Tissues that connect

Connective tissue joins and supports other tissues. Two that look nothing alike are both connective tissue: blood and bone.

"How can blood and bone be the same kind of tissue?" The difference is the stuff between their cells, the matrix:

• In blood, it is watery.
• In bone, it is hard, made of calcium and phosphorus compounds.
• In cartilage, it is soft and jelly-like.

What is in blood?

• Plasma, the liquid: 55% of blood.
• Formed elements: 45%. These are red blood cells, white blood cells and platelets.

Blood is red because of haemoglobin, an iron-rich protein in red blood cells, which carry oxygen and live about 4 months.

You can see blood at work in daily life:

• At a cut, platelets make the blood clot.
• Around an infection, white blood cells gather to fight germs, causing redness, swelling and pus.
• When you exercise, your muscles need more oxygen, so you breathe faster and blood flows faster.

Feel the tissues in your own body:

• Tap your elbow: hard and rigid. That is bone, for strength, support and protection.
• Fold your ear or press your nose: soft and bendy. That is cartilage, which also cushions the ends of bones.
• Wiggle your fingers and feel your forearm move. The muscles there pull your finger bones through tendons: tendons join muscle to bone.
• Lift your knee: it bends only so far. Ligaments join bone to bone, keep joints steady, and stop bones slipping out of place.