Blood: Plasma and the Formed Elements

Why does the body need a circulating connective tissue?

A cell deep inside a large animal cannot directly exchange enough oxygen, nutrients and metabolic waste with the external environment. Blood solves this distance problem by moving materials between organs and tissues. It carries oxygen from the lungs, nutrients absorbed from the gut, signals from endocrine glands, and waste products towards organs that remove them. Unlike epithelial tissue, blood has cells dispersed in an extracellular matrix that flows; for this reason it is a specialised connective tissue.

The liquid matrix is plasma and the suspended components are called formed elements. In a typical sample, plasma makes up approximately 55% of total volume, while red cells, white cells and platelets together account for roughly 45%, mainly due to the volume occupied by red cells. The proportions vary with hydration, age and physiological condition. The blood does not merely 'carry oxygen': different components transport, defend, clot and help maintain stable conditions.

Plasma: a transport medium with specialised proteins

About 90–92% of plasma is water. Its ability to dissolve and distribute materials makes plasma suitable for moving glucose, amino acids, salts, hormones and many waste molecules. Plasma proteins form about 6–8%: fibrinogen is a precursor of fibrin during clot formation; globulins include proteins important in defence; and albumin contributes strongly to colloid osmotic pressure, helping retain water inside blood vessels. The presence of these proteins means plasma is more than dilute water.

Common dissolved ions include Na⁺, Ca²⁺, Mg²⁺, HCO₃⁻ and Cl⁻. Calcium participates in several physiological processes, including blood coagulation. Substances listed as 'in transit' are continuously entering and leaving the circulation, so blood composition changes with food intake, metabolism, secretion and excretion.

Red blood cells: oxygen carriers shaped for efficient exchange

Erythrocytes are the most abundant formed elements. Adult red cells are usually biconcave discs, thinner at the centre and lacking nuclei in most mammals. The biconcave form provides a large surface area relative to volume and a flexible shape that helps movement through narrow capillaries. Losing the nucleus leaves more room for haemoglobin but also limits the mature cell's ability to synthesise new proteins or divide.

Haemoglobin is an iron-containing protein that binds oxygen reversibly. It enables far more oxygen to be transported than could dissolve in plasma alone. The chapter gives a typical adult male red-cell count of about 5–5.5 million per mm³ and haemoglobin concentrations around 12–16 g per 100 mL as broad textbook reference values; real clinical reference ranges vary by sex, age, altitude and laboratory. Do not assume the same number is normal for every individual.

In adults, red cells are produced in red bone marrow and circulate for roughly 120 days before ageing cells are cleared mainly by macrophages in the spleen and liver. Their components, including much of the iron, can be recycled. Because red cells carry respiratory gases, fewer functional red cells or less haemoglobin can reduce oxygen-delivery capacity.

White blood cells: specialised defenders

Leucocytes are nucleated, colourless in the sense that they do not contain haemoglobin, and much less numerous than red cells. The chapter gives roughly 6,000–8,000 cells per mm³ as a representative count. They can migrate from blood to tissues, where immune responses and debris removal often occur. Granulocytes possess conspicuous cytoplasmic granules in stained preparations; agranulocytes lack the prominent specific granules used in this classification.

Neutrophils are the most numerous white cells, typically about 60–65% in this chapter, followed by lymphocytes at about 20–25%; monocytes are about 6–8%, eosinophils about 2–3%, and basophils about 0.5–1%. These are approximate relative percentages and are not interchangeable with absolute cell counts. The shape of the stained nucleus offers a clue: a neutrophil often has several lobes; a monocyte is larger with a curved nucleus; lymphocytes often have a prominent rounded nucleus.

Platelets: fragments that help seal damaged vessels

Platelets, or thrombocytes, are fragments released from large bone-marrow cells called megakaryocytes. Their usual textbook count is about 150,000–350,000 per mm³. When a vessel is injured, platelets adhere, become activated and release substances that help recruit other platelets and the coagulation cascade. Platelets are not complete nucleated white cells, and they do not transport oxygen.

A reduction in platelet number or a defect in platelet function may increase bleeding risk, but platelet quantity by itself does not reveal the whole clotting capacity. Conversely, plasma clotting proteins and vessel-wall responses are also necessary for effective haemostasis. The next lessons connect blood-cell structure to blood compatibility and injury repair.

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