Overview
plasma
The liquid component of blood, containing proteins, electrolytes, hormones and clotting factors.haemoglobin
The iron-containing protein in red blood cells that binds and releases oxygen.Interesting facts
- • Mature red blood cells have no nucleus and no mitochondria — they cannot repair themselves or use the oxygen they carry, which is why they last only about 120 days.
- • Haemoglobin changes shape as it binds oxygen, making each of its four binding sites more receptive than the last — cooperative binding that produces the characteristic S-shaped oxygen dissociation curve.
- • The body makes roughly two to three million red blood cells every second, and destroys the same number, mostly in the spleen and liver.
- • Plasma is around 92% water, but its protein content — chiefly albumin — is what holds fluid inside blood vessels; when albumin falls, fluid leaks into tissues and causes oedema.
- • Blood type arises from sugar molecules on the red cell surface, and the distribution of types varies markedly between populations, most likely shaped in part by historical infectious disease pressure.
Common misconceptions
- Deoxygenated blood is blue.
It is dark red. Veins look blue because of how skin scatters light, not because of the blood inside them. Blood is never blue in humans. - Eating iron-rich food fixes any anaemia.
Only iron-deficiency anaemia responds to iron. Anaemia from B12 deficiency, chronic kidney disease, thalassaemia or blood loss needs a different treatment, and giving iron unnecessarily can cause harm. - Blood-type diets improve health.
Controlled research has found no relationship between blood group and the metabolic response to different diets. The apparent benefits come from the diets themselves being generally healthier, not from matching blood type. - Detoxes and cleanses purify the blood.
The liver and kidneys clear metabolic waste continuously and cannot be enhanced by a juice regimen. There is no product that cleans blood; dialysis exists precisely because nothing oral does the job. - Thicker blood means better oxygen delivery.
Above a certain point, higher red cell concentration increases viscosity and worsens flow, which is why polycythaemia raises clot and stroke risk rather than improving performance.
Anatomy & how it works
A plasma matrix carrying three cell lineages, all descended from a single stem cell population in the bone marrow.
Plasma
The liquid phase — water, albumin, clotting factors, immunoglobulins, electrolytes, hormones and nutrients.
Red blood cells (erythrocytes)
Biconcave discs packed with haemoglobin, lacking a nucleus, dedicated to oxygen and carbon dioxide transport.
Neutrophils
The most numerous white cell, first responders to bacterial infection, short-lived and abundant.
Lymphocytes
B cells, T cells and natural killer cells — the adaptive immune system's memory and targeted killing capability.
Monocytes, eosinophils and basophils
Macrophage precursors and the cells handling parasites, allergy and inflammatory signalling.
Platelets (thrombocytes)
Cell fragments from marrow megakaryocytes that adhere, aggregate and initiate clot formation within seconds of injury.
Haematopoietic stem cells
The self-renewing marrow population from which every blood cell derives — the cells transplanted in a bone marrow transplant.
Haemoglobin
The iron-containing protein in red blood cells that binds and releases oxygen.erythropoietin
The kidney hormone that stimulates red cell production in response to low oxygen.anaemia
A haemoglobin concentration below the normal range, from many different causes requiring different treatments.Primary functions
- • Transporting oxygen from the lungs to every tissue and carbon dioxide back
- • Delivering immune cells and antibodies to sites of infection
- • Forming clots to stop bleeding after injury
Secondary functions
- • Carrying nutrients from the gut and hormones from endocrine glands
- • Removing metabolic waste to the kidneys and liver
- • Distributing heat to regulate core body temperature
- • Buffering blood pH within a very narrow survivable range
Across a lifetime
- Development
- Blood formation migrates during development — from the yolk sac to the liver and spleen, and finally to bone marrow. Fetal haemoglobin binds oxygen more tightly than adult haemoglobin, allowing the fetus to extract it across the placenta.
- Childhood
- Fetal haemoglobin is replaced over the first six months, which is when sickle cell disease and thalassaemia first become apparent. Iron deficiency is common in toddlers and adolescents during rapid growth.
