Blood Vessels

Blood vessels form the network of tubes that carry blood throughout the body. There are three main types: arteries, veins and capillaries. Each has a specific structure suited to its function.

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Overview of Blood Vessels

Blood leaves the heart in arteries, passes through capillaries where exchange occurs, and returns to the heart in veins.

The order is always: Heart → Arteries → Arterioles → Capillaries → Venules → Veins → Heart

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Arteries

Arteries carry blood away from the heart.

Structure

• Thick muscular walls — the smooth muscle layer can contract and relax to help push blood along and regulate blood flow
• Thick elastic tissue layer — allows the artery wall to stretch and recoil with each heartbeat, smoothing out the surges of high pressure
• Small lumen (the hollow centre) — maintains high blood pressure
• No valves (except semilunar valves at the very start in the pulmonary artery and aorta)
• Lined with endothelium — a smooth inner layer that reduces friction

Key Features of Blood in Arteries

• Blood flows at high pressure (due to the force of the heart's contraction)
• Blood flows in pulses — you can feel this as your pulse at the wrist (radial artery) or neck (carotid artery)
• Most arteries carry oxygenated blood (exception: pulmonary artery carries deoxygenated blood to the lungs)

Exam tip: Remember "Arteries carry blood Away" — the double A helps you remember the direction.

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Veins

Veins carry blood back towards the heart.

Structure

• Thinner walls than arteries — less muscle and elastic tissue because blood is at lower pressure
• Larger lumen — allows blood to flow more easily at low pressure, reducing resistance
• Valves — prevent the backflow of blood, ensuring it flows in one direction towards the heart
• Lined with endothelium

Key Features of Blood in Veins

• Blood flows at low pressure
• Blood flow is steady (no pulse)
• Blood is helped along by the squeezing action of surrounding skeletal muscles as you move — this pushes blood through the veins
• Most veins carry deoxygenated blood (exception: pulmonary vein carries oxygenated blood from the lungs)

How Valves Work

Valves are pocket-like flaps on the inner walls of veins. When blood flows in the correct direction (towards the heart), the valves are pushed open. If blood tries to flow backwards, the valves fill with blood and close, blocking the reverse flow.

Exam tip: Varicose veins occur when valves in leg veins stop working properly, allowing blood to pool. This is a real-world example of why venous valves matter.

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Capillaries

Capillaries are tiny, thin-walled blood vessels that connect arteries to veins. They are the site of substance exchange between blood and tissues.

Structure

• Walls are only one cell thick (one endothelial cell) — this creates a very short diffusion distance, allowing rapid exchange of substances
• Very narrow lumen — red blood cells must squeeze through in single file, bringing them close to the capillary wall and slowing them down, which increases the time available for exchange
• No muscle or elastic tissue — they do not need to withstand high pressure
• Permeable walls — allow substances to pass through

Capillary Beds

Capillary beds are dense networks of capillaries that surround tissues and organs. They provide a large surface area for exchange.

How Exchange Occurs

At the capillary beds, substances are exchanged between the blood and the surrounding tissue cells by diffusion (and some by osmosis):

From blood → to cells	From cells → to blood
Oxygen	Carbon dioxide
Glucose	Other metabolic waste (e.g. urea)
Amino acids
Water
Minerals

• Oxygen and glucose move from the blood (where concentration is higher) into the cells (where concentration is lower) — they are used for respiration
• Carbon dioxide and waste products move from the cells (where concentration is higher) into the blood (where concentration is lower) — they are transported away for removal

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Comparing the Three Blood Vessels

This table is essential for the exam — learn it thoroughly:

Feature	Artery	Vein	Capillary
Direction of flow	Away from heart	Towards heart	Between arteries and veins
Wall thickness	Thick (muscle + elastic tissue)	Thinner than arteries	One cell thick
Lumen size	Small (relative to wall)	Large (relative to wall)	Very narrow
Valves	No (except semilunar at heart)	Yes (throughout)	No
Blood pressure	High	Low	Decreasing (high to low)
Blood flow	Pulsing/rapid	Steady/slow	Slow
Oxygenation	Usually oxygenated	Usually deoxygenated	Mixture (exchange occurring)
Main function	Transport blood at high pressure from heart	Return blood at low pressure to heart	Exchange substances with tissues

Exam tip: In a 6-mark question comparing blood vessels, always structure your answer with a clear comparison for each feature. Do not just describe each vessel separately — directly compare them. For example: "Arteries have thick muscular walls to withstand high pressure, whereas veins have thinner walls because blood is at lower pressure."

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Blood Vessel Diseases

Atherosclerosis

Atherosclerosis is the build-up of fatty deposits (plaques) inside artery walls. This:

• Narrows the lumen — restricting blood flow
• Reduces blood supply to tissues — cells receive less oxygen and glucose
• Can lead to blood clots forming on the plaque surface
• If this happens in the coronary arteries → heart attack
• If this happens in arteries supplying the brain → stroke

Risk Factors

• High-fat diet (especially saturated fat)
• Smoking
• Lack of exercise
• Genetic predisposition
• High blood pressure

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Stents and Bypass Surgery

If coronary arteries become severely narrowed:

• A stent (a small mesh tube) can be inserted to hold the artery open, restoring blood flow
• Coronary bypass surgery uses a blood vessel from another part of the body (often the leg) to bypass the blocked section

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Summary

Blood Vessel	Key Adaptation	Why
Artery	Thick walls, small lumen	Withstand and maintain high pressure
Vein	Thin walls, large lumen, valves	Allow easy flow at low pressure; prevent backflow
Capillary	One cell thick wall, narrow	Short diffusion distance; substances exchange with tissues