AS-LEVEL AQA BIOLOGY NOTES 
TOPIC 4: mass transport in animals

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Haemoglobin is the primary method of oxygen transportation in the blood. It is also important for carbon dioxide transportation. Haemoglobin is found in large quantities in red blood cells (RBCs). One hemoglobin molecule has four haem groups and each can bind reversibly to one oxygen molecule.

        Hb + 4O2 →  HbO8

Haemoglobin binds to oxygen in the lungs, where oxygen is present in high concentrations. It is transported by the blood, pumped by the heart, throughout the body. When it arrives at an area of high oxygen consumption, for example a muscle being used, it is arriving in an area of high carbon dioxide concentration, since respiration produces carbon dioxide.

• Haemoglobin has higher affinity to CO2 than O2
  
• Hb dissociates with O2 and binds with CO2 instead
  
• Thereby depositing oxygen in areas that are respiring a lot and getting rid of carbon dioxide which is a waste product that needs to be removed
  

Many species have haemoglobin which is specialised to their environmental and physiological needs.

Haemoglobin dissociation curve:

• S-shaped curve
  
• Low pO2 (eg respiring muscle)
  
  • Low partial pressure of Oxygen
    
  • Hb dissociates oxygen
    
  • Binds to CO2
    
• High pO2 (eg lung capillaries)
  
  • High partial pressure of Oxygen
    
  • Due to CO2 concentration gradient across capillary wall, Hb that is bound to CO2 dissociates
    
  • Hb is now free to bind to O2
    
  • First O2 molecule binds to haem group and these changes the overall shape of Hb making it easier for the other 3 molecules to bind
    
    • Called the ‘cooperative nature’ of Hb
      
• This is called the Bohr Effect
  

Blood circulation in the heart
Nervous Control of the Cardiac Cycle

• Sinoatrial node (SAN) sends signal across walls of both atria
  
• Signal causes atria to contract
  
• Signal reaches the AV node
  
• Signal conducted down the purkinje fibres (specialised conducting tissue)
  
• This signal causes the ventricles to contract from the bottom upwards
  
• This occurs 55-80 times a minute
  

Diastole

• Both atria and ventricles relaxed
  
• Pressure decreased
  
• Internal volume increases
  
• Blood flows into heart from major veins
  
• Blood flows into atria, then through open atrioventricular valves and into ventricles
  
• ‘Filling phase’
  
• Atria and ventricles have blood inside of them
  

Atrial Systole

• Both atria contract together (systole)
  
• Small increase in pressure created by contraction helps to push blood from aorta into already partially full ventricles
  
• Stretches walls of ventricles
  
• Ensures that they are already full of blood
  
• AV valves then shut
  

Ventricular Systole

• Ventricles full of blood
  
• Begin to contract
  
• Pressure increases
  
• Volume decreases
  
• Contractions start at apex, pushing blood upwards towards arteries
  
• Semilunar valves forced open
  
• Blood pushes out of both ventricles and into the arteries
  
• Semilunar valves shut

Blood circulation in the body

	Blood vessel entering	Blood vessel exiting
Heart	Coronary artery	Coronary vein
Lungs	Pulmonary artery	Pulmonary vein
Kidney	Renal artery	Renal vein

EXAM TIPS Practice drawing the structure of the heart - particularly the internal anatomy - and labelling as much of it as you can. The tricuspid and bicuspid valves are also sometimes called the right and left atrioventricular valves, respectively

Arteries

• Function
  
  • Blood pumped into arteries by the ventricles of heart
    
  • Arteries carry blood away from the heart
    
  • Blood travels to other part of body
    

• E.g. lungs, muscles
  

• Structure
  
  • Thick walls do not allow for diffusion of chemicals
    
  • Strong walls made of elastic fibre and muscle swell and contract as blood surges through them every time heart beats
    
  • Small Lumen
    
  • Blood is at high pressure
    
  • Therefore, arteries must be able to give under pressure
    
  • Blood is pumped through by the heart
    
• Reason for structure
  
  • To keep blood pressure high and to be able to withstand high pressure
    

Arterioles

• Function
  
  • Smaller branches of arteries
    
  • Deliver oxygen and nutritional supplies to tissue
    
• Structure
  
  • Similar to arteries
    
  • Smaller diameter
    
• Reason for structure
  
  • Blood moves slightly slower to allow time for diffusion across capillary wall
    

Veins

• Function
  
  • Veins bring blood back to the heart
    
  • The blood flows in only one direction
    
  • Veins do not pulse, the blood is helped back to the heart by muscles
    

• In arms and legs, veins lie between muscles
  

• • When muscles contract, they squeeze the blood back to the heart
    
  • Valves only allow blood to be pumped one way
    
• Structure
  
  • Strong walls – but thinner than artery walls
    
  • Valves – only allow blood to flow in one direction
    
  • Large Lumen
    
  • Thin outer wall
    
• Reason for structure
  
  • To allow for the transport lots of blood
    
  • Easy transport of blood to the lungs and heart
    

Capillaries

• Function
  
  • Link between the arteries and veins
    
  • Diffusion into/out of the blood only occurs here
    
• Structure
  
  • Outer wall only one cell thin – allows diffusion of substances into/out of the blood in the capillaries
    
  • Form ‘capillary beds’ in tissues
    
  • Steady blood flow
    
  • Small Lumen
    
  • Very thin – has to be small enough to fit between cells, so that it can bring blood to every cell in the body
    
• Reason for Structure
  
  • So that substances can easily diffuse into/out of the blood through the walls
    

EXAM TIP It might be useful to know that capillaries are able to alter the solidarity of the junctions between endothelial cells - making them more or less leaky. This is useful during infection, for instance, when white blood cells need to be able to leave the circulatory system and enter infected tissue to fight pathogens.

Formation of tissue fluid
Tissue fluid is the fluid that surrounds individual cells and creates their environment. Tissue fluid homeostasis is a very important process and preserves constant characteristics that constitute the optimum environment for cells so they they function correctly. Tissue fluid is formed from blood plasma:

1. Blood in the arteriolar end of the capillary has high hydrostatic pressure, meaning it has a high water potential.
   
2. Because it has a lot of water, it also has a lot of dissolved ions and small molecules like oxygen, glucose and amino acids
   
3. These molecules - including water - are small enough to be forced through the capillary lining because of this pressure
   
4. This is ultrafiltration
   
5. By the time blood arrives at the venous end of the capillary, the blood has lost some water and ions. It has a reduced hydrostatic pressure.
   

The proteins, which were too big to exit the capillary by ultrafiltration, are now at a higher concentration. They give the blood in the venous end of the capillary a high oncotic pressure, which draws back in water by osmosis, down the concentration gradient. ​