- Adulthood
- Iron deficiency is common in menstruating women and is the commonest nutritional deficiency worldwide; in men and postmenopausal women it should always prompt a search for a source of bleeding.
- Later life
- Marrow reserve declines, anaemia becomes markedly more common and is associated with worse outcomes, and the risk of both clotting and blood cancers rises.
- Sex differences
- Reference ranges for haemoglobin are meaningfully higher in men. Menstrual blood loss makes iron deficiency far more common in women, while haemophilia, being X-linked, overwhelmingly affects men.
Body connections
Blood is the body's shared infrastructure, and that makes it a uniquely powerful diagnostic window. Because every organ deposits its products and problems into the same circulating fluid, a small sample reports on kidney function, liver function, inflammation, nutritional status, hormone levels, clotting capacity and marrow health simultaneously. Blood is also where several distinct systems become interdependent: anaemia can originate in the gut, the kidney, the marrow or the genes, and telling those apart is often the first real step in a diagnosis.
Body connections
How this links to the rest of you
Blood and the vessels containing it form one functional system; vessel wall health determines whether blood flows smoothly or clots.
The heart provides the pressure that moves blood; cardiac output and blood volume together determine tissue perfusion.
Bone marrow is the factory for every blood cell, so marrow disease presents as blood abnormalities and vice versa.
The kidneys produce erythropoietin, the hormone driving red cell production — which is why chronic kidney disease reliably causes anaemia.
Gas exchange loads oxygen onto haemoglobin and unloads carbon dioxide, making lung and blood function inseparable.
The spleen removes aged and abnormal red cells and stores about a third of the body's platelets.
The liver synthesises albumin and almost all clotting factors, so liver disease causes both oedema and bleeding tendency.
Iron, folate and B12 absorption all occur here, making bowel disease a common cause of anaemia.
How lifestyle changes it
Exercise
Endurance training expands plasma volume and total haemoglobin mass, improving oxygen delivery. It can also cause a dilutional 'sports anaemia' that looks abnormal on a test but reflects adaptation rather than deficiency.
Nutrition
Iron, vitamin B12, folate and vitamin C are the direct determinants of red cell production; vitamin K underpins clotting factor synthesis. This is one of the areas where nutrition maps onto blood values very directly.
Hydration
Dehydration concentrates blood, raising apparent haemoglobin and haematocrit while increasing viscosity and clot risk. Fluid status is why the same person's results can vary between tests.
Sleep
Chronic short sleep is associated with higher inflammatory markers and altered immune cell counts, both measurable in blood.
Stress
Acute stress mobilises white cells and increases platelet reactivity — a short-term preparation for injury that becomes unhelpful when sustained.
Ageing
Marrow reserve falls, anaemia prevalence rises steeply after 65, and clonal changes in blood stem cells become common, some of which raise cardiovascular and leukaemia risk.
Environment
Altitude raises erythropoietin and red cell mass within weeks. Smoking raises carboxyhaemoglobin and haematocrit; lead and benzene exposure both damage marrow function directly.
Genetics
Sickle cell disease, thalassaemia, haemophilia, von Willebrand disease, haemochromatosis and factor V Leiden are all inherited and all common enough to matter in family history.
