Blood: Structure, Function, and Components (Anatomy & Physiology Chapter 19 Study Notes)

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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:

Blood: The fluid connective tissue that transports nutrients, gases, wastes, and hormones.
Heart: The muscular pump that circulates blood.
Blood Vessels: The network of tubes (arteries, veins, capillaries) that carry blood to and from all body tissues.

Components and Functions of Blood

Composition of Blood

Blood is a specialized connective tissue composed of cells and cell fragments (formed elements) suspended in a fluid matrix (plasma).

Plasma: The liquid component, making up 46-63% of blood volume. It is about 92% water and contains proteins, nutrients, hormones, electrolytes, and wastes.
Formed Elements: The cellular portion, including red blood cells (RBCs), white blood cells (WBCs), and platelets.

Functions of Blood

Transport: Carries dissolved gases (O2, CO2), nutrients, hormones, and metabolic wastes.
Regulation: Maintains pH, ion composition, and fluid balance in tissues.
Restriction of Fluid Loss: Clotting mechanisms prevent excessive bleeding after injury.
Defense: WBCs and antibodies protect against toxins and pathogens.
Stabilization of Body Temperature: Distributes heat generated by muscles.

Physical Characteristics of Blood

Temperature: Approximately 38°C (100.4°F).
Viscosity: About 5 times more viscous than water.
pH: Slightly alkaline (7.35–7.45).
Volume: About 7% of body weight in kilograms.

Plasma and Plasma Proteins

Albumins: Most abundant; contribute to osmotic pressure and transport substances.
Globulins: Include antibodies (immunoglobulins) and transport proteins.
Fibrinogen: Soluble protein involved in clotting; converted to insoluble fibrin during coagulation.

Most plasma proteins are synthesized by the liver, except immunoglobulins, which are produced by WBCs.

Formed Elements

Red Blood Cells (Erythrocytes): Transport oxygen and carbon dioxide.
White Blood Cells (Leukocytes): Defend against infection and disease.
Platelets (Thrombocytes): Cell fragments involved in clotting.

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

Red Blood Cells (RBCs)

Structure and Function

RBCs are biconcave discs, increasing surface area for gas exchange and allowing flexibility in capillaries.
They lack nuclei, mitochondria, and ribosomes, making them unable to divide or repair themselves.
Contain hemoglobin, a protein responsible for oxygen and carbon dioxide transport.

Hemoglobin (Hb)

Composed of four polypeptide chains (2 alpha, 2 beta), each with a heme group containing iron.
Iron binds reversibly to oxygen, forming oxyhemoglobin ().
Hemoglobin also binds carbon dioxide (as carbaminohemoglobin) at different sites than oxygen.

RBC Life Cycle and Erythropoiesis

RBCs live about 120 days; about 1% are replaced daily.
Erythropoiesis is the formation of RBCs from stem cells (hemocytoblasts) in red bone marrow.
Regulated by the hormone erythropoietin (EPO), secreted by kidneys and liver in response to hypoxia.
Essential nutrients: amino acids, iron, vitamin B12, B6, and folic acid.

Hemoglobin Recycling

Old RBCs are phagocytized in the spleen, liver, and bone marrow.
Heme is converted to biliverdin (green), then bilirubin (yellow), which is excreted in bile.
Iron is recycled, transported by transferrin, and stored as ferritin or hemosiderin.
Excessive hemolysis can cause hemoglobinuria (Hb in urine) or hematuria (RBCs in urine).

Blood Types

ABO and Rh Blood Groups

Antigens (agglutinogens) on RBC surfaces determine blood type.
Four main ABO types:
- Type A: A antigen, anti-B antibodies
- Type B: B antigen, anti-A antibodies
- Type AB: A and B antigens, no anti-A or anti-B antibodies
- Type O: No A or B antigens, both anti-A and anti-B antibodies
Rh (D) antigen: Presence (+) or absence (−) determines Rh status.

