Blood vessels: arteries, veins and capillaries

What you'll learn

Blood vessels form the transport network of the circulatory system, carrying blood to every cell in the body. This revision guide covers the three types of blood vessels—arteries, veins and capillaries—their structural adaptations, functions, and how these relate to their roles in the body. You'll learn to identify each vessel type, explain their differences, and apply this knowledge to exam-style questions.

Key terms and definitions

Artery — a blood vessel that carries blood away from the heart, usually under high pressure

Vein — a blood vessel that carries blood towards the heart, usually under low pressure

Capillary — a microscopic blood vessel with walls one cell thick where exchange of materials occurs between blood and tissues

Lumen — the internal space or cavity inside a blood vessel through which blood flows

Vasoconstriction — the narrowing of blood vessels due to contraction of smooth muscle in vessel walls

Vasodilation — the widening of blood vessels due to relaxation of smooth muscle in vessel walls

Tissue fluid — the liquid that surrounds body cells, formed from plasma that leaks through capillary walls

Valve — a structure that prevents backflow of blood, found in veins and the heart

Core concepts

Structure of arteries

Arteries carry blood away from the heart to the tissues and organs. They must withstand high pressure generated by the heart's pumping action.

Wall structure:

• Thick muscular walls containing smooth muscle and elastic fibres
• Three distinct layers: outer fibrous coat, middle muscular layer (tunica media), and inner endothelial lining (tunica intima)
• Small lumen relative to wall thickness
• No valves (except where aorta and pulmonary artery leave the heart)

Structural adaptations:

• Thick elastic tissue allows arteries to stretch when blood surges through during ventricular contraction (systole) and recoil during ventricular relaxation (diastole), maintaining blood pressure
• Thick smooth muscle layer can contract (vasoconstriction) or relax (vasodilation) to regulate blood flow and pressure
• Smooth endothelial lining reduces friction as blood flows
• Strong fibrous outer layer prevents bursting under high pressure

Main arteries in humans:

• Aorta: carries oxygenated blood from left ventricle to the body
• Pulmonary artery: carries deoxygenated blood from right ventricle to lungs (exception to the rule that arteries carry oxygenated blood)
• Carotid arteries: supply blood to the head and brain
• Renal arteries: supply blood to the kidneys
• Hepatic artery: supplies blood to the liver

Structure of veins

Veins carry blood from the tissues back to the heart. They operate under much lower pressure than arteries.

Wall structure:

• Thin walls with less muscle and elastic tissue than arteries
• Same three-layer structure but thinner middle layer
• Large lumen relative to wall thickness
• Valves present throughout to prevent backflow

Structural adaptations:

• Thin walls are adequate because blood pressure is low
• Large lumen reduces resistance to blood flow and accommodates large volume of blood (veins act as blood reservoirs)
• Valves prevent backflow, ensuring blood flows only towards the heart
• Less elastic and muscular tissue since veins don't need to withstand high pressure or regulate flow

How blood returns to the heart:

Blood in veins moves against gravity when returning from the lower body. Several mechanisms assist venous return:

• Skeletal muscle contraction: When muscles contract during movement, they squeeze veins, pushing blood towards the heart—this is especially important in the legs where cane-cutters or market vendors who stand for long periods may experience poor circulation
• Valves: Prevent blood flowing backwards between muscle contractions
• Breathing movements: Pressure changes in the chest during breathing help draw blood back to the heart
• Suction effect: As the heart relaxes, it creates slight suction that pulls blood in

Main veins in humans:

• Vena cava (superior and inferior): carry deoxygenated blood to right atrium
• Pulmonary veins: carry oxygenated blood from lungs to left atrium (exception to the rule that veins carry deoxygenated blood)
• Hepatic vein: drains blood from liver
• Renal veins: drain blood from kidneys
• Hepatic portal vein: carries blood from intestines to liver (special case—connects two capillary beds)

Structure of capillaries

Capillaries are microscopic vessels that form networks (capillary beds) in tissues where exchange of materials occurs.

Wall structure:

• Wall is one cell thick (made only of endothelial cells)
• No muscle or elastic tissue
• Extremely narrow lumen (approximately 7-10 micrometres—just wide enough for red blood cells to squeeze through in single file)
• Permeable walls allow exchange of materials

Structural adaptations:

• Thin walls provide short diffusion distance for rapid exchange of oxygen, carbon dioxide, glucose, amino acids, hormones, and waste products
• Narrow lumen brings blood cells close to tissues
• Permeable walls allow tissue fluid to form
• Extensive branching creates large surface area for exchange
• Slow blood flow (due to narrow lumen and branching) provides time for exchange

Formation of tissue fluid:

1. At the arterial end of capillaries, high blood pressure forces plasma (without proteins, which are too large) through capillary walls into tissue spaces
2. This fluid, called tissue fluid, bathes cells and allows exchange of materials by diffusion
3. Oxygen and nutrients diffuse from tissue fluid into cells; carbon dioxide and wastes diffuse from cells into tissue fluid
4. At the venous end, lower blood pressure and osmotic effects cause most tissue fluid to return to capillaries
5. Excess tissue fluid drains into lymphatic vessels to become lymph

Comparison of blood vessel types

Understanding differences between vessel types is frequently tested at CSEC level:

Pulse and blood pressure

Pulse is the rhythmic expansion and recoil of arteries caused by blood surging through after each heartbeat. You can feel pulse at pressure points where arteries are close to the skin surface:

• Radial artery (wrist)
• Carotid artery (neck)
• Temporal artery (temple)

Normal resting pulse rate: 60-100 beats per minute (slightly higher in children)

Blood pressure is the force exerted by blood against vessel walls. It's measured in mmHg (millimetres of mercury) and expressed as two values:

• Systolic pressure (higher number): pressure when ventricles contract (normally around 120 mmHg)
• Diastolic pressure (lower number): pressure when ventricles relax (normally around 80 mmHg)
• Written as 120/80 mmHg

Factors affecting blood pressure:

• Heart rate and strength of contraction
• Blood volume
• Resistance in vessels (affected by diameter)
• Age (blood pressure generally increases with age)
• Physical activity
• Stress and emotions
• Diet (high salt intake increases blood pressure)
• Conditions like diabetes, which affect many Caribbean people, can impact blood pressure

Coronary circulation

The heart muscle itself requires oxygen and nutrients. Coronary arteries branch from the aorta immediately after it leaves the heart, supplying the heart muscle with oxygenated blood.

