to the bile duct and into the gall bladder for storage
How the liver works
Blood delivered to the liver through the hepatic portal vein and hepatic artery mixed in sinusoids (the spaces that are surrounded by hepatocytes)
This allows the highly oxygenated blood can mix with the deoxygenated blood from the hepatic portal vein
This blood runs through sinusoids and drains into the hepatic vein where it is transported away
Describe the livers role of glycogen storage
In response to insulin, hepatocytes can absorb excess glucose from the blood and convert it into glycogen.
In response to glucagon, hepatocytes hydrolyse glycogen back into glucose and release it into the blood
Describe the livers role of detoxification
Detoxification is the neutralisation and break down of unwanted chemicals such as alcohol, drugs, hormones and toxins produced in chemical reactions in the body. The liver contains enzymes to break these toxins into non toxic substances
Describe the livers role in the formation of urea
(The ornithine cycle) is how urea is produced from ammonia to be transported to the kidneys and excreted.
Excess proteins from our diet can’t be stored so they are delivered to the liver to be deaminated (when the amine group in removed). Ammonia is highly toxic which is why it is converted to urea before being transported in the blood. Urea is also toxic but only in very high concentrations
Roles of the kidney
Osmotegulation
Excretion of nitrogenous waste
Parts of the kidney
Renal artery- supplies kidney with the blood to be filtered
Renal vein- carries filtered blood away from the kidney
Cortex- dark outer layer (contains many capillary networks carrying blood from renal arteries to nephrons)
Medulla- contains the nephrons
Pelvis- where the urine collects before leaving the kidney and travelling to the ureter
Structure of the nephron
Bowman’s capsule- Contains glomerulus (tangle of capillaries) where ultrafiltration occurs
Proximal convoluted tubule- Where substances needed by the body (eg glucose) are reabsorbed into the blood
Loop on henle- Sodium ions are transported out of the ascending limb into the medulla to create a low water potential (high solute conc in the tissue fluid of the medulla)
Distal convoluted tubule- Selectively secretes and absorbs different ions to maintain blood pH and electrolyte balance
Collecting ducts- Reabsorbs solutes from the filtrates passes through to the pelvis
Describe ultrafiltration as a function of the kidney
Blood enters through the agreement arterials and this splits into smaller capillaries which make up the glomerulus
This causes a high hydrostatic pressure of the blood, water and small molecules such as glucose and mineral ions are forced out of the capillaries and form the glomerulus filtrate
This fluid then passes through the basement membrane (made of a network of collagen fibres and proteins) which acts like a sieve,
Podocytes in the bowman’s capsule wall are special cells that can act as additional filters, large protein and blood cells are too big to fit through the gaps in the capillary endothelium so remain in the blood (this blood leaves through a different arteriole)
Describe selective reabsorption as a function of the kidney
After the glomerulus filtrate is formed, it enters the proximal convoluted tubule, then the filtrate is absorbed back into blood which leaves urea, excess mineral ions and water behind.
Glucose is reabsorbed using co transport, concentration of sodium ions in the PCT cell is decreased as the sodium ions are actively transported out of the PCT cells into the blood in the capillaries. Due to the concentration gradient, sodium ions diffuse down the gradient from the lumen of the PCT into the cells lining it. The proteins which transport the sodium ions carry glucose with it
What is the main role of the loop of henle
the loop of henle maintains the sodium ion gradient and therefore water being reabsorbed.
How does the loop of henle maintain a sodium ion gradient
The filtrate passes into the loop of henle from the PCT into the decending limb, mitochondria in the walls of the cells allow for the active transport of sodium ions oht of the ascending limb of the loop of henle
The accumulation of sodium ions outside the nephron lowers the water potential of the medulla (in comparison to the filtrate) therefore the water diffuses out by osmosis into the inserstitial space and then to capillaries:
At the base of the ascending limb, some sodium ions are transported out by diffusion
The filtrate reaches the top of the loop and is very dilute, the filtrate then moves into the DCT and collecting duct, this section of the medulla is very concentrated therefore even more water diffuses out of the DCT and collecting duct. What remains is transported to form urine
How does the hypothalamus control the water potential of the blood
Changes in the water potential are detected by the osmoreceptors in the hypothalamus
If water potential is too low, water leaves osmoreceptors via osmosis and they shrivel, this stimulates the hypothalamus to produce more of the hormone ADH
If water potential is too high, water enters the osmoreceptors by osmosis which stimulates the hypothalamus to produce less ADH
How does ADH work
The hypothalamus produces ADH, it then moves to the posterior pituitary and is released from there into capillaries where it then moves through the blood to the kidney.
