Blood and the Cardiovascular System

Overview of the Cardiovascular System

The cardiovascular system is essential for transporting substances throughout the body and maintaining homeostasis. It consists of three main components:

• The heart: A muscular pump that propels blood.

• Blood vessels: A network of tubes (arteries, veins, capillaries) that carry blood to and from all body tissues.

• Blood: A fluid connective tissue that transports gases, nutrients, hormones, and waste products.

Components and Functions of Blood

Blood as a Connective Tissue

Blood is a specialized connective tissue composed of cells suspended in a fluid matrix called plasma. It performs several vital functions:

• Transport: Moves dissolved gases (O2, CO2), nutrients, hormones, and metabolic wastes.

• Regulation: Maintains pH and ion composition of interstitial fluids.

• Restriction: Limits fluid loss at injury sites through clotting mechanisms.

• Defense: Protects against toxins and pathogens via immune cells.

• Stabilization: Helps regulate body temperature by redistributing heat.

Physical Characteristics of Blood

Blood has distinct physical and chemical properties:

• Temperature: Approximately 38°C (100.4°F).

• Viscosity: Blood is more viscous than water due to the presence of cells and proteins.

• pH: Slightly alkaline, with a pH range of 7.35–7.45.

• Volume: Blood volume is about 7% of body weight. For a 75-kg (165-lb) person, this equals roughly 5.4 liters (5.4 quarts).

Composition of Blood

Blood consists of two main components:

• Plasma: The fluid portion, making up about 55% of blood volume.

• Formed elements: Cells and cell fragments, including red blood cells (RBCs), white blood cells (WBCs), and platelets.

Fractionation is the process of separating whole blood into plasma and formed elements for analysis or transfusion.

Plasma

Plasma is primarily water (over 90%) and contains dissolved proteins and other solutes. Its composition is similar to interstitial fluid because water, ions, and small solutes are exchanged across capillary walls.

Plasma Proteins

Plasma contains several important proteins, each with specific functions:

• Albumins (~60%): Maintain plasma osmolarity and transport fatty acids, thyroid hormones, and steroid hormones.

• Globulins (~35%): Include antibodies (immunoglobulins) and transport globulins (hormone-binding proteins, metalloproteins, apolipoproteins, and steroid-binding proteins).

• Fibrinogen (~4%): A soluble protein involved in blood clotting; converted to insoluble fibrin during clot formation.

• Other plasma proteins (~1%): Enzymes and peptide hormones.

Most plasma proteins are synthesized in the liver, except antibodies (produced by plasma cells) and peptide hormones (produced by endocrine organs).

Formed Elements of Blood

Overview

The formed elements of blood include:

• Red blood cells (RBCs): Also called erythrocytes, responsible for oxygen transport.

• White blood cells (WBCs): Also called leukocytes, involved in immune defense.

• Platelets: Small cell fragments important for blood clotting.

Hematopoiesis is the process of producing formed elements, occurring primarily in red bone marrow.

Red Blood Cells (RBCs)

RBCs make up 99.9% of formed elements and contain hemoglobin, a red pigment that binds and transports oxygen and carbon dioxide.

• Structure: Small, biconcave discs with a thin central region and thicker outer margin. This shape increases surface area for gas exchange and allows flexibility to pass through narrow capillaries.

• Properties: RBCs are anucleate (lack nuclei), lack mitochondria and ribosomes, cannot divide or repair themselves, and have a lifespan of about 120 days.

• Main function: Transport oxygen to peripheral tissues and remove carbon dioxide.

Hemoglobin (Hb)

Hemoglobin is the protein in RBCs responsible for transporting respiratory gases.

• Normal levels: Adult males: 14–18 g/dL; adult females: 12–16 g/dL.

• Oxyhemoglobin: Hemoglobin bound to oxygen.

• Deoxyhemoglobin: Hemoglobin not bound to oxygen.

• Fetal hemoglobin: Binds oxygen more readily than adult hemoglobin, facilitating oxygen transfer from maternal blood.

