5.5 - Blood

Red blood cells and oxygen transport

• Red blood cells (also called erythrocytes) carry oxygen from the lungs to all the cells in the body.
• Each cell is packed with a red pigment called haemoglobin, which contains iron and gives blood its colour.

How the cells are adapted

• Small, with a biconcave disc shape - A disc pushed in on both sides (a little like a doughnut), giving a large surface area for absorbing and releasing oxygen.
• No nucleus - Leaves more room inside the cell to carry oxygen.
• Packed with haemoglobin - Provides the pigment that binds the oxygen.

Binding and releasing oxygen

1. In the lungs, where there is a lot of oxygen, haemoglobin binds to oxygen to form oxyhaemoglobin.
2. In the body tissues, the reverse happens: oxyhaemoglobin splits back into haemoglobin and oxygen, releasing the oxygen to the cells that need it.

White blood cells and the body's defence

• White blood cells defend the body against infection.
• They are usually larger than red blood cells and, unlike red blood cells, they do have a nucleus.
• When the body has an infection, white blood cells multiply to fight it off - so a blood sample taken during an infection shows a raised white blood cell count.

The two main types

1. Phagocytes - Change shape to engulf and digest unwelcome microorganisms, a process called phagocytosis.
2. Lymphocytes - Produce proteins called antibodies that act against microorganisms. Some also produce antitoxins that neutralise the toxins microorganisms make.

How antibodies work

• Each antibody is specific to a particular pathogen.
• It recognises and binds to that pathogen's specific antigen.

Platelets and blood clotting

• Platelets are small fragments of cells - they have no nucleus.
• Their job is to help the blood to clot.
• Platelets carry proteins on their surface that help them stick to each other and to the wound site.

How a clot forms

1. When a blood vessel is damaged, platelets gather at the wound and clump together to plug the damaged area - this is the basis of blood clotting.
2. They help to form the scab that seals the cut.

Why clotting matters

• A clot stops too much blood being lost from the wound.
• It also stops microorganisms getting in through the broken skin.
• Someone short of platelets bleeds and bruises more easily than normal.

Plasma and what it transports

• Plasma is the pale, straw-coloured liquid part of blood - essentially blood with the cells removed.
• The red blood cells, white blood cells, and platelets are suspended in the plasma.
• Plasma carries just about everything that needs transporting around the body, and also spreads heat around the body, helping to keep the internal temperature stable.

What the plasma transports

• Soluble products of digestion - Such as glucose and amino acids, carried from the gut to the body's cells
• Carbon dioxide - Carried from the organs to the lungs
• Urea - Carried from the liver to the kidneys
• Hormones - Chemical messengers
• Dissolved ions and proteins - Carried around the body