Symptoms & conditions
Common symptoms
Common conditions
Rare conditions
- • Leukaemia
- • Lymphoma
- • Myeloma
- • Aplastic anaemia
- • Polycythaemia vera
- • Haemophilia
- • Sickle cell disease
- • Thalassaemia major
- • Thrombotic thrombocytopenic purpura
Acute & chronic problems
- • Major haemorrhage from trauma
- • Acute haemolysis from transfusion reaction or drug exposure
- • Deep vein thrombosis and pulmonary embolism
- • Disseminated intravascular coagulation in sepsis
- • Iron deficiency anaemia
- • Vitamin B12 or folate deficiency
- • Anaemia of chronic disease and of kidney failure
- • Inherited clotting or bleeding disorders
- • Haemochromatosis with iron overload
Early warning signs
- • Unusual tiredness with breathlessness on mild exertion
- • Pale skin, gums or inner eyelids
- • Unusual bruising or bleeding gums
- • Recurrent infections or slow recovery from them
- • Heavy periods with fatigue
- • Craving ice or non-food substances, a recognised sign of iron deficiency
Risk factors
- • Heavy menstrual bleeding
- • Plant-based diets without B12 supplementation
- • Coeliac disease and inflammatory bowel disease
- • Chronic kidney disease
- • Family history of inherited blood disorders
- • Prolonged immobility for clot risk
- • Benzene or lead exposure
Protective factors
- • Adequate dietary iron with vitamin C to aid absorption
- • B12 supplementation on a plant-based diet
- • Investigating anaemia rather than treating it blindly
- • Regular movement during long travel or bed rest
- • Blood donation in haemochromatosis, where it is genuinely therapeutic
Optimise & recover
Prevention
- • Pair plant iron sources with vitamin C, and separate them from tea, coffee and calcium, which inhibit absorption
- • Supplement B12 reliably if you eat little or no animal food — deficiency causes irreversible nerve damage if left long enough
- • Get anaemia investigated rather than self-treating with iron; in men and postmenopausal women it can be the first sign of gut bleeding
- • Move regularly on long flights and during illness or bed rest to reduce clot risk
- • Do not smoke, which raises haematocrit and platelet reactivity while reducing oxygen-carrying capacity
Recovery
- • Iron stores take three to six months to rebuild after correcting deficiency — stopping when haemoglobin normalises leaves the tank empty
- • Alternate-day iron dosing produces better absorption than daily dosing, because a dose transiently blocks the next one
- • After significant blood loss, red cell mass takes weeks to restore even with adequate iron; exercise capacity returns gradually
Blood does not rehabilitate in the musculoskeletal sense, but recovery of exercise capacity after anaemia or blood loss follows a predictable course and responds to graded aerobic training. In sickle cell disease and haemophilia, structured physiotherapy has a specific role — protecting joints from the damage caused by repeated bleeds, and maintaining conditioning without triggering crises.
Movement library
- Beginner
Regular movement breaks during immobility
Calf and ankle movement during long travel or bed rest activates the muscle pump and reduces clot risk.
- Beginner
Joint range-of-motion work in haemophilia
Maintains range after joint bleeds, done under specialist physiotherapy guidance.
- Beginner
Graded aerobic training
Rebuilds exercise capacity after anaemia or significant blood loss, starting well below previous tolerance.
- Beginner
Low-impact resistance training
Preferred in bleeding disorders — builds muscle to protect joints without high-impact bleed risk.
- Advanced
Altitude or heat training blocks
Used by endurance athletes to expand plasma volume and haemoglobin mass; requires adequate iron status to work.
- Beginner
General flexibility routine
Supports return to activity during recovery from anaemia; no blood-specific mechanism.
Massage does not improve blood quality or 'circulation' in any lasting sense. It is specifically contraindicated over an area of suspected deep vein thrombosis and should be used cautiously in anyone with a low platelet count or on anticoagulants, where bruising risk is raised.
Habits worth building
- • Donate blood if eligible — it is safe for you and directly useful, and iron stores recover between donations
- • Know your own baseline blood results; trends are far more informative than single values
- • Take iron supplements on an empty stomach with vitamin C, and expect dark stools, which are harmless
Nutrition, devices & products
Blood is where nutrition shows up fastest and most measurably. Iron, vitamin B12 and folate are required to build red cells, and deficiency in any one produces anaemia with a distinct pattern — small pale cells with iron lack, large cells with B12 or folate lack. Vitamin C substantially increases absorption of plant iron; tea, coffee, calcium and phytates reduce it. Vitamin K is needed for clotting factor synthesis, which is why it interacts with warfarin. Consistency rather than quantity is the aim for anyone on anticoagulants.
