8.18 · Transport in Mammals
The Circulatory System — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions
5 min read 12 parts Lesson 2 of 21 1 quiz
Lesson summary
8.1 The Circulatory System — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: State that mammals have a closed double circulation, relate blood vessel and blood cell structure to function, and explain the formation of tissue fluid and lymph.
- Mass transport
- Open circulation
- Closed circulation
- Pulmonary artery
- Pulmonary vein
- Other vessels
Section 1 of 12Need for mass transport and types of circulation
Need for mass transport and types of circulation
Mass transport
- Mass transport large organisms have a small SA:V ratio and long diffusion distances,
- so exchange surfaces such as lungs and gut are linked to a system that moves blood in bulk in one direction.
Open circulation
- Open circulation in arthropods and many molluscs, blood is pumped into body cavities and bathes the organs directly.
Closed circulation
- Closed circulation vertebrates keep blood in vessels at higher pressure.
Double circulation
- Double circulation mammals' blood passes through the heart twice per circuit.
- The right side pumps deoxygenated blood to the lungs, and the left side pumps oxygenated blood to the body.
- Lung capillaries receive lower pressure, which avoids damaging them, while the body receives blood at high pressure.
Section 2 of 12Main blood vessels
1Sequence
- Sequence vena cava, right atrium, right ventricle, pulmonary artery, lungs, pulmonary vein, left atrium, left ventricle, aorta, body.
2Pulmonary artery
- Pulmonary artery it carries deoxygenated blood and is the only artery to do so.
3Pulmonary vein
- Pulmonary vein it carries oxygenated blood.
4Other vessels
- Other vessels coronary arteries, hepatic artery, hepatic portal vein, and renal artery and vein.
Section 3 of 12Structure of arteries
Three layers
- Three layers tunica intima (endothelium), tunica media and tunica externa (collagen).
- (smooth muscle and elastic fibres)
Elastic fibres
- Elastic fibres they stretch during systole and recoil during diastole, maintaining pressure and smoothing flow.
Collagen
- Collagen it resists overstretching and bursting in artery and vein walls.
Elastic arteries
- Elastic arteries those near the heart, such as the aorta, have a tunica media rich in elastic fibres.
Muscular arteries
- Muscular arteries those further away have more smooth muscle and less elastin, so they can constrict or dilate to control blood flow to organs.
Arterioles
- Arterioles they have proportionally more smooth muscle, and contraction narrows while relaxation widens them.
- (vasoconstriction)
Pressure
- Pressure arterial pressure fluctuates and falls greatly in arterioles and capillaries.
- (for example 16 kPa / 12 kPa, or 120/80 mmHg)
Section 4 of 12Capillaries and veins
Capillaries
- Capillaries the thin endothelium gives a short diffusion distance,
- and the small lumen squeezes red cells against the wall, slowing them and bringing oxygen close to tissues.
- Gaps (fenestrations) allow tissue fluid to form.
- Their large total cross-sectional area slows blood flow, giving more time for exchange.
Veins
- Veins a wide lumen gives low resistance, the wall is thin because pressure is low, and valves prevent backflow.
- Contraction of nearby skeletal muscle squeezes veins and pushes blood towards the heart.
| Feature | Artery | Capillary | Vein |
|---|---|---|---|
| Wall thickness | Thick | One cell (endothelium) | Thin |
| Elastic fibres | Many | None | Few |
| Smooth muscle | Much | None | Little |
| Lumen | Narrow | About 7 µm, one cell width | Wide |
| Valves | None (except at heart exit) | None | Semilunar valves |
| Pressure | High, pulsatile | Falling | Low, steady |
Section 5 of 12Blood cells
Red blood cells
- Red blood cells biconcave, small and flexible to pass through capillaries, and without a nucleus or organelles to hold more haemoglobin.
- (large surface area to volume ratio)
Neutrophils
- Neutrophils multilobed nucleus, and they are phagocytes.
Lymphocytes
- Lymphocytes a large round nucleus with little cytoplasm;
- B cells produce antibodies and T cells kill infected cells.
Monocytes
- Monocytes large white cells with a bean-shaped nucleus that leave the blood to become macrophages.
Section 6 of 12Water in blood
Solvent
- Solvent water is the main component of plasma and transports solutes.
Heat distribution
- Heat distribution its high specific heat capacity helps distribute heat without large temperature changes.
