Excretion definition:
The removal of metabolic waste from the body
Metabolic waste definition:
A substance that is produced in excess by the metabolic presses in the cells; it may become toxic
What are the main excretory products?
- Carbon dioxide from respiration
- Nitrogen-containing products such as urea
- Other compounds, such as the bile pigments found in faeces
Egestion definition:
The elimination of faeces from the body
What are the excretory organs?
- The lungs
- The liver
- The kidneys
- The skin
How do the lungs excrete carbon dioxide?
Carbon dioxide diffuses into the alveoli to be excreted as you breathe out.
How does the liver excrete substances?
- Some substances produced will be passed into the bile for excretion with the faeces, e.g. the pigment bilirubin
- The liver is also involved in converting excess amino acids into urea through deamination and the ornithine cycle
How are the kidneys involved in excretion?
-Urea is removed from the blood to become part of the urine
How is the skin involved in excretion?
Sweating
Why is excretion important?
Allowing the products of metabolism to build up could be fatal
Why are there hydrogen ions in the blood?
Carbon dioxide mostly travels in the blood as hydrogencarbonate ions, but forming this also creates hydrogen ions
What do hydrogen ions change the PH of?
The cytoplasm of red blood cells
What can happen if the PH of the blood drops below 7.35?
-Headaches
-Drowsiness
-Confusion
-Restlessness
-Tremor
-Rapid heart rate
-Changes in blood pressure
This is respiratory acidosis
What causes respiratory acidosis?
Blockage of the airway
Hepatocytes definition:
Liver cells
What are the blood vessels going to and from the liver?
To: -Hepatic artery -Hepatic portal vein Away: -Hepatic vein
What does the hepatic artery do?
Supplies the liver with oxygen for aerobic respiration. it needs a lot of oxygen to carry out all the metabolic processes
What does the hepatic portal vein do?
- Carries deoxygenated blood from the intestine to the liver
- The blood is rich in the products from digestion
- May also contain toxic compounds that have been absorbed from the intestine
- Important that this blood does not circle the rest of the body before the concentrations of these substances have been adjusted.
What does the hepatic vein do?
- Blood leaves liver via hepatic vein
- Joins up with vena cava
What does the bile duct do?
- Carries bile from the liver to the gall bladder where it is stored until required to aid the digestion of fats in the small intestine
- Bile also contains some excretory products such as bile pigments like bilirubin which will leave the body with the faeces
How is the liver arranged?
- To ensure the greatest possible contact between the blood and the hepatocytes
- The liver is divided into lobes which are further divided into cylindrical lobules
What happens to the hepatic artery and hepatic portal vein as they enter the liver?
- They split into smaller and smaller vessles
- The vessels run between and parallel to the lobules
- They are known as inter-lobular vessels
What is a sinusoid?
- A chamber lined with liver cells
- At intervals in the lobules, blood from the hepatic artery and the hepatic portal vein enter, mix, and then pass down a sinusoid
- As the blood flows along a sinusoid it is in close contact with the liver cells, which are able to remove substances from the blood, and return other substances to the blood
- The concentrations of many components are modified along the sinusoid
Kupffer cells definition:
Specialised macrophages
What do kupffer cells do?
- Move about in sinusoids
- Break down and recycle old RBCs
Why is bilirubin formed?
It is one of the products of the breakdown of haemoglobin
Where is bile made?
- Liver cells
- Is then released into the bile canaliculi
- The bile canaliculi join together to form the bile duct which transports the bile to the gall bladder
Where do the sinusoids empty into?
The hepatic vein (intra-lobular vessel)
Ornithine cycle definition:
A series of biochemical reactions that convert ammonia to urea
Functions of the liver:
- Control of blood glucose levels, amino acid levels, lipid levels
- Synthesis of bile, plasma proteins, cholesterol
- Synthesis of RBCs in the foetus
- Storage of vitamins A, D and B12, iron, glycogen
- Detoxification of alcohol and drugs
- Breakdown of hormones
- Destruction of RBCs
Where is glycogen stored?
The liver
What are the enzymes in the liver that can render toxic molecules less toxic?
- Catalase
- Cytochrome P450
What does catalase do in the liver?
