5.1.2 Excretion Flashcards
What is excretion?
-the removal of metabolic waste(i.e CO2, nitrogenous waste) that are toxic if allowed to build up
-excretion maintains homeostasis by helping to keep levels of certain substances in the blood roughly constant
Examples of substances that are excreted
-CO2= waste product of respiration that is excreted from the lungs
-bile= from breakdown of haemoglobin from old red blood cells in the liver, excreted from the liver into the small intestine via gall bladder + bile duct(colours the faeces)
-nitrogenous waste= formed from the breakdown of excess amino acids in the liver, excreted by kidneys in urine
-(mammals produce urine, birds produce uric acid + fish produce ammonia)
Structure of the liver: hepatocytes
-liver cells(can regenerate by mitosis)
-have large nucleus + golgi
-lots of mitochondria
Structure of the liver: hepatic artery
-supplies liver with oxygenated blood from the heart(liver has a good supply of oxygen for hepatocytes’ functions i.e mitosis)
Structure of the liver: hepatic portal vein
-brings blood from small intestine to the liver
-carries glucose, amino acids, CO2, ingested substances i.e drugs + alcohol)
Structure of the liver: hepatic vein
-carries deoxygenated blood away from the liver to the heart
-delivers excess amino acids which are converted into urea to the kidney
Structure of the liver: Bile duct
-carries bile(a substance produced by liver to emulsify lipids and neutralise stomach acids in small intestine) to the gall bladder to be stored
The histology/make up of the liver
-made up of liver lobules–} cylindrical structures made up of hepatocytes
-each lobule has a central vein that connects to the hepatic vein
Sinusoids
-hepatic artery + portal vein connect to central vein and mix in *capillaries called sinusoids
-act as canals carrying the blood
*single layer of cells that allows substances to diffuse into hepatocytes
Why is it important that blood mixes in sinusoids?
-it increases O2 concentration in the blood from the hepatic portal vein–} supplies hepatocytes with sufficient oxygen
Hepatocytes
-remove any harmful substances and oxygen from the blood in the sinusoids via diffusion
-substances are broken down by hepatocytes and less harmful substances re-enter the blood
-i.e excess amino acids are broken down into urea by hepatocytes
Central vein
-blood travels through sinusoids to central vein
-the central vein from all of the lobules connect up to form hepatic vein
Kupffer cells
-within the sinusoids
-act as resident macrophages of the liver–} phagocytosis for pathogens and ingested toxins
Bile canaliculi
-hepatocytes secrete bile from the breakdown of blood into this space
-bile can drain into the bile duct from here to be transported to gall bladder
Identifying histology of liver tissue
-hepatic portal vein has a larger lumen than the hepatic artery
-hepatic artery has narrow vessels + hepatic portal vein has branched vessels
-bile canaliculi is lined with hepatocytes(sweetcorn)
What are the 3 functions of the liver?
-storage of glycogen
-detoxification
-formation of urea
Storage of glycogen
-the liver responds to insulin and glucagon to control BGC levels
-when blood glucose levels rise, insulin levels increase —} cause hepatocytes to convert glucose into glycogen via glycogenesis
-glycogen is stored in the hepatocytes until glucose is released for energy
(glycogenolysis)
-amino acids + glycerol can be converted into glucose(gluconeogenesis)
Detoxification
-liver breaks down harmful substances like alcohol, drugs, unwanted hormones–} become less harmful compounds that can be excreted
-hydrogen peroxide(by product of metabolic process) is broken down by catalase into O2 + water
-alcohol= broken down by alcohol dehydrogenase into ethanal, then ethanoate(used for respiration or builds up fatty acids)
-drugs= i.e paracetamol are broken down(excess can lead to liver/kidney failure)
-excess hormones= i.e insulin is broken down(excess can lead to issues with BGC)
Why do excess amino acids need to be broken down?
-AA’s contain nitrogen in their amino groups
-nitrogenous substances cannot usually be stored in the body
-excess AA’s= excess nitrogenous substances
Deamination
-amine group is removed from amino acid as nitrogenous waste
-forms ammonia(NH3) and organic acids(can become pyruvate + be respired to produce ATP/converted to carbohydrate + stored as glycogen)
The ornithine cycle
-ammonia is too toxic for mammals to excrete directly
-is combined with CO2 to produce water and urea(not toxic in small concentrations) which can be excreted
-require ATP
What are the kidneys made up of?
-millions of nephrons that act as filtering units
Explain the structure of the kidney?
