Blood (Chapter 16)

Components of Blood

Blood is a specialized connective tissue composed of plasma and formed elements. Understanding its composition is essential for grasping its functions in transport, regulation, and protection.

• Plasma: The liquid matrix of blood, making up about 55% of total blood volume. It consists of water (90%), proteins (albumin, globulins, fibrinogen), electrolytes, nutrients, gases, hormones, and waste products.

• Functions of Plasma: Maintains osmotic balance, transports substances, and plays a role in clotting and immunity.

• Formed Elements: Cellular components including erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (platelets).

• Hematocrit: The percentage of blood volume occupied by erythrocytes; normal range is 40–45%.

• Buffy Coat: Thin layer between plasma and erythrocytes containing leukocytes and platelets.

Erythrocytes (Red Blood Cells)

Erythrocytes are the most numerous formed elements, specialized for oxygen transport.

• Shape: Biconcave discs, increasing surface area for gas exchange and flexibility for passage through capillaries.

• Function: Transport oxygen via hemoglobin and assist in carbon dioxide transport.

• Chemical Structure of Hemoglobin: Hemoglobin is a protein with four globin chains, each containing a heme group with an iron ion that binds oxygen.

• Erythropoiesis: The process of erythrocyte formation, stimulated by erythropoietin (EPO) from the kidneys in response to hypoxia. Occurs in red bone marrow.

• Breakdown/Recycling: Erythrocytes are broken down in the spleen and liver. Iron is recycled, globin is broken into amino acids, and heme is converted to bilirubin for excretion.

Abnormal Conditions

• Anemias: Reduced oxygen-carrying capacity due to low RBC count or hemoglobin.

• Polycythemia: Excess RBCs, increasing blood viscosity.

• Hypoxia: Deficiency in oxygen reaching tissues.

Blood Types

• ABO Blood Types: Determined by presence of A and/or B antigens on RBCs and corresponding antibodies in plasma.

• Rh Factor: Presence (Rh+) or absence (Rh-) of D antigen; important in pregnancy and transfusions.

Leukocytes (White Blood Cells)

Leukocytes are immune cells with diverse functions and characteristics.

• Overall Function: Defense against pathogens, removal of debris, and immune regulation.

• Shared Characteristics: Nucleated, capable of diapedesis (movement out of blood vessels).

• Granular Leukocytes: Neutrophils (phagocytosis, most numerous), eosinophils (combat parasites, allergies), basophils (release histamine, rare).

• Agranular Leukocytes: Lymphocytes (B and T cells, immune response; mature in lymphoid tissues), monocytes (phagocytosis, become macrophages in tissues).

Thrombocytes (Platelets)

• Function: Essential for blood clotting.

• Precursor Cell: Megakaryocyte fragments form platelets.

Hemostasis and Coagulation

Hemostasis is the process of stopping bleeding, involving three phases:

• Vascular Spasm: Vasoconstriction reduces blood flow.

• Platelet Plug Formation: Platelets adhere to exposed collagen and aggregate.

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

Coagulation involves two pathways:

• Intrinsic Pathway: Initiated by damage inside vessel.

• Extrinsic Pathway: Initiated by external trauma.

• Both converge at the common pathway leading to conversion of prothrombin to thrombin, then fibrinogen to fibrin.

Clot Resolution: Plasmin enzyme dissolves fibrin.

Blood Disorder Terminology

• Thrombosis: Formation of unwanted clots.

• Hemophilia: Genetic disorder affecting clotting factors.

• Leukemia: Cancer of white blood cells.

The Heart (Chapter 17)

Structure of the Heart

The heart is a muscular organ with specialized layers and chambers for pumping blood.

• Layers of Heart Wall: Epicardium (outer), myocardium (muscle), endocardium (inner).

• Base vs Apex: Base is superior, apex is inferior and points left.

• Interventricular Septum: Separates right and left ventricles.

• Coverings: Pericardium (fibrous and serous layers).

Chambers and Valves

• Four Chambers: Right atrium (receives oxygen-poor blood), right ventricle (pumps to lungs), left atrium (receives oxygen-rich blood), left ventricle (pumps to body).

• Four Valves: Tricuspid, pulmonary, mitral (bicuspid), aortic.

Heart Sounds and Murmurs

• Lub-dup: 'Lub' is closure of AV valves; 'dup' is closure of semilunar valves.

