Excretion Flashcards
define the term excretion;
the removal of waste products of metabolism from the body
these waste products are harmful/toxic and would damage cells if allowed to accumulate
How is a build up of CO2 dangerous if not removed from the body?
forms carbonic acid which lowers the pH of the blood slowing down enzyme controlled reactions
removed excreted by the lungs
What is the importance of removing nitrogenous waste from the body?
amino acids from protein foods cannot be stored as they become toxic and would lower the pH.
deamination occurs to remove the NH2 group to form very soluble but very toxic ammonia which id combined with CO2 in the ornithine cycle to form the much less toxic and fairly soluble urea which is excreted in urination by the kidneys
describe the histology and gross structure of the liver
lobules, hepatic artery, hepatic portal vein, sinusoid, bile canaliculus
Liver is made up of several lobes.
Hepatocytes are arranged in chains radiating from the centre of hexagonal lobules surrounding the interlobular vein which carries blood to the hepatic vein which then takes it to the vena cava.
At the corners of each lobule are branches of the hepatic artery brings oxygenated blood from the aorta and the hepatic portal vein brings blood from the gut. Oxygenated and deoxygenated blood from the two vessels is mixed in the sinusoid where it passes many hepatocytes which take up waste and nutrients from it. The sinusoid is lined with phagocytic macrophages which take up bacteria from the blood.
The bile canaliculus carries bile secreted by liver cells to the bile duct –> stored in the gall bladder –> released into small intestine.
describe the formation of urea in the liver
proteins from food are digested into amino acids.
Excess amino acids are deaminated in the liver - the amino group, NH2, is removed to for ammonia NH3
The remainder of the amino acid forms keto acid which can be respired or converted to fat and stored.
The toxic ammonia is detoxified into less harmful urea via the ornithine cycle in the liver cells - it combines with CO2. ATP required. The urea then leaves the liver via the hepatic vein and is carried to the kidneys where water is added and it is excreted as urine.
Detoxification occurs in the liver. What is detoxification? give two examples that take place in the liver.
Detoxification is the breakdown of toxins into less harmful products.
- Ammonia is detoxified into urea
- Alcohol is detoxified by being broken down by enzymes into produces that can be respired ethanol –(ethanol dehydrogenase)–> ethanal –(ethanal dehydrogenase)–> acetate –> krebs cycle (respiration) Ethanol is oxidised to ethanal by NAD.
- Old hormones detoxified, limiting the time they are functional in the body
- drugs such as antibiotics are detoxified in the liver
How can the detoxification of alcohol lead to damage?
The detoxification of alcohol requires NAD to oxidise ethanol to ethanal. Reduced NAD accumulates if too much alcohol is detoxified, meaning less NAD is available for the metabolism (oxidation) of fatty acids. Fatty acids thus accumulate and form fats,
Excess fat in the liver cells causes damage leading to cirrhosis which is eventually fatal as the liver cells can no longer break down toxins and carry out its vital functions.
describe the gross structure of the kidneys
The kidney has an other cortex region, a middle medulla and an inner pelvis.
Each kidney is made up of about a million nephrons
Each kidney receives blood from a renal artery which branches in the cortex to form mamy afferent arterioles which supply the nephrons. A narrower efferent arteriole branches into capillaries which wrap around each nephron and join into venues and then to the renal in which carries blood out of the kidney
describe the histology of the nephrons
Glomerulus walls made of squamous endothelium with pores/
Bowman’s capsule of each nephron is lined with podocytes cells which have foot like processes.
Between bowman’s and glomerulus the basement membrane acts as the main filter.
Walls of the proximal convoluted tubules are made of cuboidal epithelium, which have many microvilli on their inner surfaces and contain many mitochondria.
describe the process of ultrafiltration in the nephrons
High b.p in the glomerulus (due to high b.p in renal arteries, narrow glomerular capillaries, wider afferent arteriole and narrower efferent arteriole) leads to high hydrostatic pressure the the glomerulus which forces water and solutes from blood into Bowman’s capsule via the basement membrane.
Solutes have to be small enough to pass through basement membrane, leaving red and white blood cells, plasma proteins and thrombocytes behind.
Filtrate consists of water, dissolved ions, glucose, vitamins, amino acid,s fatty acids, glycerol, urea, ammonia, uric acid.
describe the process of selective reabsorption in the nephrons
occurs in the proximal convoluted tubules in the cortex if the kidneys, and occurs to conserve nutrients.
Means of reabsorption:
- diffusion down conc. gradient
- active transport of sodium ions (thus creating a diffusion gradient)
- co-transport: sodium ions are linked to glucose/amino acids and carry them with them down their conc. gradient
- osmosis of water: the reabsorption of solutes lowers the water potential in blood so some water is reabsorbed by osmosis
Wastes remain in filtrate and increase in concentration as water leaves.
How is the proximal convoluted tubule adapted for effective reabsorption?
- large surface area thanks to long length and presence of microvilli on cuboidal epithelial cells
- convolutions to slow down flow rate thus increasing time for reabsorption
- many blood capillaries close to tubule to maintain conc. gradient and provide short diffusion distance
- single layer of cuboidal epithelial cells for short diffusion distance
- many mitochondria = ATP = active transport
- specific carrier and transporter proteins for nutrients in the cell membrane
describe how a steep diffusion and osmotic gradient between the ascending and descending limbs of the loop of Henle is maintained
Occurs in the collecting duct and makes use of the counter-current system in the loop of Henle: fluids flow in opposite directions maintaining a steep diffusion and osmotic gradient between them - filtrate in ascending limb flows upwards and blood in surrounding flows upwards, whilst urine in the collecting duct flows downwards and blood in surrounding capillaries flows upwards.
describe the sequence of events that leads to the further reabsorption of water from the collecting duct
- Na+ and Cl- actively transported out of filtrate from ascending limb of loop of Henle into tissue fluid and diffuse iinto the blood aided by the counter-current flow
- This lowers the water potential of the blood in the medulla causing water to move by osmosis from the filtrate in the descending limb/urine in the collecting duct, aided by the counter current flow
How much water moves out by osmosis depends on the presence of ADH
Describe the sequence of events that occurs in the kidneys when the blood water potential decreases
- Decreased water potential detected by osmoreceptors in the hypothalamus
- the posterior pituitary releases more ADH into the blood
- ADH binds ro membrane receptors on the cells of the collecting duct walls, making them more permeable to water as more aquaporins move to the cell membranes
- More water moves through aquaporins into blood, increasing blood water potential
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