- Converts hydrogen peroxide into oxygen and water
- High turnover number of 5 million (per second)
What does cytochrome P450 do in the liver?
- Group of enzymes used to break down drugs
- The role in metabolising drugs can interfere with other metabolic roles and cause the unwanted side effects of some medicinal drugs
How does the detoxification of alcohol work?
- EthanOl dehydrogenase breaks down ethanOl into ethanAl and hydrogen is released and combined with the enzyme NAD to form reduced NAD
- EthanAl dehydrogenase breaks down ethanAl into athanoic acid and hydrogen is released and combined with the enzyme NAD to form reduced NAD
- Ethanoic acid becomes acetyl coenzyme A which enters aerobic respiration
Why might NAD run out?
- Used to detoxify alcohol and break down fatty acids
- If a lot of alcohol is drunk, there won’t be enough to break down the fatty acids
- The fatty acids are then converted back into lipids and stored as fats in the hepatocytes, causing the liver to become enlarged (fatty liver)
- This can lead to alcohol-related hepatits or to cirrhosis
What is the word equation for the formation of urea?
amino acid - ammonia + keto acid - urea
What processes are used in the formation of urea?
- Deamination
- The ornithine cycle
How does deamination work?
- Removes the amino group from an amino acid and produces ammonia
- Ammonia is very soluble and highly toxic so can’t accumulate
- Keto acid (an organic compound) is also produced and can enter respiration directly
How does the ornithine cycle work?
- Ammonia is combines with carbon dioxide and ornithine to produce citrulline and water
- Citrulline then combines with ammonia to produce arginine and water
- Arginine then combines with water, and urea is removed, to form ornithine again
Nephron definition:
The functional unit of a kidney
Ultrafiltratin definition:
Filtration of the blood on a molecular level
What are the regions of the kidney?
- Capsule
- Cortex
- Medulla
- Pelvis
What are the three layers between the blood in the capillary and the lumen of the Bowman’s capsule?
- The endothelium of the capillary
- The basement membrane
- The epithelial cells of the Bowman’s capsule
How does the endothelium of the capillary enable ultrafiltration?
- Narrow gaps between the cells of the endothelium of the capillary wall
- The cells of the endothelium also contain pores
- Allows blood plasma and the substances dissolved in it out of the capillary
How does the basement membrane enable ultrfiltration?
- Fine mesh of collagen fibres and glycoproteins
- Prevents the passage of molecules with a molecular mass of 69,000 or greater
How do the epithelial cells of the Bowman’s capsule enable ultrafiltration?
- Specialised shape with finger like projections (major processes)
- On the major processes there are minor processes that hold the cells away from the endothelium of the capillary
- These projections ensure there are gaps between the cells
- Fluid can pass through
How are glucose and amino acids selectively reabsorbed in the kidney?
- Sodium ions are actively pumped out of the cells lining the tubule
- Concentration of sodium ions in cell cytoplasm decreases, creating a concentration gradient
- Sodium ions diffuse through a contransport protein, carrying a glucose molecule or amino acid with them
- Water moves into the cell by osmosis
- Glucose and amino acids diffuse into the blood
What happens in the loop of Henle?
Descending limb:
-Diffusion of sodium and chloride ions into the descending limb
-Water moves out by osmosis and enters capillaries
Ascending limb:
-Sodium and chloride ions diffuse out of the lower part of the ascending limb
-Sodium and chloride ions are actively transported out of the higher parts of the ascending limb
Collecting duct:
-Water moves out of collecting duct by osmosis and enters capillaries
The concentration in the medulla increases the further into the medulla, which causes water to keep travelling out by osmosis
Antiduretic hormone (ADH) defintion:
A hormone that controls the permeability of the collecting duct walls
Osmoreceptor definition:
A sensory receptor that detects changes in water potential
Where does the body gain water from?
- Food
- Drink
- Metabolism
Where does the body lose water from?
- Urine
- Sweat
- Water vapour in exhaled air
- Faeces
What happens to the walls of the collecting duct if you need to conserve less water?
They become less permeable
What happens to the walls of the collecting duct if you need to conserve more water?
They become more permeable
How is the permeability of the walls of the collecting duct altered if the water potential in the collecting duct is too high?
