5.1.2 - Excretion as an example of homeostatic control

Question 1

Give the definition of excretion.

Answer 1

The removal of metabolic waste from body.

Question 2

What is metabolic waste?

Answer 2

The unwanted products of cell metabolism.

Question 3

Why is excretion so important?

Answer 3

The waste products could:

• build up & become toxic
• act as inhibitors & reduce activity of enzymes.
• could interfere with cell process by altering the pH.

Excretion maintains:
> normal metabolism
> homeostasis = keeps levels of some substances in blood constant.

Question 4

What are the main excretory products?

Answer 4

• CD. from resp.
• nitrogen-containing compounds, e.g. urea (i.e. nitrogenous waste)
• other compounds, e.g. the bile pigments found in faeces.

Question 5

What are the 4 excretory organs?

Answer 5

• lungs
• liver
• kidneys
• skin

Question 6

Outline the role of the lungs in excretion.

Answer 6

• every cell in body produces CD. from resp.
• CD. is passed from cells of respiring tissues into bloodstream
• it is transported (mostly in the form of hydrogencarbonate ions) to lungs.
• in lungs: CD. diffuses into alveoli to be excreted as you exhale.

Question 7

Outline the roles of the liver in excretion.

Answer 7

• has many metabolic roles
• some substances produced will be passed into bile for excretion with faeces, e.g. pigment bilirubin.
• liver converts excess amino acids to urea.

> amino acids: broken down by DEAMINATION. Nitrogen-containing part of molecule is then combined w CD. to make urea.

Question 8

Outline to roles of the kidneys in excretion.

Answer 8

• urea: passed into bloodstream to be transported to kidneys
• urea is transported in solution - dissolved in the plasma
• in kidneys: urea is removed from b. to come a part of the urine.
  > urine is stored in bladder before being excreted from body via urethra.

Question 9

Outline the role of the skin in excretion.
(substances sweat contains)
(name 3 of these that are excretory products)
(why is the loss of water & salts via skin important?)

Answer 9

Sweat contains many substances inc.: 
> salts
> urea
> water
> uric acid
> ammonia

Urea, uric acid & ammonia are all excretory products.

Loss of water & salts: important part of homeostasis - maintaining the body temp. & Ψ of b.

Question 10

Give two metabolic products that are toxic and how they interfere with cell processes in the body.

Answer 10

• CD. & ammonia.

- interfere by altering the pH, so that normal metabolism is prevented.

Question 11

In what form is CD. transported in the blood?

Answer 11

As hydrogencarbonate ions.

Question 12

Forming hydrogencarbonate ions also forms…

Answer 12

hydrogen ions.

Question 13

Give the equation of carbonic acid formation.

Give the equation of carbonic acid dissociation.

Answer 13

CO2 + H2O > H2CO3

H2CO3 > (H+) + (HCO3-)

Question 14

Where does carbonic acid dissociation occur?

Answer 14

• inside the RBC., under the influence of the enzyme carbonic anhydrase.
• can also occur in the b. plasma.

Question 15

Give two things that H+ ions affect.

Answer 15

• H+ ions affect pH of cyt. in RBC.

- H+ ions interact w bonds within haem.,changing its 3D shape.

Question 16

How does the change in haemoglobin’s 3D shape affect its function?

Answer 16

• reduces the affinity for O., affecting O., transport.

- the H+ ions can then combine w haem. forming haemoglobinic acid.

Question 17

What happens to the CD. that is not converted to hydrogencarbonate ions?

Answer 17

It can combine directly w haem. producing carbaminohaemoglobin.

Question 18

Can haemoglobinic acid and carbaminohaemoglobin combine with oxygen as normal?

Answer 18

NO. They are unable to combine with O as normal. This reduces O transport further.

Question 19

How do H+ ions affects things in the blood plasma?

Answer 19

• excess H+ ions can reduced the pH of the plasma.
• maintain b.plasma pH is ESSENTIAL = changes could alter the structure of many proteins in the blood that help to transport different substances around body.
• proteins in b. act as buffers to resist change in pH.

Question 20

If the change in blood pH is small, what happens?

Answer 20

• the extra H+ ions are detected by respiratory centre in the MEDULLA OBLONGATA of brain.

> this causes an increase in breathing rate to help remove excess carbon dioxide.

Question 21

If the change in blood pH is big, what happens?

Answer 21

• if b. drops below 7.35 = may cause:

> headaches, drowsiness, restlessness, tremor & confusion. Rapid heart rate & change in blood pressure = RESPIRATORY ACIDOSIS.

• can be caused by diseases: emphysema, chronic bronchitis, asthma, or sever pneumonia.
• blockage of airway due to swelling, foreign object, vomit = can also induce.

Question 22

How does the body deal with excess amino acids?

Answer 22

• bod. cannot store excess AA. contain almost as much energy as carbs. Wasteful to excrete them.
• AA. are transported to liver & potentially toxic amino group is removed (DEAMINATION).
• amino group initially forms v soluble & toxic ammonia.
  > converted to urea. (Can be transported to kidneys for excretion.)
• remaining keto acid can be used directly in respiration to release energy or may be converted to carb or fat for storage.

Question 23

Give the equation for the formation of keto acid and ammonia. (Deanimation)

Then give the equation for the formation of urea.

