Excretion Flashcards
What is metabolic waste?
Waste substances that have been produced by metabolic reactions inside cells, and may be toxic, or unwanted by the cells
What is excretion?
The removal of metabolic waste from the body
What are the two main substances that need to be excreted?
Carbon dioxide from respiration
Nitrogen containing compounds such as urea
Where is carbon dioxide produced?
In every living cell in the body as a result of respiration
Where is urea produced?
In the liver from amino acids
What is deamination?
The removal of the amine group from an amino acid, to produce ammonia
Where is carbon dioxide excreted?
It is passed from the cells of restoring tissues to the bloodstream. It is transported in the blood to the lungs, where it diffuses to the alveoli to be excreted as we breathe out
Where is urea excreted?
Urea is produced in the liver and then passed into the bloodstream to be transported to the kidneys. In the kidneys it is removed from the blood to become part of the urine, which is stored in the bladder before being excreted
Why must excess carbon dioxide be excreted?
Excess carbon dioxide is toxic. A high level of carbon dioxide has two main effects:
•it reduces the ability of haemoglobin to carry oxygen
•it decreases the pH of the blood, making it more acidic
Why are the amine groups removed from excess amino acids in the liver?
The body cannot store amino acids, but they contain almost as much energy as carbohydrates. Therefore they are transported to the liver and the potentially toxic amine group is removed, allowing the remaining organic acids to be used directly in respiration or to be converted to a carbohydrate or fat for storage
What are hepatocytes?
Liver cells
What are all of the veins, arteries and ducts connected to the liver?
The hepatic artery
The hepatic vein
The hepatic portal vein
The bile duct
What does he hepatic artery do?
It supplies the liver with oxygenated blood from the heart, so it has a good supply of oxygen for respiration
What does the hepatic vein do?
It takes deoxygenated blood away from the liver
What does the hepatic portal vein do?
It brings blood from the small intestine, which is rich in the products of digestion. This means that concentrations of various compounds are uncontrolled, and the blood may contain toxic compounds from the intestine
What does the bile duct do?
It takes bile to the gall bladder to be stored
What is bile?
A substance produced by the liver to emulsify fats
What are the structures that the liver is divided into?
Liver lobules
What is the structure of a liver lobule?
Each lobule is made up of rows of hepatocytes radiating out from a central vein, which connects to the hepatic vein. This is called the intra-lobular vessel
What are inter-lobular vessels?
Branches of the hepatic artery and hepatic portal vein which run between different liver lobules. The inter-lobular vessels enter the lobules at intervals, and merge into chambers called sinusoids, allowing the oxygenated blood from the hepatic artery to mix with the blood rich in digestion products from the hepatic portal vein. These sinusoids drain into the intra-lobular vessel
What is the function of the sinusoids?
The sinusoids are lined with hepatocytes, which remove oxygen and harmful compounds from the blood flowing through the sinusoids, breaking down the harmful substances to make them less harmful
Where are kupffer cells found and what do they do?
They are found in the sinusoids inside liver lobules and they move around within the sinusoids, removing bacteria and breaking down old red blood cells
Where does the bile produced in the hepatocytes go?
It is released into the bile canaliculi, which are found inside liver lobules and all join together to form the bile duct, which transports the bile to the gall bladder
What are the metabolic functions of hepatocytes?
- Protein synthesis
- Transformation & storage of carbohydrates
- Synthesis of cholesterol and bile salts
- detoxification
How are hepatocytes specialised?
They have may microvilli on their surfaces, and very dense cytoplasm, with specialised amounts of certain organelles
What are the metabolic functions of the liver
Control of: blood glucose levels, amino acid levels, lipid levels
Synthesis of: red blood cells in the fetus, bile, plasma proteins, cholesterol
Storage of: vitamins A,D and B12, iron, glycogen
Detoxification of: alcohol, drugs
Breakdown of hormones
Destruction of red blood cells
How are excess amino acids broken down by the liver?
By the process of deamination followed by the ornithine cycle
What is the equation for the process of deamination?
Amino acids->ammonia+organic acids
What is the equation that sums up the ornithine cycle?
Ammonia+CO2->urea+water
What does the ornithine cycle do?
It combines ammonia with CO2 to break it down into urea and water
Why must ammonia be converted to urea?
