What is excretion
The removal of metabolic waste from the body
What are the 2 main metabolic waste products excreted by the body?
Carbon dioxide. Nitrogenous compounds (including urea)
Where is carbon dioxide produced and excreted from the body
Carbon dioxide is produced by every living cell as a by-product of respiration.
It diffuses out of cells and into the bloodstream where most is transported to the lungs as hydrogencarbonate ions and diffuses into alveoli from which it is removed by expiration.
Where is urea produced and excreted from the body
Urea is produced in the liver as a by-product of the deamination of amino acids.
It dissolves in blood plasma and carried from the liver to the kidneys where a majority is removed through filtration, and leaves the body dissolved in urine
Why is carbon dioxide toxic to the body
- severely decreases oxygen affinity and carrying capacity of blood, which may result is some cells suffering from oxygen starvation
- binds directly with haemoglobin to form carbaminohaemoglobin, which also has lower oxygen affinity
- Respiratory acidosis is a result of too high a concentration of H+ ions in the blood which breaks the buffering capacity of blood.
- lowers the pH of active tissues
Why is an excess of nitrogenous compounds toxic to the body
- Cannot be stored as the amine group is toxic
- Deamination produces ammonia- highly toxic and soluble
What is the arrangement of vessels in the liver?
- Oxygenated blood from the heart is directly pumped to the liver via the hepatic artery.
- Deoxygenated blood from the small intestines enters the liver via the hepatic portal vein.
- Deoxygenated blood leaves the liver via the hepatic vein
- The bile duct connects the liver to the gall bladder,
What does the liver use oxygenated blood for
Supplies the hepatocytes (which are active and require ATP for metabolism) with oxygen for respiration
What is contained within the deoxgenated blood in the liver
contains every substance that has been absorbed by the small intestines into the bloodstream in unregulated amounts, even toxic substances
What is the bile duct used for
used to carry bile made in the liver by hepatocytes to the gall bladder to be stored for later use; when it is secreted into the duodenum to emulsify fat.
What is the principle behind the arrangement of cells in the liver
To ensure best possible contact with blood vessels
What is the structural arrangement of hepatocytes in the liver
Hepatocytes are divided into lobes which are subdivided into lobules
Give 2 structural features of a hepatocyte
- Many microvilli on the surface to increase surface area for secretion and absorption of substances
- Very dense cytoplasm with specialised amounts of certain organelles, specialised to perform a range of metabolic functions
What are inter-lobular vessels
Smaller branches which run parallel to the lobules
What are sinusoids
Chambers for mixing of the oxygen-rich blood from the hepatic artery and the nutrient-rich blood from the portal vein
What do al the intra-lobular vessels join to form in the liver
What are the structures which drain into branches of the bile duct
What are intra-lobular vessels
Sinusoids empty into the intra-lobular vessels which run through the middle of every lobule
What are kupffer cells
specialised macrophages responsible for breaking down old erythrocytes
What is billrubin a waste product of
The breakdown of old erythrocytes
Give 3 functions of the liver
- Control of substance levels in body including: Blood glucose, amino acids
- Synthesis of substances in the body including: bile, plasma proteins, cholesterol
- Storage of substances in the body including: Vitamins (A,D and B12), iron, glycogen… 4. Detoxification of alcohol and drugs.
- Breakdown of hormones.
- Breakdown of old erythrocytes.
Summarise by an equation deamination
Amino acid + oxygen –> ammonia + keto acid
What is the benefit of converting ammonia to urea
Ammonia is very toxic to the body whereas urea is much less toxic.
Ammonia is much more soluble than urea so is harder to remove.
Urea can be safely transported in the bloodstream and removed by the kidneys whereas ammonia cannot.
Define the ornithine cycle
The process by which ammonia combines with CO2 to form urea.
Give the stages of the ornithine cycle
- NH3 + CO2 + Ornithine –> Citrulline + H2O.
- Citrulline + NH3 –> Arginine + H2O.
- Arginine + H2O –> Urea + Ornithine.
What role do the hepatocytes play in detoxification
Hepatocytes contain a number of enzymes used in the process of detoxification, including catalase for breaking down hydrogen peroxide
What is the process of detoxifying ethanol
- Ethanol is oxidised by the enzyme ethanol dehydrogenase to form ethanal.
- Ethanal is further oxidised by the enzyme ethanal dehydrogenase to form ethanoic acid.
- Ethanoic acid combines with coenzyme A which can be respired.
- Hydrogen atoms lost during both oxidation processes combine with coenzyme NAD to form reduced NAD
What may be a problem with excess alcohol consumption
NAD is required for respiring fatty acids to release energy.
When too much alcohol is consumed, a large amount of NAD is converted to reduced NAD and there may not be sufficient NAD left to respire fatty acids.
