Body Fluids and Circulation

Plasma

A straw-coloured, viscous fluid constituting nearly $55\%$ of the blood, composed of $90\text{--}92\%$ water and $6\text{--}8\%$ proteins (mainly fibrinogen, globulins, and albumins).

Serum

Blood plasma from which the clotting factors have been removed.

Formed Elements

The collective cellular components of blood—comprising erythrocytes, leucocytes, and platelets—that constitute approximately $45\%$ of whole blood.

Erythrocytes (Red Blood Cells / RBCs)

The most abundant cells in blood, biconcave and enucleated in most mammals, formed in red bone marrow, carrying haemoglobin, and possessing an average lifespan of $120\text{ days}$ before destruction in the spleen.

Leucocytes (White Blood Cells / WBCs)

Nucleated, colourless blood cells lacking haemoglobin, numbering on average $6000\text{--}8000\text{ mm}^{-3}$ of blood, categorized into granulocytes and agranulocytes.

Neutrophils

The most abundant leucocytes ($60\text{--}65\%$ of total WBCs) that function as phagocytic cells to engulf and destroy foreign organisms.

Basophils

The least abundant granulocytes ($0.5\text{--}1\%$ of total WBCs) that secrete histamine, serotonin, and heparin, and are actively involved in inflammatory reactions.

Eosinophils

Granulocytes constituting $2\text{--}3\%$ of all WBCs that resist infections and play a key role in allergic reactions.

Platelets (Thrombocytes)

Cell fragments produced by megakaryocytes in the bone marrow ($1,500,00\text{--}3,500,00\text{ mm}^{-3}$ of blood) that release factors essential for blood coagulation.

ABO Blood Grouping

A classification system based on the presence or absence of two inherited surface antigens (A and B) on RBCs and corresponding natural antibodies in the plasma.

Rh Grouping

A blood categorization based on the presence (Rh-positive, found in nearly $80\%$ of humans) or absence (Rh-negative) of an antigen first identified in Rhesus monkeys on the surface of RBCs.

Erythroblastosis Foetalis

A potentially fatal haemolytic condition occurring in subsequent pregnancies when Rh antibodies from an Rh-negative mother leak across the placenta into an Rh-positive foetus, destroying foetal RBCs and causing severe anaemia and jaundice.

Thrombokinase

An enzyme complex produced by an enzymatic cascade mechanism in response to injury that catalyzes the conversion of inactive prothrombin into active thrombin.

Lymph (Tissue Fluid)

A colourless interstitial fluid drained by the lymphatic network, containing specialized lymphocytes, nutrients, and hormones, and facilitating substance exchange between blood vessels and body tissues.

Open Circulatory System

A circulatory pattern found in arthropods and molluscs where blood pumped by the heart passes through large vessels into open spaces or body cavities termed sinuses.

Closed Circulatory System

A circulatory arrangement characteristic of annelids and chordates in which blood pumped by the heart is confined to a continuous network of blood vessels, permitting precise regulation of flow.