Because it is very soluble and highly toxic, so must be converted into the less soluble and less toxic urea.
Why does alcohol need to be broken down by the liver?
It is a toxic substance that can damage cells, however it also contains a lot of chemical potential energy.
How is alcohol broken down by the liver?
Alcohol (ethanol) is broken down in the hepatocytes into ethanal. This releases two hydrogen ions which are accepted by NAD, reducing it. Ethanal is then dehydrogenated further into ethanoic (acetic) acid, releasing two more hydrogen ions which reduce two more molecules of NAD. The final compound produced is acetate, which is combined with coenzyme A and used in respiration
How can excess alcohol consumption lead to liver cirrhosis?
NAD is also required to oxidise and break down fatty acids for use in respiration. If the liver has to detoxify too much alcohol it has insufficient NAD to deal with the fatty acids. This means that the fatty acids are converted back to lipids and are stored in hepatocytes, causing the liver to become enlarged. This is a condition known as fatty liver, and can lead to liver cirrhosis
Why does paracetamol need to he broken down by the liver?
Excess paracetamol in the blood can lead to liver and kidney failure
Why does insulin need to be broken down by the liver?
Excess insulin can cause problems with blood sugar levels
What are nephrons?
Microscopic tubules in the kidney that receive fluid from the blood capillaries in the cortex and convert it to urine which drains into the ureter
What is the glomerulus?
A fine network of capillaries that work to increase local blood pressure, meaning fluid is squeezed out of the blood to be filtered in the kidney. The glomerulus is surrounded by a capsule which collects the fluid and leads to the nephron
Where is the kidney found?
Most people have two kidneys and they are positioned on either side of the spine just below the lowest rib. Each kidney is supplied with blood from a renal artery and is drained by a renal vein.
What is the role of the kidney?
To remove waste products from the blood and produce urine
What is the structure of the kidney?
The kidney consists of an inner region, called the medulla and an outer region, called the cortex. It the centre is the pelvis, which leads to the ureter. The whole of the kidney is surrounded by a tough capsule
What does the bulk of the kidney consist of?
Tiny tubules called nephrons. There are about one million of them in each kidney
Where does each nephron start?
In the cortex, where the capillaries form a knot called the glomerulus, which is surrounded by a cup shaped structure called the bowman’s capsule. Fluid from the blood is pushed into the bowman’s capsule by the process of ultrafiltration. This fluid then drains into the nephron
What is ultrafiltration?
Filtration at a molecular level, such as in the glomerulus, where large molecules and cells are left in the blood and smaller molecules pass into the bowman’s capsule
What are the four parts of the nephron?
- the proximal convoluted tubule
- the loop of henle
- the distal convoluted tubule
- the collecting duct
What causes the composition of the fluid inside the nephrons to change as it moves along?
selective reabsorption of substances back into the tissue fluid and blood capillaries surrounding the nephron tubule
Where does most of the fluid in the nephron get absorbed from?
The proximal convoluted tubule, where 85% of the fluid is reabsorbed
How does the composition of the fluid inside the nephrons change?
- in the proximal convoluted tubule all sugars, most salts and some water are reabsorbed - about 85% of the total fluid
- in the descending limb of the loop of henle the water potential of the fluid is decreased by addition of salts & removal of water
- in the ascending limb salts are removed by active transport
- in the collecting duct the water potential is decreased again by the removal of water, so the urine produced has a low water potential&a high concentration of solutes
How is a high blood pressure maintained in the glomerulus?
The afferent arteriole, leading to the glomerulus, is wider than the efferent arteriole, leading away. This ensures that the blood in the glomerulus is under increased pressure
Where does the fluid pushed out of the blood in the glomerulus go?
Into the bowman’s capsule
What are the three layers of barriers between the blood in the capillaries of the glomerulus and the lumen of the bowman’s capsule?
- the endothelium of the capillaries
- the basement membrane
- the epithelial cells of the bowman’s capsule
How is the endothelium of the capillaries in the glomerulus adapted to allow ultrafiltration?
There are narrow gaps between the cells that blood plasma, and the substances dissolved in it, can pass through
How is the basement membrane between the glomerulus and the bowman’s capsule adapted to allow ultrafiltration?
