Blood

Blood is a vital tissue in mammals that is part of a closed double circulatory system. This system ensures efficient movement of substances throughout the organism. Blood is continuously circulated through blood vessels.

Components of Blood

Blood is comprised of a liquid component called plasma, in which various types of blood cells are suspended.

• Plasma

  • Plasma is a straw-coloured liquid that makes up about 55% of the blood.

  • It is mainly water (around 90-95%), which acts as a powerful solvent for polar substances. This property is crucial for the continuous exchange of substances by cells and tissues.

  • Plasma transports a wide range of dissolved substances, including nutrients (e.g., glucose, amino acids, lipids, vitamins), excretory products (e.g., urea, carbon dioxide), and hormones.

  • It also contains dissolved proteins, known as plasma proteins, such as albumin, antibodies, and components of the blood clotting mechanism. Albumin, for instance, plays a role in regulating the water potential of the blood.

  • Due to water's high specific heat capacity, plasma also helps in the distribution of heat throughout the body, contributing to temperature regulation.

• Blood Cells

  • These constitute about 45% of the blood volume.

  • Red Blood Cells (Erythrocytes)

    • They are the most numerous blood cells, about 5 million per mm³ of blood.

    • They are biconcave disc-shaped and lack a nucleus in their mature state to maximize space for haemoglobin. They also lose mitochondria and endoplasmic reticulum during development.

    • Their primary function is the transport of oxygen around the body. This is achieved by the haemoglobin protein they contain, which reversibly binds to oxygen. Each haemoglobin molecule can carry four oxygen molecules (eight oxygen atoms).

    • They have a lifespan of about 120 days before being broken down and replaced.

  • White Blood Cells (Leukocytes)

    • These cells are crucial for the immune system and fighting disease.

    • They are generally larger than red blood cells and possess a nucleus.

    • Main types include:

      • Phagocytes (e.g., neutrophils, monocytes/macrophages) which engulf and destroy pathogens.

      • Lymphocytes (B-cells and T-cells) which have specific roles in the immune response, including producing antibodies (B-cells) and attacking infected cells (T-killer cells).

  • Platelets

    • These are small cell fragments.

    • Their primary function is blood clotting. This process involves a series of reactions converting soluble fibrinogen into insoluble fibrin, which traps blood cells to form a clot, preventing blood loss and pathogen entry.

Blood Transport Functions

Blood is essential for mass transport due to the large distances substances need to travel in larger organisms. Its key transport roles include:

• Supplying all body parts with oxygen and glucose for respiration.

• Removing carbon dioxide and other metabolic waste products.

• Transporting hormones from endocrine glands to target organs, contributing to communication and coordination within the body.

• Transporting nutrients (sugars, amino acids, lipids, vitamins, inorganic ions) from the gut or liver to all cells.

• Distributing heat to all tissues, which helps in temperature regulation.

Blood Pressure

Blood pressure varies significantly throughout the circulatory system.

• It is highest in the aorta and arteries due to the direct pumping action of the heart.

• There is a steep drop in pressure as blood flows through the arterioles and into the capillaries due to increased total cross-sectional area and resistance to flow.

• Blood flows at low pressure in venules and veins, with pressure no longer pulsatile. Blood flow in veins is aided by the contraction of surrounding body muscles and the presence of valves to prevent backflow.

Relationship with Tissue Fluid and Lymph

Blood plays a central role in the formation and circulation of tissue fluid and lymph.

• Tissue fluid is formed when small molecules from blood plasma leak out of capillaries. This occurs primarily at the arteriole end of a capillary bed due to the higher hydrostatic pressure inside the capillaries.

• Tissue fluid is similar to plasma but does not contain red blood cells or large plasma proteins, as these are too large to pass through the capillary walls.

• Tissue fluid serves as the immediate environment for body cells, facilitating the exchange of oxygen and nutrients from the blood to cells, and the removal of metabolic waste from cells back into the fluid.

• Approximately 90% of the tissue fluid re-enters the capillaries at the venule end by osmosis, due to the lower water potential within the capillaries (caused by the higher concentration of plasma proteins).

• Any excess tissue fluid that doesn't return to the capillaries is drained into the lymphatic system, where it becomes lymph. The lymphatic system returns lymph to the circulatory system near the heart.

• Lymph is compositionally similar to tissue fluid but may contain more large proteins (those that did not return to blood capillaries) and white blood cells. Lymphatic vessels contain valves to prevent backflow.

• An imbalance in fluid exchange can lead to oedema, which is swelling of the tissues caused by an increased volume of tissue fluid. This can be linked to conditions like low plasma protein levels (e.g., hypoalbuminemia).

Blood and Health

The proper functioning of blood is fundamental to health. Cardiovascular diseases are often initiated by issues within blood vessels, such as atheroma formation, which can restrict blood flow and lead to conditions like myocardial infarction (heart attack) and thrombosis. Blood clotting is a vital protective mechanism against excessive bleeding and pathogen entry. The composition of blood is routinely monitored through blood tests to diagnose diseases and assess treatment success.