-fibrous capsule= fairly tough outer layer
-cortex=dark outer layer where filtering takes place, has dense capillary network carrying blood from renal artery to nephrons
-medulla= (lighter colour) contains the tubules of nephrons that form the pyramids of the kidney + collecting ducts
-renal pelvis= central chamber where urine collects before passing out of ureters(tubes)
Explain the journey of blood into the kidney
-blood leaves the liver via hepatic vein, feeds into vena cava and reaches renal artery
-renal artery splits into lots of arterioles + capillaries within cortex and medulla of the kidney
-nephrons are running alongside the capillaries–} substances can move between capillaries and nephron tubes during filtering process
-any substances left in capillary will flow out of kidney through renal vein
-any substances left in nephron tubules will enter pelvis and leave the body through the ureters as urine
What are nephrons?
-long nephron tubules alongside the bundle of capillaries where the blood is filtered
-around 1 million per kidney
-majority of filtered material is returned to the blood, removing nitrogenous waste + controls water potential via osmoregulation
-along the entire nephron it carries filtrate(yellow substance)
The structure of the nephron: Bowman’s capsule
-cup-shaped structure that contains glomerulus(bundle of capillaries)
-more blood goes into glomerulus than leaves it due to ultrafiltration
The structure of the nephron: Proximal convoluted tubule
-first coiled region after bowman’s capsule
-found in the cortex where many substances needed by the body are reabsorbed
-surrounded by capillaries
The structure of the nephron: Loop of Henle
-long loop of tubule that drops from cortex into medulla
-creates a very high solute conc in the tissue fluid deep in medulla
-U shaped(travels back up through the medulla to cortex)
The structure of the nephron: Distal convoluted tubule
-second coiled region where the permeability of its walls to water varies in response to ADH
-further regulation of ion balance and pH in blood takes place
The structure of the nephron: collecting duct
-urine(containing excess filtrate + water) passes down the collecting duct through the medulla to pelvis
-more water balance takes place here
-walls are sensitive to ADH
Nephron capillaries
-the network of capillaries around the nephron leads to venule, then the renal vein
-the blood that leaves the kidney has reduced levels of urea but levels of glucose, amino acids, mineral ions and other necessary substances are roughly the same
Functions of the nephron: Ultrafiltration
-the glomerulus is supplied with blood from a short, wide afferent arteriole from the renal artery and the blood leaves through a narrower efferent arteriole–} creates a pressure gradient in the capillaries of glomerulus(hydrostatic pressure builds up in glomerulus)
-this forces blood out through capillary walls and it passes through 3 layers:
- a gap in the capillary wall/endothelium
- basement membrane(made up of network of collagen fibres + other proteins)–} contains pores which allows them to act as sieves so blood cells + most proteins retain in capillary as they are too big to pass through
-cells in the lining of these bowman’s capsule called podocytes(have ‘claw-clip’ extensions called pedicels) that wrap around membrane + form slits so cells, platelets + large plasma proteins that got through both layers don’t enter into tubule
-any molecule small enough(glucose, urea, water, ions) will pass into bowman’s capsule via gaps between the podocyte cells as glomerular filtrate
How is selective reabsorption maintained throughout the nephron?
-through the loop of Henle, DCT + collecting duct, water is continuously reabsorbed via osmosis
-ion movement to help maintain steep water potential gradient–} remaining substances in filtrate i.e water, dissolved salts, urea, excess hormones + vitamins are excreted as urine
-urine doesn’t usually contain proteins or blood cells as they are too big to be filtered out
Functions of the nephron: Selective reabsorption
-in the PCT, all of the glucose, amino acids, vitamins + hormones moved back from filtrate into blood
-sodium ions are moved by active transport from epithelial cell to capillary first to produce a conc gradient + then chloride ions follow via diffusion(Cl- to balance Na2+ charge)
-conc gradient allows sodium ions and useful substances like glucose, amino acids, vitamins, ions + some salts to move from the PCT lumen to the epithelial cell via facilitated diffusion and co-transport through transport proteins
-can diffuse down conc gradient into capillaries from the epithelial cells
-water will then follow via osmosis because water potential of the blood is lower then filtrate–} moves from PCT to capillaries down water potential gradient
How is the PCT specialised for selective reabsorption?