Each RBC contains about 280 million Hb molecules, allowing it to carry over 1 billion molecules of O2.

• In peripheral capillaries (low O2): Hemoglobin releases O2 and binds CO2 (forming carbaminohemoglobin).

• In the lungs (high O2): Hemoglobin binds O2 and releases CO2.

Anemia

Anemia occurs when the hematocrit or hemoglobin content of RBCs is reduced, interfering with oxygen delivery to tissues.

Red Blood Cell Formation (Erythropoiesis)

Erythropoiesis is the process of RBC formation, occurring in myeloid tissue (red bone marrow) from hematopoietic stem cells (HSCs).

• Erythropoietin (EPO): A hormone secreted by the kidneys and liver in response to low oxygen (hypoxia), stimulating RBC production.

Blood Types

Surface Antigens and Blood Typing

Blood type is determined by the presence or absence of specific surface antigens (A, B, and Rh/D) on RBC membranes. These antigens are recognized by the immune system.

• Type A: Surface antigen A

• Type B: Surface antigen B

• Type AB: Both antigens A and B

• Type O: Neither antigen A nor B

• Rh factor: Rh positive (Rh+) has the Rh antigen; Rh negative (Rh–) does not.

Blood Compatibility and Transfusion Reactions

Transfusion reactions (cross-reactions) can occur if donor and recipient blood types are incompatible. Plasma antibodies react with specific surface antigens, causing RBCs to agglutinate (clump) and possibly hemolyze.

White Blood Cells (WBCs)

Overview and Functions

White blood cells, or leukocytes, are immune cells that defend the body against pathogens, remove toxins and wastes, and attack abnormal or damaged cells. Most WBCs are found in connective tissue and lymphatic organs; only a small fraction circulates in the blood (5,000–10,000 per microliter).

Characteristics of Circulating WBCs

• All can migrate out of the bloodstream.

• All are capable of amoeboid movement.

• All are attracted to specific chemical stimuli (positive chemotaxis).

• Some are phagocytic (engulf pathogens).

Types of White Blood Cells

WBCs are classified into several types based on structure and function:

• Neutrophils: 50–70% of WBCs; phagocytic, attack and digest bacteria, release prostaglandins and leukotrienes, contribute to pus formation.

• Eosinophils: 2–4% of WBCs; engulf bacteria and debris, attack parasites, release toxic compounds, sensitive to allergens, reduce inflammation.

• Basophils: <1% of WBCs; release histamine (vasodilation) and heparin (prevents clotting), accumulate in damaged tissues.

• Monocytes: 2–8% of WBCs; become macrophages in tissues, aggressive phagocytes, attract other immune cells and fibroblasts.

• Lymphocytes: 20–40% of WBCs; migrate between blood and lymphatic tissues, part of specific immune defense.

Classes of Lymphocytes

• T cells: Cell-mediated immunity; attack foreign cells or regulate other lymphocytes.

• B cells: Humoral immunity; differentiate into plasma cells that produce antibodies.

• Natural killer (NK) cells: Detect and destroy abnormal cells.

White Blood Cell Disorders and Regulation

• Leukopenia: Low WBC count.

• Leukocytosis: High WBC count.

• Leukemia: Cancer of WBCs, indicated by extreme leukocytosis.

Leukopoiesis is the formation of WBCs from hematopoietic stem cells in bone marrow, producing myeloid and lymphoid progenitors.

Regulation of WBC Production

Colony-stimulating factors (CSFs) are hormones that regulate WBC populations:

• Multi-CSF: Accelerates production of granulocytes, monocytes, platelets, and RBCs.

• GM-CSF: Stimulates granulocyte and monocyte production.

• G-CSF: Stimulates granulocyte production.

• M-CSF: Stimulates monocyte production.

Table: Major Plasma Proteins and Their Functions

Key Equations

• Blood Volume Calculation:

• Hematocrit: The percentage of whole blood volume occupied by formed elements (mainly RBCs).

Additional info: Some details, such as the full list of plasma proteins and the exact mechanisms of hematopoiesis, were expanded for academic completeness.