Describe the structure and functions of arteries, veins and capillaries, and explain how their structure relates to blood pressure, pulse generation and the main vessels that connect the heart, lungs and kidneys.
Key blood vessels
Aorta – the main artery carrying oxygen‑rich blood away from the left ventricle to the systemic circulation.
Vena cava (superior & inferior) – the large veins that return deoxygenated blood from the body to the right atrium.
Pulmonary artery – carries deoxygenated blood from the right ventricle to the lungs.
Pulmonary vein – carries oxygen‑rich blood from the lungs back to the left atrium.
Renal artery – supplies the kidneys with oxygen‑rich blood.
Renal vein – drains deoxygenated blood away from the kidneys.
Suggested diagram: Simple schematic showing the heart, aorta, vena cava, pulmonary artery & vein, and renal artery & vein.
Arteries
Structure (outer → inner)
Tunica externa – dense connective tissue that anchors the vessel to surrounding tissues.
Tunica media – thick layer of smooth muscle and abundant elastic fibres; provides strength, elasticity and the ability to constrict or dilate.
Tunica intima – thin endothelium lining the lumen, reducing friction.
Relatively small lumen** and **thick wall** (wall ≫ lumen).
Highly elastic – expands during systole and recoils during diastole.
No one‑way valves.
Why this structure matters
Thick tunica media + elastic fibres → withstands the high pressure generated by the heart and stores elastic energy for pulse propagation.
Small lumen reduces the cross‑sectional area, helping to maintain a high pressure gradient.
Function
Carry oxygen‑rich blood away from the heart (except the pulmonary artery).
Maintain blood pressure through elastic recoil of the tunica media.
Regulate blood flow to tissues by vasoconstriction and vasodilation of the smooth muscle.
Generate the palpable pulse: during systole the wall stretches, storing potential energy; during diastole the stored energy is released, keeping blood moving even when the heart is relaxed.
Relevance to health
Atherosclerosis – plaque builds up in the tunica intima, narrowing the lumen and raising arterial pressure.
Hypertension – chronic high pressure places extra strain on the arterial wall, leading to thickening of the tunica media.
Suggested diagram: Cross‑section of an artery showing the three tunics, elastic laminae and the absence of valves.
Veins
Structure (outer → inner)
Tunica externa – relatively thick connective‑tissue layer; contains one‑way valves (especially in the limbs).
Tunica media – thin layer of smooth muscle and elastic fibres; provides limited contractility.
Tunica intima – endothelium with a thin sub‑endothelial connective‑tissue layer.
Relatively large lumen** and **thin wall** (wall ≪ lumen).
One‑way valves prevent backflow toward the feet.
Low elasticity – walls are more compliant than arteries.
Why this structure matters
Thin wall and large lumen allow veins to expand and act as a blood reservoir at low pressure.
Valves and the skeletal‑muscle pump together ensure unidirectional flow back to the heart despite the low pressure.
Function
Return deoxygenated blood to the heart (except the pulmonary vein).
Serve as a **blood reservoir** (≈ 60 % of total blood volume is stored in veins).
Assist venous return by:
the skeletal‑muscle pump (muscle contraction squeezes veins), and
valve action that prevents backflow.
Relevance to health
Varicose veins – valve failure leads to blood pooling and vein dilation, commonly seen in the legs.
Deep‑vein thrombosis (DVT) – clot formation in deep veins can obstruct return flow and be life‑threatening.
Suggested diagram: Cross‑section of a vein showing thin walls, a large lumen and a typical bicuspid valve.
Capillaries
Structure
Diameter ≈ 5–10 µm – just wide enough for a red blood cell to pass in single file.
Wall = one endothelial cell thick plus a thin basement membrane.
Types:
Continuous – uninterrupted endothelium; most common (skin, skeletal muscle).
Fenestrated – pores (≈ 60 nm) in the endothelium; found in kidneys, endocrine glands, intestinal villi.
Sinusoidal (discontinuous) – large gaps and a discontinuous basement membrane; located in liver, spleen, bone marrow.
Highly permeable – permeability can be altered by endothelial contraction or the presence of pores/fenestrations.
Why this structure matters
One‑cell‑thick wall minimises diffusion distance, allowing rapid exchange of gases, nutrients and wastes.
Specialised types (fenestrated, sinusoidal) increase permeability where rapid transfer of larger molecules is required (e.g., glomerular filtration).
Function
Site of exchange of O₂ ↔ CO₂, nutrients, waste products, hormones and heat between blood and tissues.
In kidneys, specialised fenestrated capillaries (glomeruli) enable filtration of plasma.
Regulation of exchange by changing endothelial permeability (e.g., histamine‑induced widening of gaps during inflammation).
Relevance to health
Edema – increased capillary permeability or elevated hydrostatic pressure causes fluid to leak into inter‑stitial spaces.
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