Overview
Blood is a fluid connective tissue that transports gases, nutrients, hormones and waste, buffers pH, defends the body and stops bleeding. This topic covers the composition and biomedical importance of blood, the formation of blood cells, erythrocytes (erythropoiesis, erythropoietin, hematocrit, metabolism, enzymopathies and anemias), leukocytes (metabolism and phagocytosis) and the metabolic regulation of T lymphocytes and the T-cell receptor complex.
Biochemistry and biomedical importance of blood; blood cells and blood formation
Blood makes up about 7-8% of body weight (about 5 L in an adult). It consists of plasma (about 55% of the volume) and formed elements (about 45%). Plasma is about 92% water; it contains proteins (60-80 g/L), electrolytes (Na⁺, K⁺, Ca²⁺, Cl⁻, HCO₃⁻, phosphate), nutrients (glucose, amino acids, lipids), metabolic waste (urea, creatinine, bilirubin, lactate), hormones and dissolved gases. The main plasma proteins are albumin (35-50 g/L; oncotic pressure, transport of fatty acids, bilirubin, drugs), globulins (transport proteins, immunoglobulins, clotting and complement factors) and fibrinogen (2-4 g/L). Serum is plasma without fibrinogen and the other clotting factors.
Functions of blood: transport of O₂, CO₂, nutrients, hormones and metabolites; acid-base buffering (bicarbonate, hemoglobin, plasma proteins keep pH at 7.35-7.45); defence; hemostasis; heat distribution; and maintenance of water and osmotic balance. Because blood reflects the state of all tissues and is easy to obtain, its biochemical analysis is the basis of laboratory diagnosis (glucose, enzymes, electrolytes, lipids, hormones).
Hematopoiesis, also known as hemopoiesis, is the process of blood cell formation. It involves the differentiation and maturation of hematopoietic stem cells (HSCs) into various blood cell lineages, including:
- Red blood cells (erythrocytes)
- White blood cells (leukocytes)
- Platelets (thrombocytes)
This process is crucial for establishing and maintaining the blood cell populations required for the proper functioning of the circulatory and immune systems. Hematopoiesis is dynamically regulated to ensure a balance between the production and removal of blood cells, maintaining homeostasis.
Hematopoiesis occurs in different locations throughout an individual's lifespan. It can be broadly categorised into two main periods: prenatal hematopoiesis, occurring before birth, and postnatal hematopoiesis, occurring after birth.
Prenatal Hematopoiesis
Prenatal hematopoiesis takes place in specific organs during fetal development and is vital for establishing the initial blood cell populations. It involves three distinct phases:
- Mesoblastic Phase: Begins around 14 days after fertilisation in the yolk sac mesoderm. This phase primarily produces primitive erythrocytes, which differ from adult erythrocytes in structure and function.