9.4 Blood

Learning objectives

Blood consists of plasma with red blood cells, white blood cells and platelets suspended in it.

Component	What it is
plasma	the liquid part of blood
red blood cells	specialised cells present in very large numbers
white blood cells	immune cells
platelets	small cell fragments involved in clotting

Plasma is the transport medium; the three formed components travel within it.

Plasma is part of blood. It is not the same as tissue fluid or lymph.

Use relative number, size, shape and presence of a nucleus to distinguish red and white blood cells in diagrams or photomicrographs.

Feature	Red blood cell	White blood cell
number in a typical field	many	far fewer
shape	small biconcave disc with a pale centre	larger and less regular
nucleus	absent	present and dark-staining

First find the numerous small pale-centred discs: these are red cells. Larger cells with dark nuclei are white cells.

A pale centre in a red blood cell is not a nucleus; mature red blood cells have no nucleus.

Each blood component has a distinct transport, defence or clotting role.

Component	Function
red blood cells	transport oxygen; haemoglobin binds oxygen
white blood cells	defend by phagocytosis and antibody production
platelets	promote blood clotting
plasma	transports blood cells, ions, nutrients, urea, hormones and carbon dioxide

Match the substance or process to its carrier: oxygen mainly with haemoglobin in red cells; the listed dissolved substances and cells in plasma.

Plasma transports the blood cells; red blood cells do not transport every substance in blood.

Blood clotting seals a damaged blood vessel.

Role	Consequence
prevents blood loss	limits bleeding from the wound
prevents pathogen entry	forms a barrier between damaged tissue and the environment

Both roles follow from closing the break in the skin and blood vessel.

Clotting prevents entry of pathogens at a wound; it does not destroy every pathogen already inside the body.

Lymphocytes and phagocytes are white blood cells distinguished mainly by nuclear shape and relative amount of cytoplasm.

Cell	Recognition cue
lymphocyte	large, round nucleus occupying most of the cell; thin rim of cytoplasm
phagocyte	lobed or irregular nucleus; more cytoplasm and a less regular outline

In a blood image, first confirm the cell is a nucleated white cell, then use the round-versus-lobed nucleus distinction.

Cell size alone is not reliable; use nuclear shape and cytoplasm together.

Lymphocytes produce antibodies; phagocytes engulf pathogens by phagocytosis.

White blood cell	Function
lymphocyte	produces antibodies that act against specific antigens
phagocyte	surrounds and engulfs pathogens by phagocytosis

Antibody production is a chemical defence by lymphocytes; engulfment is a cellular process carried out by phagocytes.

Lymphocytes do not engulf pathogens in this syllabus model, and phagocytes do not produce antibodies.

At a damaged blood vessel, platelets help trigger conversion of soluble fibrinogen into insoluble fibrin.

Stage	Event
1	the vessel is damaged and platelets become involved
2	soluble fibrinogen is converted to insoluble fibrin
3	fibrin forms a mesh across the wound
4	the mesh traps blood cells, forming a clot that seals the wound

The fibrin mesh produces the physical barrier that reduces blood loss and pathogen entry.

Fibrinogen is the soluble precursor; fibrin is the insoluble mesh. Do not reverse the conversion.