9700 · 8.1
The circulatory system
Ever wondered how every single cell in your body gets the oxygen it needs to survive, or how waste products are efficiently removed? It’s all thanks to your incredible **circulatory system**, a vital transport network that keeps you alive and thriving. In this lesson, we'll dive deep into the **mammalian circulatory system**, exploring the intricate **structure and function** of your heart, the diverse roles of **blood vessels**, and the precise rhythm of the **cardiac cycle**. Key points align with **A-Level Notes** 9700 Biology chapter **8** (topic **8.1**). Notes and diagrams from **A-Level Notes** chapter **8**, topic **8.1** (1 pages).
Need to know
What you need to know
- **Artery walls, inside out:** tunica intima (interna) — a single, very smooth layer of flat endothelial cells; tunica media — smooth muscle, collagen and elastic fibres, and the thickest layer in an artery; tunica externa — collagen and elastic fibres.
- The tunica media of an artery is rich in elastic fibres, so the wall stretches as blood surges through at high pressure and recoils as the pressure drops — widening to reduce the pressure slightly and narrowing again, which smooths the flow. The thickness of the wall varies with the pressure the artery carries.
- The pulmonary artery and the aorta have semilunar valves at their base, so blood leaving the ventricles cannot flow back.
- Arteries branch into **arterioles**, whose walls have proportionally more smooth muscle: contracting it narrows the lumen and reduces blood flow to the tissues beyond.
- **Vein walls** have the same three layers — a smooth tunica intima, a thin tunica media of smooth muscle, collagen and elastic fibres, and a tunica externa of elastic and collagen fibres that is the thickest layer in a vein. The wide lumen and thin muscular wall suit the low pressure of returning blood.
Explanation
The circulatory system
- **Artery walls, inside out:** tunica intima (interna) — a single, very smooth layer of flat endothelial cells; tunica media — smooth muscle, collagen and elastic fibres, and the thickest layer in an artery; tunica externa — collagen and elastic fibres.
- The tunica media of an artery is rich in elastic fibres, so the wall stretches as blood surges through at high pressure and recoils as the pressure drops — widening to reduce the pressure slightly and narrowing again, which smooths the flow. The thickness of the wall varies with the pressure the artery carries.
- The pulmonary artery and the aorta have semilunar valves at their base, so blood leaving the ventricles cannot flow back.
- Arteries branch into **arterioles**, whose walls have proportionally more smooth muscle: contracting it narrows the lumen and reduces blood flow to the tissues beyond.