- ADH binds to membrane bound receptors on the walls of the collecting duct, causing a chain of enzyme controlled reactions in the cell
- The end result of the reactions is to cause vesicles containing water permeable channels (aquaporins) to fuse with the cell surface membrane, making the walls more permeable to water
When are more water permeable channels inserted into the walls of the collecting duct?
When the levels of ADH in the blood rise
How is the permeability of the walls of the collecting duct altered if the water potential in the collecting duct is too low?
- The level of ADH in the blood falls
- The cell surface membrane folds inwards to create new vesicles that remove water-permeable channels from the membrane
Where are osmoreceptors located?
The hypothalamus
What happens to the osmoreceptors when the water potential of the blood is low?
They lose water by osmosis and shrink, stimulating neurosecretory cells in the hypothalmus
Neurosecretory cells definition:
-Specialised neurones that produce and release ADH
Where is ADH manufactured?
In the cells bodies of neurosecretory cells. The cell body lies in the hypothalamus
How is ADH secreted?
- Moves down the axon to the terminal bulb in the posterior pituitary gland where it is stored in vesicles
- When the neurosecretory cells are stimulated by the osmoreceptors, they carry action potentials down their axons and cause the release of ADH by exocytosis
How long is the half life of ADH
20 minutes
Glomerular filtrate rate (GFR) definition:
The rate at which fluid enters the nephrons
Monoclonal antibodies definition:
Antibodies made from one type of cell- they are specific to one complementary molecule
Renal dialysis definition:
A mechanism used to artificially regulate the concentrations of solutions in the blood
How can kidney failure be assessed?
Glomerular filtrate rate. If it is too low, it indicates a problem
Causes of kidney failure:
- Diabetes (types 1 and 2)
- Heart disease
- Hypertension (high blood pressure)
- Infection
How does renal dialysis work?
- Waste products, excess fluids and mineral ions are removed from the blood by passing it over a partially permeable dialysis membrane which allows the exchange of substances between the blood and the dialysis fluid
- Patients must have a carefully monitored diet
What does dialysis fluid contain?
The correct concentrations of the substances found in the blood
How does haemodialysis work?
- Blood from an artery or vein is passed into a machine that contains an artificial dialysis membrane shaped to form many capillaries to increase the surface area for exchange
- Herapin is added to avoid clotting
- The dialysis fluid flows in the opposite direction to the blood (countercurrent system) which increases the efficiency of the exchange
- Any bubbles are removed before the blood is returned to the body via a vein
Where is haeomodialysis performed?
Usually at a clinic for a few hours a week, but some patients are trained to carry it out at home
How does peritoneal dialysis (PD) work?
- The dialysis membrane is the body’s own abdominal membrane (peritoneum)
- A surgeon implants a permanent tube in the abdomen.
- Dialysis fluid is poured through the tubes and fills the space between the abdominal wall and the organs
- After several hours, the solution is then drained
Where is PD performed?
At home or at work. The patient can walk around while it is being carried out, so it is sometimes called ambulatory PD
Advantaged of kidney transplant:
- Freedom from time consuming renal dialysis
- Feel fitter
- improved quality of life- able to travel
- Improved self-image- no longer chronically ill
Disadvantages of kidney transplant:
- Major operation
- Need to take immunosuppressant drugs
- Need for regular checks and signs of rejection
- Side effects of immunosuppressant drugs- fluid retention, high blood pressure, susceptibility to infections
How small does a molecule have to be to enter the nephron?
Relative molecular mass of less than 69,000
What can urine be tested for?
- Diabetes
- Alcohol
- Recreational drugs
- Human chorionic gonadotrophin (hCG) in pregnancy testing
- Anabolic steroids
What do pregnancy testing kits use?
Monoclonal antibodies that bind to hCG in urine
How does a pregnancy test work?
- Urine poured onto stick
- hCG binds to mobile antibodies attached to a blue bead
- Mobile antibodies move down test stick
- If hCG is present, it binds to fixed antibodies, holding blue bead in place
- Mobile antibodies with no hCG bind to another fixed site to show the test is working
How are anabolic steroids tested for?
Analysing urine using gas chromatography
