Module 2 - Excretion

8/6/2019 Module 2 - Excretion

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Excretion

1) Excretion

y Dont confuse with egestion which is the removal of undigested food by the process of defecationy Carbon dioxide is a waste product from respiration and needs to be removedy It is passed out of cells of respiring tissues into the bloodstream where it is transported to the lungs

where it diffuses into the alveoli and is breathed out

y It must be removed becauseo It is carried in the blood by hydrogen carbonate ions which also form H + ions which bind with

haemoglobin reducing oxygen transport

o Carbon dioxide also directly combines with haemoglobin resulting in haemoglobin having alower affinity for oxygen

o Excess carbon dioxide combines with H 2O to form carbonic acid which lowers the blood pHwhich can result in respiratory acidosis

y Nitrogen containing compounds such as ureay Urea is produced in the liver from the breakdown of excess of amino acids , this isdeaminationy Transported to the kidneys where it is formed into urine

y The liver removes the amine group from the amino acid to produce ammoniao Amino acid + oxygen Keto acid + ammonia

y Ammonia is converted into a les soluble and less toxic compound called ureao 2NH3 + CO2 CO(NH2)2 + H2O

Ammonia carbon dioxide urea water

y The remaining Keto acid can be used directly in respiration or stored as a carbohydrate

Syllabus statements:

4.2.1 (a)(b)

Excretion the removal of metabolic waste from the body

Metabolic waste consists of waste substances that may be toxic

or are produced in excess by the reactions inside cells

Deamination the removal of the amine group from an amino acid

to roduce ammonia


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Excretion

2) Liver structure

* See pages 38 and 39 of OCR text book for diagrams ofliver*

y Liver cells (hepatocytes) carry out many metabolic functionso Microvilli to increase surface area for absorptiono Very dense containing many organelles for numerous functions

y Kupffer cells are specialised macrophages that break down and recycle old erythrocyteso Bilirubin is a waste product of the breakdown of haemoglobin, brown pigment found in faeces

Arteries, Veins andducts

y The hepatic artery supplies the liver with oxygenated blood from the heart via the aorta (hepatocytesneed oxygen as they are very active and respire rapidly)

y The hepatic portal vein delivers deoxygenated blood from the digestive system, the blood is rich in theproducts of digestion any harmful substances are filtered out and broken down

y Blood leaves the liver via the hepatic vein and rejoins the vena cavay The bile duct carries bile produced by the liver to the gall bladder where it is stored

o Bile is used to emulsify fatso Connected to the central vein via tubes called canaliculi

GeneralStructure

y Liver consists ofliver lobules - cylindrical structures made of hepatocytes that are arranged in rowsradiating out from the centre

y Each lobule has a central vein in the middle that connects to the hepatic veiny Many branches of the hepatic artery, hepatic portal vein and bile duct are also connected to each

lobule

y Hepatic artery and hepatic portal vein combine and are connected to the central vein by capillariescalled sinusoids

y Blood runs through the sinusoids, past the hepatocytes which remove harmful substances and oxygenfrom the blood

y Hepatocytes break down the harmful substances into less harmful ones which re -enter the bloody The blood runs to the central vein and the cen tral veins of all the lobules connect up to form the

hepatic vein


4.2.1 (c)


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Excretion

3) Liver function

y The liver has many functions including:o storage of vitamins, control of blood glucose levels, detoxification of alcohol, breakdown of

erythrocytes

Formation of urea

y Excess amino acids cannot be stored in the body the amine group makes them toxicy However amino acid molecules contain a lot of chemical potential energy so it would be wasteful to

excrete the whole molecule

y Therefore it undergoes treatment in the liver involving 2 steps1) Deamination2) The ornithine cycle

y Deamination produces ammonia which is highly toxic and highly soluble therefore must not be allowedto accumulate

o Amino acid + O2 Keto acid + Ammoniay Ammonia is very soluble and very toxic it must be converted into a less soluble and less toxic form

before it can be excreted

y Ammonia is combined with CO2 in the ornithine cycle to produce urea o 2NH3 + CO2CO(NH2)2 + H2O

y Urea is less soluble and less toxic than ammonia and can be passed into the blood to be transported tothe kidneys where it is excreted via urine

Detoxification

y The liver is also responsible for the breakdown of harmful substances like alcohol, drugs and unwantedhormones

y Alcohol (ethanol) is a toxic substance that can damage cells but it still has a lot of chemical potentialenergy

y It is broken down in the hepatocytes by the enzyme ethanoldehydrogenase forming ethanaly This is dehydrogenated further by the enzyme ethanaldehydrogenase to form acetate (ethanoic acid)y This is combined with acetyl coenzyme A and enters the Krebs cycley NAD is reduced by the hydrogens released forming reducedNADy NAD is also required to oxidise and break down fatty acids for use in respirationy If the liver has to detoxify too much alcohol it has insufficient NAD to deal with the fatty acids which

are then converted back to lipids and stored in the liver

y This causes the liver to become enlarged (fatty liver) which can lead to alcohol related hepatitis orcirrhosis