Foods to prioritise
- • Haem iron from red meat, or plant iron from legumes, tofu and fortified cereals paired with vitamin C
- • Vitamin B12 from animal foods, fortified products or a supplement on a plant-based diet
- • Folate from leafy greens, legumes and fortified flour
- • Vitamin C alongside plant iron sources to improve uptake
- • Adequate protein for haemoglobin and plasma protein synthesis
Foods to limit
- • Tea and coffee with iron-rich meals, which markedly reduce absorption
- • Calcium supplements taken at the same time as iron
- • Alcohol, which impairs folate use and marrow function and worsens bleeding risk
- • Iron supplementation without confirmed deficiency, especially in haemochromatosis
| Supplement | Evidence | Note |
|---|---|---|
| Iron | Strong | Definitive treatment for iron-deficiency anaemia. Alternate-day dosing improves absorption. Should follow a test, not a guess. |
| Vitamin B12 | Strong | Essential where intake or absorption is inadequate; untreated deficiency causes irreversible neurological damage, so it should never be left to chance on a vegan diet. |
| Folic acid | Strong | Prevents neural tube defects preconception and treats folate-deficiency anaemia; also needed in chronic haemolysis. |
| Vitamin C | Moderate | Reliably enhances non-haem iron absorption when taken with the meal; no benefit taken separately. |
| Blood-building herbal tonics | Limited | Products marketed to 'build blood' rarely contain therapeutic iron doses and delay proper diagnosis of the cause of anaemia. |
Devices & wearables
- • Pulse oximeters, which estimate haemoglobin oxygen saturation non-invasively
- • Home INR monitors for people on warfarin
- • Compression stockings to reduce venous clot risk
- • Glucose meters and continuous glucose monitors, which sample blood or interstitial fluid
- • Optical heart rate and SpO2 sensors, useful for trends but less accurate on darker skin tones and in poor perfusion
- • Continuous glucose monitors, increasingly used outside diabetes with limited evidence of benefit
Professional treatments
- • Full blood count and blood film examination
- • Iron studies, B12, folate and haemoglobinopathy screening
- • Blood transfusion
- • Bone marrow aspiration and biopsy
- • Venesection for haemochromatosis and polycythaemia
- • Stem cell and bone marrow transplantation
Educational mention only, not a recommendation: Oral and intravenous iron for deficiency anaemia, B12 injections or high-dose oral replacement, Erythropoiesis-stimulating agents in kidney-related anaemia, Anticoagulants including warfarin and direct oral anticoagulants for clot prevention, Antiplatelet drugs such as aspirin and clopidogrel, Clotting factor concentrates in haemophilia.
When to seek medical care
Persistent fatigue with breathlessness, pallor or heavy periods warrants a full blood count — it is inexpensive and highly informative. Iron deficiency in a man or a postmenopausal woman should never simply be treated with iron; it needs a search for a bleeding source, because it can be the first sign of a gut cancer. Unexplained bruising, bleeding gums, a non-blanching rash, or recurrent infections with weight loss and night sweats all need prompt assessment. Sudden breathlessness with chest pain, or a swollen painful calf, may indicate a clot and needs same-day care.
Seek care promptly if you notice
- • Bleeding that will not stop, or vomiting blood or passing black tarry stools
- • A non-blanching rash with fever or feeling very unwell
- • Widespread unexplained bruising or bleeding from gums and nose
- • Sudden breathlessness with chest pain, or a swollen tender calf
- • Unexplained weight loss with night sweats, itching or lumps
- • Fainting or confusion with marked pallor
Research & frequently asked questions
Current research
- Clonal haematopoiesis of indeterminate potential — age-related mutations in blood stem cells — has emerged as a substantial independent cardiovascular and leukaemia risk factor, and its clinical management is being actively worked out.
1
The Lancet · 2021
Iron deficiency
Review covering global prevalence, diagnosis using ferritin with awareness of inflammation, and the requirement to identify a bleeding source in men and postmenopausal women.
- Gene therapy for sickle cell disease and beta thalassaemia has moved from trials into approved treatment, including the first approved CRISPR-based therapy, transforming the outlook for inherited haemoglobin disorders.
2
The Lancet Haematology · 2017
Iron absorption from oral iron supplements given on consecutive versus alternate days
Trial demonstrating that a dose of iron raises hepcidin and blocks absorption of subsequent doses, so alternate-day single doses deliver more absorbed iron than divided daily regimens.
