1. Overview
The mammalian circulatory system is a closed double circulation: blood is always held inside the heart and blood vessels, and it passes through the heart twice on each complete circuit of the body. The system is made of a heart, blood, and a series of vessels — arteries, arterioles, capillaries, venules and veins. Blood moves around two linked loops: the pulmonary circulation (heart to lungs and back) and the systemic circulation (heart to the rest of the body and back). Each vessel type has a wall structure that is matched to its job, and substances are exchanged between the blood and the body's cells through tissue fluid formed in capillary networks.
Key Definitions
- Closed circulatory system: a system in which blood is always contained within blood vessels and the heart, rather than flowing freely through body cavities.
- Double circulation: a system in which blood passes through the heart twice for every complete circuit of the body, made up of a pulmonary circuit and a systemic circuit.
- Pulmonary circulation: the part of the circulation that carries blood between the heart and the lungs.
- Systemic circulation: the part of the circulation that carries blood between the heart and the rest of the body.
- Artery: a blood vessel that carries blood away from the heart, usually at high pressure.
- Vein: a blood vessel that carries blood back towards the heart, at low pressure, and contains valves.
- Capillary: a microscopic vessel with a wall one cell thick where exchange between blood and tissue fluid occurs.
- Tissue fluid: the fluid that surrounds and bathes body cells, formed from blood plasma forced out of capillaries.
- Hydrostatic pressure: the pressure exerted by a fluid, which at the arteriole end of a capillary pushes fluid out of the blood.
Content
A closed double circulation
In a mammal, blood never leaves the vessels — it is a closed system, which lets the blood be kept under pressure and directed precisely to where it is needed. It is also a double circulation: blood passes through the heart twice for each full circuit of the body. The two loops are:
- the pulmonary circulation, carrying blood from the heart to the lungs and back, and
- the systemic circulation, carrying blood from the heart to all other organs and tissues and back.
The advantage of a double circulation is that blood pressure can be restored at the heart between the two loops. Blood leaving the lungs has lost a lot of pressure passing through the lung capillaries; returning it to the heart allows it to be pumped out again at high pressure to the rest of the body, so oxygen reaches the tissues quickly and a high metabolic rate can be supported.
The general path of blood through the vessels is: arteries → arterioles → capillaries → venules → veins. Arterioles are small arteries that feed the capillary beds and can constrict or dilate to control blood flow into a tissue; venules are small veins that collect blood from capillaries and join to form veins.
The main blood vessels
Four large vessels connect the heart to the two circuits:
- Pulmonary artery: carries deoxygenated blood from the right side of the heart to the lungs. (It is unusual in being an artery that carries deoxygenated blood.)
- Pulmonary vein: carries oxygenated blood from the lungs back to the left side of the heart. (It is unusual in being a vein that carries oxygenated blood.)
- Aorta: the main artery carrying oxygenated blood from the left side of the heart to the systemic circulation (the rest of the body).
- Vena cava: the main vein returning deoxygenated blood from the body to the right side of the heart.
Recognising and drawing vessels
Under the microscope, the three main vessel types can be told apart from a transverse section (TS):
- Artery: a relatively small, rounded lumen with a thick wall; the wall has visible elastic tissue and a thick layer of smooth muscle.
- Vein: a large, often irregular lumen with a thin wall; valves may be seen in longitudinal section (LS).
- Capillary: a tiny vessel whose wall is a single layer of endothelium, often only just wide enough for a red blood cell to pass through.
When you make a plan diagram of an artery or vein in TS, draw the boundaries of the tissue layers as clear lines — do not draw individual cells. Show the lumen, the inner endothelium, the layer of smooth muscle and elastic tissue, and the tough outer collagen coat, and keep the proportions correct (a thick wall and small lumen for an artery; a thin wall and large lumen for a vein).
You may also be asked for a longitudinal section (LS), which shows the vessel cut along its length so the wall layers run lengthways as parallel bands and the lumen appears as a long open channel. The point of drawing a vein in LS is to show a pocket (semilunar) valve: two flap-like pockets attached to the wall that open towards the heart to let blood through and close to prevent backflow when pressure behind them drops.
Scale and magnification care: if a question gives a scale bar or asks you to measure a vessel from a micrograph, take your time over the units. Convert carefully along the chain , and always sanity-check that your final size is a sensible order of magnitude (a capillary is only a few across, not a few nm).
