Heart and Circulation Lecture

Where is the heart located and what is it protected by?

Thoracic Cavity, Pericardium

Features of the Pericardium

Fibrous pericardium, Serous pericardium (Parietal Layer & Visceral Layer)

Fibrous Pericardium

outer layer, dense regular connective tissue

Serous Pericardium

double layered (Parietal & Visceral), pericardial fluid-filled membrane

Heart’s multi-layers (Outer to inner)

Epicardium, Myocardium, Endocardium

Epicardium

outmost layer, simple squamous epithelia, loose areolar connective and adipose tissue

Myocardium

thickest layer, contains cardiomyocytes and cardiac muscle

Endocardium

deepest layer, made of simple squamous endothelial tissue

Cardiac Anatomy

Heart four chambers

• Two upper is atria

• Two lower is ventricles

Left/Right separated by Cardiac septum

Right atrium + right ventricle = pulmonary pump (Deoxygenated blood)

Left atrium + left ventricle = systemic pump (Oxygenated blood)

Heart Valves

create one-directional blood flow

Atrioventricular Valves (AV)

• Tricuspid valve (3 cusps)

• Bicuspid (Mitral) valve (2 cusps)

Semilunar Valves (SL)

• Aortic valve

• Pulmonary valve

Heart’s Two Major Divisions of Circulatory System

Pulmonary Circulation

• Right side > lungs (picks up oxygen and release carbon dioxide) > blood vessels > left side

Systemic Circulation

• Left side > systemic cells (exchanges gases, nutrients, and waste) > blood vessels > right side

Both

• Work parallel

• connected and highly coordinated

• contract and relax together

• pump same volume of blood

Blood flow through heart and major vessels

Deoxygenated blood > superior/inferior vena cava > right atrium > tricuspid valve > right ventricle > lungs > left atrium > mitral valve > left ventricle > systemic circulation > blood vessels > right side

Fetal Development blood flow exceptions

Foramen Ovale: small hole that allows blood to bypass the right ventricle, moving directly between right atrium and left atrium

Ductus Arteriosus: connects pulmonary trunk to aorta

Features of cardiomyocytes

single, centrally- located nucleus, short and wide, striation present, myofilaments arranged into sarcomeres, myofibrils branched and variable in size

Heart Contractions

Conduction system initiates and propagates an action potential

Cardiac muscle cells intiate action potentials and contract

Heart’s Conduction System

Sinoatrial (SA) node: initiates heartbeat/action potential (pacemaker), located high in posterior wall of right atrium

Atrioventricular (AV) node: located in floor of right atrium (near right AV valve)

Atrioventricular (AV) bundle: extends from AV node through interventricular septum, divides into left and right bundles

Purkinje fibers: extend from left and right bundles at heart’s apex, through walls of ventricles

Action Potential in contractile Cardiomyocytes

• Resting membrane typically -90 mV

• Voltage gated channels closed when at rest, leaky K channels maintain general RMP

• Depolarization opens fast voltage gated Na channels, depolarizes to -30 mV

• causes K & Ca comes in depending on plateau effect

• Rapid repolarization calcium channels close, K channels remain open, membrane potential repolarizes to resting

Electrical Conduction of Heart

Depolarization moves rapidly through ventricular conducting system to the Apex of the heart > ensure blood is efficiently ejected upward to vessels

Electrocardiogram (ECG)

provides an electrical picture of heart function, common diagnostic tool

EKG Waves

P wave, QRS complex, T wave

P wave

reflects electrical changes of atrial depolarization originating in SA node

QRS complex

electrical changes associated with ventricular DEpolarization, atria also simultaneously repolarizing

T wave

electrical change associated with ventricular REpolarization

Cardiac Cycle

all events in the heart from the start of one heart beat to start of the next, includes both systole (contractions) and diastole (relaxation)

Ventricular Activity

Ventricular contraction RAISES ventricular pressure , AV valves pushed closed

Ventricular relaxation LOWERS ventricular pressure, semilunar valves

Cardiac Cycle Step 1

Atrial Systole

• corresponds with contraction of the atria

• Atrial pressure: > ventricle

• AV valves: open

• Ventricles: blood volume increasing, eventually reaching maximum hold (end diastolic volume/ EDV)

