Erythrocytes Diagram: Red Blood Cells and Their Function
Explore an erythrocytes diagram with labelled red blood cell structures, functions, blood components, plasma and how red blood cells are produced.
An erythrocytes diagram helps you visualise one of the most important cells in the human body — the red blood cell. Erythrocytes are the tiny, disc-shaped cells that carry oxygen from your lungs to every tissue in your body, and help return waste gases back to be exhaled. Understanding their structure is key to understanding how blood keeps you alive. This article explains the labelled features of a red blood cell diagram, the four components of blood, where red blood cells are made, and what makes these cells so remarkably well suited to their work.
What Is Blood?
Blood is a specialised connective tissue that circulates throughout the body via the cardiovascular system. It performs four essential functions:
- Transport — carrying oxygen, nutrients, hormones and waste products
- Immune defence — delivering white blood cells and antibodies to sites of infection
- Clotting — preventing excessive blood loss through platelets and clotting proteins
- Homeostasis — helping regulate body temperature, pH and fluid balance
Blood is made up of both liquid and cellular components. The liquid portion is called plasma, and it suspends the cellular elements — red blood cells, white blood cells and platelets — throughout the circulatory system.
The Four Components of Blood
Understanding blood and its cells begins with the four principal components:
| Component | Type | Primary Function |
|---|---|---|
| Plasma | Liquid | Transports cells, proteins, nutrients and waste |
| Red blood cells (erythrocytes) | Cell | Carry oxygen and support CO₂ transport |
| White blood cells (leucocytes) | Cell | Immune defence and infection response |
| Platelets (thrombocytes) | Cell fragment | Blood clotting and wound repair |
It is worth noting that platelets are not complete cells — they are small fragments shed from larger cells called megakaryocytes in the bone marrow. Together, these four blood components form a dynamic system that maintains the body’s internal environment.
Erythrocytes Diagram: Structure and Labels
A typical red blood cell diagram shows a cell that looks quite unlike most cells you might study in biology. Here is what a labelled erythrocytes diagram would identify:
Key Labelled Features
- Biconcave disc shape — The cell is flattened with a depression on both sides, like a donut that has not been fully hollowed out. This shape increases the surface area available for gas exchange.
- Cell membrane — A thin, flexible outer layer that allows the cell to deform as it squeezes through narrow capillaries.
- Haemoglobin-containing interior — The cell is packed with haemoglobin, the iron-containing protein responsible for binding and transporting oxygen.
- Central pallor (depression) — The thinner central region visible under a microscope, caused by the biconcave shape.
- No nucleus — Mature human red blood cells do not contain a nucleus, which is a defining feature covered in detail below.
Image Brief: A scientifically accurate labelled diagram should depict a mature human erythrocyte in biconcave disc form, shown in cross-section and surface view. Labels should indicate: (1) biconcave shape, (2) flexible cell membrane, (3) haemoglobin-filled cytoplasm, (4) central depression, (5) absence of nucleus. The cell should not be depicted with a nucleus, as this would misrepresent the mature human erythrocyte.
What Do Red Blood Cells Do? Function of Erythrocytes
Erythrocytes are almost entirely dedicated to a single purpose: gas transport.
Main functions of red blood cells:
- Bind oxygen in the lungs via haemoglobin and release it in body tissues
- Help transport carbon dioxide away from tissues (though most CO₂ travels as bicarbonate dissolved in plasma)
- Maintain the blood’s capacity to deliver oxygen efficiently throughout the body
- Contribute to blood viscosity and flow characteristics
The biconcave shape is not arbitrary — it gives the red blood cell a larger surface area relative to its volume than a sphere of the same size would have. This maximises the efficiency of gas diffusion across the membrane.
The Role of Haemoglobin
Haemoglobin is the protein that makes red blood cells red. Each molecule contains four iron-containing haem groups, each capable of binding one molecule of oxygen. In oxygen-rich environments (the lungs), haemoglobin readily picks up oxygen to form oxyhaemoglobin. In oxygen-poor tissues, it releases the oxygen and picks up some carbon dioxide to form carboxyhaemoglobin — though the majority of CO₂ is converted to bicarbonate ions and carried in the plasma.
Practical Insight: The flexibility of the red blood cell membrane allows erythrocytes to squeeze through capillaries smaller than their own diameter. This, combined with their flat shape and haemoglobin content, means they can deliver oxygen even to the most remote tissues efficiently.