The fatty acids are converted back into lipids and stored in hepatocytes, which causes fatty liver.
What are the functions of the kidney
Excretion: Removal of nitrogenous waste.
Osmoregulation: Control of blood/body water potential.
Regulation of blood pH.
What is the structure of the kidney
Outside of kidney surrounded by fibrous layer called the capsule.
The outermost layer is the cortex and is where the renal corpuscules, PCTs (Proximal Convoluted Tubules) and DCTs (Distal Convoluted Tubules) are located.
The second layer is the medulla and is where the Loop of Henle and collecting ducts are located.
The innermost layer is the pelvis, which is where the renal artery and vein connects. It leads to the ureter.
What is the structure of a nephron
- A nephron starts with the Bowman’s capsule, which contains the glomerulus
- Filtrate then enters the PCT,
- Filtrate then flows into the descending limb of of the loop of Henle.
- Filtrate flows into ascending limb
- Filtrate then flows into DCT
- Filtrate flows into the collecting ducts
- Collecting ducts join at pelvis which leads into ureter.
What happens in the glomerulus
the glomerular filtrate is formed
What happens in the PCT
selective reabsorption occurs and most solutes are reabsorbed
What happens in the descending limb
water is removed by osmosis into surroundings
What happens in the ascending limb
The thin section is permeable to water but the thick is not, and is where active transport of NaCl happens
What happens in the DCT
more adjustments are made to the salt concentrations
What happens in the collecting duct
Reabsorbs more water (as needed)
What is the structure of blood vessels in the kidneys
- Blood enters kidneys through renal artery
- Renal artery branches off into smaller, afferent arterioles
- Afferent arterioles branch further to form a network of fine capillaries called the glomerulus
- Capillaries recombine to form efferent arterioles which leads away from Bowman’s capsule.
- Blood then joins network of capillaries which wrap around the nephron and reabsorb substances
- Capillaries join together to form venules, which combine to form renal vein
How does the water potential change along the nephron
- Water potential decreased along descending limb as salts diffuse in and water diffuses out.
- Water potential increases again along ascending limbs as water is prevented from leaving and NaCl is removed
- Water potential decreases agains as water leaves the collecting ducts by osmosis
How is high pressure generated in the glomerulus
The afferent arteriole is much wider than the efferent arteriole, which creates a bottleneck effect in the glomerulus. As blood enters at a higher rate than can be removed, pressure increases beyond pressure inside Bowman’s capsule, which creates pressure gradient for ultrafiltration.
What are the filtration barriers during ultrafiltration?
Endothelium of capillaries
What is the content of glomerular filtrate?
- Water. - Glucose. - Amino acids. - Vitamins. - Hormones. - Urea. - Inorganic ions (salts). - Small proteins.
How much of each substance is reabsorbed into the blood from PCT?
Glucose - All Amino acids - All Water - Most Urea - Some Inorganic ions - some Small proteins - All Vitamins - All Hormones - Some
3 ways the epithelial cells lining the PCT is adapted for selective re absorption?
Microvilli increases the surface area which results in larger exchange surface and greater rate of diffusion.
Plasma membrane contains lots of co-transporter proteins used to reabsorb glucose and salts.
Contains lots of mitochondria to produce energy as ATP for use in active transport.
One layer thick to minimise diffusion distance.
Nuclei contain instructions to manufacture transport membranes.
Basal membranes on capillary side of cells also folded to increase surface area.
Tight junction between cells to force filtrate through epithelial cells to control absorption rather than leaking into capillaries directly.
How is water reabsorbed in the PCT?
Due to ultrafiltration, there is a very low water potential in the capillaries next to epithelial cells, much lower than the glomerular filtrate (especially due to Na+ and glucose being removed).
Water potential gradient set up across the epithelial cells between capillaries and PCT which results in water moving out of PCT, across epithelial cells and into blood by osmosis.
How are glucose and Na+ ions reabsorbed?
- Sodium-potassium pump in basal membrane pumps Na+ ions out of epithelial cells by active transport, which maintains low Na+ concentration in cell.
- Na+ ions diffuse into epithelial cells through co-transporter proteins, down concentration gradient.
- Movement of Na+ ions provides energy for glucose to be pumped into epithelial cells, even against concentration gradient (indirect active transport).
- Concentration of glucose increases in cell so it diffuses out and into tissue fluid. It then diffuses into blood and is carried away.
How are amino acids reabsorbed?
Enter the cells through co-transporter proteins with Na+ ions and then diffuse into blood down concentration gradient.
How are larger molecules like small proteins reabsorbed?
What is the headpin countercurrent multiplier effect?