How vessel structure relates to function
Each vessel's wall is built for the pressure it must handle and the job it does. The table below summarises the differences:
| Vessel | Wall thickness | Lumen | Elastic vs muscle tissue | Valves | Main function |
|---|---|---|---|---|---|
| Elastic artery (e.g. aorta, near heart) | Thick | Narrow | Mostly elastic fibres | No | Wall stretches with each surge of blood and recoils to smooth the flow and maintain pressure |
| Muscular artery (further from heart) | Thick | Narrow | More smooth muscle | No | Muscle contracts/relaxes to change lumen diameter and direct blood to organs |
| Vein | Thin | Wide | Little of either | Yes | Low resistance to low-pressure return; valves prevent backflow |
| Capillary | One endothelial cell thick | Very narrow | Neither | No | Short diffusion distance and large surface area for exchange |
A few points to add to the table:
- All arteries have thick walls and a narrow lumen so they can withstand and maintain the high pressure of blood pumped from the heart.
- Veins rely on valves plus the squeezing action of surrounding skeletal muscles to push the low-pressure blood back towards the heart.
- Capillaries also have gaps between the endothelial cells that let plasma leak out to form tissue fluid, and the slow flow through them gives more time for exchange of oxygen, carbon dioxide, glucose and other substances.
Blood cells you must recognise and draw
Identify each cell by its shape and especially the shape and relative size of its nucleus:
| Cell | Relative size | Nucleus | Function |
|---|---|---|---|
| Red blood cell (erythrocyte) | Small | No nucleus; biconcave disc | Carries oxygen (packed with haemoglobin) |
| Monocyte | Largest white cell | Large kidney- or bean-shaped | Develops into a macrophage; phagocytosis |
| Neutrophil | Medium white cell | Lobed (multi-lobed), granular cytoplasm | Engulfs pathogens by phagocytosis |
| Lymphocyte | Small white cell | Large and round, almost fills the cell (thin rim of cytoplasm) | Specific immune response |
When drawing these cells, show the outline of the cell and the nucleus clearly, since these features are how each one is identified.
Water and its role in transport
Water is the main component of both blood plasma and tissue fluid. Two properties of water make it ideal for transport:
- Solvent action: water is a polar molecule, so it dissolves many ions (such as Na⁺ and Cl⁻), glucose, amino acids and other polar substances. These dissolved substances can then be transported in solution around the body.
- High specific heat capacity: a large amount of energy is needed to change water's temperature, so the watery blood can absorb and distribute heat with only a small change in its own temperature. This helps keep the body's core temperature stable.
Tissue fluid: functions and formation
Tissue fluid bathes the body's cells and is the medium through which they exchange substances with the blood. Its functions are to supply cells with oxygen, glucose, amino acids, ions and other nutrients, and to carry away waste products such as carbon dioxide — cells cannot exchange directly with the blood, so tissue fluid acts as the go-between.
At the arteriole end of the capillary
- The hydrostatic pressure of the blood, created by the pumping of the heart, is high — higher than the pressure outside the capillary.
- This ultrafiltration pushes plasma out through the gaps between the endothelial cells: water and small dissolved molecules (glucose, ions) leave.
- Plasma proteins and blood cells are too large to pass through, so they stay inside the capillary. The fluid that escapes is tissue fluid.
At the venule end of the capillary
- By this point the hydrostatic pressure has dropped (fluid has been lost and the vessel offers resistance), so it no longer dominates.
- The plasma proteins left behind give the blood a low (more negative) water potential. Tissue fluid contains far fewer proteins, so it has a relatively higher water potential than the blood.
- Water therefore moves down the water potential gradient, from the tissue fluid back into the capillary by osmosis, returning much of the tissue fluid to the blood.
Not all of the fluid returns directly. The remaining excess drains into the lymphatic system and is eventually returned to the blood.
The pressure profile along the circulation
Blood pressure is not constant around the body — it falls steadily as blood moves further from the heart:
- Highest in the aorta and arteries, where blood is pumped directly from the ventricle.
- It drops sharply across the arterioles and capillaries, because these vast, narrow, branching vessels offer huge resistance to flow.
- It is lowest in the veins, and lowest of all where blood arrives back at the right atrium from the vena cava, ready to be moved into the ventricle.
Worked example
Exam-style question: Explain how the structure of an artery is related to its function. [3]
Model answer:
- The thick wall contains elastic fibres that stretch and recoil with each surge of blood, so that the artery can withstand the high pressure from the heart and keep blood moving smoothly between beats.
- The narrow lumen helps maintain the high pressure of the blood as it is carried away from the heart.
- Smooth muscle in the wall can contract or relax to alter the lumen diameter, which allows the flow of blood into different organs to be controlled.
(Each mark needs a structural feature and its functional consequence; naming the feature alone — "thick wall", "narrow lumen" — scores nothing.)