ADH will bind to complementary receptors that are only located on target cells in the DCT and collecting duct
When it binds it activates adenyl cyclades to make cAMP which activates an enzyme which causes vesicles containing aquaporins(channel proteins) to fuse with the membrane which allow water to transport across the membrane so the membrane becomes more permeable to water and more will leave to be reabsorbed back into the blood
Summarise the negative feedback loop in the kidneys (too much water)
Water potential of blood increases
Detected by osmoreceptors in the hypothalamus
Water moves via osmosis into the osmoreceptors
Hypothalamus detects this and produces less ADH which is released by the posterior pituitary gland
ADH moves into the blood and travels to the kidneys
Less ADH binding to the DCT and collecting duct walls mean less vesicles containing aquaporin and therefore the walls are less permeable to water
Less water is reabsorbed into the blood therefore there is more in the urine.
Back to normal
Summarise the negative feedback loop in the kidneys (not enough water)
Water potential of blood is too low
Osmoregulators detect this and water moves out and into the blood via osmosis
Osmoregulators shrivel and hypothalamus detects this and releases more ADH
The ADH goes from the posterior pituitary gland and into the blood, this travels to the kidneys
More ADH binds to the DCT and the collecting duct which means that more vesicles with aquaporin are released so the walls are more permeable to water.
More water will be reabsorbed and less will come out as urine
How is urine used for diagnosis
Can be used to test for diabetes, pregnancy, anabolic steroids and drugs
A pregnancy test uses monoclonal antibodies to detect the presence of the human growth hormone which is produced by pregnant woman.
A urine sample is tested, the absorbent end is submerged in urine
First mobile antibody is complimentary to HGH has a coloured dye attached
A second antibody complimentary to the antigen is immobilised in the test
A third antibody is immobilised and is complimentary to the first antibody
Causes of kidney failure
Infection
High blood pressure
Genetic conditions
Physical damage
Consequences of kidney failure
Can damage tubules, podocytes, epithelial cells and basement membranes of the bowman’s capsule. So large molecules will filter out of the blood (such as proteins and red blood cells)
If the kidney completely fails, blood will not be filtered correctly which can lead to a build up of urea in the blood and an imbalance of electrolytes
The glomerular filtrate rate is affected by kidney failure and can be measured as an indication of disease (measured by testing for creative levels of blood, if it increases it indicates that the kidneys aren’t filtering properly)
Treatments of kidney failure
Haemodialysis (involves a dialysis machine)
Peritoneal dialysis (occurs inside the body)
Transplant
Describe the process of haemodialysis
Blood enters machine on the opposite side to the dialysis fluid which is separated from the blood by a partially permeable membrane
The dialysis fluid contains no urea to ensure a strep concentration gradient so all urea diffuses out
Dialysis fluid has normal plasma levels for glucose and minerals to ensure no net movement of glucose out of the blood and that only mineral ions are removed from the body
This process takes hours many times a week and a careful diet must be put in place to keep blood levels as constant as possible
Describe the process of peritoneal dialysis
Utilises the peritoneum (the lining of the abdomen) which acts as a natural dialysis membrane.
Dialysis fluid is introduced into the abdomen using a catheter, the fluid is left inside for several hours during which the patient can carry on with daily activities. During this time, urea and excess mineral ions move out of the blood and into the tissue fluid then out across the peritoneal membrane and into the dialysis fluid
At the end the fluid is drained out of the abdomen and discarded, leaving the blood in the body filtered.
Pros and Cons of transplant
Best treatment
Lack of available donors
Risk of organ rejection (immunosuppressant drugs would need to be taken for life which makes the patient more prone to illness)
Most transplanted kidneys only work around 10 years