It consists of a fine mesh of collagen fibres and glycoproteins. These act as a filter to prevent the passage of molecules with a relative molecular mass of greater than 69000. This means that all blood cells and most proteins are held in the capillaries of the glomerulus
How are the epithelial cells of the bowman’s capsule adapted to allow ultrafiltration?
The epithelial cells are called podocytes. Podocytes have a very specialised shape, with many finger-like projections called major processes. These ensure that there are gaps between the cells, so that fluid from the blood in the glomerulus can pass between the cells into the lumen of the bowman’s capsule
What is filtered out of the blood into the bowman’s capsule?
- water
- amino acids
- glucose
- urea
- inorganic ions (sodium, chloride, potassium)
What substances are not filtered out of the blood and into the bowman’s capsule during ultrafiltration?
The blood cells and proteins, which are too large to pass through the basement membrane. The presence of the proteins means that the blood has a very low water potential. This causes some of the fluid that could have been filtered into the bowman’s capsule to remain in the blood. This includes some of the water and some of the substances dissolved in it
How are the cells lining the proximal convoluted tubule specialised for reabsorption?
The cell surface membrane in contact with the tubule fluid is folded into microvilli, for increased surface area.
This membrane also contains co-transporter proteins that transport glucose or amino acids, in association with sodium ions, into the cell (facilitated diffusion).
The opposite cell membrane is also folded to increase surface area. This membrane contains sodium potassium pumps that pump sodium ions out of the cell & potassium ions in.
The cell cytoplasm has many mitochondria for active processes.
What substances are reabsorbed in the proximal convoluted tubule?
All the glucose and amino acids, some salts and some water
How does reabsorption of substances from the tubule fluid in the proximal convoluted tubule occur?
sodium potassium pumps remove sodium from the cells lining the PCT. This reduces the concentration of sodium ions in the cytoplasm.
Sodium ions diffuse down their concentration gradient from the tubule fluid into cells. Glucose and amino acid molecules are transported in association, by facilitated diffusion.
As glucose &amino acid concentrations rise in the cell, these substances can diffuse out of the back of the cell into tissue fluid, and then into the blood where they are carried away.
Reabsorption of salts, glucose and amino acids reduces the water potential of the cell + increases water potential in the tubule, so water enters the cells & is reabsorbed into the blood by osmosis.
Large molecules such as small proteins will be reabsorbed by endocytosis
What are microvilli?
Microscopic folds of the cell surface membrane that increase the surface area of the cell
What are co-transporter proteins?
Proteins in the cell surface membrane that allow the facilitated diffusion of simple ions to be accompanied by transport of a larger molecule such as glucose
What is facilitated diffusion?
Diffusion that is enhanced by the action of proteins in the cell membrane
What are sodium potassium pumps?
Special proteins in the cell surface membrane that actively transport sodium and potassium ions against their concentration gradients
How is glucose reabsorbed from the tubule fluid into cells in the proximal convoluted tubule?
There is a concentration gradient for sodium ions to move from the tubule fluid into cells, so sodium ions diffuse through co-transporter proteins by facilitated diffusion. These co-transporter proteins are associated with glucose molecules, meaning they are transported into the cells along with the sodium.
Why is there a concentration gradient of sodium ions from the tubule fluid into the cells lining the proximal convoluted tubule?
Because the sodium potassium pumps on the opposite membrane of the cells lining the PCT are actively transporting sodium ions out of the cells and into the blood
How are amino acids reabsorbed from the tubule fluid into cells in the proximal convoluted tubule?
There is a concentration gradient for sodium ions to move from the tubule fluid into cells, so sodium ions diffuse through co-transporter proteins by facilitated diffusion. These co-transporter proteins are associated with amino acid molecules, meaning they are transported into the cells along with the sodium.
Why do cells lining the proximal convoluted tubule have a lower water potential than the tubule fluid?
Because salts, glucose and amino acids have been reabsorbed from the tubule fluid into the cells
What is osmoregulation?
The control and regulation of the water potential of the blood and body fluids. In humans the kidney controls the water potential of the blood
What is a hairpin countercurrent multiplier?