Worked example
Exam-style question: A capillary runs through a block of muscle tissue. Explain how tissue fluid is formed at the arteriole end of this capillary. [3]
Model answer:
- At the arteriole end the hydrostatic pressure of the blood is high (generated by the contraction of the heart), higher than the pressure of the fluid outside the capillary.
- This forces plasma, containing water and small dissolved molecules such as glucose and ions, out through the gaps between the endothelial cells.
- Plasma proteins and blood cells are too large to pass through, so they remain in the capillary; the fluid that leaves is tissue fluid.
Worked example
Exam-style question: The table shows the mean blood pressure measured in different vessels of the systemic circulation.
| Vessel | Mean blood pressure / kPa |
|---|---|
| Aorta | 12.5 |
| Small arteries | 11.0 |
| Arterioles | 8.0 |
| Capillaries | 4.0 |
| Veins | 1.0 |
Using the data, describe and explain how blood pressure changes from the aorta to the veins. [4]
Model answer:
- Describe: blood pressure falls continuously along the system, dropping only slightly from the aorta to the small arteries (12.5 to 11.0 kPa) but steeply across the arterioles and capillaries (11.0 down to 4.0 kPa), reaching its lowest value in the veins (1.0 kPa).
- The high pressure near the heart is produced by the contraction of the ventricle, and the elastic walls of the arteries recoil to maintain it.
- The sharp drop occurs because the arterioles and capillaries are numerous and have a very narrow lumen, giving a high total resistance to blood flow and using up pressure energy.
- By the veins the pressure is very low, so blood return relies on valves and the squeezing of skeletal muscles rather than on pressure from the heart.
Key Equations
This is a qualitative topic, so no equations are required. The one quantitative skill is converting units when measuring vessels or cells from micrographs: The key conceptual idea is that hydrostatic pressure is highest at the arteriole end of a capillary and falls towards the venule end, while the water potential of the blood stays low because plasma proteins remain inside — together these two factors decide whether fluid leaves or re-enters the capillary.
Common Mistakes to Avoid
- Defining double circulation as just "blood going to the lungs and the body". You must mention the heart: blood passes through the heart twice for every complete circuit, or name the pulmonary and systemic circuits. Leaving the heart out loses the mark.
- Saying "concentration of water" when explaining why fluid returns to the capillary. Use the correct term water potential, and describe water moving from a region of higher water potential to a region of lower water potential by osmosis.
- Forgetting that blood pressure falls along the system. Pressure is highest in the arteries, drops sharply across the arterioles and capillaries, and is lowest in the veins — with the very lowest point at the right atrium as it fills from the vena cava. Don't describe veins as carrying high-pressure blood.
- Mixing up the pulmonary vessels. The pulmonary artery carries deoxygenated blood and the pulmonary vein carries oxygenated blood — the opposite of every other artery and vein.
- Drawing individual cells in a plan diagram. A plan diagram shows the boundaries between tissue layers only; never draw single cells in the wall of an artery or vein.
- Saying plasma proteins leave the capillary. Proteins (and blood cells) are too large to be filtered out; only water and small solutes form tissue fluid, and the retained proteins are what give blood its low water potential.
- Confusing the white blood cells. Identify cells by their nucleus shape: lobed for neutrophils, large and round for lymphocytes, kidney-shaped for monocytes.
- Muddling units when measuring from a micrograph. Convert carefully between mm, µm and nm and never mix them up — a capillary is a few micrometres wide, not a few nanometres. Always check your answer is a sensible order of magnitude before writing it down.
Exam Tips
- When asked to compare vessels, write a single comparative sentence (for example, "arteries have a thicker wall and a narrower lumen than veins") rather than describing each vessel separately.
- Always link structure to function explicitly — e.g. "elastic tissue recoils to maintain pressure" or "the wall is one cell thick to give a short diffusion distance" — don't just state the structure.
- When the command word is "explain" in a structure-function question, every mark needs a structural feature joined by "so that" / "which allows" to a functional consequence (feature + reason). A list of features on its own answers "describe", not "explain", and scores no marks.
- For tissue-fluid questions, name the two opposing factors precisely: hydrostatic pressure (pushes fluid out at the arteriole end) and water potential (pulls water back in at the venule end).
- Quote vessel sizes the right way round: arteries have a thick wall, small lumen; veins have a thin wall, large lumen; capillaries have a one-cell-thick wall.
- When identifying blood cells under the microscope, base your answer on the shape and relative size of the nucleus, and remember red blood cells have no nucleus.
- Use precise terms throughout: endothelium for the inner lining, lumen for the space inside, and ultrafiltration for the formation of tissue fluid.