5.1.2 Excretion

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    What is excretion?

    The removal of metabolic waste from the body

    Why is passing stool not an example of excretion?

    It is not metabolic waste but food waste

    What is stool an example of?

    Egestion

    Is excretion part of homeostasis?

    Yes, it regulates internal environments by removing excess of abnormal substances such as CO2 and nitrogenous waste.

    Why is a build up of CO2 dangerous?

    Increased acidity of the blood due to the Bohr effect

    What are the functions of the liver?

    -Detoxification of blood including the breakdown of alcohol -Regulation of blood sugar levels -Conversion of amino acids to ammonia to urea -Production of cholesterol -Production of blood proteins -Production of bile

    Describe and explain how the gross structure of the liver is adapted to its functions.

    - Hepatic artery supplies liver with a good supply of oxygen for increased respiration rate, for plenty of energy - Hepatic vein takes deoxygenated blood away from the liver - Hepatic portal vein brings blood from the duodenum and ileum so it is rich in products of digestion. Any harmful substances are filtered out and broken down. - The bile duct takes bile from the gall bladder to emulsify fats

    What is a liver lobule?

    A hexagon shaped structure with a triad at each corner (bile duct, portal arteriole, portal venuole)

    What is a hepatocyte?

    a parenchymal liver cell that performs all functions ascribed to the liver

    What is a Kupfer cell?

    macrophage in liver

    What is a sinusoid?

    Capillary space in the liver between hepatocytes.

    What is the central vein?

    Veins found at the centre of a "classic" hepatic lobule. They receive the blood from the sinusoids and return it to circulation via the hepatic vein

    How does the liver detoxify blood?

    - Blood runs through the centre of the lobules through both the hepatic artery and portal vein connected to the central vein by sinusoids - Blood runs through sinusoids, past hepatocytes and Kufper cells on the walls which remove harmful substances. - Less harmful substances re-enter the blood and runs through the central veins of all the liver lobules and is returned to the hepatic vein.

    What produces bile?

    hepatocytes in the liver

    what is the bile canaliculi's function

    collects bile produced by hepatocytes and drains into the bile duct takes

    On a stained micrograph, how would you identify the central vein?

    Large, white circle shape

    On a stained micrograph, how would you identify a sinusoid?

    Smaller, frequent white spaces

    On a stained micrograph, how would you identify a hepatocyte?

    Red blobs which radiate out from the central vein.

    Describe the process of deamination of excess amino acids to form urea by the liver via the ornithine cycle.

    - Amine groups are removed from excess amino acids forming ammonia and organic acids, this is deamination. - Organic acids are respired to make ATP or converted to carbohydrates and stored as glycogen. - Ammonia is too toxic to directly excrete so is combined with CO2 via the ornithine cycle (no detail required) to form urea and water. - Urea is released into the blood and the kidneys filter urea out to the bladder which is secreted as urine.

    Describe the liver's role in detoxification.

    - Breaks down ethanol into ethanal and then acetic acid (excess alcohol consumption may lead to cirrhosis) - Breaks down drugs such as paracetamol (excess drug consumption can cause liver failure) - Insulin is broken down in the liver when blood sugar is too low

    Describe the liver's role in energy storage.

    The liver converts excess glucose to glycogen via glycogenesis. Glycogen is stored in the liver cells until glucose is needed for energy.

    Describe the gross structure of the kidney.

    - cortex - medulla - renal calyx - renal capsule - renal artery and vein - pelvis - ureter - bladder - urethra

    What is the function of the renal artery and vein?

    Renal artery (branch of abdominal artery) brings oxygenated blood to the nephron (functional unit of the kidney) Renal vein takes de-oxygenated blood away from the kidney to the caudal vena cava

    What is the function of the cortex of the kidney?

    Outer layer, site of glomerular filtration

    What is the function of the medulla of the kidney?

    Inner layer, contains the loop of Henle

    Outline the movement of water through the kidney.

    - Blood enters via the renal artery and substances are filtered out via ultrafiltration - Useful substances re-enter the blood via selective reabsorption - Remaining unwanted substances pass along the tubules, into the ureter, into the bladder and out of the urethra. - Blood exits via the renal vein

    What is a nephron?

    structural and functional unit of the kidney

    Describe the structure of a nephron.