• Murmur: Caused by turbulent blood flow, often due to valve defects.

Major Vessels

• Returning Blood: Superior/inferior vena cava, pulmonary veins.

• Carrying Blood Away: Pulmonary arteries, aorta.

Blood Flow Pathways

• Pulmonary Circulation: Right ventricle → lungs → left atrium.

• Systemic Circulation: Left ventricle → body → right atrium.

• Coronary Circulation: Supplies heart muscle with blood.

Potential Problems

• Angina Pectoris: Chest pain from reduced coronary blood flow.

• Myocardial Infarction: Heart attack due to blocked coronary artery.

Conduction System of the Heart

• Autorhythmicity: Heart generates its own electrical impulses.

• Sympathetic Influence: Increases heart rate (HR) and stroke volume (SV).

• Parasympathetic Influence: Decreases HR.

• Commands: Originate from medulla oblongata, reach heart via vagus (parasympathetic) and sympathetic nerves.

• Pacemaker Pathway: SA node → AV node → bundle of His → bundle branches → Purkinje fibers.

• Contractile Cell Impulses: Depolarization (Na+ influx), plateau (Ca2+ influx), repolarization (K+ efflux).

• Functional Syncytium: Cardiac muscle cells are electrically connected via gap junctions, allowing coordinated contraction.

Cardiac Cycle and Output

• Systole: Contraction phase; chambers eject blood, valves open/close accordingly.

• Diastole: Relaxation phase; chambers fill with blood.

• Cardiac Output (CO): Amount of blood pumped per minute. Formula: Where HR = heart rate, SV = stroke volume.

• Factors Influencing CO: Preload, afterload, contractility, autonomic input.

Blood Vessels (Chapter 18)

Structure of Blood Vessel Walls

Blood vessels are organized into three layers, each with distinct tissue composition and function.

• Tunica Intima: Innermost layer; endothelium and connective tissue.

• Tunica Media: Middle layer; smooth muscle and elastic fibers.

• Tunica Adventitia (Externa): Outermost layer; connective tissue.

Types of Blood Vessels

• Arteries: Carry blood away from heart; thick tunica media.

• Elastic Arteries: Large, near heart; stretch and recoil.

• Muscular Arteries: Distribute blood to organs.

• Arterioles: Smallest arteries; "resistance vessels" due to control over blood flow.

• Capillaries: Site of exchange; types include continuous (tight junctions), fenestrated (pores for filtration).

• Veins: Carry blood to heart; thinner walls, larger lumen.

• Venules: Smallest veins; collect blood from capillaries.

Blood Flow and Resistance

• Resistance: Opposes blood flow; affected by vessel diameter, blood viscosity, and vessel length.

• Vasoconstriction: Narrowing of vessels; increases resistance. Triggered by chemicals like norepinephrine, angiotensin II.

• Vasodilation: Widening of vessels; decreases resistance. Triggered by nitric oxide, histamine.

• Velocity: Blood flow velocity decreases from arteries to capillaries, then increases in veins.

Blood Pressure Regulation

• Baroreceptors: Detect changes in blood pressure; located in carotid sinuses and aortic arch.

• Vasomotor Center: Controls vessel diameter.

• Cardioacceleratory Center: Increases HR.

• Cardioinhibitory Center: Decreases HR.

Venous Return and Blood Flow Shifts

• Factors Affecting Venous Return: Skeletal muscle pump, respiratory pump, venous valves.

• Blood Flow Shifts: Regulated by local and systemic factors (e.g., exercise, hormones).

Blood Volume Regulation

• ANH (ANP): Atrial natriuretic peptide decreases blood volume by promoting sodium excretion.

• Aldosterone: Increases blood volume by promoting sodium retention.

• ADH: Antidiuretic hormone increases water reabsorption.

Capillary Exchange

Capillary exchange is governed by hydrostatic and osmotic pressures.

• NFP (Net Filtration Pressure): Determines movement of fluid across capillary walls. Formula: Where CHP = capillary hydrostatic pressure, BCOP = blood colloid osmotic pressure.

• Positive NFP: Fluid moves out of capillaries (filtration).

• Negative NFP: Fluid moves into capillaries (reabsorption).

• Role of Plasma Proteins: Maintain BCOP, preventing excessive fluid loss.

Example: In systemic capillaries, filtration occurs at the arterial end, reabsorption at the venous end.