Blood: transport, defence and haemostasis

Blood is a circulating tissue

The liquid component of blood is plasma, containing water, electrolytes, nutrients, hormones and many proteins. Red cells, white cells and platelets are suspended within it. Red cells are the most numerous formed elements. Platelets are fragments released from bone-marrow megakaryocytes; white cells include several populations with different immune roles. Calling blood a tissue emphasises coordinated activity between cells and their fluid environment.

Albumin contributes to oncotic effects and transports selected substances. Clotting-related proteins participate in repair, while immunoglobulins can function as antibodies. Serum is the liquid obtained after blood has clotted and is not interchangeable with plasma. Whether a collection tube contains an anticoagulant affects the sample and the tests that can be performed. Concentrations also depend on water balance: dehydration can raise a measured concentration without increasing the total amount of that substance in the body.

OpenStax: an overview of blood

Red cells transport oxygen within biological limits

Mature red cells lack a nucleus. Their biconcave shape supports a large relative surface area and the flexibility needed to pass through narrow microvessels. Haemoglobin binds oxygen reversibly, allowing blood to carry far more oxygen than plasma could dissolve alone. Red cells also contribute to carbon dioxide transport and acid–base buffering, although haemoglobin does not carry every carbon dioxide molecule.

Production occurs mainly in bone marrow and requires resources including iron, vitamin B12 and folate. Erythropoietin, produced predominantly by the kidneys, helps regulate the response to oxygen availability. Producing replacement cells takes time. Anaemia means a haemoglobin concentration below the reference threshold for the relevant population. It can arise from inadequate production, blood loss or accelerated destruction; it is not synonymous with iron deficiency. Someone who interprets fatigue as a reason to take iron has skipped several questions. Fatigue is nonspecific, and even demonstrated anaemia requires an explanation of its mechanism before it can be reduced to a nutritional problem.

OpenStax: erythrocytes

White cells and platelets have distinct roles

Neutrophils participate rapidly in many acute inflammatory responses. Lymphocytes support specific recognition, antibody responses and cellular immunity. Monocytes entering tissues can develop into macrophages and related cells. Their functions overlap and cooperate, so a total white-cell count is not a general score of immune competence. Elevation may accompany infection, inflammation, stress or other processes; a normal count does not exclude illness. Cell differentials, symptoms and the time course provide necessary context.

Platelets help establish a barrier at an injured vessel. Intact endothelium provides signals that limit platelet adhesion and clotting. Injury exposes underlying structures, allowing platelets to adhere, activate, change shape, release mediators and aggregate. Amplification is useful locally but dangerous if it spreads. More platelets or greater activation are therefore not automatically beneficial. A platelet count measures quantity, not every aspect of platelet function, and says little directly about the activity of all coagulation proteins.

OpenStax: leukocytes and platelets

Haemostasis is a coordinated local process

Haemostasis limits blood loss from an injured site. Local vessel constriction can reduce escape, platelets form an initial plug, and coagulation generates a fibrin network that stabilises it. These processes overlap rather than waiting in a strict queue. Many coagulation proteins circulate as inactive precursors and are activated through enzyme reactions that amplify the response. Thrombin converts soluble fibrinogen into fibrin capable of forming a supporting mesh.

The traditional intrinsic, extrinsic and common pathways help explain laboratory assays, but do not fully reproduce interactions among cell surfaces, blood flow and tissue factor in a living wound. Normal haemostasis also depends on more than the liver or a single protein. A platelet-and-fibrin barrier after a skin cut is useful repair; an obstructive clot inside a vessel can be harmful. Shared molecular machinery produces different consequences depending on the trigger, location, extent and adequacy of regulation.

OpenStax: hemostasis

Why clotting must stop and be cleared

During repair, clotting must remain confined to the needed region. Intact endothelium, removal of activated substances by flowing blood and natural anticoagulant systems restrain extension. Fibrinolysis gradually degrades fibrin, with plasmin acting as an important effector. Coagulation and fibrinolysis are coordinated in space and time rather than functioning as mutually exclusive switches. A tube of blood clotting outside the body does not reproduce the complete regulation of living circulation.

Everyday descriptions of “thin” or “thick” blood often mix viscosity, red-cell proportion, platelet behaviour, coagulation activity and clot risk. These are different properties. Antiplatelet and anticoagulant medicines target different steps and do not simply dilute blood with water. A measure that lowers a particular clotting risk may increase bleeding risk. Bruising or fear of thrombosis therefore cannot independently determine treatment. For understanding, identify the precise biological step being altered and the other functions that depend on that step.

OpenStax: hemostasis

Interpret tests within a clinical situation

Suppose a person has a normal platelet count but recurrent mucosal bleeding. Normal haemostasis has not been established: platelet function, vessel-related factors and certain protein abnormalities remain possible. Another person may have reduced haemoglobin with unremarkable white-cell and platelet counts. The other two results cannot dismiss a red-cell problem. Measuring these cell populations separately is useful precisely because they can change independently.

Prothrombin time and activated partial thromboplastin time measure clot formation under specified reagent conditions. They answer selected questions and do not encompass every bleeding mechanism or thrombotic risk. Begin reasoning with the observation: ongoing bleeding, local swelling, fatigue or an incidental laboratory finding. Next separate structural, numerical and functional possibilities. Finally state which interpretation each test supports or leaves unresolved. This avoids treating one normal value as certification of the entire system, or one abnormal value as a definitive diagnosis.

OpenStax: erythrocytes; MedlinePlus: Partial Thromboplastin Time (PTT) Test

Bilingual terms

血浆 · Plasma

The liquid component of unclotted blood.

血红蛋白 · Haemoglobin

The red-cell protein that binds oxygen reversibly.

血小板 · Platelet

A megakaryocyte fragment involved in haemostasis.

纤维蛋白 · Fibrin

A protein forming the clot mesh.

纤溶 · Fibrinolysis

The regulated degradation of fibrin.

Sources and further reading

• OpenStax: an overview of blood
• OpenStax: erythrocytes
• OpenStax: leukocytes and platelets
• OpenStax: hemostasis
• MedlinePlus: Partial Thromboplastin Time (PTT) Test

Original course source-check record: 9 September 2026. Full Chinese and English sentence-by-sentence language review: 14 September 2026. AI editing and language review are not human clinical review. Linked institutions have not participated in or endorsed this course.