Lifespan of a Red Blood Cell
A mature erythrocyte lives for approximately 120 days before it becomes too fragile and is removed from circulation. Ageing red blood cells are broken down primarily in the spleen and liver, and components such as iron are recycled for use in new haemoglobin.
Do Red Blood Cells Have a Nucleus?
No — mature human red blood cells do not have a nucleus. This is one of the most frequently asked questions in human biology, and the answer is clear: by the time an erythrocyte enters the bloodstream, it has already ejected its nucleus as part of the maturation process.
Developing red blood cells in the bone marrow — known as erythroblasts — do contain a nucleus in their early stages. As maturation progresses, the nucleus is expelled, along with most other organelles including mitochondria and ribosomes. The resulting cell is essentially a membranous sack of haemoglobin, optimised purely for gas transport.
The absence of a nucleus has two important consequences: the cell cannot replicate itself, and it cannot produce new proteins. This is why erythrocytes have a fixed lifespan and must be continuously replaced.
Where Are Red Blood Cells Manufactured?
Red blood cells are produced in the red bone marrow — a spongy tissue found inside certain bones, including the sternum, vertebrae, pelvis and the ends of long bones such as the femur. In healthy adults, this is the primary site of blood cell production, a process known as haematopoiesis.
Red blood cell production specifically is called erythropoiesis. It is regulated by a hormone called erythropoietin (EPO), which is produced mainly by the kidneys. When blood oxygen levels fall — for example at high altitude or during anaemia — the kidneys release more EPO, which stimulates the bone marrow to produce more red blood cells.
From the initial stem cell stage to a fully mature erythrocyte ready to enter circulation typically takes around seven days.
What Is Blood Plasma and What Does It Contain?
Plasma is the fluid component of blood — the straw-coloured liquid in which all blood cells are suspended. It makes up approximately 55% of total blood volume.
Plasma contains:
- Water (~90%) — the universal solvent that allows substances to dissolve and move through the body
- Plasma proteins — including albumin (maintaining osmotic pressure), fibrinogen (clotting), and immunoglobulins (immune defence)
- Electrolytes — such as sodium, potassium, calcium and chloride, which support nerve and muscle function
- Nutrients — glucose, amino acids, fatty acids and vitamins
- Hormones — chemical messengers travelling between organs
- Dissolved gases — including oxygen and carbon dioxide
- Waste products — such as urea and creatinine, en route to the kidneys for excretion
Plasma vs serum: Serum is what remains after blood has clotted and the clotting proteins (including fibrinogen) have been removed. Plasma retains these proteins and is obtained by preventing blood from clotting before separating the cells.
Red Blood Cells vs White Blood Cells
| Feature | Red Blood Cells (Erythrocytes) | White Blood Cells (Leucocytes) |
|---|---|---|
| Primary function | Oxygen transport | Immune defence |
| Contains haemoglobin | Yes | No |
| Nucleus in mature cells | No | Yes |
| Relative abundance | ~4.5–5.5 million per µL | ~4,000–11,000 per µL |
| Appearance | Biconcave disc, uniformly red | Variable; some granular, some with lobed nuclei |
| Lifespan | ~120 days | Hours to years, depending on type |
White blood cells are not a single type of cell. They include neutrophils, lymphocytes, monocytes, eosinophils and basophils, each with a distinct structure and role. A white blood cell diagram would need to show several different cell types to be accurate — unlike the more uniform appearance of red blood cells.
Red Blood Cell Counts and Blood Testing
Red blood cell levels are typically measured as part of a full blood count (FBC), a routine laboratory test that reports values including red blood cell count, haemoglobin concentration, haematocrit (the proportion of blood made up of red blood cells) and mean cell volume.
Reference ranges can vary between laboratories and depend on individual factors including age and sex. Results should always be interpreted in the context of a person’s clinical history by a qualified professional — an educational diagram alone cannot determine whether any individual’s results are healthy.
If you are interested in assessing your broader health markers, you may wish to explore a full health screening package to understand what routine testing can cover. Those with concerns about cardiovascular health may also find it useful to look at chest pain screening as part of a wider assessment. For a comprehensive overview, a total body check up can bring together a range of health markers in a structured report.
