| 1.1.2 The Heart and Circulation |
|---|
The mammalian circulatory system overcomes the limits of diffusion through mass transport.
Diffusion is too slow to supply large, active organisms, so a heart pumps blood through a network of vessels.
Arteries
- Carry blood at high pressure away from the heart
- Have thick, muscular, elastic walls that withstand and help maintain the high pressure
- Have a narrow lumen
Veins
- Return blood at low pressure back to the heart
- Have thinner walls, with less muscle and elastic tissue
- Have a wider lumen
- Have valves to prevent the backflow of blood
Capillaries
- Have walls only one cell thick (a single layer of endothelium), giving a short diffusion distance for rapid exchange of substances between the blood and the tissue cells
- Have a very narrow lumen, so red blood cells pass in single file, which slows the flow and aids exchange
The four-chambered heart
- Contains two atria (upper chambers) and two ventricles (lower chambers)
- Contains four valves that prevent the backflow of blood
- Has a thicker left ventricle wall than the right ventricle, to pump blood at high pressure around the whole body
The cardiac cycle
- Consists of atrial systole (contraction of the atria), ventricular systole (contraction of the ventricles), and cardiac diastole (relaxation of the heart)
- Valves open and close because of pressure differences between the chambers and the major vessels
At a Glance
| Vessel | Pressure | Walls | Lumen |
|---|---|---|---|
| Arteries | High pressure | Thick, muscular, elastic walls | Narrow lumen |
| Veins | Low pressure | Thinner walls | Wider lumen |
| Capillaries | Lower than arteries (falls along their length) | Walls one cell thick | Very narrow (about one red blood cell wide) |
Oxygen is transported by haemoglobin, a protein with four polypeptide chains and four haem groups, each able to bind one oxygen molecule.
The oxygen dissociation curve is sigmoid because of cooperative binding, allowing efficient loading at high pO₂ in the lungs and unloading at low pO₂ in the tissues.
The Bohr effect shifts the curve to the right when CO₂ rises or pH falls, releasing more oxygen where respiration is most active.
Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin, ensuring oxygen transfer from mother to fetus across the placenta.
Carbon dioxide travels mainly as hydrogencarbonate ions in the plasma, with smaller fractions dissolved or bound to haemoglobin as carbaminohaemoglobin.