P1 Mass Transport in Animals

Question 1

Why do animals require transport systems?

Answer 1

1. Meet high metabolic demands
2. Maintain steep concentration gradients
3. Overcome low SA:V ratio
4. Allow diffusion across large distances

Question 2

What is an open circulatory system?

Answer 2

Blood is pumped into cavities surrounding organs

Question 3

What is a closed circulatory system?

Answer 3

Blood is contained in blood vessels

Question 4

Describe a single closed circulatory system and a double closed circulatory system

Answer 4

• In a single closed circulatory system blood passes through the heart once.
• In a double closed circulatory system blood passes through the heart twice.

Question 5

Describe the structure of blood vessels

Answer 5

Arteries, arterioles and veins all have the same basic structure: an outer layer, muscle layer, elastic layer, endothelium and a lumen.
- All outer layers contain collagen to contain structural support. The thickness of each layer depends on the blood vessels function.

Question 6

Structure of arteries

Answer 6

• A thick muscle layer that contracts and relaxes to control blood flow.
• A thick elastic layer which stretches and recoils to maintain high blood pressure.
• Overall wall is thick to prevent arteries bursting from high pressure.

Question 7

Structure of arterioles

Answer 7

• Thicker muscle layer than arteries, which contracts to reduce blood flow into capillaries.
• Thinner elastic layer than arteries as they don’t need to maintain a high blood pressure.

Question 8

Structure of veins

Answer 8

• Relatively thin muscle layer as blood flow does not need to be controlled.
• Thin elastic layer as only low blood pressure is needed.
• Overall venal wall is thin.
• Contain valves to prevent the back flow of blood.

Question 9

Structure of venules

Answer 9

The same structure as veins, but are much smaller:
- thin muscle layer
- thin elastic layer
- thin walls
- contain valves

Question 10

Structure of capillaries

Answer 10

• Only contain a thin endothelium and narrow lumen, to provide a short diffusion distance.
• There are a large number of capillaries and they are highly branched providing a large surface area for exchange.

Question 11

What is tissue fluid?

Answer 11

• A liquid surrounding cells that consists of a range of substances, including water, glucose and ions.
• Its role is to control the exchange of substances between blood and cells.

Question 12

How is tissue fluid formed?

Answer 12

Tissue fluid is formed from blood plasma:
- blood moving into the capillaries has a high hydrostatic pressure (pressure exerted by a fluid) at the arteriole end, which is greater than the oncotic pressure (tendency of water to move into the blood by osmosis).
- this difference in pressure forces water (and small dissolved substances) out of the capillaries, forming tissue fluid.
- this loss of fluid and the remaining plasma proteins causes the oncotic pressure to be greater than the hydrostatic pressure in the capillaries, forcing fluid out of the tissue fluid and back into the venus end of the capillary.

Question 13

Which other way does tissue fluid return from the tissues to the circulatory system?

Answer 13

A small amount of tissue fluid returns back to the circulatory system by the lymphathic system:
- the fluid there is called lymph, it has a similar composition to blood plasma, but less oxygen and nutrients
- however it contains more proteins due to antibody production by lymphocytes

Question 14

Describe the cardiac cycle:

Answer 14

1. During diastole the atria and ventricles are relaxed, and blood enters the atria.
2. When the blood pressure in the atria is greater than the ventricles, the atrioventricular valves open.
3. During atrial systole, the atria contract, decreasing the volume of the atria, increasing the pressure even more and all remaining blood enters the ventricles.
4. During ventricular systole, the ventricles contract, decreasing the volume and increasing the pressure.
5. When the pressure in the ventricles is greater than the atria, the atrioventricular valves close, preventing the back flow of blood into the atria. Meanwhile, the semi lunar valves open, and blood is pushed into the blood vessels. When pressure in the blood vessels is greater than the ventricular pressure, the semi lunar valves close, preventing back flow.

Question 15

Where are atrioventricular valves located?

Answer 15

Between the atria and the ventricles

Question 16

Where are semi lunar valves located?

Answer 16

Between the ventricles and the blood vessels

Question 17

How is resting heart rate maintained?

Answer 17

1. The sinoatrial node (SAN) sends out a wave of depolarisation, causing the atria to contract.
2. The wave of depolarisation reaches the atrioventricular node (AVN) where it is delayed.
3. When the wave of depolarisation is released, it is conducted along the bundle of His (a collection of specialised muscle fibres called Purkyne fibres) and causes the ventricles to contract.

Question 18

What does a normal ECG look like?

Answer 18

• It has 3 main peaks.
  1. When the atria are depolarised, there is a small peak - the P wave.
  2. When the ventricles are depolarised the ECG shows a large peak - the QRS complex.
  3. When the ventricles repolarise, there is a final small peak - the T wave.

Question 19

What is tachycardia?

Answer 19

The heart beats too fast

Question 20

What is bradycardia?

Answer 20

The heart beats too slow

Question 21

What is fibrillation?

Answer 21

The heart beats irregularly:
1. Atrial fibrillation shows a large peak followed by many small peaks.
2. Ventricular fibrillation shows many medium sized peaks (all the same size).

Question 22

What is an ectopic heartbeat?

Answer 22

When there are extra heart beats outside of the normal rhythm.