Frequently Asked Questions
1. What is an erythrocytes diagram?
An erythrocytes diagram is a labelled scientific illustration showing the structure of a red blood cell. It typically identifies the biconcave disc shape, the flexible cell membrane, the haemoglobin-rich interior, the central depression and the absence of a nucleus in mature human red blood cells. These diagrams are used widely in biology education to explain how red blood cell structure supports its function.
2. What are erythrocytes and what do they do?
Erythrocytes are red blood cells — the most abundant cells in human blood. Their primary role is to transport oxygen from the lungs to body tissues using a protein called haemoglobin. They also play a supporting role in carrying carbon dioxide back towards the lungs for exhalation, though most CO₂ travels as bicarbonate dissolved in plasma.
3. Do mature human red blood cells have a nucleus?
No. Mature human red blood cells do not have a nucleus. The nucleus is ejected during the final stages of red blood cell maturation in the bone marrow. This distinguishes erythrocytes from almost all other human cells and allows more space for haemoglobin, improving the cell’s oxygen-carrying capacity.
4. What are the four components of blood?
The four main components of blood are plasma (the liquid portion), red blood cells (erythrocytes), white blood cells (leucocytes) and platelets (thrombocytes). Each plays a distinct role. Plasma transports substances; red blood cells carry oxygen; white blood cells defend against infection; and platelets assist with clotting. Platelets are fragments of cells rather than complete cells.
5. What is the fluid component of blood called?
The fluid component of blood is called plasma. It is a straw-coloured liquid that makes up roughly 55% of blood volume. Plasma contains water, proteins, electrolytes, nutrients, hormones, dissolved gases and waste products. If the clotting proteins are removed after blood clots, the remaining fluid is called serum.
6. Where are red blood cells manufactured?
Red blood cells are produced in the red bone marrow of certain bones, including the sternum, vertebrae, pelvis and ends of long bones. This process is called erythropoiesis and is regulated by the hormone erythropoietin, which is released by the kidneys when blood oxygen levels are low. Maturation from stem cell to circulating red blood cell takes approximately seven days.
7. What is the function of haemoglobin in red blood cells?
Haemoglobin is the iron-containing protein that fills red blood cells. It binds to oxygen in the lungs to form oxyhaemoglobin and releases that oxygen in body tissues where it is needed. Each haemoglobin molecule can carry four oxygen molecules. Haemoglobin also binds some carbon dioxide, though most CO₂ is carried as bicarbonate dissolved in plasma.
8. How are red blood cells different from white blood cells?
Red blood cells carry oxygen using haemoglobin and lack a nucleus when mature. White blood cells, by contrast, do contain a nucleus and are involved in immune defence. There are several types of white blood cells, each with different structures and roles. Red blood cells are far more numerous, but white blood cells are essential for protecting the body from infection and disease.
9. What is the difference between plasma and red blood cells?
Plasma is the liquid matrix of blood — a protein-rich fluid that suspends all blood cells and transports substances around the body. Red blood cells are solid, disc-shaped cells contained within that liquid. Plasma constitutes approximately 55% of blood volume, while red blood cells make up most of the remaining 45%.
10. Which blood test measures red blood cell counts?
Red blood cell counts are measured as part of a full blood count (FBC), a standard laboratory test. It reports values including red blood cell count, haemoglobin levels and haematocrit. Reference ranges vary by laboratory and individual characteristics. Results should always be interpreted alongside relevant clinical information by a qualified healthcare professional, not assessed in isolation.
Sources and Further Reading
- NHS. Blood tests. Available at: https://www.nhs.uk/conditions/blood-tests/ (accessed October 2026)
- NHS. Blood. Available at: https://www.nhs.uk/conditions/blood/ (accessed October 2026)
- Hoffbrand, A.V. and Moss, P.A.H. (2016) Hoffbrand’s Essential Haematology. 7th edn. Wiley-Blackwell.
- Sherwood, L. (2015) Human Physiology: From Cells to Systems. 9th edn. Cengage Learning.
- British Society for Haematology. Guidelines and resources. Available at: https://b-s-h.org.uk/ (accessed October 2026)
Educational Disclaimer: This article is intended to provide general educational information about blood biology, erythrocyte structure and the components of blood. It is not a substitute for personalised medical advice. If you have concerns about your blood health or test results, please consult a qualified healthcare professional. Health Screening Clinic provides testing and reporting services only and does not offer GP consultations, diagnosis, treatment or prescriptions.
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