Blood composition
CIE A-Level BiologyΒ· 35 min read
1. Plasma: The Liquid Transport Matrixβ βββββ± 10 min
Plasma
The liquid component of blood that makes up approximately 55% of total blood volume in healthy humans
Example:
Contains dissolved solutes including glucose, amino acids, urea, hormones, mineral ions and carbon dioxide
Plasma is the medium that carries all other blood components around the body. Its key roles include transporting nutrients to cells, removing waste products from metabolism, distributing heat to regulate body temperature, and transporting hormones between endocrine glands and target organs.
A student claims plasma only transports waste products like urea. Evaluate this claim.
- 1
The student's claim is incorrect, because plasma transports a wide range of substances beyond waste products.
- 2
Plasma carries nutrients like glucose and amino acids, absorbed from the small intestine, to all body cells for respiration and growth.
- 3
It also transports hormones from endocrine glands to target organs, carries most carbon dioxide away from tissues, and distributes heat around the body to maintain core temperature.
- 4
While plasma does transport urea and other metabolic wastes, it has many other critical roles, so the claim is false.
Exam tip:
Remember most carbon dioxide is transported in plasma, not bound to hemoglobin in red blood cells.
2. Erythrocytes (Red Blood Cells): Oxygen Transport Specialistsβ β ββββ± 15 min
Erythrocytes
Specialized anucleate cells adapted to transport oxygen bound to the pigment hemoglobin
Example:
Human blood contains ~5 million erythrocytes per cubic millimetre
Erythrocytes have multiple key adaptations for their role: they lose their nucleus and most organelles during maturation to maximise space for hemoglobin; they have a biconcave disc shape that increases surface area for gas exchange and reduces diffusion distance; and they are flexible to squeeze through the narrow lumens of capillaries.
Explain how the biconcave disc shape of erythrocytes adapts them for oxygen transport.
- 1
A biconcave disc has a much larger surface area to volume ratio than a spherical cell of the same total volume.
- 2
A larger surface area allows much faster diffusion of oxygen into and out of the cell, which is required to meet the high oxygen demand of respiring body tissues.
- 3
The thin central region of the disc also reduces the diffusion distance that oxygen molecules must travel to reach hemoglobin deep in the cell, further speeding up gas exchange.
3. Leukocytes (White Blood Cells): Immune Defenseβ β β βββ± 15 min
Leukocytes
Nucleated blood cells that function in immune defense against invading pathogens
Example:
Leukocytes are much less numerous than erythrocytes, making up less than 1% of total blood volume
Leukocytes are divided into two main functional groups: phagocytes and lymphocytes. Phagocytes (including neutrophils and macrophages) carry out non-specific defense by engulfing and digesting any pathogen via phagocytosis. Lymphocytes mediate specific immune responses: B lymphocytes produce antibodies that bind and neutralize specific pathogens, while T lymphocytes destroy infected host cells and coordinate the overall immune response.
Distinguish between the roles of phagocytes and lymphocytes in mammalian blood.
- 1
Phagocytes provide non-specific immune defense, meaning they target any type of pathogen that enters the body.
- 2
Their core role is phagocytosis: they engulf pathogens and break them down with digestive enzymes.
- 3
Lymphocytes provide specific immune defense, where each lymphocyte targets only one specific type of antigen on a pathogen.
- 4
B lymphocytes secrete antibodies that bind and inactivate specific pathogens, while T lymphocytes kill infected body cells. This means phagocytes give general protection, while lymphocytes give targeted, long-term immunity.
4. Thrombocytes (Platelets): Blood Clottingβ β ββββ± 10 min
Thrombocytes (Platelets)
Small anucleate cell fragments produced from large megakaryocyte cells in bone marrow
Example:
Healthy human blood has 150,000β450,000 platelets per microlitre
When a blood vessel wall is damaged, platelets become activated, stick to the damaged site, and release clotting factors that trigger a cascade of reactions. This cascade leads to the formation of a mesh of fibrin, which traps red blood cells to form a solid clot that stops blood loss and prevents pathogens from entering the wound.
Explain why platelets are classified as cell fragments, not true cells.
- 1
True cells have a nucleus and full set of organelles to carry out independent metabolism and division. Platelets have neither.
- 2
Platelets form when large precursor cells called megakaryocytes in bone marrow break apart into many small, membrane-bound fragments.
- 3
Because they are derived from broken down parent cells and lack the structure of a complete, independent cell, they are classified as fragments, not true cells.
5. Common Pitfalls
Wrong move:
Claiming all oxygen is transported bound to hemoglobin in red blood cells
Why:
A small but significant proportion of oxygen (~1.5%) is dissolved directly in plasma
Correct move:
State that ~98.5% of oxygen is bound to hemoglobin in erythrocytes, with ~1.5% dissolved in plasma
Wrong move:
Stating mature mammalian red blood cells have a nucleus to carry hemoglobin
Why:
Mature erythrocytes lose their nucleus to maximise space for hemoglobin
Correct move:
Confirm that mature mammalian erythrocytes are anucleate, an adaptation to maximise hemoglobin storage
Wrong move:
Confusing phagocytes and lymphocytes, claiming lymphocytes carry out phagocytosis
Why:
Only phagocytes perform phagocytosis; lymphocytes have specific immune roles
Correct move:
Remember: phagocytes = non-specific engulfing of pathogens, lymphocytes = specific antibody-mediated immunity
Wrong move:
Claiming platelets are true nucleated cells
Why:
Platelets are anucleate cell fragments, not full independent cells
Correct move:
Always describe platelets as cell fragments involved in blood clotting
Wrong move:
Stating most carbon dioxide is transported bound to hemoglobin
Why:
Only ~23% of COβ is bound to hemoglobin; ~70% is transported as hydrogencarbonate ions in plasma
Correct move:
Recall that most COβ is transported as hydrogencarbonate ions dissolved in plasma
6. Quick Reference Cheatsheet
Blood Component | Approximate Proportion | Core Function(s) |
|---|---|---|
Plasma | ~55% | Transport of solutes, heat, COβ |
Erythrocytes | ~45% | Transport Oβ and some COβ |
Leukocytes | <1% | Immune defense against pathogens |
Platelets | <1% | Blood clotting and wound repair |
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2022 Β· 1
Function of plasma multiple choice
- 2023 Β· 2
Describe RBC adaptations
- 2021 Β· 1
Role of platelets in clotting
Going deeper
What's Next
Mastering blood composition is the foundation for all further topics in transport and immunity in CIE A-Level Biology. You will build on this knowledge when studying how respiratory gases are loaded, transported and unloaded in different tissues, and how the blood clotting cascade works to prevent blood loss. This topic also connects directly to the study of the specific immune system, where you will explore the roles of phagocytes and lymphocytes in more detail. Exam questions commonly combine knowledge of blood component structure and function, so a strong grasp of this topic will help you access full marks on both multiple choice and extended response questions.