Question 23

Describe the structure of haemoglobin

Answer 23

• A protein with a quaternary structure (made up of 4 polypeptide chains).
• Each chain contains a Fe2+ haem group, enabling it to bind to oxygen (one haemoglobin can bind to 4 oxygen molecules).

Question 24

What happens after the first oxygen molecule binds to haemoglobin?

Answer 24

• Haemoglobin’s quaternary structure changes, making it easier for a second oxygen molecule to bind. The same happens when the second oxygen molecule binds, as more of the Fe2+ ions are uncovered.
• This is called positive cooperativity.
• The same occurs when oxygen molecules are released, when one is released the tertiary structure changes and makes it easier to release the next oxygen molecule.
• Therefore oxygen association/dissociation is an example of positive cooperatively. This speeds up the transfer of oxygen from alveoli to haemoglobin in the capillaries, and speeds up the transfer of oxygen from haemoglobin to respiring cells.

Question 25

What causes the association and dissociation of oxygen?

Answer 25

• Oxygen moves down a pressure gradient from a high partial pressure in the alveoli to a low partial pressure in the capillaries, and this causes oxygen to associate with the Fe2+ ions on haemoglobin (a low partial pressure causes oxygen dissociation).
• Similarly when oxygen arrives at aerobically respiring cells, there is a pressure gradient and oxygen moves from a high partial pressure in the capillaries, to a lower partial pressure in the cells, causing the dissociation of oxygen (a high partial pressure causes oxygen association).

Question 26

How does partial pressure affect the number of oxygen molecules bound to haemoglobin?

Answer 26

Increasing partial pressure of oxygen, increases the number of oxygen molecules that bind to haemoglobin - percentage saturation of oxygen.

Question 27

Describe the reasons for the oxyhemoglobin dissociation curve

Answer 27

1. Initially the gradient is not steep, because it is hardest/slowest for the initial oxygen atom to bind to haemoglobin.
2. There is then a steep gradient due to positive cooperactivity.
3. The curve then rolls off, not reaching 100% oxygen saturation. This is because once the majority of Fe2+ ions have oxygens attached, it is more difficult for the remaining oxygen molecules to locate the Fe2+ ions.

Question 28

How does carbon dioxide concentration affect the oxyhemoglobin dissociation curve?

Answer 28

Decreasing carbon dioxide concentration shifts the curve left, and increasing carbon dioxide concentration shifts the curve to the right.
- Therefore at lower carbon dioxide concentrations, oxygen associates more readily, and dissociates less readily (haemoglobin has a HIGHER AFFINITY for oxygen).
- At higher carbon dioxide concentrations, oxygen associates less readily, and dissociates more readily (haemoglobin has a LOWER AFFINITY for oxygen).
- This is known as the Bohr effect.

Question 29

Why does carbon dioxide affect the oxyhemoglobin dissociation curve?
(Bohr effect)

Answer 29

• Increasing the concentration of carbon dioxide increases blood acidity (decreases pH).
• This is because carbon dioxide reacts with water, forming H+ ions, which affect blood pH.
  1. CO2 +H2O –> H2CO3 (carbonic acid) - catalysed by carbonic anhydrase
  2. H2CO3 –> H+ + HCO3-
• This affects the oxygen dissociation curve because haemoglobin is a protein, and changes in pH changes the shape of the protein.
• At a lower pH the shape changes so it is easier for oxygen to dissociate, and at a higher pH the shape changes so it is easier for oxygen to associate.

Question 30

Describe the chloride shift

Answer 30

• If the concentration of carbon dioxide is too high in the blood, there will not be a concentration gradient for CO2 to diffuse down, and CO2 will be trapped in tissues/cells.
• Therefore red blood cells are permeable to HCO3- ions, which diffuse into the blood plasma to prevent them from reacting with H+ ions to result in carbon dioxide.
• For every HCO3- ion that diffuses into the blood plasma, a Cl- ion diffuses out of the blood plasma and into the red blood cells to maintain a constant charge in the blood plasma.
• However, one red blood cells reach the alveoli the opposite occurs in order to release CO2: Cl- ions diffuse out of the red blood cells, and HCO3- ions diffuse back in. They react with H+ to form H2CO3, which splits into H2O and CO2.
• CO2 then diffuses down the concentration gradient to out lungs and is exhaled.

Question 31

What are the two ways of maintaining blood pH (reducing H+ ion concentration)?

Answer 31

1. H+ + HCO3- –> H2CO3
2. H+ ions react with haemoglobin, forming haemoglobinic acid (then if blood pH gets too high, haemoglobinic acid can release the H+ ion).

Question 32

How does foetal haemoglobin differ to adult haemoglobin?

Answer 32

• Foetal haemoglobin has a higher affinity for oxygen (associates more readily and dissociates less readily) and therefore lies to the left of the adult haemoglobin curve on an oxygen dissociation curve.
• This is because foetuses receive their oxygen from their mothers blood at the placenta. The placenta has a low partial pressure of oxygen, so when the mothers blood arrives this causes oxygen to dissociate. This oxygen travels to the foetus, and since the foetal haemoglobin has a higher affinity for oxygen, it can associate with oxygen at the lower partial pressure, ensuring the foetus has enough oxygen for respiration.