Tissue fluid role
- Tissue fluid role it supplies cells with oxygen, glucose and amino acids and removes carbon dioxide and wastes,
- and its volume is kept roughly constant.
Section 7 of 12Formation of tissue fluid
1
At the arterial end of a capillary, hydrostatic pressure of blood is higher than the pressure of tissue fluid.
2
Water and small solutes (glucose, amino acids, ions, oxygen) are forced out through gaps between endothelial cells by pressure filtration.
3
Large plasma proteins and red blood cells stay in the capillary, and the proteins give the blood a lower water potential.
4
As fluid leaves, hydrostatic pressure falls along the capillary.
5
At the venous end the water potential gradient returns much water to the blood by osmosis, carrying carbon dioxide and urea.
- Oedema
- high blood pressure or a low plasma protein concentration makes more tissue fluid form and accumulate.
Section 8 of 12Lymph
1
Excess tissue fluid drains into blind-ended lymph capillaries, which have valves.
2
Squeezing of surrounding muscles moves the lymph along.
3
Lymph passes through lymph nodes.
4
It returns to the blood in the subclavian veins near the heart.
| Component | Plasma | Tissue fluid | Lymph |
|---|---|---|---|
| Red blood cells | Present | Absent | Absent |
| Plasma proteins | Many | Very few | Few |
| White cells | Present | Some (phagocytes) | Many lymphocytes |
| Oxygen and glucose | High | Lower | Low |
Section 9 of 12More on this topic
Size
- Size mass about 300 g, roughly the size of a closed fist.
Pressure graph
- Pressure graph the left atrium produces low pressure, enough for a short distance,
- and the left ventricle produces much higher pressure so blood reaches the body.
Section 10 of 12More from the specification
More from the specification
Arterial Structure for High Pressure
- Systemic arteries contain relatively thick muscular and elastic tissue layers in their tunica media, enabling them to withstand and smooth out high pulsing pressures from ventricular systole.
Capillary Architecture for Material Exchange
- Capillary walls are one-cell thick composed solely of flattened endothelial cells, creating an extremely short diffusion distance for respiratory gases, glucose,
- and waste substances.
Structural Comparison of Blood Vessels
- While arteries contain narrow lumens and thick elastic walls, veins have relatively wide lumens, thinner walls,
- and internal valves to prevent backflow under low venous pressure.
Exchange of Dissolved Substances
- The narrow caliber of capillaries forces red blood cells to travel in single file, slowing flow velocity and optimizing the exchange of dissolved substances between blood plasma and tissue fluid.
Arterial Structure for High Pressure
- Systemic arteries contain relatively thick muscular and elastic tissue layers in their tunica media, enabling them to withstand and smooth out high pulsing pressures from ventricular systole.
Capillary Architecture for Material Exchange
- Capillary walls are one-cell thick composed solely of flattened endothelial cells, creating an extremely short diffusion distance for respiratory gases, glucose,
- and waste substances.
Structural Comparison of Blood Vessels
- While arteries contain narrow lumens and thick elastic walls, veins have relatively wide lumens, thinner walls,
- and internal valves to prevent backflow under low venous pressure.
Exchange of Dissolved Substances
- The narrow caliber of capillaries forces red blood cells to travel in single file, slowing flow velocity and optimizing the exchange of dissolved substances between blood plasma and tissue fluid.
Section 11 of 12Key terms
| Term | Meaning |
|---|---|
| Closed circulation | Blood is always contained within vessels, allowing higher pressure and faster flow. |
| Double circulation | Blood passes through the heart twice in one circuit of the body. |
| Artery | A vessel carrying blood away from the heart at high pressure. |
| Capillary | The smallest vessel, with a wall one endothelial cell thick, where exchange takes place. |
| Vein | A vessel carrying blood towards the heart at low pressure, with valves. |
| Tissue fluid | Fluid surrounding body cells, formed by pressure filtration from capillaries. |
| Hydrostatic pressure | The pressure a fluid exerts on the walls of its container. |
| Lymph | Tissue fluid that has drained into lymphatic vessels. |
Section 12 of 12Exam technique
1
Saying arteries always carry oxygenated blood — the pulmonary artery does not.
2
Saying veins have thick walls — they have thin walls with a wide lumen and valves.
3
Confusing tissue fluid with plasma — tissue fluid lacks large proteins and red blood cells.
4
Saying oedema is caused by too much plasma protein — a low protein concentration causes it.
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