Lesson 3 of 7 / 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 state blood components and all listed transport and defence roles.Syllabus
Syllabus K224 / K225, Topic 5(c). State blood components and all listed transport and defence roles.
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 called plasma. Red blood cells, white blood cells and platelets float in the plasma. Each part has a different job.
Red blood cells contain haemoglobin. Haemoglobin joins with oxygen to form oxyhaemoglobin, and can let it go again. In the lungs, where there is a lot of oxygen, haemoglobin picks oxygen up. In tissues that are respiring, where there is less oxygen, it lets oxygen go.
A red blood cell is shaped like a disc that dips in on both sides. This gives it a large surface area for its volume and a short distance for oxygen to diffuse. A mature red blood cell has no nucleus, which leaves more room for haemoglobin. It can bend to squeeze through narrow capillaries.
Plasma carries many dissolved substances: ions, glucose, amino acids, vitamins, hormones, urea and proteins. Most carbon dioxide is also carried in the plasma.
Each substance has a destination. Glucose goes from the gut to cells, hormones go to their target organs, and urea goes from the liver to the kidneys. Oxygen is carried mainly by haemoglobin, not dissolved in plasma.
Step by step
- 1
Pick the part of the blood
For oxygen, name red blood cells and haemoglobin. For most dissolved substances, name plasma.
- 2
Say where it loads and unloads
Say where the substance enters the blood and where it is used or removed.
- 3
Link shape to job
For red cells, mention a large surface area, a short distance for diffusion, or more 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?