The arrangement of tubules in a tight hairpin shape, meaning fluid is always flowing in the opposite direction to each other and there is always a concentration gradient between the fluid in the two limbs of the loop of Henle. It lowers the wp of the medulla
What is the process of water reabsorption?
- Water moves, by osmosis, out of the descending limb and into surrounding tissue fluid. This concentrates filtrate.
- The fluid reaching the bottom of the loop of Henle is very concentrated due to loss of water. Medulla is also very concentrated due to accumulation of Na+ and Cl- ions by diffusion and active transport.
- Fluid enters ascending limb. As it moves up, Na+ and Cl- ions are pumped out and into medulla by active transport.
- Active transport of Na+ and Cl- out of filtrate increasing water potential in filtrate and decreasing the water potential in medulla
- Water is prevented from leaving by osmosis as walls of ascending limb are impermeable.
- Fluid at top of ascending limb very dilute and empties into collecting duct.
- Reabsorption of water in the collecting duct is controlled by ADH which makes the walls more/less permeable.
How does the loop of Henle aid in the reabsorption of water?
Low water potential in the medulla created by the countercurrent multiplier effect in the loop of Henle. This helps produce very concentrated (hypertonic) urine.
What is osmoregulation?
Control and maintenance of constant water levels in the body.
How is water gained?
- Food. - Drinks. - Metabolism + respiration
How is water lost?
Urine. - Sweat. - Expiration. - Faeces
What happens when there is too little water in the body?
- Low water potential of blood causes osmoreceptor cells in hypothalamus to shrink.
- This stimulates the neurosecretory cells and an action potential is created.
- Action potential causes more ADH to be released into the bloodstream from the posterior pituitary gland.
- ADH travels to the collecting ducts via blood.
- ADH binds to complementary receptors on the plasma membrane of cells lining collecting ducts.
- Walls of collecting ducts are made more permeable.
- More water is reabsorbed from collecting ducts.
- Blood water potential increases to normal while small volume of concentrated urine is produced.
What happens when there is too much water in the body?
- High water potential of blood causes osmoreceptor cells in the hypothalamus to swell.
- This inhibits the neurosecretory cells.
- Less ADH is released into the bloodstream from posterior pituitary gland due to lack of action potentials. 4. Less ADH travels to the collecting duct via blood.
- Reduced level of ADH in the blood causes the walls of the collecting duct to become less permeable.
- Less water is reabsorbed from the collecting ducts.
- Blood water potential decreases to normal while large volume of dilute urine is produced.
What hormone is responsible for controlling reabsorption of water from the kidneys?
Anti-Diuretic Hormone (ADH).
How does ADH make the walls of collecting duct more/less permeable?
- When ADH binds to receptors on cells lining the collecting ducts, a series of enzyme-catalysed reaction occur.
- This causes vesicles containing aquaporins (water channels) to be moved to membrane lining collecting ducts.
- These vesicles fuse with membrane so that aquaporins are present in walls of collecting ducts which allows water to be absorbed at quicker rate, so more is reabsorbed.
- As ADH is broken down and concentration drops, collecting duct membranes fold inwards, removing aquaporins from walls and reducing amount of water reabsorbed.
- If less ADH is released from pituitary gland, the net concentration of ADH drops in blood, so less aquaporins are present on walls, making walls of collecting ducts less permeable.
What are the 3 most common causes of kidney failure
- Diabetes mellitus. - Hypertension. - Infection
What is dialysis
Process of using partially permeable membrane and dialysis fluid to filter blood.
How is urine used in pregnancy tests?
During pregnancy, human Chorionic Gonadotrophin (hCG) is excreted from urine.
- A dipstick containing monoclonal antibodies specific to hCG is dipped in urine.
- hCG acts as the antigen and attaches to antibodies to form antigen-antibody complex. Antibodies also have coloured markers attached to other end.
- As urine is drawn up stick by capillary action, it brings hCG-antibody complexes with it.
- As the complexes move up, they encounter line of immobilised antibodies also specific to hCG.
- Immobilised antibodies attach to hCG on hCG-antibody complexes, holding complexes in position so that coloured markers make coloured line.
- Above test line is a control line with immobilised antibodies specific to monoclonal antibodies themselves. These hold any antibodies not attached to test line.
- Coloured markers form a line indicating the test is working.
How is urine used to test for anabolic steroids?
Gas chromatography is used.
- Urine vaporised and dissolved in gaseous solvent.
- Mixture passed though tube lined with absorption agent.
- Different molecules come out of gas at different times and is absorbed by agent at different places.
- This allows for creation of chromatograms.
- Urine chromatograms compared against drug chromatogram to test for any drugs.
Summarise the ornithine cycle as an equation
Ammonia + CO2 –> urea + water
Summarise with an equation the formation of urea
Amino acid –> ammonia –> urea