The Cardiovascular System: Blood Composition and Function

Exam Tip: If a question asks about the "buffy coat," remember it refers specifically to the thin middle layer of centrifuged blood that contains the leukocytes and platelets.

Primary Functions of Blood

The functions of blood can be grouped into three vital categories: transportation, maintenance of homeostasis, and defense.

1. Transportation

Blood acts as a highway for the body, moving oxygen, nutrients, hormones, and heat to various tissues, while simultaneously carrying metabolic waste products to organs for elimination.

2. Maintenance of Homeostasis

Blood is crucial for maintaining a stable internal environment through several mechanisms:

• Osmotic Pressure: By regulating the concentration of solutes (primarily through albumin), blood helps maintain the proper water content of cells.
• Temperature Regulation: Blood helps regulate core body temperature. For example, when the body is hot, blood vessels in the skin dilate to dissipate heat; on a cold day, blood is diverted away from the skin to preserve warmth in the body core.
• pH Regulation: Blood contains proteins and other compounds that act as buffers. These buffers help maintain the body's chemical balance within a narrow pH range of 7.35 to 7.45.

3. Defense

Blood protects the body from injury and disease. It can clot to prevent excessive blood loss following an injury. Furthermore, leukocytes and specific plasma proteins (such as antibodies, interferons, and complement) defend the body against infectious diseases.

Deep Dive: The Erythrocyte (Red Blood Cell)

Because they are the most numerous formed elements, erythrocytes deserve special attention.

Structure and Hemoglobin

Mature erythrocytes are unique because they lack a nucleus and most organelles, such as mitochondria and endoplasmic reticulum. This lack of internal structures provides more space for hemoglobin, the protein responsible for gas transport.

Hemoglobin is a large molecule composed of four protein chains called globin, each bound to a pigment molecule called heme which contains iron ($Fe^{2+}$). Each iron atom can bind to one oxygen molecule; therefore, a single erythrocyte can transport up to 1.2 billion oxygen molecules.

The shape of the erythrocyte is a biconcave disk. This specific geometry increases the surface area available for rapid gas exchange.

The Life Cycle and Hematocrit

Erythrocytes have a relatively short lifespan of about 120 days. Because they lack the organelles necessary to repair themselves, they eventually wear out and are removed by macrophages in the spleen, liver, and red bone marrow.

The production of new blood cells is called hematopoiesis, which occurs primarily in the red bone marrow. This process is regulated by growth factors:

• Erythropoietin (EPO): Stimulates RBC production in response to low oxygen levels (hypoxia).
• Thrombopoietin: Stimulates platelet production.
• Cytokines: Stimulate white blood cell formation.

Hemostasis: Stopping the Bleed

When a blood vessel is damaged, the body initiates hemostasis, a sequence of responses to stop bleeding. This occurs in three main stages:

1. Vascular Spasm: The smooth muscle in the vessel walls contracts to reduce blood flow.
2. Platelet Plug Formation: Platelets adhere to the exposed collagen of the damaged vessel, become "sticky," and clump together to form a temporary seal.
3. Blood Clotting (Coagulation): This is a complex cascade of enzymatic reactions that results in the conversion of the soluble protein fibrinogen into insoluble fibrin threads. These threads create a mesh that traps platelets and blood cells, forming a durable clot.

Note on Coagulation Pathways: There are two ways to trigger this cascade: the intrinsic pathway (triggered by factors inside the vessel) and the extrinsic pathway (triggered by tissue damage outside the vessel). The extrinsic pathway is much faster, completing in seconds, whereas the intrinsic pathway takes minutes.

Leukocytes: The Immune Defenders

White blood cells (leukocytes) are larger than red blood cells and possess a nucleus and organelles. They are categorized into two groups based on the presence of visible granules in their cytoplasm:

Granular Leukocytes

• Neutrophils: The most common type (50–70%); they are rapid responders and efficient phagocytes.
• Eosinophils: Contain molecules toxic to parasitic worms; high counts often indicate allergies or parasites.
• Basophils: These intensify the inflammatory response.

Agranular Leukocytes

• Lymphocytes: Crucial for adaptive immunity; includes B cells, T cells, and Natural Killer (NK) cells.
• Monocytes: These develop into macrophages, which phagocytize debris and pathogens.

When leukocytes move from the bloodstream into the tissues to reach an infection site, the process is called diapedesis.