Cardiac Muscle + Circulatory System

A&P 335 Exam 3

Cardiac muscle structure

Striated and uni-nucleated

Components in cardiac muscle

Intercalated disks - provide mechanical and electrical connections

Desmosomes - keeps cells together as organ changes dimension

Gap junctions - protein channels linking cytosols of adjacent cells, allows small molecules to pass quickly from cell-to-cell

Functional syncytium

All cells in the cardiac muscle operate together

Explain contractile cell AP

• Very negative resting membrane from lots of K+ leak

• Rapid opening of voltage-gated Na+ channels

• Prolonged “plateau” due to slow opening of voltage-gated Ca2= channels, L-type channels

• Slower opening of v-g K+ channels back to repolarization

• Long in duration to allow for refilling of ventricles

L-type Ca2+ channels

Long-lasting Ca2+ channels that are responsible for plateaus in APs after depolarization

The same thing as DHPRs

Walk through cardiac EC-coupling

Excitation

• Membrane is depolarized by Na+ entry as AP begins

• Depolarization opens DHPR (L-type Ca2+ channels) in T-tubules

• CICR (calcium-induced calcium release) - entry of trigger Ca causes Ca to bind to and open Ryr —> Ca goes into the cytosol

Contraction

• Binding of Ca exposes cross-bridge binding sites on thin filaments

• Cross bridge cycling generates force, filaments slide

Relaxation

• SERCA pump returns Ca to SR

• Na/Ca exchangers

• Ca ATPase pump

• Membrane repolarizes

Cardiac muscles and tetanus

Tetanus is a sustained contraction. Can’t occur in cardiac muscles due to the prolonged refractory period and lack of summation in cardiac muscle

Occurs to allow ventricles to refill

All muscle cells…

• Have myosin thick and actin thin filaments

• Have a cross-bridge cycle

• Have sliding filament mechanism

• Use ATP to generate force

• Include elevated cytosolic Ca to initiate contraction

These muscle cells are small and uninucleated

Smooth and cardiac

These muscle cells have troponin and tropomyosin

Skeletal and cardiac

What role does the cardiovascular system have on homeostasis

It’s the main transport system

Average total blood volume

5.5 L

Plasma (centrifuge)

Averages 3 L or 55-58%

Is like the ISF but has plasma proteins

Buffy coat (centrifuge)

Insignificant volume

Contains leukocytes and platelets

Hematocrit (centrifuge)

Averages 2.5 L or 42-45%

Contains erythrocytes/RBCs

Explain erythrocytes

Biconcave discs with a large surface area and small volume for rapid diffusion

Carries hemoglobin

Doesn’t contain DNA

4 commitments of blood cells

Reticulocyte —> RBC —> O2 transport

Megakaryocyte —> platelets —> clotting

Monocyte, neutrophil, eosinophil, basophil —> WBC —> immunity defense

B, T-lymphocytes —> immunity defense

Bone marrow

Produces erythrocytes

Define arteries

Carries blood away from heart

Define veins

carries blood toward the heart

Define perfusion

Passage of blood through a vascular bed, moves by bulk flow from high to low pressures

Why are vascular beds parallel and not in a series?

1. Blood quality - ensures every organ gets the same amount of O2, nutrients, CO2 waste

2. Flow regulation - reducing blood flow in one organ will still have lots of flow to other organs

3. Initial pressure - pressure is lost through each organ, but being parallel allows for 110/70 pressure for all organs

Follow the flow of blood through the pulmonary and systemic circulation (start at vena cava)

Vena cava —> right atrium —> right AV (tricuspid) valve —> right ventricle —> pulmonary semilunar valve —> pulmonary arteries —> arterioles —> lung capillaries —> pulmonary veins —> venules —> left atrium (pulmonary)

Left atrium —> left AV (bicuspid/mitral) valve —> left ventricle —> aortic semilunar valve —> aorta —> systemic arteries —> arterioles —> capillary beds —> systemic veins —> venules —> vena cava (systemic)

List the 3 types of cardiac muscle cells

Pacemaker, conducting, and contractile cells

Describe pacemaker cells

Has automaticity, SA node usually determines the heart rate as 100-120 APs/min

Uses nodal cell APs

Describe conducting cells

All are conducting (due to gap junctions) but few are specialized to rapidly spread electrical stimulus through chambers

Includes the bundle of his, bundle branches, and Purkinje fibers

Use conducting APs

Describe contractile cells

99% of cardiac muscle cells whose activity allows blood to be pumped out of the heart

Use fast-conducting APs

Describe coronary circulation

Blood supply to the heart through coronary arteries that branch off the aorta and coronary veins that return to the right atrium

Pressure, flow, and resistance equations

n = viscosity

Flow and resistance are inversely related

Radius and resistance are inversely related

Radius and flow are directly related

Relationship between resistance and radius

The smaller the radius (vasoconstriction), ↑ resistance, ↓ flow

The bigger the radius (vasodilation), ↓ resistance, ↑ flow

Purpose of heart valves

To promote a one-way direction of blood flow

How do pressure gradients affect valves

If the pressure is greater in 1 than in 2, blood will flow from 1 to 2. If pressure is greater in 2 than in 1, the valve will be closed and blood from 1 and 2 will bounce off the valve

Define prolapse

Heart valve flops backward onto the chamber behind it when it should be closed

Where are the chordae tendineae and papillary muscles located

Ventricular chambers

How do the chordae tendineae and papillary muscles prevent prolapse?