-epithelial cell covered with microvilli which increases SA for substances for diffusion so more substances can be reabsorbed
-many mitochondria to provide ATP for active transport
-large no. of co-transport proteins
-substances only need to pass through a single epithelial cell—} short diffusion distance
Regulating water potential
-the amount of water potential in the blood needs to be kept constant
–} is lost during excretion, sweating and is gained through eating and drinking
-changing the conc of urine helps maintain water potential
Describe the negative feedback system to regulate water potential
-water potential is monitored by the osmoreceptors in the hypothalamus(sensitive to conc of ions)
-if water potential is too low, nerve impulses are sent to posterior pituitary gland–} ADH is released into bloodstream
-increases the permeability of the DCT and collecting duct to water–} can pass into the blood in surrounding capillaries
-if water potential is too high, it is picked up by osmoreceptors and nerve impulses are sent to posterior pituitary to inhibit ADH release(walls of collecting duct remain impermeable)
Explain the mechanism of ADH
-to enter tubule cells, it binds to receptors on the cell membrane and triggers the formation of cyclic AMP(secondary messenger)–} leads to cascade of events:
-vesicles in the cell lining the collecting duct fuse with plasma membrane
-aquaporins(protein channels) are inserted into plasma membrane–} allow water to be passed into the membrane via osmosis, making DCT and the collecting duct more permeable to water
-more water is reabsorbed from these tubules into medulla + into blood via osmosis
-small amount of concentrated urine produced(some water lost from the body)
How does ADH contribute to negative feedback?
-the more ADH that is released, the more water channels are inserted into plasma membrane
-makes it easier for water to leave tubules by diffusion
-when ADH levels fall, levels of cAMP fall so the water channels are removed from the tubule cell membranes and enclosed in vesicles + collecting duct remains impermeable to water
The loop of Henle
-enables mammals to produce urine that is more concentrated than their blood
-it is made up of two ‘limbs’
-the descending limb(cortex to medulla) is impermeable to ion movement + the ascending limb(medulla to cortex) is impermeable to water
-counter current multiplier mechanism= helps to reabsorb water back into blood via osmoregulation
Counter current multiplier mechanism: ascending limb
-near the top of the ascending limb, Na+ and Cl- ions are actively transported into the medulla
-ascending limb is impermeable to water so the water stays inside the tubule
-high conc of ions in the tissue of the medulla creates a low water potential
Counter current multiplier mechanism: descending limb
-due to low water potential in the medulla, water leaves the descending limb via osmosis
-this increases the conc of filtrate in the tubule as the descending limb is impermeable to ions
-the water in the medulla is reabsorbed into the blood via the capillary network
-the concentrated ions diffuse down the limb until they reach near the bottom of the ascending limb–} Na+ and Cl- ions diffuse* out into the medulla, lowering water potential
-this causes water to move out of the collecting duct via osmosis–} permeability of the collecting duct is controlled by the level of ADH
*diffuse as ions are concentrated enough to form gradient
What impact does the length of the loop of Henle have on its function?
-the longer an animal’s loop of Henle, the more water they can reabsorb from filtrate
-longer ascending limb= lower water potential in the medulla, more water moves out of the nephron and collecting duct into the capillaries–} very concentrated urine
-animals who live in areas where there is little water usually have long loops to retain more water
What is kidney failure and how is it detected?
-when the kidneys cannot carry out their usual function
-can be detected using glomerular filtration rate (GFR)–} a measure of the vol of blood flowing through kidney/rate of filtering
-measured indirectly by the level of creatinine in the blood(breakdown product of muscles)
-GFR(cm3/min) can be impacted by certain factors i.e age(lower), muscle mass(more= higher)
Why can creatinine be used to determine GFR?
-it enters the bowman’s capsule and none of it gets reabsorbed–} if there are higher levels in the blood it is a signal the kidneys aren’t working properly
Common causes of kidney failure
-kidney infections(the structure of podocytes + tubules may be damaged due to swelling–} impacts filtering in the Bowman’s capsule + reabsorption)
-high BP(can damage the basement membrane + epithelial cells in Bowman and the capillaries within the glomeruli–} larger molecules i.e proteins and blood can get through capillary walls and into urine)
-genetic conditions(i.e polycystic kidney disease= healthy tissue replaced by fluid-filled cysts/damaged by pressure from cysts)
Common effects of kidney failures
-loss of electrolyte balance= body cannot excrete sodium, potassium + chloride ions–} leads to osmotic imbalances in blood
-build-up of toxic urea in the blood= can poison cells
-high blood pressure= can cause heart problems, strokes
-weakened bones as the calcium/phosphorous balance in blood is lost
-anaemia= reduced production of red blood cells–} tiredness/lethargy
How is kidney failure detected?
-normal GFR’s don’t fall below 70
-GFR of below 60 for more than 3 months may indicate severe chronic kidney disease—} little filtering of the blood in the kidney
What is renal dialysis?
