Bio241_chapter13_blood_lecture

Overview

• Chapter focuses on Blood, Heart, and Circulation (BIOL 241 – Ch. 13)
• Sub-topics: Blood • Pulmonary & Systemic Circulations • Heart Valves • Cardiac Cycle • Electrical Activity • Structure of Vessels

Functions of the Circulatory System

• Transport
  • \mathrm{O_2}, nutrients, enzymes, hormones ⟶ tissues
  • \mathrm{CO_2} & metabolic wastes ⟶ excretory organs
• Heat distribution → regulation of body temperature
• Fluid, electrolyte & pH homeostasis
• Protection
  • Immune surveillance (microbes, foreign Ag, tumors)
  • Hemostasis → prevents blood loss

Components of the Circulatory System

• Cardiovascular system: heart + blood vessels (arteries, arterioles, capillaries, venules, veins)
• Lymphatic system: returns excess interstitial fluid to venous blood; lymph nodes = immune organs

Composition of Whole Blood

• Total volume ≈ 5 L
• Two compartments:
  • Plasma (≈ 55 %)
  • 92 % H₂O, 8 % proteins + solutes (ions, nutrients, gases, wastes)
  • Serum = Plasma – clotting factors
  • Formed Elements (≈ 45 %)
  • \approx 95\% RBCs (erythrocytes) – anucleate
  • <1\% WBCs (leukocytes) – nucleated
  • \approx 5\% Platelets (thrombocytes) – anucleate

Hematocrit (Hct)

• % RBCs in whole blood
  • Women: 36\text{–}46\%
  • Men: 41\text{–}53\%
• Example: 80 mL sample → 30 mL RBCs ⇒ \text{Hct}=\frac{30}{80}\times100=37.5\% (within female range)

Plasma Proteins (7–9 % of plasma)

• Albumin (60–80 %)
  • Creates colloid osmotic pressure → draws \mathrm{H_2O} into capillaries, maintains BP & volume
• Globulins
  • Alpha/Beta: lipid carriers
  • Gamma: antibodies (immunoglobulins)
• Fibrinogen → Fibrin (clotting)

Formed Elements Details

Erythrocytes (RBCs)

• Flattened biconcave discs → large SA for diffusion
• Lack nuclei & mitochondria; contain ~280 million Hb molecules/RBC
• Production rate: \approx 3\times10^{11}\ \text{day}^{-1}

Leukocytes (WBCs)

• Have nucleus, mitochondria, amoeboid motility, diapedesis capability
• Granular (detox + heparin): neutrophils, eosinophils, basophils
• Agranular (phagocytosis/antibodies): lymphocytes, monocytes

Platelets (Thrombocytes)

• Smallest, anucleate fragments of megakaryocytes
• Majority of clot mass; live 5–9 days
• Release serotonin (vasoconstriction) & growth factors for vessel repair

Hematopoiesis

• Occurs in myeloid (red marrow) & lymphoid tissue
• Output: \approx5\times10^{11} cells/day
• Fetal site = liver
• Erythropoiesis
  • Stimulus: renal \mathrm{EPO}
  • Rate: 2.5\times10^{6}\ \text{sec}^{-1}; lifespan 120 days; old cells removed by liver, spleen, marrow; Fe recycled
• Leukopoiesis
  • Regulated by cytokines (autocrine factors)

Blood Groups & Typing

• ABO antigens on RBC membrane
  • A, B, AB, O (none)
• Agglutination / Transfusion Reactions
  • Type A → anti-B Abs; Type B → anti-A; Type AB → none; Type O → both
  • Universal donor = O (no antigens); universal recipient = AB (no anti-A/B Abs)
• Rh Factor
  • Rh⁺: D antigen; Rh⁻: none
  • Erythroblastosis fetalis: Rh⁻ mother sensitized by Rh⁺ fetus → anti-D Abs hemolyze next Rh⁺ fetus

Hemostasis (Bleeding Cessation)

1. Platelet Plug (small tears)
   • Intact endothelium secretes prostacyclin (PGI₂) & NO → inhibit platelet stickiness & vasodilate
   • Damage exposes collagen; von Willebrand factor bridges collagen ↔ platelets
   • Platelet Release Reaction: ADP, serotonin, thromboxane A₂ (TxA₂)
   • Serotonin & TxA₂: vasoconstriction; ADP & TxA₂: recruit more platelets → positive feedback until plug forms
   • Aspirin inhibits TxA₂ synthesis ⇒ antiplatelet effect
2. Coagulation (Fibrin Clot) (large tears)
   • Intrinsic Pathway (contact activation)
     • Negatively charged surface → activates factor XII ⇒ cascade ⇒ factor X (prothrombin activator)
   • Extrinsic Pathway (tissue factor)
     • Tissue thromboplastin (factor III) from damaged tissue directly activates factor X (shortcut)
   • Common Pathway: \mathrm{Prothrombin\ (II) \xrightarrow{Xa+Ca^{2+}+PL} Thrombin}
     \mathrm{Fibrinogen\ (I) \xrightarrow{Thrombin} Fibrin \rightarrow polymer}

