Blood and the Cardiovascular System

Overview of the Cardiovascular System

The cardiovascular system is essential for transporting substances throughout the body. It consists of the heart (a pump), blood vessels (conducting hoses), and blood (a fluid connective tissue). Blood plays a critical role in maintaining homeostasis and supporting cellular function.

Components and Functions of Blood

Main Components of Blood

• Plasma: The liquid matrix, making up about 55% of blood volume. Over 90% of plasma is water, with dissolved proteins and solutes.

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

Major Functions of Blood

• Transport: Carries gases (O2, CO2), nutrients, hormones, and wastes.

• Regulation: Maintains pH, ion composition, and body temperature.

• Protection: Restricts fluid loss, defends against toxins/pathogens, and stabilizes temperature.

Physical Characteristics of Blood

• Temperature: 38°C (100.4°F)

• Viscosity: High

• pH: Slightly alkaline (7.35–7.45)

• Volume: ~7% of body weight (e.g., 5.25 L in a 75-kg adult)

Plasma Proteins

• Albumins (60%): Maintain osmotic pressure, transport fatty acids and hormones.

• Globulins (35%): Include antibodies (immunoglobulins) and transport proteins.

• Fibrinogen (4%): Functions in clotting; converted to fibrin during coagulation.

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

Formed Elements of Blood

Red Blood Cells (Erythrocytes)

Red blood cells (RBCs) are the most abundant formed elements, responsible for oxygen transport. They are biconcave discs, which increases surface area for gas exchange and allows flexibility in capillaries.

• Count: Adult males: 4.5–6.3 million/μL; females: 4.2–5.5 million/μL

• Hematocrit: Percentage of RBCs in blood (males: 46%, females: 42%)

• Structure: Anucleate, lack mitochondria and ribosomes, lifespan ~120 days

Hemoglobin (Hb)

Hemoglobin is a protein in RBCs that binds and transports oxygen and carbon dioxide. Each molecule consists of four globular subunits (two alpha, two beta), each with a heme group containing iron.

• Normal Hb: Males: 14–18 g/dL; Females: 12–16 g/dL

• Oxyhemoglobin (HbO2): Hemoglobin bound to oxygen

• Deoxyhemoglobin: Hemoglobin without oxygen

• Carbaminohemoglobin: Hemoglobin bound to CO2

RBC Life Cycle and Erythropoiesis

RBCs are produced in red bone marrow through erythropoiesis, regulated by erythropoietin (EPO). Old RBCs are recycled by macrophages in the spleen, liver, and bone marrow.

White Blood Cells (Leukocytes)

White blood cells (WBCs) are involved in defense against pathogens, removal of toxins, and attacking abnormal cells. They are classified as granulocytes or agranulocytes based on the presence of cytoplasmic granules.

• Neutrophils: Phagocytic, attack bacteria, most abundant (50–70%)

• Eosinophils: Attack parasites, reduce inflammation (2–4%)

• Basophils: Release histamine and heparin (<1%)

• Monocytes: Become macrophages, phagocytize large particles (2–8%)

• Lymphocytes: Specific immunity (T cells, B cells, NK cells) (20–40%)

Platelets (Thrombocytes)

Platelets are small, membrane-bound cell fragments essential for blood clotting. They circulate for 9–12 days and are removed by the spleen.

• Functions: Release clotting chemicals, form temporary plugs, reduce vessel break size

• Production: Formed from megakaryocytes in red bone marrow (thrombocytopoiesis)

Blood Types and Transfusion Compatibility

ABO and Rh Blood Groups

Blood type is determined by the presence or absence of surface antigens (A, B, Rh) on RBCs. The four main types are A, B, AB, and O. The Rh factor determines if blood is positive or negative.

• Type A: Surface antigen A, anti-B antibodies

• Type B: Surface antigen B, anti-A antibodies

• Type AB: Both antigens, no anti-A/B antibodies

• Type O: No antigens, both anti-A and anti-B antibodies

Transfusion Reactions

If incompatible blood is transfused, antibodies in the recipient's plasma attack donor RBCs, causing agglutination and hemolysis.

Blood Type Testing

Blood samples are mixed with antibodies to determine blood type by observing agglutination patterns.

Hemostasis: The Control of Blood Loss

Phases of Hemostasis

Hemostasis is the process of stopping bleeding and involves three phases: vascular, platelet, and coagulation.

• Vascular Phase: Vascular spasm constricts the vessel after injury.

• Platelet Phase: Platelets adhere to exposed surfaces and aggregate to form a plug.

• Coagulation Phase: Cascade of reactions leads to the formation of fibrin, stabilizing the clot.

Clot Retraction and Fibrinolysis

After clot formation, platelets contract to reduce the size of the break (clot retraction). Fibrinolysis is the gradual breakdown of the clot by plasmin.

Summary Table: Main Components of Blood

Key Equations and Concepts

• Blood Volume Estimate:

• Hematocrit:

Additional info:

Blood disorders such as anemia, leukopenia, leukocytosis, and leukemia can be diagnosed by analyzing the formed elements and their counts. Vitamin K and calcium ions are essential for proper clotting. The universal donor is O−, but cross-matching is always required due to other minor antigens.