intro to Excretion & LIVER

Question 1

define excretion, metabolic waste and state 3 main types

Answer 1

excretion = removal of metabolic waste from body
a substance produced in excess by metabolic processes; build up can become toxic

Eg. CO2 from resp, nitrogenous waste (urea from excess amino acids), bile pigments eg. bilirubin found in faeces, produced by breakdown of RBCs

Question 2

State the 4 excretory organs & briefly explain how each is involved in excretion

Answer 2

1. Lungs - CO2 excreted via expiration
2. Skin - sweat secreted contains urea & NH3
3. Kidneys - remove urea from blood
4. Liver - converts excess AAs to urea, breaks down old RBCs to bile pigments (ejected in faeces)

Question 3

What happens to excess CO2 in blood?

Answer 3

combines with H2O (carbonic anhydrase)in RBC to form carbonic acid
dissociates to H+ and HCO3-

H+ affects pH of cytoplasm; changes tertiary structure reducing affinity for O2

So now H+ can combine with Hb to form haemoglobinic acid
& CO2 not converted to acid can combine with Hb to form carbaminohaemoglobin

So oxygen transport further reduced

Question 4

Harmful effect of excess Co2 ?

Answer 4

excess H+ from carbonic acid dissociation; reduced pH
can alter tertiary structure of proteins in the body

small change in pH detected by respiratory centre in medulla oblongata of brain; increases breathing rate to remove CO2

if pH drops excessively low:
RESPIRATORY ACIDOSIS = lungs can’t remove enough CO2
rapid heart rate, high BP, headaches, drowsiness, restlessness, tremors,

Question 5

What causes respiratory acidosis?

Answer 5

bronchitis, asthma, emphysema, pneumonia, blockage of airway due to swelling or foreign object

Question 6

How are excess nitrogenous compounds, eg. amino acids from ingesting too much protein, dealt with?

Answer 6

DEANIMATION (removal of potentially toxic amino group): combines with oxygen to form keto acid & ammonia
keto acid enters Krebs cycle

NH3 combines with CO2 in ornithine cycle to form urea & water

Question 7

Blood supply to liver

Answer 7

Hepatic artery: oxygenated blood from heart- liver is highly metabolically active so has high O2 demand for respiration

Hepatic Vein: returns deoxygenated blood back to heart

Hepatic portal vein: deoxygenated blood from duodenum (part of small intestine from digestive system) rich in products of digestion to be detoxified if necessary before being allowed to circulate around the body

Question 8

4 vessels associates with liver lobules

Answer 8

hepatic artery
branch of hepatic vein (interlobular vessel)
hepatic portal vein
BILE DUCT

Question 9

purpose of bile duct

Answer 9

carries bile secreted by liver to the gall bladder to be stored until needed for use in small intestine; useful for fat digestion
also contains excretory products like bilirubin to be excreted

Question 10

what are kupffer cells & where are they found?

Answer 10

specialised macrophages that inhabit sinusoids of liver lobules
breakdown & recycle old RBCs
Bilirubin is a product of Hb breakdown

Question 11

How does blood reach the liver & what happens as it flows through?

Answer 11

blood from hepatic portal vein and hepatic artery reaches the lobules & mix in sinusoids
sinusoids are chambers between hepatocytes, so the blood is in close contact with the hepatocytes which detoxify any toxic compounds (from HPV), and exchange their waste products for fresh O2 & nutrients (from HA)

blood then leaves the sinusoid via a branch of the HV which eventually connects to the central HV to return to the heart, as it is now rendered safe to enter body circulation

Question 12

What happens to bile made in the liver?

Answer 12

travels down bile canaliculi to be fed into the bile duct, where it is fed into the gall bladder to be stored until required to aid digestion of fats in the small intestine

Question 13

What are the 3 main functions of the liver?

Answer 13

1. detoxification of alcohol and drugs
2. storage of glycogen
3. formation of urea to detoxify nitrogenous waste

Question 14

how are sinusoids adapted to maximise exchange in the sinusoids?

Answer 14

they have many microvilli so increased surface area for diffusion of substances in / out of the cell

the cytoplasm is very dense as inside are many mitochondria, and organelles involved in protein synthesis as they use lots of energy & enzymes for their metabolic processes.

Question 15

what shape are hepatocytes

Answer 15

cuboidal - so many can pack together in a line along the sinusoid walls

Question 16

explain the storage role of liver

Answer 16

it stores glucose as the polysaccharide glycogen (highly branched)
= insoluble granules in cytoplasm

Question 17

state the 2 terms for converting glucose to & from glycogen

Answer 17

Glycogenolysis = glycogen --> glucose
Glycogenesis = glucose --> glycogen

Question 18

state the 4 mechanisms by which the liver detoxifies harmful substances ingested, from the duodenum

Answer 18

oxidation, methylation, reduction OR combining with another molecule

Question 19

outline the process of hepatocytes detoxifying alcohol

Answer 19

ethanol dehydrogenated by ethanol dehydrogenase to ETHANAL (2NAD accepts 2H to 2NADH)

ethanal dehydrogenated by ethanal dehydrogenase to ETHANOIC ACID; AKA acetate (2NAD accepts 2H)

ethnic acid (acetate combines with CoA to form acetyl CoA for use in respiration

Question 20

what happens when excess ethanol is ingested?

Answer 20

NAD used up to NADH to break it down, so no NAD left for respiration; fatty acids can be used for respiratory so build up in the hepatocytes as fat; can lead to alcohol hepatitis / cirrhosis

Question 21

what are the 2 main processes involved in the formation of urea from excess amino acids

Answer 21

1. deamination

2. ornithine cycle

Question 22

outline how the liver deals with excess amino acids (too much protein ingested for example) (7 marks)

Answer 22

1. Deamination: AA + oxygen –> NH3 + keto acid
   keto acid used in respiration
2. Ornithine cycle: NH3 combines with CO2 to form urea & H2O

Question 23

why do we convert NH3 to urea?

Answer 23

urea is less toxic & less soluble

Question 24

Outline the ornithine cycle

Answer 24

1. NH3 and CO2 combine with ornithine to make citrulline
2. another NH3 combines with citrulline to produce arginine & H20
3. arginine combines with water to produce urea & ornithine
4. ornithine is regenerated so can re-enter cycle