Blood

Blood is a vital circulatory fluid found in humans and other vertebrates, responsible for transporting oxygen, nutrients, hormones and metabolic waste throughout the body. As a specialised connective tissue, it contains cellular components suspended in plasma, enabling essential physiological processes such as immunity, homeostasis, gas exchange and temperature regulation. Its composition and function underpin the operation of the cardiovascular and immune systems, making it central to maintaining life.

Composition and General Characteristics

Blood consists of two principal components: plasma, a straw-coloured fluid forming about 55 per cent of total blood volume, and formed elements, which include red blood cells, white blood cells and platelets. Plasma is composed of approximately 92 per cent water, with the remainder consisting of proteins, electrolytes, nutrients, dissolved gases and hormones. The proteins include albumin, clotting factors, immunoglobulins and a range of transport molecules.
In adult humans, blood normally accounts for around 7 per cent of total body weight, with an average volume of approximately five litres. Its density is close to that of water, and it exhibits non-Newtonian fluid characteristics because its viscosity changes with flow conditions.

Red Blood Cells

Red blood cells, or erythrocytes, constitute the most abundant formed element, making up roughly 45 per cent of total blood volume. A single microlitre of human blood contains between 4.2 and 6.1 million erythrocytes depending on sex. These cells are specialised for oxygen transport and contain large quantities of haemoglobin, a protein capable of reversibly binding oxygen to increase its solubility.
Mature erythrocytes in mammals lack nuclei and organelles, maximising space for haemoglobin and improving flexibility within narrow capillaries. The proportion of blood occupied by red cells is known as the haematocrit, a key indicator of oxygen-carrying capacity. The surface area of all erythrocytes combined is exceptionally large, supporting efficient gas exchange.

White Blood Cells

White blood cells, or leukocytes, form part of the defensive and adaptive immune functions of vertebrates. Typical concentrations range from 4,000 to 11,000 per microlitre. These cells identify and neutralise pathogens, remove damaged cells and contribute to inflammatory responses. Vertebrates with jaws (Gnathostomata) possess complex adaptive immune mechanisms largely mediated by these cells. Cancers of white blood cells, such as leukaemia, disrupt normal immune function and are diagnosed through abnormalities in circulating leukocyte counts.

Platelets

Platelets, or thrombocytes, are small cell fragments derived from megakaryocytes in the bone marrow. Concentrations typically fall between 200,000 and 500,000 per microlitre in humans. They initiate and support blood clotting by adhering to damaged vessel walls and interacting with clotting factors to form a fibrin mesh. In non-mammalian vertebrates, nucleated thrombocytes perform analogous roles.

Plasma and Dissolved Components

Blood plasma contains essential solutes including glucose, amino acids, fatty acids, hormones and electrolytes such as sodium and chloride. It also transports metabolic waste products like urea, carbon dioxide and lactic acid from tissues to excretory organs. Plasma minus its clotting factors is known as serum, used in diagnostic testing.
Plasma pH is tightly regulated between 7.35 and 7.45. Significant deviations can be life-threatening, with values below 6.9 or above 7.8 generally incompatible with survival. Respiratory and renal systems maintain acid–base homeostasis through compensation mechanisms that adjust carbon dioxide levels and bicarbonate concentrations.

Circulation and Heart Function

Blood circulates through an extensive network of arteries, veins and capillaries driven by the heart’s rhythmic contractions. Oxygenated blood leaves the left ventricle through the systemic arteries to supply body tissues, and deoxygenated blood returns to the right atrium through veins. From the right ventricle it is pumped to the lungs for gas exchange, returning oxygenated to the left atrium before entering systemic circulation again.
In addition to cardiac pumping, skeletal muscle contractions assist venous return by compressing veins and promoting movement toward the heart. This dual mechanism ensures continuous distribution of gases and nutrients and removal of metabolic by-products.

Blood Cell Production and Life Cycle

Blood cells are produced in the bone marrow, a process termed haematopoiesis. During childhood, many bones participate in this process, while in adults production becomes concentrated in the pelvis, sternum, ribs, vertebrae and long bones. Red blood cells are generated through erythropoiesis, while white blood cells and platelets arise through various myeloid pathways. The thymus plays an important developmental role in T-lymphocyte maturation, particularly in early life.
Red blood cells have an average lifespan of about 120 days, after which they are removed by macrophages in the spleen and liver. The liver also metabolises plasma proteins, detoxifies substances and contributes to lipid and amino-acid processing.

Blood in Non-Mammalian Vertebrates

In contrast to mammals, the erythrocytes of non-mammalian vertebrates are typically nucleated and oval in shape. There is also greater variation in white blood cell types and proportions among these groups. Platelets, unique to mammals, are replaced by nucleated thrombocytes in birds, reptiles, amphibians and fish.

Physiological Roles and Functions

Blood supports an array of essential bodily functions:

• Gas transport: oxygen to tissues and carbon dioxide to the lungs
• Nutrient distribution: including glucose, fats and amino acids
• Waste removal: conveying metabolic by-products to kidneys, lungs and liver
• Immune surveillance: circulating antibodies and immune cells
• Clot formation: responding rapidly to vessel injury
• Hormonal signalling: distributing endocrine messages
• Thermal regulation: redistributing heat through vasoconstriction and vasodilation