-when a patient’s blood is filtered artificially
Haemodialysis
-the patient’s blood is passed through a dialysis machine—} blood flows from patient’s artery into machine on one side of a partially permeable membrane + dialysis fluid flows on the other side
-blood + dialysis fluid flow in opposite directions to maintain a steep conc gradient between the 2 fluids—} increases rate of diffusion(counter current exchange system)
-the membranes mimic the basement membrane of the bowman’s capsule–} waste products + excess water and ions are removed whereas blood cells and larger molecules i.e proteins remain in the blood
Why is haemodialysis important?
-it is vital that patients lose the excess mineral ions + urea that have built up whilst maintaining levels of glucose and some ions
-dialysis fluid contains normal plasma levels of glucose to ensure no net movement of glucose out of the blood
-has normal plasma levels of mineral ions, so any excess ions in the blood move out by diffusion down a conc gradient
-fluid contains no urea= steep conc gradient, so urea moves from blood into fluid
Disadvantages of haemodialysis
-patients may feel increasingly unwell between sessions because waste products + fluid start to build up in the blood
-each session takes around 3-5 hours and patients who rely on it have to remain attached to a dialysis machine several times a week
-must manage diets carefully i.e eat little protein + salts
-expensive
Kidney transplants
-a new kidney is implanted into a patient’s body to replace a damaged one(blood vessels are joined and new ureter is inserted into bladder)
-must be from a person with the same blood and tissue type
-often donated from a living relative or an organ donor
Advantages of kidney transplants
-a transplant is cheaper than long term dialysis
-more convenient and comfortable for a patient than constant dialysis sessions
Disadvantages of kidney transplants
-major operation is risky
-risk of rejection–} antigens on the donor organ may differ from antigens of the patient, which will be rejected by the immune system and may lead to destruction of the kidney
-transplants don’t last forever(average of around 10 years)–} must return to dialysis or wait for another donor
Dialysis vs Transplant
-dialysis is more readily available than donors
-patients have to monitor diet and have regular sessions on dialysis whereas a transplant frees them from this
-long term dialysis is more expensive than a transplant and can eventually cause damage to the body
How is urine a diagnostic tool?
-urine contains water, urea, mineral salts as well as breakdown products
-when affected by a disease, new substances will appear in the urine i.e presence of glucose= diabetes/large amounts of creatinine= muscle damage
What is hCG?
-a hormone only found in the urine of pregnant women
-pregnancy tests detect this
How do pregnancy tests work?
-when urine is applied to the wick, any hCG will bind to the antibody on mobile monoclonal antibodies that have small coloured beads attached to them–} form hCG-antibody complex
-urine moves up the test strip carrying the beads with it
-it reaches a window where there are immobilised monoclonal antibodies that only bind to hCG
-if a woman is pregnant, the hCG turns the strip blue/forms a line because the immobilised antibodies binds to the complex which has small coloured beads attached
-if no hCG is present the beads will pass through the test area without binding to anything
(regardless of hCG presence or not, urine moves to a second window where the immobilised antibodies bind to mobile antibodies to form a coloured line–} must be 2 lines to indicate pregancy)
Monoclonal antibodies
-antibodies from a single clone of cells that are produced to target particular cells/chemicals in the body
How are monoclonal antibodies made for pregnancy tests?
- a mouse is injected with hCG which then secretes B cells
-the cells are removed from the mouse and fused with a myeloma(fast dividing cancer cell)–} forms a hybridoma
-each hybridoma reproduces rapidly to produce clones of the desired antibody and these monoclonal antibodies are collected and purified for usage
Anabolic steroids
-drugs that build up muscle tissue i.e testosterone and Nandrolone
-athletes are banned from using these steroids= increases performance i.e strength + power BUT has dangerous side-effects i.e liver damage
-they are excreted in the urine which can be used to test for the presence of them via gas chromatography/mass spectrometry(GC/MS)
How does GC/MS work?
-urine sample is vaporised with a known solvent and passed through a column containing a polymer
-different substances move through the column at different speeds so the substances in the urine separate
- a mass spectrometer converts them into ions and separates them depending on mass + charge
-results are analysed and compared to the results of known substances to decipher which substances were in the urine sample
Testing for drugs
-urine is used for the testing of many recreational drugs as well as alcohol
-it is possible to find drug traces in the urine as it is filtered through the kidneys and stored in the bladder
-drug testing is usually a two step process:
1) urine sample is applied to the test strip and with the presence of a drug, a colour change occurs to indicate a positive result
2) a sample is usually run through a GC/MS to confirm the presence of a drug