4.2.1 (d)(e)

Detoxification the conversion of toxic molecules to less toxic or non

toxic molecules


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Excretion

4) KidneyStructure

* See pages 14 and 15 of OCR revision guide for diagrams of kidney and nephron*

The kidney

y The main function of the kidneys is to excrete waste products (e.g. urea) produced by the liver producingurine which passes down the ureter and is stored in the bladder

y The kidneys also play a role in osmoregulation y The outer region of the kidney is called the cortexy The inner region of the kidney is called the medullay The centre is the pelvis which leads to the uretery Blood enters the kidneys through the renal arteryy The process of ultra filtration and selective re -absorption occursy The filtered blood passes out of the kidney through the renal vein

The nephron

y The nephron is the functional unit of the kidneyy It is a microscopic tubule that receives fluid from the blood capillaries in the cortex and conver ts this to

urine

y Each nephron starts in the cortex y The capillaries in the cortex form a knot called the glomerulusy The glomerulus is surrounded by a cup shaped structure called the bowmans capsule

o (Fluid from the blood is pushed into the bowmans (renal) capsule during ultra filtration)y The capsule leads into the nephron which is divided into 4 parts

o Proximal convoluted tubuleo Distal convoluted tubuleo Loop of Henleo Collecting duct

y The proximal tubule is the part closest to the glomerulusy The distal tubule is the part furthest from the glomerulusy A convoluted tubule is one that is bent and coiled to increase surface area for absorptiony The glomerulus is a fine network of capillaries that increase the local blood pressure to squeeze fluid

out of the blood

y It is surrounded by a cup (bowmans capsule) which collects the fluid and leads into the nephrony Afferent vessels bring blood to the organy Efferent vessels take blood away from the organ


4.2.1 (f)(g)


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Excretion

5) Ultra filtration

y Blood from the renal artery enters small arterioles in the cortexy Blood flows into the glomerulus from the afferent arterioley Filtered blood is taken away from the glomerulus by the efferent arteriole

o Efferent arterioles has a smaller diameter than the afferent arterioleo This difference in diameters means the blood in the glomerulus is under high pressure

y This high pressure forces liquid and small molecules in the blood out of the capillary and into thebowmans capsule

y The liquid and small molecules pass through 3 layers to get into the bowmans capsule and enter thenephron tubules

1) Endothelium of capillaries y Has narrow gaps between its cells where blood plasma and substances dissolved in it

can pass through

2) Basement membrane (main filter)y Consists of a fine mesh of collagen fibres and glycoproteins that prevent molecules with

RMM > 69000 passing through

y This means most proteins and red blood cells are held in capillaries3) Epithelial cells of bowmans capsule (podocytes)

y Podocytes have finger like projections these ensure there are gaps between the cellsy Fluid from the blood in the glo merulus can pass through these gaps into the lumen of

the bowmans capsule

y The following substances are filtered out of the bloodo Watero Amino acidso Glucoseo Ureao Inorganic ions (sodium, chloride, potassium)

y The following substances are kept in the capillaryo Blood cellso Large proteins (RMM > 69000)o (This results in a very low water potential)


4.2.1 (h)

Ultra filtration filtration at a molecular level


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Excretion

6) Selective re-absorption

y Many useful substances (e.g. water, glucose etc) were filtered out during ultra filtrationy Most of these useful substances will be reabsorbed in the proximal convoluted tubule

o 85% of filtrate reabsorbed hereo All of the glucose and amino acids and some of the water reabsorbed

y 65% of the water that was filtered is reabsorbed at the proximal convoluted tubule via osmosisbecause the filtrate has a higher water potential than the blood

Process

y Sodium-Potassium pumps removed sodium ions from the cells lining the proximal convoluted tubuley This reduces the concentration of sodium ions in the cell cytoplasmy Sodium ions are then transported into the cell along with glucose or aminoacid molecules via

facilitated diffusion

y Glucose and amino acid concentrations rise within the cell resulting in them diffusing out of theopposite side of the cell into the tissuefluid

o Process may be aided by active removal of glucose and amino acidso From the tissue fluid they diffuse into the blood and are carried away

y Larger molecules (e.g. small proteins) will be reabsorbed by endocytosisy As this happens the water potential of the cell drops resulting in osmosis of water into the celly The filtrate that remains is urine which passes along the ureter to the bladderAdaptation of the cells lining the proximal convoluted tubule

y The cell surface membrane is folded forming microvilli to increase surface area for re-absorptiony Membrane contains co-transporter proteins that transport glucose/amino acid in association with

sodium ions

y Cell cytoplasm contains many mitochondria needed to produce lots ofATP for active transport y Opposite side of cell is folded into microvilli to increase movement into tissue fluid


