Biology 112

Chapter 13: Cardiovascular System

Introduction

Cardiovascular system: a pump (the heart) that delivers blood to vessels (arteries) that circulate blood throughout the body; materials in blood are exchanged with cells through tiny vessels (capillaries); blood is returned to the heart by another group of vessels (veins)

pulmonary circuit: blood vessels that carry blood to & from the lungs

sends deoxygenated blood to lungs to pick up oxygen & unload carbon dioxide

systemic circuit: blood vessels that carry oxygenated blood to body tissues & return deoxygenated blood with wastes to heart

Structure of Heart

heart size varies; an average adult heart is about 14 cm long x 9 cm wide
heart is within mediastinum; bordered laterally by lungs, posteriorly by vertebral column & anteriorly by sternum
extends 12-14 cm within mediastinum, from 2nd rib to 5th intercostal space
base: attaches to large vessels; superior/posterior aspect of heart
apex: pointed inferiolateral aspect of heart; pointed slightly to left
pericardium: double-walled sac enclosing heart
fibrous pericardium: outer dense connective tissue layer

anchors heart to surrounding structures (diaphragm, vessels)

serous pericardium: deep to fibrous pericardium

parietal layer: lines internal surface of fibrous pericardium
visceral layer (epicardium) – deep to parietal layer; outer layer of heart wall

skeleton of the heart: fibrous connective tissue anchored to vessels & septum; strengthens & holds tissue together

Layers of heart wall:

epicardium: visceral layer of serous pericardium; often accumulates fat
myocardium: cardiac muscle deep to epicardium; bulk of heart tissue

branched cardiac muscle cells linked by connective tissue fiber bundles (collagen & elastin) – fibrous skeleton of heart

endocardium: thin inner myocardial surface; sheet of endothelium (squamous epithelium) resting on connective tissue
lines chambers & valves; continuous with endothelial linings of major vessels

Heart Chambers & Valves

atria: upper chambers; receiving chambers for blood

auricles: small protruding appendages that slightly increase atrial volume
right atrium receives deoxygenated blood from superior vena cava (from areas above diaphragm), inferior vena cava (from areas below diaphragm), & coronary sinus (from myocardium)
left atrium receives oxygenated blood from pulmonary veins (4, from lungs)

ventricles: lower chambers; discharging (pumping) chambers for blood

papillary muscles: conelike muscle bundles in ventricular cavity; attached to tendon (chordae tendineae) that play a role in valve function
right ventricle pumps blood into pulmonary trunk (to lungs)
left ventricle pumps blood into aorta (to systemic circulation/body tissues)

septum: separates atria & ventricle on left side from right side
atrioventricular (AV) valves: prevent backflow of blood from ventricles to atria

tricuspid valve: right AV valve; has 3 cusps (flaps of endocardium reinforced with CT)
bicuspid (mitral) valve: left AV valve
chordae tendineae: collagen cords attached to AV valve flaps; anchor cusps to papillary muscles

semilunar (SL) valves: prevent backflow of blood from great vessels to ventricles

aortic valve: prevents blood from flowing back into left ventricle following ventricular contraction
pulmonary valve: prevents blood from flowing back into right ventricle following ventricular contraction

Pathway of blood through heart:

pulmonary circuit: blood vessels that carry blood to & from the lungs
systemic circuit: blood vessels that carry oxygenated blood to & from body tissues
superior & inferior vena cava and coronary sinus ® right atrium ® (tricuspid valve) ® right ventricle ® (pulmonary semilunar valve) ® pulmonary trunk ® pulmonary arteries ® lungs ® pulmonary veins ® left atrium ® (bicuspid valve) ® left ventricle ® (aortic semilunar valve) ® aorta ® body tissues

Coronary Circulation: functional blood supply of heart (myocardium)

coronary arteries: arise from the base of the aorta; carry oxygenated blood to myocardium of atria & ventricles
cardiac veins: carry deoxygenated blood from myocardium to coronary sinus, which empties into right atrium
proper circulation (constant blood supply) to myocardium is critical; blockage of coronary arterial circulation can be serious/fatal

collateral circulation: smaller branches of coronary arteries have connections (anastomoses) for alternate blood pathways
angina pectoris: chest pain due to short block of blood supply to myocardium
myocardial infarct (MI, heart attack or coronary): can result from prolonged blockage