    - Bowman's capsule at start of nephron: cup-shaped, surrounds glomerulus, inner layer of podocytes. - Proximal convoluted tubule (PCT): series of loops surrounded by capillaries, walls made of epithelial cells with microvilli. - Loop of Henle: hairpin loop extends from cortex into medulla. - Distal convoluted tubule : similar to PCT but fewer capillaries. - Collecting duct: DCT from several nephrons empty into collecting duct, which leads into pelvis of kidney.

    Describe the process of ultrafiltration.

    - Blood enters a bundle of dense capillaries called the glomerulus via the afferent arteriole - Due to the decrease in the diameter of the lumen, extremely high pressure is generated in the glomerulus, forcing liquid and small molecules out of the capillary into the Bowman's capsule. - To enter, the liquid and small molecules must pass through the capillary endothelium, basement membrane and the epithelium of the Bowman's capsule. - Larger molecules like proteins and RBCs stay in the blood. - The liquid and substances in the Bowman's capsule is now called filtrate, which pass through the rest of the nephron.

    What substances are ultrafiltrated?

    - water - glucose - amino acids - mineral ions - urea

    Describe the process of selective reabsoprtion of glucose, amino acids, vitamins and salts.

    - Takes place in the PCT - The epithelium of the proximal convoluted tubule has microvilli to provide a large surface area for the reabsorption of useful substances such as glucose, amino acids and salts. - This is reabsorbed by facilitated diffusion and active transport.

    Describe the process of selective reabsorption of water.

    - Takes place in the loop of Henle, DCT and collecting duct. - water moves out of the filtrate into the blood via osmosis due to a water potential gradient. - The filtrate left is called urine which travels along the ureter to the bladder

    What are the components of urine?

    urea, water, salt, excess hormones, drugs

    How would you identify the cortex under a microscope?

    - contains glomeruli - contains Bowman's capsules - contains DCT and PCT

    How would you identify the medulla under a microscope?

    Contains loops of Henle and capillaries

    Outline a method for kidney dissection.

    1. Observe the renal capsule 2. Notice the indents on the kidney 3. Try to identify the blood vessels, the ureter will be the most fatty 4. Draw a labelled sketch of the external kidney 5. Cut the kidney open using a scalpel in half lengthways 6. Notice that the cortex appears grainy and lighter than the medulla 7. Cone-shaped structures should be visible in the medulla 8. The renal calyces should lead to the pelvis 9. Draw a sketch to show the inside structures and add labels

    What hormone is responsible for water regulation?

    Anti diuretic hormone - ADH

    What is the loop of Henle responsible for?

    concentrating urine

    Describe the structure of the loop of Henle

    Has a descending limb, thin(attached to the PCT) and then curves round where it forms the ascending limb, thick, which joins the DCT.

    What is the countercurrent multiplier system in the context of the loop of Henle?

    Water is selectively reabsorbed into the blood via osmosis due to the set up of a water potential gradient in the ascending and descending limbs.

    How does the loop of Henle establish a water potential gradient?

    - Near the top of the ascending limb, sodium and chloride ions are actively pumped into the medulla. - The ascending limb is impermeable to water, creating a very low water potential in the medulla. - As there is a lower water potential in the medulla than the DESCENDING limb, water moves out of the descending limb via osmosis and is reabsorbed into the blood by capillaries in the medulla - This increases filtrate concentration as the descending limb is impermeable to ions. - Near the base of the ascending limb, more ions diffuse out further lowering the water potential of the medulla. - Water diffuses out of the collecting duct via osmosis and is again absorbed by capillaries in the medulla.

    Why does the length of the loop of Henle differ in different animals?

    If animals live in a dry environment, they have longer loops of Henle to ensure maximum water reabsorption. If animals live in wet environments, they may not even have loops of Henle.

    What receptors detect the water potential in the blood? Where are they located?

    Osmoreceptors in the osmoregulatory centre of the hypothalamus

    How does ADH regulate water reabsorption?

    ADH increases the permeability of principal cells in the DCT and collecting duct, allowing more water to be reabsorbed into the blood.