Coronary heart disease occurs when coronary arteries become narrowed or blocked (usually by fatty deposits called atherosclerosis), reducing blood supply to heart muscle. This is increasingly common in the Caribbean due to dietary factors.

Risk factors include:

• High-fat diet (excessive fried foods, fatty meats)
• High salt intake
• Lack of exercise
• Smoking
• Excessive alcohol consumption
• Obesity
• Diabetes
• Family history

Worked examples

Example 1: Structure-function question (6 marks)

Question: Explain how the structure of an artery is adapted to its function. [6 marks]

Model answer:

1. Arteries have thick walls containing elastic tissue [1 mark] which allows them to stretch when blood surges through and recoil between heartbeats, maintaining blood pressure [1 mark]

2. Arteries have thick smooth muscle in their walls [1 mark] which can contract or relax to regulate blood flow to different organs [1 mark]

3. Arteries have a small lumen relative to wall thickness [1 mark] which helps maintain high blood pressure [1 mark]

Mark scheme notes: Award one mark for each structural feature and one mark for each correct functional explanation. Accept equivalent phrasing.

Example 2: Comparison question (4 marks)

Question: State two structural differences between arteries and veins. [4 marks]

Model answer:

1. Arteries have thicker walls than veins / veins have thinner walls than arteries [1 mark]

2. Arteries have more elastic tissue/muscle than veins / veins have less elastic tissue/muscle than arteries [1 mark]

3. Arteries have a smaller lumen relative to wall thickness than veins / veins have a larger lumen relative to wall thickness [1 mark]

4. Veins have valves but arteries do not (except at heart) [1 mark]

Mark scheme notes: Award one mark for each valid structural difference. Any two are required for full marks. Do not accept functional differences.

Example 3: Application question (5 marks)

Question: A market vendor stands for 8-10 hours daily selling produce. By evening, she notices swelling in her ankles.

(a) Name the fluid that causes this swelling. [1 mark]

(b) Explain why standing for long periods leads to this problem. [4 marks]

Model answer:

(a) Tissue fluid / lymph [1 mark]

(b) Standing reduces muscle contraction in the legs [1 mark], which normally helps push blood back up through veins to the heart [1 mark]. This causes blood to pool in leg veins [1 mark], increasing capillary blood pressure and forcing more tissue fluid out, which accumulates in tissues causing swelling [1 mark]

Mark scheme notes: Accept equivalent terminology. For part (b), award marks for explaining the mechanism linking inactivity to fluid accumulation.

Common mistakes and how to avoid them

• Confusing which vessels carry oxygenated vs deoxygenated blood: Remember the rule "arteries away from heart, veins towards heart" not "arteries carry oxygenated blood." The pulmonary artery carries deoxygenated blood and pulmonary veins carry oxygenated blood—these are exceptions.

• Stating arteries have valves: Only the aorta and pulmonary artery have valves where they leave the heart. Arteries throughout the body do not have valves. Veins have valves.

• Describing capillary walls as "thin": Use precise terminology—capillary walls are "one cell thick" or "one endothelial cell thick," not just thin.

• Confusing blood pressure values: Remember systolic (higher value, around 120) is when ventricles contract; diastolic (lower value, around 80) is when ventricles relax. The higher number always comes first.

• Not explaining how structure relates to function: Simply listing structural features without explaining their functional significance loses marks. Always connect structure to function using words like "allows," "enables," or "which means."

• Mixing up tissue fluid and blood plasma: Tissue fluid is formed from blood plasma but does not contain large proteins or blood cells. Be specific about which fluid you're describing.

Exam technique for "Blood vessels: arteries, veins and capillaries"

• Command words matter: "State" requires brief factual answers (1-2 words often sufficient). "Explain" requires reasons or mechanisms (use "because" or "which allows"). "Compare" means identify similarities AND differences. "Describe" requires detailed account of features or processes.

• Comparison questions: Set up clear comparisons using comparative language ("thicker than," "more than," "larger than"). Avoid describing each vessel separately—directly compare them. Tables work well for planning but write in sentences for full marks.

• Diagram labelling: Use a ruler for label lines. Lines should touch the structure being labelled. Never use arrows unless showing direction of flow. Write labels horizontally. Make sure labels don't overlap.

• Mark allocation guides detail: A 4-mark question typically requires four distinct points. If explaining, you usually need both a feature AND its significance. Plan your answer to match available marks—don't write paragraphs for 1-mark questions.

Quick revision summary

Blood vessels form three types with distinct structures matching their functions. Arteries have thick elastic muscular walls with small lumens, carrying blood away from the heart under high pressure. Veins have thinner walls, larger lumens, and valves, returning blood to the heart under low pressure with help from skeletal muscles. Capillaries have walls one cell thick enabling exchange of materials between blood and tissues through tissue fluid formation. Know the exceptions: pulmonary artery carries deoxygenated blood; pulmonary veins carry oxygenated blood. Understanding structure-function relationships is essential for CSEC success.