Circulatory Systems and Blood Vessels

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. This content matches OCR A-Level Biology A specification 3.1.2 (a)–(c).

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).

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Open vs Closed Circulation

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.

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Single vs Double Circulation

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.

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Blood Vessels

Blood vessels fall into five types: arteries, arterioles, capillaries, venules and veins.

General Structural Layout

Most vessels share a three-layered wall:

• Tunica externa (adventitia) — outer layer, mainly collagen and elastic fibres; provides strength and anchors vessel in place.
• Tunica media — middle layer, smooth muscle and elastic tissue; controls vessel diameter and handles pressure.
• Tunica intima — inner layer, endothelium (single layer of squamous cells) on a thin basement membrane; smooth, reducing friction.

Arteries

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.

• Thick tunica media with abundant elastic fibres and smooth muscle.
• Narrow lumen relative to wall thickness.
• No valves (except at the base of the aorta and pulmonary artery, where they are sometimes classed as part of the heart).
• Elastic recoil during diastole (heart relaxation) helps maintain blood pressure and smooth out the flow — this is called the Windkessel effect.

Arterioles