4.2.1 (j)


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Excretion

7) Loop of Henle

* Refer to diagram in page 24 of CGP revision guide and page 47 of OCR textbook*

y The role of the loop of Henle is to create a low water potential in the tissue of the medulla to ensurethat even more water can be reabsorbed from the fluid in the collecting duct

y The loop of Henle is made up of two limbs the descending limb and the ascending limby This sets up a mechanism called the countercurrent multiplier mechanismProcess

1) Near the top of the ascending limb Na+and Cl-ions are actively pumped out of the loop of Henleinto the medulla

2) The ascending limb is impermeable to water so the water stays inside the tubule creating a lowwater potential in the medulla due to a high concentration of ions

3) Due to a lower water potential in the medulla than in the descending limb water moves out of thedescending limb into the medulla via osmosis and is reabsorbed into the blood through the capillary

network

4) The ions cannot diffuse out as the descending limb is impermeable to them this makes the filtratemore concentrated

5) Near the bottom of th e ascending limbNa+and Cl-diffuse out into the medulla and water remainsfurther lowering the water potential of the filtrate

6) The water potential in the medulla is extremely low due to the massive increase in ion concentrationin the medulla

7) This causes water to move out of the collecting duct via osmosis into the medulla and as before thewater in the medulla is reabsorbed into the blood through the capillary network

y Different animals have different length loops of Henley The longer an animals loop of Henle the more water they can reabsorb from the filtratey When there is a longer ascending limb more ions are actively pumped out into the medulla creating a

really low water potential in the medulla

y This means more water is moved out of the nephron and collecting duct into the capillariesy This results in a very concentrated uriney Longer loops of Henle are common in desert animals where water is sparse to conserve water


4.2.1 (i)


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Excretion

8) Osmoregulation

y The water potential of the blood is monitored by osmoreceptors in the hypothalamusy When the osmoreceptors are stimulated by low water potential they lose water via osmosis causing

them to shrink

y This stimulates neurosecretory cells in the hypothalamus which produce the anti diuretic hormone(ADH)

y ADH is stored in the pituitary gland until it is neededy The ADH enters the blood capillaries and acts on the cells of the collecting ducty Its effect is to increase the number ofaquaporins in the walls of the collecting duct and DCT resulting

in more water to be reabsorbed

y Once the water potential of the blood rise again lessADH is releasedy When your dehydrated

o Water content of your blood drops therefore water potential dropso Osmoreceptors in the hypothalamus detects this changeo Posterior pituitary gland is stimulated releasing more ADH into the bloodo More ADH means the DCT and collecting duct are more permeable so more water is reabsorbed

into the blood via osmosis

o A small amount ofhighly concentrated urine is produced andless water is lost

y When your hydratedo The water content of your blood rises so its water potential riseso This is detected by the osmoreceptors in the hypothalamus o The posterior pituitary gland releases lessADH into the blood o Less ADH means DCT and collecting tube are less permeable so less w ater is reabsorbed into the

blood by osmosis

o A large amount ofdilute urine is produced and water is lost


4.2.1 (i)

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


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Excretion

9) Urine Sampling and kidney failure

y Kidney failure is when the kidneys cant carry out their normal functionsy Can be caused by many things e.g.

o Diabetes mellituso Hyper tensiono Infection

y Kidney failure can cause a lot of problems o Waste products such as urea build up in the blood which can cause vomiting and weight loss o Fluid starts to accumulate causing parts of the body to swell

y There is two options for treatment kidney transplants or dialysisy Dialysis is the use of a partially permeable membrane to filter the blood and there is two types

o Haemodialysis - blood from the vein is passed in to a machine that contains an artificial dialysismembrane must be performed at clinic expensive and time consuming

o Peritonealdialysis the filter is the bodys own abdominal membrane (peritoneum) tube isconnected and the peritoneum is filled with dialy sis fluid after several hours fluid is drained can

be done at home

y Kidney transplants have many advantages and disadvantagesAdvantages Disadvantages

Freedom from time consuming dialysis Need immunosuppressants for the life of the

kidney

Diet is less limited Need major surgery under general anaesthetic

One feels physically better Risks of surgery include infections and internal

bleeding

A better quality of life e.g. ability to travel Frequent checks for signs of organ rejection

No longer seen as chronically ill Immunosuppressants can increase susceptibility to

infections

y Some small substances

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Module 2 - Excretion