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Spec Mapping — OCR H420 Module 3.1.2 — Transport in animals, content statements covering the principles of mass transport in animals, the distinction between open and closed circulations and between single and double circulations, and the histology and function of arteries, arterioles, capillaries, venules and veins (refer to the official OCR H420 specification document for exact wording). This lesson begins Module 3.1.2 and lays out the architectural plan of the mammalian circulation that the heart (next lessons) drives.
A multicellular organism's exchange surfaces would be useless without a transport system to carry materials between them and the cells in the interior. This lesson examines the principles of mass transport systems in animals — specifically open versus closed and single versus double circulations — then looks in detail at the structure of each type of blood vessel (arteries, arterioles, capillaries, venules, veins) and explains how each is adapted for its function.
The conceptual breakthrough was William Harvey's Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus (1628), which demonstrated by careful dissection and quantitative argument that blood circulates in a closed loop. Harvey calculated (paraphrasing his school of thought) that the heart ejects more blood in half an hour than the entire body contains, so the same fluid must be returning to the heart repeatedly — overturning Galen's view that the liver continually manufactured blood and the tissues consumed it. The actual capillary connection between arteries and veins was not seen directly until Marcello Malpighi observed it under the microscope in 1661, completing the picture. Ernest Starling (1896 and onwards) then formalised the dynamics of capillary fluid exchange — the framework we still use today (next lesson on tissue fluid).
Key Definitions:
- Mass transport — the bulk flow of fluid carrying substances over relatively long distances in an organism.
- Open circulation — blood (haemolymph) is pumped into a body cavity (haemocoel) and bathes the tissues directly.
- Closed circulation — blood is contained within vessels throughout, separate from tissue fluid.
- Single circulation — blood passes through the heart once per complete circuit (e.g., fish).
- Double circulation — blood passes through the heart twice per circuit (e.g., mammals).
| Feature | Open (e.g., insects) | Closed (e.g., mammals, fish) |
|---|---|---|
| Blood vessels | Few; tissues bathed directly | Continuous network |
| Blood pressure | Low | Higher; can be maintained |
| Flow control | Little direction control | Precise distribution possible |
| Transport pigments | Usually none or haemocyanin in haemolymph | Haemoglobin |
| Rate of delivery | Slow | Fast |
Open systems are adequate for small, slow-moving or sedentary animals such as most arthropods, in which the tracheal system handles gas exchange separately. Closed systems are essential for active vertebrates that need rapid delivery of large amounts of oxygen.
In a single circulation, such as that of bony fish, blood passes through the heart once per full circuit:
flowchart LR
H[Heart] --> G[Gills]
G --> B[Body tissues]
B --> H
The downside is that after passing through the fine capillaries of the gills, blood pressure is much reduced, so flow to the body is slower and less efficient.
In a double circulation, such as that of mammals, blood passes through the heart twice per circuit:
flowchart LR
RH[Right heart] --> L[Lungs]
L --> LH[Left heart]
LH --> BT[Body tissues]
BT --> RH
The right side pumps deoxygenated blood to the lungs (pulmonary circulation, low pressure to protect the delicate alveolar capillaries), while the left side pumps oxygenated blood to the rest of the body (systemic circulation, high pressure to reach distant tissues). Because blood is re-pressurised between the pulmonary and systemic circuits, delivery to active tissues is fast and efficient.
Exam Tip: A common OCR question asks why mammals have a double circulation. Answer: because the left side of the heart re-pressurises blood after it has passed through the lungs, enabling rapid delivery of oxygen to metabolically active tissues.
Blood vessels fall into five types: arteries, arterioles, capillaries, venules and veins.
Most vessels share a three-layered wall:
Arteries carry blood away from the heart. They must withstand high pressure and convert the pulsatile flow from the heart into a more even flow downstream.
Arterioles are smaller vessels (typically 10–100 µm) that link arteries to capillaries. They play the key role in distributing blood to tissues.
Capillaries are the exchange vessels of the circulation. Their structure is purpose-built for diffusion:
Venules and veins return blood to the heart. Pressure here is low, so the structural priorities are different.
Veins act as a capacitance reservoir: at any moment, roughly 70% of total blood volume is contained within the venous system. When the body needs more circulating blood (during exercise, haemorrhage, or sympathetic activation), venoconstriction shifts blood out of this reservoir into the active circulation. This is why blood loss is initially compensated without any drop in arterial pressure — the body simply mobilises venous reserves until they are exhausted.
The valves of the veins are pocket-shaped: their cusps line the inner vessel wall when blood is flowing forward (toward the heart), but flip into the lumen and meet when pressure attempts to drive blood backward, sealing off retrograde flow. Failure of these valves causes varicose veins — visibly bulging, tortuous superficial veins, most common in the legs where venous pressure is highest under gravity. The valves can be replaced by surgery in severe cases, but the underlying weakness of the vein wall often persists.
| Feature | Artery | Arteriole | Capillary | Vein |
|---|---|---|---|---|
| Pressure | High (mean ~13 kPa) | Moderate (~7 kPa) | Falling (~5 → 2 kPa) | Low (~1–2 kPa) |
| Wall thickness | Thick | Moderate | Single endothelial cell | Thin |
| Elastic tissue | Abundant | Some | None | Little |
| Smooth muscle | Present | Abundant (proportionally) | None | Little |
| Lumen size | Narrow | Narrow | Very narrow (5–10 µm) | Wide |
| Valves | No | No | No | Yes |
| Function | Carry blood at high pressure from heart | Distribute blood to tissues | Exchange | Return blood to heart |
| Cross-sectional area (total) | ~5 cm² | ~50 cm² | ~5000 cm² | ~500 cm² |
| Velocity of blood | High (~50 cm s⁻¹ aorta) | Falling | Slowest (~0.05 cm s⁻¹) | Rising again |
Blood pressure falls progressively from the aorta (~120/80 mmHg) through the arteries and arterioles (largest fall, due to high resistance), then falls more slowly across capillaries, and reaches its lowest value (~0–10 mmHg) in the vena cava. The arterioles account for most of the drop in pressure and are therefore the primary site of peripheral resistance control.
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