Blood Type	Surface Antigens	Plasma Antibodies
A	A	Anti-B
B	B	Anti-A
AB	A, B	None
O	None	Anti-A, Anti-B

Blood Transfusions and Compatibility

Transfusion reactions (cross-reactions) occur if donor and recipient blood types are incompatible, causing agglutination and hemolysis.
Type O− is the universal donor; AB+ is the universal recipient.
Compatibility testing (cross-matching) is essential before transfusions.

Hemolytic Disease of the Newborn (HDN)

Occurs when an Rh− mother is sensitized to Rh+ fetal blood, producing anti-Rh antibodies that can attack a subsequent Rh+ fetus.
Prevention: Administration of anti-Rh immunoglobulin (RhoGAM) to the mother.

White Blood Cells (WBCs)

Types and Functions

Leukocytes are nucleated cells involved in defense against pathogens, removal of toxins, and attacking abnormal cells.
Most WBCs are found in connective tissues and lymphatic organs; only a small fraction circulates in blood.

Classification of WBCs

Granular Leukocytes:
- Neutrophils: 50–70%; phagocytic, first responders to infection, form pus.
- Eosinophils: 2–4%; attack parasites, involved in allergic responses.
- Basophils: <1%; release histamine (vasodilation) and heparin (anticoagulant).
Agranular Leukocytes:
- Monocytes: 2–8%; become macrophages, phagocytize large particles.
- Lymphocytes: 20–40%; include T cells (cell-mediated immunity), B cells (antibody production), and NK cells (immune surveillance).

WBC Production and Regulation

Leukopoiesis: Formation of WBCs from stem cells in bone marrow.
Regulated by colony-stimulating factors (CSFs) and exposure to antigens.
WBC disorders include leukopenia (low count), leukocytosis (high count), and leukemia (cancer of WBCs).

Platelets

Structure, Function, and Production

Platelets (thrombocytes) are cell fragments involved in blood clotting.
Circulate for 9–12 days; removed by phagocytes in the spleen.
Functions:
- Release clotting chemicals
- Form temporary platelet plugs
- Reduce the size of vessel breaks
Produced by megakaryocytes in red bone marrow (thrombocytopoiesis), stimulated by thrombopoietin (TPO), interleukin-6 (IL-6), and multi-CSF.

Hemostasis

Phases of Hemostasis

Hemostasis is the process of stopping bleeding and involves three phases:

Vascular Phase: Vascular spasm constricts blood vessels; endothelial cells release factors for repair and become sticky.
Platelet Phase: Platelets adhere to exposed collagen, aggregate to form a plug, and release chemicals (ADP, thromboxane A2, serotonin, clotting factors, PDGF, Ca2+).
Coagulation Phase: Cascade of reactions involving clotting factors leads to the conversion of fibrinogen to fibrin, forming a stable clot.

Coagulation Pathways

Extrinsic Pathway: Triggered by tissue factor from damaged cells; activates Factor X.
Intrinsic Pathway: Triggered by exposure of blood to collagen; activates Factor X via a series of steps.
Common Pathway: Factor X activates prothrombin activator, which converts prothrombin to thrombin. Thrombin converts fibrinogen to fibrin.

Key Equations:

Regulation of Clotting

Positive feedback by thrombin accelerates clotting.
Anticoagulants (antithrombin-III, heparin, thrombomodulin, prostacyclin) prevent excessive clotting.
Calcium ions and vitamin K are essential for clotting factor synthesis and function.

Clot Retraction and Fibrinolysis

Clot Retraction: Platelets contract, pulling torn vessel edges together for repair.
Fibrinolysis: Plasminogen is converted to plasmin (by thrombin and t-PA), which digests fibrin and dissolves the clot.

Clinical Correlations

Thrombocytopenia: Low platelet count, leading to bleeding risk.
Hemophilia: Inherited deficiency of clotting factors, causing excessive bleeding.
Thrombophilia: Increased tendency to form clots (e.g., deep vein thrombosis, pulmonary embolism).