8.1.18 · Transport in Mammals
Circulatory Systems — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
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Lesson summary
8.1.1 Circulatory Systems — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Compare open and closed, single and double circulatory systems and explain the advantages of a double circulation in mammals.
- Small organisms
- Larger, active animals
- Mass transport
- Components
- Limitation
- Pulmonary circulation
Part 1 of 8Why a transport system is needed
Why a transport system is needed
Small organisms
- Small organisms flatworms rely on diffusion because of a large SA:V ratio and short diffusion distances.
Larger, active animals
- Larger, active animals they have a small SA:V ratio, high metabolic rates and cells too far from the exchange surface for diffusion to supply oxygen and nutrients fast enough.
Mass transport
- Mass transport exchange surfaces such as the lungs and digestive system are connected to a system that moves gases or liquids in bulk in one direction through vessels.
Components
- Components a pump (the heart), vessels and a transport fluid.
- (blood or haemolymph)
Functions
- Functions it carries respiratory gases, nutrients, hormones and wastes, and distributes heat.
Part 2 of 8Open and closed circulation
| Feature | Open circulation | Closed circulation |
|---|---|---|
| Fluid | Haemolymph, which leaves the vessels and bathes the tissues directly | Blood, which stays within vessels |
| Pressure | Low | High, and can be adjusted |
| Speed and control | Slow, with little control of distribution | Fast, with flow directed to organs that need it |
| Examples | Insects, arthropods, most molluscs | All vertebrates, annelid worms, squid |
Insects
- Insects they have an open system but do not use it for oxygen, because tracheae carry air directly to the tissues.
Active animals
- Active animals closed systems suit them because blood is delivered quickly at a controlled rate.
Part 3 of 8Single circulation
1Fish
- Fish blood passes through the heart once in each complete circuit.
2Heart
- Heart a two-chambered heart pumps blood to the gills, where it is oxygenated.
- (one atrium and one ventricle)
3Flow
- Flow blood leaves the gills and goes on to the tissues without returning to the heart.
4Limitation
- Limitation pressure is low after the narrow gill capillaries, so blood flows slowly, limiting oxygen delivery and how active a fish can be.
Part 4 of 8Double circulation in mammals
Double circulation in mammals
Pulmonary circulation
- Pulmonary circulation deoxygenated blood goes from the right side of the heart to the lungs and back to the left side.
Systemic circulation
- Systemic circulation oxygenated blood goes from the left side to all body organs, and deoxygenated blood returns to the right side.
Pressure boost
- Pressure boost blood returns to the heart after the lungs, where pressure has dropped, and is pumped again to a higher pressure for the body.
Effects of high pressure
- Effects of high pressure faster flow, more efficient delivery of oxygen and removal of wastes, a high metabolic rate and maintenance of body temperature in endotherms.
Lungs
- Lungs they receive blood at lower pressure, which prevents damage to delicate capillaries and stops fluid being forced into the alveoli.
Separation
- Separation oxygenated and deoxygenated blood do not mix, so blood leaving to the body is fully saturated.
Part 5 of 8Direction of flow in a mammal
- Vena cava.
- Right atrium.
- Right ventricle.
- Pulmonary artery.
- Lungs.
- Pulmonary vein.
- Left atrium.
- Left ventricle.
- Aorta, then arteries to organs, capillaries, venules, veins and back to the vena cava.
1Organs in parallel
- Organs in parallel each organ has its own arterial supply from the aorta, so each receives fresh oxygenated blood.
2Exceptions
- Exceptions the pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood.
Part 6 of 8Heart chambers across vertebrates
| Group | Heart | Circulation |
|---|---|---|
| Fish | Two chambers | Single, with low-pressure delivery to tissues |
| Amphibians | Three chambers (two atria, one ventricle) | Partly double, with blood partially mixed |
| Reptiles (many) | Ventricle partly divided | Incomplete double |
| Birds and mammals | Four chambers | Complete double, allowing very high metabolic rates and constant high body temperature |
- Ventricle walls
- the left ventricle wall is about three times thicker than the right because it pumps to the whole body.
Part 7 of 8More from the specification
More from the specification
Arterial wall elasticity and lumen
- Arterial wall elasticity and lumen elastic arteries contain thick smooth muscle and elastic fibres surrounding a narrow lumen to withstand pulsed blood pressure.
Collagen and valves in veins
- Collagen and valves in veins large veins contain tough collagen fibres to prevent overstretching and possess relatively wide lumens with internal pocket valves.
Capillary exchange of dissolved substances
- Capillary exchange of dissolved substances thin capillary walls permit rapid diffusion of dissolved respiratory gases, water,
- and glucose between blood plasma and tissues.
Blood Vessel Histology
- Arteries vs Veins — Arteries possess thick tunica media walls containing smooth muscle cells and elastic fibres to withstand and smooth out high pulsatile pressure, surrounding a relatively narrow lumen;
- veins have thinner walls with less muscle and elastic tissue, a wider lumen,
- and semilunar valves to prevent backflow of blood.
Structural Role of Collagen and Elastic Fibres
- Arterial walls contain an outer tunica externa rich in fibrous collagen to provide tensile strength and prevent vessel rupture,
- while elastic fibres in the tunica media stretch during ventricular systole and recoil during diastole to maintain continuous blood flow.
Blood Plasma Composition and Transport of Dissolved Substances — Blood plasma is an aqueous fluid composed of over 90% water,
- which acts as a versatile biological solvent transporting dissolved substances—including glucose, amino acids, urea, mineral ions, hormones,
- and plasma proteins—between organs.
Tunica media
- The middle layer of the wall of a blood vessel containing muscle and elastic fibres.
Hydrostatic pressure
- The pressure a fluid exerts on the walls of its container.
Water potential
- A measure of the tendency of water to move; pure water has the highest.
Concentration gradient
- The difference in concentration of a substance between two regions.
Collagen
- A protein in skin, gums and bones that needs vitamin C to form.
Plasma
- The liquid part of blood.
Smooth muscle
- Involuntary muscle that narrows or widens airways.
Part 8 of 8More on this topic
- Pressure in atrium and ventricle then rises slowly as both keep filling.
Key terms
| Term | Meaning |
|---|---|
| Closed circulation | A system in which blood stays inside vessels. |
| Open circulation | A system in which fluid leaves the vessels and bathes the tissues. |
| Single circulation | Blood passes through the heart once in each full circuit. |
| Double circulation | Blood passes through the heart twice in each full circuit. |
| Pulmonary circulation | The circuit from the heart to the lungs and back. |
| Systemic circulation | The circuit from the heart to body organs and back. |
| Endotherm | An animal that maintains body temperature using internal metabolic heat. |
Avoid these mistakes
1
Saying all arteries carry oxygenated blood — the pulmonary artery carries deoxygenated blood.
2
Saying that in a double circulation blood goes around the body twice — it passes through the heart twice per circuit.
3
Saying insects use the open circulation to transport oxygen.
4
Saying fish blood returns to the heart after the gills — in a single circulation it goes on directly to the body.
5
Describing the pulmonary circuit as carrying blood at high pressure.
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