Lesson 4 of 9 / Transport in humans
What blood carries
Why does blood need both red cells and liquid plasma?
I can match oxygen and other transported materials to the relevant blood component.
I can explain blood components in transport, defence and clotting.Syllabus
Syllabus K325, Topic 5(d). Explain blood components in transport, defence and clotting.
About 4 min
Make a guess. You are not marked.
In diagrams, blood returning to the heart in the veins is drawn in blue. What colour is it really?
Show the answer
Dark red, because it still has some oxygen.
Deoxygenated blood holds less oxygen and looks darker red. Blue is just a diagram convention.
The key idea
Haemoglobin in red blood cells carries most oxygen; plasma carries the cells and many dissolved substances.
Explanation
Blood has a liquid part, plasma, with red blood cells, white blood cells and platelets suspended in it. Each component contributes a different function; blood is not just a liquid containing oxygen.
Red blood cells contain haemoglobin, which combines reversibly with oxygen to form oxyhaemoglobin. In the lungs, abundant oxygen favours loading. In respiring tissues, a lower oxygen concentration favours release.
A red blood cell has a biconcave shape, giving a large surface area relative to its volume and a short diffusion distance. It has no nucleus when mature, leaving more space for haemoglobin. Its flexibility helps it pass through narrow capillaries.
Plasma transports blood cells and dissolved ions, soluble food substances such as glucose and amino acids, vitamins, hormones, urea and plasma proteins. Much carbon dioxide is carried in the plasma after conversion into dissolved bicarbonate ions; the detailed chemistry is not needed here.
These journeys have destinations: glucose moves from absorption sites towards cells, hormones towards target organs, and urea from the liver towards kidneys. Oxygen transport is mainly associated with haemoglobin, not simply dissolved in plasma.
Step by step
- 1
Choose the component
For most oxygen, name red blood cells and haemoglobin. For many dissolved materials, name plasma.
- 2
Locate loading and unloading
Explain where the substance enters the blood and where it is used or removed.
- 3
Link structure to function
For red cells, explain surface area, diffusion distance or room for haemoglobin.
Worked example
Two different deliveries
Compare how blood carries oxygen from the lungs and urea from the liver.
One way to explain it
Most oxygen is carried bound to haemoglobin in red blood cells and released in tissues. Urea is dissolved in plasma and transported from the liver to the kidneys for excretion.
Why this answer works
- Name a different carrier for each substance.
- Urea is a nitrogenous waste, not an oxygen carrier.
- Transport does not mean the substance is used at its destination: kidneys remove urea.
Watch out for this
A student says: "Deoxygenated blood contains no oxygen and turns blue." What is wrong with this?
Show the answer
It contains less oxygen and is dark red. Blue in diagrams is a convention that makes routes easier to distinguish.
Check your understanding
A hormone and urea are travelling in blood. Which component mainly transports both?
Show the answer
Plasma
Plasma transports hormones, urea and many other dissolved substances as well as the blood cells.
Quick recall
Why does blood need both red cells and liquid plasma?