Emerging therapies
- • Gene editing and gene addition therapies for haemoglobinopathies
- • Lab-grown red blood cells, now entering early human trials for people with rare blood types
- • Non-factor therapies for haemophilia including bispecific antibodies and RNA interference
- • Universal donor blood produced by enzymatically removing surface antigens
Scientific controversies
- • Optimal transfusion thresholds remain debated, though the accumulated evidence favours restrictive strategies in most stable patients over liberal transfusion.
- • Whether to screen for and how to act on clonal haematopoiesis is unresolved, since the finding carries risk but no established intervention.
- • Direct-to-consumer blood testing panels are expanding rapidly, with real concerns that testing dozens of markers without clinical context generates false alarms and unnecessary investigation.
Blood transfusion was lethally unpredictable until Karl Landsteiner identified the ABO blood groups in 1901, work that made transfusion routine and earned a Nobel Prize. The discovery of the Rhesus factor four decades later explained haemolytic disease of the newborn and led to anti-D prophylaxis, one of the clearest examples of a single intervention nearly eliminating a disease. Blood is also where molecular medicine began: sickle cell disease was the first condition traced to a specific single amino acid substitution in a protein.
Frequently asked questions
Why am I always tired — could it be my blood?
It could, and a full blood count is one of the most useful first tests. Anaemia from iron, B12 or folate deficiency is common and treatable. But most people investigated for fatigue have normal counts, so a normal result should redirect the search rather than end it.
Should I just take iron if I feel tired?
Better to test first. Iron only helps iron-deficiency anaemia, taking it unnecessarily causes side effects and can be harmful in conditions like haemochromatosis, and — importantly — unexplained iron deficiency in men and postmenopausal women can be the first sign of bleeding that needs investigating.
Is blood really blue inside the body?
No. It ranges from bright red when oxygenated to dark red when not. Veins appear blue because of how light scatters in skin, which is a trick of optics rather than anything about the blood.
Does donating blood harm your health?
For eligible donors, no. You lose about 470 ml, plasma volume restores within a day or two, and red cells over a few weeks. Donation intervals are set to let iron stores recover. In haemochromatosis, regular removal of blood is actually the treatment.
What does a high white cell count mean?
Most often an infection or inflammation, sometimes just physical stress or steroid treatment. Persistently high or very high counts, particularly with weight loss, night sweats or abnormal cells on a blood film, need proper haematological assessment.
Can you thin your blood naturally?
Some foods and supplements have mild antiplatelet effects, but none are reliable substitutes for prescribed anticoagulation, and combining them with medication increases bleeding risk. If you need blood thinning, it needs to be dosed and monitored.
Explore further
Conditions affecting this
Keep exploring
Glossary
- Haemoglobin
- The iron-containing protein in red blood cells that binds and releases oxygen.
- Haematocrit
- The proportion of blood volume made up of red cells; rises with dehydration and at altitude.
- Plasma
- The liquid component of blood, containing proteins, electrolytes, hormones and clotting factors.
- Anaemia
- A haemoglobin concentration below the normal range, from many different causes requiring different treatments.
- Platelet
- A cell fragment from bone marrow that initiates clotting by adhering and aggregating at sites of injury.
- Erythropoietin
- The kidney hormone that stimulates red cell production in response to low oxygen.
- Haematopoiesis
- The production of blood cells from stem cells in the bone marrow.
- Ferritin
- The iron storage protein; a blood ferritin level is the usual measure of iron stores, though it also rises with inflammation.
Trusted organisations & further reading
- NHS — Iron deficiency anaemia
- NHS Blood and Transplant
- Blood Cancer UK
- World Health Organization — Anaemia
- Nine Pints — Rose George. Wide-ranging and consistently interesting account of blood, donation, transfusion and the industries built around them.
- The Immortal Life of Henrietta Lacks — Rebecca Skloot. Not about blood specifically, but essential context on cell biology, consent and the history of human tissue in research.
Medical disclaimer
This page is for general education and does not replace personalised medical advice. If you have concerning symptoms, or before starting a new supplement, medication or exercise programme, speak with a qualified healthcare professional.