Chapter 19: The Circulatory System – Blood

The Circulatory System: Blood

Overview of Blood Circulation

Blood is a vital connective tissue responsible for transporting substances throughout the body. It circulates via arteries, capillaries, and veins, facilitating the exchange of gases, nutrients, and wastes between tissues and the bloodstream.

• Arteries carry blood away from the heart, branching into capillaries.

• Capillaries allow diffusion of oxygen and nutrients into tissues, and removal of carbon dioxide and wastes.

• Veins return oxygen-deficient blood to the heart, which is then sent to the lungs for gas exchange.

Composition of Blood

Blood consists of a liquid matrix called plasma and formed elements. It is the only fluid tissue in the body and is classified as connective tissue.

• Plasma: Straw-colored fluid, ~90–92% water, ~8% proteins, ~1% other solutes.

• Formed elements: Erythrocytes (RBCs), leukocytes (WBCs), and platelets.

Components of Whole Blood

Blood is separated into plasma and formed elements by centrifugation. The hematocrit is the percentage of RBCs in total blood volume.

• Normal hematocrit: Males: 46%±5%, Females: 42%±5%

Physical Characteristics and Volume

Blood is a sticky, opaque fluid with a metallic taste. Its color varies from bright scarlet (oxygen-rich) to dark red (oxygen-poor).

• pH: 7.35–7.45 (slightly alkaline)

• Temperature: ~38°C (100.4°F)

• Volume: Males: ~5–6 L, Females: ~4–5 L

Functions of Blood

Blood performs three main functions: distribution, regulation, and protection.

• Distribution: Transports oxygen, nutrients, hormones, and wastes.

• Regulation: Maintains pH, temperature, and fluid balance.

• Protection: Hemostasis (clotting) and immune defense.

Blood Plasma

Plasma is the fluid matrix of blood, containing water, proteins, nutrients, electrolytes, gases, and hormones.

• Proteins: Albumin (60%), globulins (35%), fibrinogen (4%)

• Electrolytes: Sodium, potassium, calcium, chloride, bicarbonate

• Organic nutrients: Glucose, amino acids, fatty acids, vitamins

Plasma Proteins

Plasma proteins are essential for osmotic balance, transport, and immune function.

• Albumin: Maintains osmotic pressure, transports substances, buffers pH.

• Globulins: Transport proteins (α, β) and antibodies (γ).

• Fibrinogen: Essential for blood clotting.

Formed Elements

Formed elements include erythrocytes, leukocytes, and platelets. Only WBCs are complete cells; RBCs lack nuclei, and platelets are cell fragments.

• Lifespan: RBCs: ~120 days, Platelets: ~7–10 days, WBCs: hours to years

• Origin: All formed elements arise from hematopoietic stem cells in red bone marrow.

Erythrocytes (Red Blood Cells)

Structure and Function

Erythrocytes are small, biconcave, anucleate cells specialized for gas transport. Their shape increases surface area for efficient diffusion.

• Diameter: ~7.5 μm

• Hemoglobin: Major protein for oxygen and carbon dioxide transport

• Flexibility: Allows passage through narrow capillaries

Hemoglobin Structure

Hemoglobin is a protein composed of four polypeptide chains (2 α, 2 β) and four heme groups, each containing an iron atom that binds oxygen.

• Oxyhemoglobin: Hemoglobin bound to O₂ (bright red)

• Deoxyhemoglobin: Hemoglobin after O₂ release (dark red)

• Carbaminohemoglobin: Hemoglobin bound to CO₂

Production and Regulation of Erythrocytes

Erythrocyte production (erythropoiesis) occurs in red bone marrow and is regulated by erythropoietin (EPO), a hormone released by the kidneys in response to hypoxia.

• Stem cell: Hemocytoblast differentiates into proerythroblast, then reticulocyte, then mature RBC.

• Regulation: Negative feedback via EPO; increased RBCs suppress EPO production.

Life Cycle and Fate of Erythrocytes

RBCs have a lifespan of ~100–120 days. Old RBCs are removed by macrophages in the spleen, liver, and bone marrow. Hemoglobin is broken down and recycled.

• Globin: Broken into amino acids

• Heme: Iron recycled; remainder converted to bilirubin and excreted

Erythrocyte Disorders

Anemia is a condition of reduced oxygen-carrying capacity, caused by decreased RBC production, hemoglobin deficiency, or increased destruction.