Answer 23

Deanimation: amino acid + oxygen > keto acid & ammonia.

Formation of urea: ammonia + carbon dioxide > urea + water.

symbol: 2NH3 + CO2 > (NH2)2CO + H2O.

Question 24

What is another word for liver cells?

Answer 24

Hepatocytes.

Question 25

Why does the liver need a good supply of blood and an internal structure that ensures as much blood as possible flows past as many liver cells as possible?

Answer 25

Because the liver has an important role in homeostasis. good b. supply & internal structure = enables liver cells to remove excess or unwanted substances from b. & return substances to b. to ensure concs. are maintained.

Question 26

The liver is supplied with blood from which two sources?

Answer 26

The HEPATIC ARTERY & HEPATIC PORTAL VEIN.

Question 27

Give the function of the hepatic artery.

Answer 27

- oxygenated b. from heart travels from aorta via the HA. into liver. - this supplies the O that is essential for aerobic resp. - liver cells v active & carry out many metabolic processes. Many of these need energy, in form of ATP, so liver needs good supply of O for aerobic resp.

Question 28

Give the function of the hepatic portal vein.

Answer 28

- deoxygenated b. from digestive system enters liver via HPV. > b. is rich in products of digestion. - concs. of various substances will be uncontrolled = just entered bod. from products of digestion in intestines. - b. may contain toxic compounds that have been absorbed from intestine. > important that these substances do not continue to go round bod before their concs. have been altered!

Question 29

What is the role of the hepatic vein?

Answer 29

- b. leaves liver via HEPATIC VEIN | - HV. rejoins vena cava and b. returns to bods. normal circulation.

Question 30

What is the fourth vessel connected to the liver and what is its role?

Answer 30

- the BILE DUCT. - bile is a secretion from liver which has functions in digestion & excretion. - BD. carries bile from liver >GALL BLADDER, = stored until needed to help digestion of fats in small intestine. - bile also contains some excretory products e.g. bile pigments > bilirubin = will leave bod w. faeces.

Question 31

Why do the cells, blood vessels and chambers in the liver need to be arranged in a certain way?

Answer 31

to ensure the greatest possible contact between the b. and the liver cells.

Question 32

What are lobes and what are lobules in the liver? | Give the shape of these lobules.

Answer 32

- liver is divided into LOBES which are further divided into lobules. - the LOBULES are cylindrical.

Question 33

Describe in detail how the blood ends up being in close contact with the liver cells. (hint: hepatic artery, hepatic portal vein, inter-lobular vessels, sinusoid)

Answer 33

- as HA. and HPV. enter liver, they split into smaller and smaller vessels > these vessels run between and parallel to the lobules: INTER-LOBULAR VESSELS. - at intervals, branches from HA. and HPV. enter the lobules. > b. from the two blood vessels is mixed & passes along chamber: SINUSOID = lined with liver cells. > as b. flows along SINUSOID it is in close contact with the liver cells. - the liver cells: able to remove substances from b. & return other substances to b.

Question 34

What are Kupffer cells and what is their primary function? | Give an example of one of the products of haemoglobin breakdown.

Answer 34

Kupffer cells move about within the sinusoids. PRIMARY FUNCTION: to breakdown and recycle old b. cells. > one of the products of haem. breakdown is BILIRUBIN: one of the bile pigments excreted as part of the bile.

Question 35

What happens to bile made in the liver cells?

Answer 35

1) bile is made in liver cells & released into the BILE CANICULI. 2) the BC. join together to form the BD., which transports bile to GB.

Question 36

What happens when the blood reaches the end of the sinusoid?

Answer 36

1) b. reaches end of sinusoid - the concs. of many of its components have been modified and regulated 2) at centre of each lobule is a branch of the HV. known as the INTRA-LOBULAR VESSEL. > sinusoids empty into this vessel. 3) branches of the HV., from different lobules, join together to form HV., which drains b. from liver.

Question 37

Why are portal vessels unusual?

Answer 37

- they have capillaries at each end | - there are also portal vessels in part of the brain to facilitate hormonal communication.

Question 38

Why do liver cells (hepatocytes) appear to be relatively unspecialised?

Answer 38

- they have a simple cuboidal shape | - have many microvilli on their surface.

Question 39

The liver cells' many metabolic functions include.. How is the cytoplasm adapted to these functions?

Answer 39

- protein synthesis - transformation & storage of carbs - synthesis of cholesterol & bile salts - detoxification MANY OTHER PROCESSES. this means the CYTOPLASM: - must be v dense - is specialised in the nos. of certain organelles that it contains.

Question 40

Give the many functions of the liver.

Answer 40

- control of b. glucose levels, amino acid levels, lipid levels - synthesis of bile, plasma proteins, cholesterol - synthesis of red b. cells in the fetus - storage of vitamins A, D & B12, iron, glycogen - detoxification of alcohol, drugs - breakdown of hormones - destruction of red b. cells.

Question 41

Describe how the liver stores glycogen.

Answer 41

- liver stores sugars in the form of glycogen - able to store approx. 100-120g of glycogen - the glycogen forms granules in the cytoplasm of the hepatocytes. - the glycogen can be broken down to release glucose into b. as required (e.g. when blood glucose levels drop)