• Hemophilia: absence of factor VIII (intrinsic)
• Vitamin K–dependent factors; Heparin blocks thrombin
• Dissolution: Factor XII → kallikrein → plasminogen → plasmin → digests fibrin
• Anticoagulants: Ca²⁺ chelators (citrate, EDTA), Heparin (activates antithrombin III), Coumarins (vit K antagonism ↓ Ca²⁺ availability)

Heart Anatomy & Circulations

• Four chambers: 2 atria (receive) • 2 ventricles (pump)
• Two pumps separated by muscular septum
• Fibrous skeleton (dense CT between atria & ventricles): electrical insulation + valve rings (annuli fibrosi)
• Pulmonary circuit: RV → pulmonary arteries → lungs → pulmonary veins → LA
• Systemic circuit: LV → aorta → tissues → veins → RA
  • Systemic resistance > pulmonary; LV workload ≈ 5–7× RV ⇒ LV wall 3–4× thicker
  • Flow rates through both circuits must be equal

Heart Valves

• Atrioventricular (AV): Right = tricuspid; Left = bicuspid/mitral
  • Chordae tendineae + papillary muscles prevent prolapse during systole
• Semilunar: Aortic & Pulmonary; open in ventricular systole, close in diastole

Cardiac Cycle (Repeat every heartbeat)

• Systole = contraction; Diastole = relaxation
• Sequence
  1. Isovolumetric Contraction: ventricles contract, all valves closed, pressure ↑
  2. Ejection: when P{vent}>P{aorta/pulmon} → semilunars open → stroke volume ejected
  3. Isovolumetric Relaxation: ventricles relax, semilunars shut (dub), all valves closed
  4. Ventricular Filling: when P{atria}>P{vent} → AV valves open; final filling by atrial systole
• Pressures: Systemic ≈ 120/80\ \text{mmHg}; Pulmonary ≈ 25/10
• Volumes
  • End-diastolic (EDV) ("preload")
  • End-systolic (ESV)
  • Stroke Volume \mathrm{SV = EDV - ESV}; Example: \mathrm{SV}=80-20=60\,mL; \mathrm{EDV>ESV}

Heart Sounds & Murmurs

• "Lub" (S₁): AV closure just after QRS
• "Dub" (S₂): Semilunar closure at T-wave onset
• Murmurs = turbulent flow (valve defects or septal defects)
  • Mitral stenosis → pulmonary hypertension
  • Incompetent valves → regurgitation
  • Septal defects (e.g., patent foramen ovale) ⇒ left→right shunt due to higher LV pressure

Electrical Activity & Conduction

Myocardium

• Cardiac muscle cells connected by gap junctions → functional syncytium (atria & ventricles separate via fibrous skeleton)

Pacemaker (SA Node)

• Resting V_m=-60\,mV → slow depolarization (pacemaker potential) via HCN Na⁺ channels (open on hyperpolarization)
• Threshold -40\,mV → \mathrm{Ca^{2+}} influx (L-type) → AP upstroke; repolarization via \mathrm{K^{+}} efflux
• Autonomic modulation alters slope of pacemaker potential

Ectopic Pacemakers

• AV node, Purkinje, etc., have slower intrinsic rates; suppressed by SA overdrive; take over if SA blocked

Contractile Myocardial AP

• RMP -90\,mV; rapid Na⁺ upstroke
• Plateau (200–300 ms): balance of slow Ca²⁺ influx vs K⁺ efflux → prolonged refractory period (prevents tetany)

Conduction Pathway

• SA node → atrial myocardium → AV node (delay 0.1 s) → Bundle of His → R/L bundle branches → Purkinje fibers (5 m/s) → ventricular myocardium (contr. starts 0.1–0.2 s after atria)

Excitation–Contraction Coupling

• Extracellular Ca²⁺ via L-type channels triggers SR Ca²⁺ release (Ca²⁺-induced Ca²⁺-release); Ca²⁺ binds troponin; relaxation via uptake/outflow pumps

Refractory Period

• ≈ 300 ms (≈ duration of AP) → heart cannot summate contractions

Electrocardiogram (ECG/EKG)

• Surface recording of electrical events (not mechanical)
  • P wave: atrial depolarization
  • QRS complex: ventricular depolarization (+ atrial repolarization)
  • T wave: ventricular repolarization
• Correlation with sounds: S₁ just after QRS; S₂ at start of T

Structure of Blood Vessels

• Tunica interna: endothelium + basement membrane + elastin
• Tunica media: smooth muscle (thicker in arteries)
• Tunica externa: CT
• Only capillaries = endothelium only → site of exchange

Arteries

• Elastic (conducting): elastin; expand (systole) & recoil (diastole) → dampen pulse pressure
• Muscular & arterioles: resistance vessels; greatest pressure drop; precapillary sphincters regulate flow; some arteriovenous anastomoses bypass capillaries

Capillary Types

• Continuous: tight; small pores; muscle, lung, adipose
• Fenestrated: wide pores; kidneys, endocrine, intestines
• Discontinuous (sinusoids): large gaps; liver, spleen, marrow

Veins