Heart Actions & Cardiac Cycle

Cardiac cycle: all events associated with blood flow through heart

systole: contraction
diastole: relaxation (dilation or expansion)
sequence:

ventricular filling (mid to late ventricular diastole)/atrial systole
ventricular systole/atrial diastole
early ventricular diastole
relaxation

Heart sounds: lub-dup sound

AV valves close (lub); SL valves close (dup)
murmurs: sounds often indicative of valve problems

Cardiac Conduction System

sequence of excitation:

sinoatrial (SA) node: autorhythmic cells here are the fastest to generate impulses (~75/min, called sinus rhythm); hence, this is the heart’s pacemaker
atrioventricular (AV) node: receives impulses from SA node; fewer connections between cells delay impulse long enough for atria to complete contraction; also autorhythmic cells here, but slower impulses (~50-60/min, called junctional rhythm), so these cells do not set the pace unless there is damage to SA node cells
atrioventricular (AV) bundle (bundle of His): electrical connection between atria & ventricles; transmits impulse to ventricles
right & left bundle branches: sends impulse along cells of interventricular septum toward apex
Purkinje fibers: extend from inferior aspect of interventricular septum to apex & into outer walls of ventricles

Electrocardiography

Electrocardiograph: measures electrical currents generated during heart contraction with a series of electrodes placed on 12 body regions
Electrocardiogram (ECG or EKG): recording from electrocardiograph

P wave: atrial depolarization
QRS complex: ventricular depolarization
T wave: ventricular repolarization
P-Q interval: beginning of atrial depolarization until beginning of ventricular depolarization
Q-T interval: beginning of ventricular depolarization until beginning of ventricular repolarization

Regulation of heart rate:

Sympathetic division of ANS: increases heart rate

stimulated by cardioacceleratory center in medulla oblongata

Parasympathetic division of ANS (vagus nerve): decreases heart rate

stimulated by cardioinhibitory center in medulla oblongata

hormones:

epinephrine & norepinephrine: increases heart rate
thyroxine: slow sustained increase in heart rate

ions: calcium, sodium & potassium

Blood Vessels
Structure of Blood Vessel Walls:

tunica interna (tunica intima): innermost tunic (layer)

endothelium (simple squamous epithelium) lining lumen of all vessels

tunica media: middle tunic

mostly smooth muscle cells & sheets of elastin fibers
generally thickest layer in arteries
smooth muscle innervated by vasomotor fibers of sympathetic division of ANS

vasoconstriction: reduced lumen diameter due to smooth muscle contraction
vasodilation: increase in lumen diameter due to smooth muscle relaxation

tunica externa (tunica adventitia): outermost tunic

mostly loose collagen fibers; protect & reinforce vessel wall & anchor it to surrounding structures
contains nerve fibers, lymphatic vessels, & elastin in larger veins

Arteries: transport blood away from the heart

elastic (conducting) arteries: thick-walled arteries near heart (aorta & branches)
muscular (distributing) arteries: branch from elastic arteries to distribute blood to body organs; includes most named arteries
arterioles: vary in size; lead from muscular arteries to capillary beds

blood flow into capillary beds determined by arteriole diameter
if arterioles constrict, tissue is largely bypassed

Capillaries: smallest blood vessels; exchange materials (gases, nutrients, hormones, etc.) in blood with tissues

only tunica interna (endothelium)
microcirculation: flow of blood from arteriole to venule
precapillary sphincter: at root of metarteriole & capillary; acts as valve to regulate blood flow into capillary (constricts to send blood through bed (using vascular shunt))
capillary exchange of respiratory gases & nutrients: oxygen, carbon dioxide, most nutrients & cellular wastes pass between blood & interstitial fluid by diffusion
fluid movement: pressure & capillary pores allow fluid to leave capillaries at arterial end of capillary bed, but most returns at venous end

hydrostatic pressure: force exerted by fluid pressing against a wall

in capillaries, same as capillary blood pressure
capillary hydrostatic pressure tends to force fluids through capillary walls
interstitial fluid hydrostatic pressure pushes fluid back in

colloid osmotic pressure: large non-diffusible molecules (proteins) draw fluid toward them through osmosis

capillary colloid osmotic pressure: large molecules cannot move through capillary membrane; draw fluid in