• Symptoms: Fatigue, pallor, shortness of breath, cold intolerance

Leukocytes (White Blood Cells)

Types and Functions

Leukocytes are complete cells essential for immune defense. They are classified as granulocytes or agranulocytes.

• Granulocytes: Neutrophils, eosinophils, basophils

• Agranulocytes: Lymphocytes, monocytes

Granulocytes

Granulocytes contain cytoplasmic granules and have lobed nuclei. They are involved in phagocytosis and inflammation.

• Neutrophils: Most numerous, highly phagocytic, first responders to infection

• Eosinophils: Target parasites and participate in allergic responses

• Basophils: Release histamine and heparin, involved in inflammation

Agranulocytes

Agranulocytes lack visible granules. Lymphocytes are key to adaptive immunity, while monocytes differentiate into macrophages for phagocytosis.

• Lymphocytes: B cells (antibody production), T cells (cell-mediated immunity), NK cells (destroy abnormal cells)

• Monocytes: Largest WBCs, become macrophages in tissues

Leukocyte Production (Leukopoiesis)

Leukopoiesis is regulated by cytokines (interleukins and colony-stimulating factors) and occurs in bone marrow. All WBCs originate from hemocytoblasts.

• Myeloid stem cells: Give rise to granulocytes and monocytes

• Lymphoid stem cells: Give rise to lymphocytes

Platelets (Thrombocytes)

Structure and Function

Platelets are cell fragments essential for hemostasis. They form temporary plugs and initiate the clotting cascade at sites of vascular injury.

• Lifespan: ~7–10 days

• Granule contents: Serotonin, Ca²⁺, ADP, enzymes, PDGF

Platelet Formation (Thrombopoiesis)

Platelets are produced in the bone marrow from megakaryocytes, which fragment to release platelets into circulation.

• Lineage: Hemocytoblast → Megakaryocyte → Platelets

Hemostasis (Blood Clotting)

Phases of Hemostasis

Hemostasis is the process of stopping blood loss through a series of rapid, localized reactions.

• Vascular spasm: Immediate vasoconstriction after vessel injury

• Platelet plug formation: Platelets adhere to exposed collagen, activate, and aggregate

• Coagulation: Cascade of enzymatic reactions leading to fibrin clot formation

Coagulation Pathways

Coagulation involves intrinsic and extrinsic pathways, both converging to form a stable fibrin clot.

• Intrinsic pathway: Initiated within blood

• Extrinsic pathway: Triggered by tissue damage

• Common pathway: Formation of prothrombin activator, conversion of prothrombin to thrombin, and fibrinogen to fibrin

Clot Retraction and Fibrinolysis

After clot formation, platelets contract to strengthen the clot and pull vessel edges together. Fibrinolysis dissolves the clot once healing is complete.

• Clot retraction: Occurs within 30–60 minutes

• Fibrinolysis: Plasmin digests fibrin, restoring normal blood flow

Blood Types and Transfusion

ABO and Rh Blood Groups

Blood type is determined by antigens (agglutinogens) on RBC membranes. The ABO system includes A, B, AB, and O types; the Rh system is based on the D antigen.

• Type A: A antigen, anti-B antibody

• Type B: B antigen, anti-A antibody

• Type AB: A and B antigens, no antibodies

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

• Rh+: D antigen present

• Rh-: D antigen absent

Transfusion Reactions

Transfusion reactions occur when mismatched blood is transfused, leading to agglutination and hemolysis. This can cause decreased oxygen delivery, kidney damage, and acute renal failure.

Hemolytic Disease of the Newborn (HDN)

HDN occurs when an Rh-negative mother becomes sensitized to Rh-positive fetal blood, producing antibodies that attack fetal RBCs in subsequent pregnancies. Prevention involves administration of RhoGAM.

Developmental Aspects

Blood Cell Formation Before Birth

Hematopoiesis occurs in the yolk sac, liver, and spleen before birth, then shifts to red bone marrow. Fetal hemoglobin (HbF) has a higher affinity for oxygen than adult hemoglobin (HbA).

Age-Related Blood Disorders

Blood disorders increase with age due to changes in the cardiovascular and immune systems, including higher risk of leukemia, thrombus, and embolus formation.