    Describe ADH action when someone is dehydrated.

    - Low water potential of the blood is detected by osmoreceptors in the hypothalamus. - The hypothalamus sends an action potential to the posterior pituitary gland - The posterior pituitary gland secretes ADH into the blood - ADH travels to target cells on the surface of the collecting duct - Aquaporins are inserted into the cell surface membrane of the cells in collecting duct which increases water permeability - More water is reabsorbed into the blood and urine is more concentrated

    What is kidney failure?

    When the kidneys don't work properly, so waste substances build up in the blood and you lose the ability to control the levels of ions and water in your body - this will eventually result in death

    How can kidney failure be detected?

    By measuring the glomerular filtration rate (GFR)

    What is the glomerular filtration rate (GFR)? What indicates kidney failure?

    The rate at which blood is filtered through the glomeruli of the kidneys. When the GFR is lower than normal

    Describe and explain 2 things that can cause kidney failure?

    - Kidney infections: inflammation can damage cells interfering with ultrafiltration and selective reabsorption. - High blood pressure: damage glomeruli, as too high a pressure can cause proteins to be forced out

    What are some examples of problems which arise from kidney failure?

    1. Build up of toxic chemicals in the blood causing extreme nausea and vomiting 2. Fluid will accumulate in tissues causing excessive swelling 3. The balance of electrolytes will be off. Blood will become acidic and bones will become brittle 4. Kidney failure eventually leads to a very unpleasant and slow death

    What are two ways kidney failure can be treated?

    - Renal dialysis (haemodialysis for OCR spec) - Kidney transplants

    Describe and explain how haemodialysis works to treat kidney failure.

    - Blood is passed through a dialysis machine. - Blood flows along one side of a partially permeable membrane and dialysis fluid flows on the other side, in opposite directions, maintaining a conc. gradient - harmful products in the blood diffuse out of the blood into the dialysis fluid. - filtered blood re-enters the body

    What are the issues of dialysis?

    - patients become increasingly sick between dialysis treatments which is very unpleasant - incredibly time consuming, each session is 3-5 hours and is needed 2-3 times a week. - There will eventually be no more safe areas to let blood flow out of the body as too many damaged blood vessels accumulate from previous dialysis sessions.

    What is a kidney transplant?

    Where a new kidney is implanted into a patient's body to replace a damaged kidney

    What must the donor kidney have?

    - matching blood type - matching tissue type

    Where do donor kidneys come from?

    Deceased organ donors or living organ donors (people can survive with only one kidney)

    What are the advantages of kidney transplants?

    The patient does not need to have dialysis treatment, which is time-consuming. Transplants are cheaper than dialysis in the long run.

    What are the risks associated with kidney transplants?

    - major operation - long recovery time - risk of rejection - must take immunosuppressants

    Give 3 things excretory products can be used to test for in urine.

    - pregnancy - anabolic steroids - recreational drugs

    How does a pregnancy test work?

    - hCG is a hormone produced by the placenta and is found in the mother's urine during pregnancy. - the test stick contains monoclonal antibodies for hCG bound to a blue bead dye. All monoclonal antibodies are identical. - When urine is applied, any hCG will bind to antibodies on the beads. - The hCG-antibody complex travels up the test strip carrying the beads with it - The test strip has immobilised antibodies which bind to hCG and release the blue dye concentrated along a line. - Regardless of hCG presence, a control line will appear to confirm the antibodies have travelled down correctly.

    What is an anabolic steroid?

    a hormone like testosterone that promotes muscle growth, usually artificially produced when referred to as anabolic steroids

    What are some side effects of anabolic steroids?

    Increased risk of heart disease, kidney damage, testicular atrophy, and aggressive behavior.

    How can you test for anabolic steroids?

    - urine testing by gas chromatography - the urine sample is vaporised and passed through a column containing a polymer. - Like TLC, different substances move through at different speeds in accordance with the mobile and stationary phases -a mass spectrometer is used to convert substances into ions and results are analysed by a computer, which is then compared with known values to determine what is present in the urine.

    How can you test for recreational drugs?

    - urine testing - usually a stick test analogous to that of a pregnancy test - if stick test is positive, gas chromatography and mass spectrometry is used to confirm.