Veins: transport blood toward heart

venules: smallest veins; range from postcapillary venules (only tunica interna) to larger venules with additional one or two layers of smooth muscle cells & thin tunica externa
veins: large vessels with all 3 tunics; vessel walls smaller & larger lumens than corresponding arteries

capacitance vessels (blood reservoirs): at any given time, most blood in the body is within veins
venous valves: formed from folds of tunica externa; flaps that prevent backflow of blood, especially in limbs

Systemic Blood Pressure: blood pressure = cardiac output x peripheral resistance

measured from brachial artery with sphygmomanometer

uses auscultatory method (listening for filling of artery as pressure in cuff drops below arterial pressure)
normal resting ranges: systolic BP: 110-140 mm Hg; diastolic BP: 75-80 mm Hg
hypotension: low blood pressure (systolic BP below 100 mm Hg)
hypertension: high blood pressure (sustained arterial pressure > 140/90)

systolic pressure: pressure generated in aorta following (left) ventricular systole (~120 mm Hg)
diastolic pressure: pressure in aorta following ventricular diastole (70-80 mm Hg)
pulse pressure: systolic pressure – diastolic pressure
cardiac output (CO): CO = Stroke Volume (SV) x Heart Rate (HR)

normal resting values: SV = 70 ml/beat; HR = 75 beats/min; CO = 5250 ml/min

resistance: opposition to blood flow (friction)

peripheral resistance (PR): resistance in systemic circulation; contributed by blood vessel diameter, blood viscosity & blood vessel length
blood viscosity: thickness of blood; more viscosity = greater resistance
blood vessel length: longer vessel length = greater resistance

blood vessel diameter: most likely to alter resistance; decreased diameter = greater resistance
smooth muscle fibers control blood vessel diameter

Maintaining Blood Pressure:

Short Term Regulation

neural controls:

vasomotor center: sympathetic neurons in medulla oblongata integrate blood pressure control by altering cardiac output & blood vessel diameter
baroreceptor-initiated reflexes: baroreceptors = pressure-sensitive mechanoreceptors that respond to changes in arterial pressure & stretch
chemoreceptor-initiated reflexes: chemoreceptors respond to changing blood levels of oxygen, carbon dioxide & acidity
higher brain centers: hypothalamus (e.g.: fight or flight response) via medulla

chemical controls:

adrenal medulla hormones; atrial natriuretic peptide (ANP); antidiuretic hormone; angiotensin II; nitric oxide (NO); inflammatory chemicals; alcohol

Long Term Regulation: Renal regulation

direct renal mechanism: blood volume altered through filtration in kidneys
indirect renal mechanism: renin-angiotensin mechanism leads to aldosterone production

Systemic Circulation:

major systemic arteries:

aorta, aortic arch, descending aorta (thoracic & abdominal aorta)
3 branches from aortic arch: brachiocephalic artery, left common carotid artery & left subclavian artery
common carotid arteries: serve head

vertebral & internal carotid arteries give off branches to form Circle of Willis in brain (several paths for blood to brain tissue)

subclavian arteries: serve arms
common iliac arteries: branch to internal iliac arteries (serve pelvic organs) & external iliac arteries (serve legs)

major systemic veins:

external & internal jugular veins: drain blood from brain, head & neck
subclavian arteries: drain blood from arms
brachiocephalic arteries: receive blood from jugular & subclavian veins & enter superior vena cava
hepatic portal vein: receives blood from abdominal (digestive) organs & enters liver (metabolism & detoxification)
common iliac veins: receive blood from internal iliac veins (from pelvic organs) & external iliac veins (from legs)

common iliac veins merge to form inferior vena cava