The arrangement of a tubule in a sharp hairpin so that one part of the tubule passes close to another part of the tubule with the fluid flowing in opposite directions. This allows exchange between the contents and can be used to create a very high concentration of solutes
What is the role of the loop of henle?
To create a very low water potential in the tissue of the medulla. This ensures that even more water can be reabsorbed from the fluid in the collecting duct.
What is the structure of the loop of henle?
It is arranged in a hairpin countercurrent multiplier, with a descending limb that descends into the medulla, and an ascending limb that ascends back out to the cortex
How does the arrangement of the loop of henle assist its function?
It allows salts to be transferred from the ascending limb to the descending limb
How does the water potential of the fluid in the descending limb of the loop of henle change as it moves deeper into the medulla?
It becomes more negative due to loss of water by osmosis to the surrounding tissue fluid and diffusion of sodium and chloride ions into the tubule from the surrounding tissue fluid
What happens to the water potential of the fluid in the ascending limb of the loop of henle as it moves back towards the cortex?
It becomes less negative because:
•at the base of the tubule sodium and chloride ions diffuse out into the tissue fluid
•higher up in the tubule, sodium and chloride ions are actively transported out into the tissue fluid
•the wall of the ascending limb is impermeable to water, so water cannot leave the tubule
•the fluid therefore loses salts but not water as it moves up the ascending limb
How does the loop of henle create a build up of salt concentration in the surrounding tissue fluid?
It’s arrangement in a hairpin countercurrent multiplier system increases the efficiency of salt transfer from the ascending limb to the descending limb, meaning the salt concentration in the surrounding tissue is increased
What is the water potential in the medulla of the kidney like?
It is very negative, becoming even lower deeper in the medulla
What is the composition of the tubule fluid like once it reaches the top of the ascending limb of the loop of henle?
It has a relatively high concentration of water, and low concentrations of sodium and chloride ions, due to the removal of these ions from the ascending limb of the loop of henle
What is the part of the nephron following the loop of henle?
The distal convoluted tubule
What happens to the tubule fluid in the distal convoluted tubule?
Active transport is used to adjust the concentrations of various salts
What happens to the tubule fluid in the collecting duct?
The collecting duct carries the tubule fluid down through the medulla to the pelvis. As the tubule fluid passes through the medulla, which has a very low water potential, water moves out of the fluid by osmosis. It enters the blood capillaries by osmosis and is carried away
What is the composition of the tubule fluid like once it reaches the pelvis of the kidney?
It has a very low water potential, and its concentration of urea and salts is higher than that of the blood plasma
What does the amount of water reabsorbed in the collecting duct depend on?
The permeability of the walls in the collecting duct
What is osmoregulation?
The control of water levels and salts in the body.
Why must the correct water balance between cells and the surrounding fluids be maintained?
To prevent problems with osmosis
Where is water gained from?
- food
- drink
- metabolism (e.g. Respiration)
How is water lost?
- urine
- sweat
- water vapour in exhaled air
- faeces
What is ADH?
Anti diuretic hormone is released from the pituitary gland and acts on the collecting ducts in the kidneys to increase their reabsorption of water
What are osmoreceptors?
Receptor cells that monitor the water potential of the blood. If the blood has a low water potential then water is moved out of the osmoreceptor cells by osmosis, causing them to shrink. This stimulates the neurosecretory cells
What is the hypothalamus?
A part of the brain that contains neurosecretory cells and various receptors that monitor the blood
What are neurosecretory cells?
Specialised cells that act like nerve cells but release a hormone into the blood.
What is the posterior pituitary gland?
The hind part of the pituitary gland, which releases ADH
How does the permeability of the walls of the collecting duct change according to the needs of the body?
On a cool day when you need to conserve less water, the walls of the collecting duct are less permeable, so less water is reabsorbed and more urine is produced.
On a hot day when you need to conserve more water, the walls are more permeable, so more water is reabsorbed and the urine becomes more concentrated.
How is the permeability of the walls in the collecting duct adjusted?
The walls of the collecting duct respond to the level of ADH (antidiuretic hormone) in the blood.
How do high levels of ADH affect the permeability of the collecting duct?
Cells in the walls of the collecting duct have membrane bound receptors for ADH, and when it binds to these a chain of enzyme controlled reactions is triggered inside the cell. This causes vesicles containing water permeable channels to be inserted into the cell membrane, meaning it is more permeable to water.
