5. Blood & Blood Vessels

1. Blood

I. Functions of Blood

Blood acts as the body's transport system and performs several important functions:

  • Transport → carries nutrients, hormones, enzymes, gases and waste products.

  • Thermoregulation → helps regulate body temperature.

  • Chemical balance → helps maintain the body's pH.

  • Clotting → prevents excessive blood loss.

  • Immune protection → protects against toxins, bacteria and other harmful organisms..




II. Components of Blood

Component

Key Features

Main Function

Red Blood Cells (Erythrocytes)

Produced in red bone marrow and contain haemoglobin.

Transport oxygen (O₂) around the body.

White Blood Cells (Leukocytes)

Transparent and much fewer than RBCs; produced in red bone marrow.

Immune defence → fight infections and destroy harmful organisms. Also help remove damaged or diseased tissue.

Platelets (Thrombocytes)

Small cell fragments, not complete cells.

Help prevent blood loss by forming a platelet plug and initiating blood clotting.

Plasma

Straw-coloured liquid; makes up approximately 55% of blood volume. Mainly water.

Transports proteins, electrolytes, nutrients, gases, hormones, enzymes, vitamins and waste products.

🧠 Red Blood Cells & Haemoglobin

  • Haemoglobin is a protein inside red blood cells.

  • It binds to oxygen, allowing oxygen to be transported through the blood.

  • Haemoglobin is also responsible for the red colour of blood.




III. Blood Flow

Blood moves through blood vessels because of pressure differences.

⭐ Key Principle

Blood flows from HIGH pressure → LOW pressure.

  • Blood pressure is highest in the aorta.

  • Pressure decreases as blood travels through the arterial system.

  • Pressure continues to decrease as blood returns to the heart through the venous system.




IV. Blood Pressure

Blood pressure = the pressure exerted by blood against the walls of the arteries.

Measurement

  • Measured using a sphygmomanometer.

  • Usually measured at the brachial artery, just above the elbow.


Two Measurements

Systolic pressure

→ Pressure when the left ventricle contracts and forces blood into the aorta.

Diastolic pressure

→ Pressure in the arteries when the left ventricle relaxes.

Blood pressure is written as:

Systolic / Diastolic

Example: 120/80 mmHg


Factors affecting blood pressure

  • Age

  • Weight

  • Sex

  • General health

  • Physical activity










2. Blood Vessels

I. Types of Blood Vessels

The main blood vessels are:

Arteries → Arterioles → Capillaries → Venules → Veins

Vasodilation & Vasoconstriction

  • Vasodilation → blood vessel widens → diameter increases.

  • Vasoconstriction → blood vessel narrows → diameter decreases.

These changes help regulate blood flow and blood pressure.



II. Characteristics of Arteries and Arterioles:

Arteries & Arterioles

  • Carry blood away from the heart.

  • Usually carry oxygenated blood.

  • Exception: pulmonary arteries carry deoxygenated blood.

  • Have thick, strong walls.

  • Walls contain:

    • Smooth muscle

    • Elastic tissue

  • Can contract to help regulate and maintain blood flow to body tissues.

  • Carry blood at a higher pressure than veins.

  • Arteries branch into smaller vessels called arterioles.

🧠 Remember:

A = Arteries → Away from the heart



III. Veins & Venules

Veins

  • Carry blood towards the heart.

  • Usually carry deoxygenated blood.

  • Exception: pulmonary veins carry oxygenated blood.

  • Have thinner walls than arteries.

  • Usually have a larger lumen (inner space) than arteries.

  • Contain valves that prevent the backflow of blood.

  • Help maintain one-way blood flow towards the heart.

  • Venous return is helped by the contraction and relaxation of surrounding muscles.

  • Veins are generally located more superficially in the body than arteries.


Varicose Veins

  • Can occur when veins lose their elasticity.

  • Blood can then pool above the valves.

  • This causes the veins to become enlarged and visible.


🧠 Remember:

V = Veins → towards the heart



IV. Capillaries

  • Microscopic blood vessels.

  • Connect arterioles to venules.

  • Have extremely thin walls.

  • Their thin walls allow substances to pass between the blood and body tissues.


Exchange through capillaries includes:

  • Oxygen → into cells

  • Nutrients → into cells

  • Carbon dioxide → into blood

  • Waste products → into blood