What Is Blood?

Blood is a tissue made up of different types of cells and substances suspended in a liquid called plasma.

💡 Key Concept: Blood transports oxygen, nutrients, hormones, and waste, and plays a vital role in defence and regulation.

Main Components of Blood

Red Blood Cells (Erythrocytes)

✔ Carry oxygen from lungs to tissues
✔ Contain haemoglobin, which binds to oxygen → oxyhaemoglobin
✔ No nucleus – more space for haemoglobin
✔ Biconcave shape – increases surface area for oxygen diffusion

💡 Adaptations:

• Small and flexible to fit through capillaries
• Large surface area to volume ratio

White Blood Cells (Leukocytes)

✔ Protect the body from pathogens (bacteria, viruses, etc.)
✔ Can change shape to engulf pathogens (phagocytosis)
✔ Others produce antibodies and antitoxins
✔ Have a nucleus

💡 White blood cells are part of the immune system.

Platelets

✔ Small cell fragments
✔ Trigger blood clotting at wounds
✔ Help form a scab to prevent pathogens entering
✔ No nucleus

❌ A low platelet count can lead to excessive bleeding.

Plasma

✔ Straw-coloured liquid
✔ Carries:

• Red and white blood cells
• Platelets
• Glucose, amino acids
• Carbon dioxide (from organs to lungs)
• Urea (to kidneys for excretion)
• Hormones
• Antibodies and antitoxins