How do low levels of ADH in the blood affect the permeability of the collecting duct?
The cell surface membranes of the cells in the walls of the collecting duct fold inwards to create new vesicles, which remove the water permeable channels from the membrane. This makes the walls less permeable, so less water is reabsorbed
How is the concentration of ADH in the blood adjusted?
The water potential of the blood is measured by osmoreceptors in the hypothalamus of the brain. These cells respond to osmosis, and shrink when the water potential of the blood is low. This stimulates neurosecretory cells in the hypothalamus to produce and secrete ADH. When the water potential of the blood rises again, less ADH is released
How is ADH removed from the blood?
It is slowly broken down, with a half life of about 20 minutes
What are the three most common causes for kidney failure?
Diabetes
Hypertension
infection
What effect does kidney failure have on the body?
The body is unable to remove excess water and certain waste products from the blood, including urea and excess salts. It is also unable to regulate the levels of water and salts in the body. This will quickly lead to death.
What is dialysis?
The use of a partially permeable membrane to filter the blood. It is the most common treatment for kidney failure. It removes wastes, excess fluid and salt from the blood by passing the blood over a dialysis membrane.
What is the dialysis membrane?
A partially permeable membrane that separates the dialysis fluid from the patients blood in a dialysis machine.
What is dialysis fluid?
A complex solution that contains the correct concentrations of salts, urea, water and other substances as blood plasma, meaning that any of these substances in excess in the blood will flow across the membrane into the fluid, and any substances that are too low in concentration will diffuse into the blood from the dialysis fluid.
What is renal dialysis?
Blood from a vein is passed into a machine that contains an artificial dialysis membrane. Heparin is added to avoid clotting and any bubbles are removed before the blood is returned to the body. Renal dialysis is usually performed at a clinic three times a week for several hours each session, but some patients learn to carry it out at home.
What is peritoneal dialysis?
In peritoneal dialysis the body’s abdominal membrane is used as the dialysis membrane. A permanent tube is inserted into the abdomen, and dialysis fluid is poured in through the tube, to fill the space between the organs and abdominal walls. After several hours the fluid is drained from the abdomen. The patient can walk around whilst having peritoneal dialysis
What are the the two main treatments for kidney failure?
Dialysis and transplant
What are the advantages of kidney transplants?
- freedom from time consuming dialysis
- diet is less limited
- feeling better physically
- better quality of life, e.g. able to travel
- no longer seeing oneself as chronically ill
What are the disadvantages of kidney transplants?
- Need immunosuppressants for the life of the kidney
- need major surgery under general anaesthetic
- risks of surgery including infection, bleeding, and damage to surrounding organs
- frequent checks for signs of organ rejection
- side effects: anti-rejection medicine causes fluid retention and high blood pressure; immunosuppressants increase susceptibility to infections
What other substances could be found in the urine?
- hCG, a glycoproteins secreted by human embryos in pregnancy
- anabolic steroids, which increase protein synthesis in cells, leading to a build up of cell tissue
How do pregnancy tests work?
Human embryos secrete hCG, which can be found in urine as early as 6 days after conception. Pregnancy tests contain monoclonal antibodies specific to hCG. When a pregnancy test is taken, any hCG in the urine attaches to an antibody that is tagged with a blue bead. This hCG-antibody complex moves up the strip until it sticks to a band of immobilised antibodies. As a result, all the antibodies attached to a blue bead and hCG are held in one place, forming a blue line
What can anabolic steroids be used for?
They increase protein synthesis within cells, leading to a build up of cell tissue, particularly in muscles. This means they can give unfair advantages in competitive sport. The use of anabolic steroids is banned by all major sporting bodies
How can urine samples be used to test for misuse of anabolic steroids?
Anabolic steroids remain in the blood for many days. They are relatively small molecules, and enter the nephron easily. They can be tested for by analysing a urine sample using gas chromatography
What is gas chromatography?
A technique used to separate substances in a gaseous state. A urine sample is vaporised and passed through a column continuing a liquid. The time taken for a substance in the sample to pass through the column is compared to the time taken for a steroid to pass through. If the times are the same, the sample contains the steroid.