Cardiac Cycle Step 2

Early Ventricular Systole

• corresponds with contraction of the ventricles

• Ventricular pressure: > atria, < great vessels

• Blood volume: constant at EDV

• AV valves: closed

• SL valves: closed

Cardiac Cycle Step 3

Late Ventricular Systole

• Ventricular pressure: > atria/great vessels

• Blood volume: decreasing

• AV valves: closed

• SL valves: open

Cardiac Cycle Step 4

Early Ventricular Diastole

• corresponds to relaxation in ventricles

• ventricular pressure > atria, < great vessels

• Blood volume: constant

• AV valves: closed

• SL valves: closed

Cardiac Cycle Step 5

Atrial Diastole

• Atrial pressure: < ventricles

• ventricular blood volume: decreasing

• AV valves: closed

Cardiac Cycle Step 6

Late Ventricular Diastole

• Ventricular pressure: < atria, < great vessels

• Ventricular blood volume: increasing through passive filling

• AV valves: open

• SL valves: closed

Three types of Blood Vessels

Arteries (heart to capillaries), Capillaries (exchange between blood and tissues), and Veins (transport blood from capillaries to heart)

Structure and function of Arteries

Elastic Arteries

• conducting arteries, large volume of elastin dispersed

• includes the aorta & vessels that branch off of the arch

• large lumens to collect large volumes of blood from the heart

Muscular Arteries

• two prominent bands of elastic tissue = internal and external elastic lamina

• includes coronary artiers and ateries further away from the heart (brachial, femoral, etc)

Structure and function of Arterioles

• Smallest artery type

Largest arterioles

• contains all three tunics

• resemble small muscular arteries

Tiniest arterioles

• simply endothelium, surrounded by 1-2 fiber layers of smooth muscle

Structure and function of Capillaries

• smallest blood vessel type, erythrocytes flow in single file

• site of gas and nutrient exchange: only unica intima present/permeable

Continuous

• most common type

• no pores in lining

Fenestrated

• endothelial cells dotted with pores

• allows passage of fluid and larger molecules

Sinusoid

• larger fenestrations and discontinuous basement membrane

• allow entire cell movement through barrier

Structure and function of Veins

Venules: smallest of vessels carrying blood back to heart, very thin three tunics, converge to form Veins which has large lumens

Veins contain Valves

• experience lower pressure than arteries

• ensure blood returns to heart, larger lumens offer less resistance to blood flow, Venous Valves (specialized foldings of tunica intima) ensure unidirectional blood flow

Varicose Veins

• failure of venous valves, blood pools in peripheral veins of legs

• caused by anything that impedes venous return

• dilation and distension of veins, causing discomfort or pain

What impacts movement of blood?

Blood Flow (F), Blood Pressure (BP or P), Resistance (R)

Three variables influence resistance: blood viscosity, blood vessel length, blood vessel radius

Blood Flow (F)

volume of blood moving through a vessel, tissue, organ, or entire circulation per unit of time

Blood Pressure (BP or P)

force exerted onto a given area of the vessel wall by the blood contained within it, measured in mm Hg

Resistance (R)

friction encountered by blood, impeding flow

Three variables influence resistance

Blood viscosity, blood vessel length, blood vessel radius

Blood pressure

force of blood against vessel wall

Blood pressure gradient

change in pressure from one end of vessel to other, properls blood through vessels, pressure is highest in arteries and lowest in veins

Arterial blood pressure

blood flow in arteries pulses with cardiac cycle

Systolic pressure

occurs when ventricle contracts (systole), higjest pressure generated in arteries (stretched), numerator of blood pressure ratio

Diastolic pressure

occurs when ventricles relax (diastole), lowest pressure generated in arteries, denominator of blood pressure ratio

Pulse pressure

pressure in arteries added by heart contractions, the difference between systolic and diastolic blood pressure, reflects elasticity and recoil of arteries, pulse pressure allows for palpation of a throbbing pulse in elastic and muscular arteries

Capillary blood pressure

pressure no longer fluctuates between systolic and diastolic (flow/pressure is smooth)

Importance of capillary blood pressure

needs to be HIGH enough for exchange

needs to be LOW enough not to damage vessels

arterial end of capillary at about 40 mm Hg

Venous end of capillary BELOW 20 mm Hg

accounts for filtration and reabsorption at respective ends