Ventricles get filled —> valves want to flip the other way —> papillary muscles contract and pull downward, pulling the chordae tendineae to keep the cusps closed against ventricular pressure

Define stenoic valve

A valve that doesn’t open all the way, turbulent flow sounds like a whistle and affects the lub/dub after it

Define insufficient valve

A valve that leaks backward due to poor closing, turbulent flow sounds like a gurgle and affects the lub/dub before it

Explain the “lub” and “dub”

Lub - AV valve closing

Dub - SL valve closing

Define sinoatrial (SA) node

A few cells collected in the right atrium that reach threshold first and are the true pacemaker

Define atrioventricular (AV) node

Electrical connection between atria and ventricles, slows AP propagation

Define bundle of his, bundle branches, Purkinje fibers

All made of conducting cells so AP propagation is fastest, all in the ventricle

Implications of electrical syncytium

• One cell starts AP

• One diseased cell can cause fatal arrhythmia

• Can install artificial pacemakers

• No recruitment

Explain the Lead I ECG trace

P wave - atrial depolarization of both SA and AV nodes

• AV node is too small, is a flat line after P wave

QRS - ventricular depolarization/systole

• Atrial repolarization still occurs, isn’t shown

• Q - septum, R - ventricular mass, S - base

T wave - ventricular repolarization/diastole

Define systole

Excitation/contraction of ventricles

Define diastole

Ventricular relaxation

Explain the nodal cell AP

• Starts at -60 mV

• Na enters through funny Na channels, then Ca entry through T (transient) type channels, bringing membrane to threshold (-40mV)

• Depolarization upstroke due to v-g L-type Ca channels

• Repolarization by v-g K+ channels

• Causes pacemaker potential

Explain fast-conducting cell APs

A mix of contractile and nodal APs

List the 4 periods in mechanical pumping

Ventricular filling (diastole), isovolumetric contraction (systole), ejection (systole), isovolumetric relaxation (diastole)

How much blood is in the left ventricle on average

135 L

Explain ventricular filling

There is an atrial kick where the atria first fill the ventricle with blood passively, then actively through contraction

Reaches end-diastolic volume

Diastole

End-diastolic volume

The final volume in the ventricle after filling, at least 135 mLs

Explain isovolumetric ventricular contraction

Ventricles contract

Hear “lub” (AV valves close), all valves are closed

Systole

Explain ventricular ejection

There is a stroke volume

Reach end-systolic volume

ESV = EDV - SV

SL valve is open

Systole

Stroke volume

The volume of blood ejected from each ventricle at rest, ~70mLs

End-systolic volume

The volume of blood remaining in ventricle after ejection

Explain isovolumetric ventricular relaxation

Hear the “dup”, SL valves close

Volume is constant at ESV

Diastole

Understand the Wigger’s diagram

Shows electrical events, pressures, volumes, valve events, and sounds of the heart

Understand Einthoven’s triangle

(+) is the recording electrode

(-) is the reference electrode

Explain the QRS axis and its shifts

QRS axis depicts a net direction of depolarization towards the left apex.

Left shift - directed left but up towards the base

Right shift- directed right

Define sinus rhythm

Typical cardiac excitation-contraction sequence beginning at SA node

Define latent pacemaker

Not active potential pacemakers, such as AV node and purkinje fibers

Define ectopic pacemaker

Abnormal, any site driving ventricular EC that isn’t the SA node. “If… then… has become…”

Define tachycardia, bradycardia, and fibrillation

Tachycardia - HR greater than 100 bpm

Bradycardia - HR slower than 60 bpm

Fibrillation - totally irregular and chaotic AP propagation

Define cardiac output

The volume of blood coming out of each ventricle per min. Is 5 L/min at rest

At rest, which autonomic nervous system dominates

Parasympathetic

How does increased physical activity affect autonomics?

↓ Parasympathetic output, ↑ sympathetic output

Cardiac output formula

= Heart rate x stroke volume

Intrinsic rate of pacemaker cells normal + rest (APs/min)

SA node - 100 → 50-70

AV node - 70 → 40-50

His/Purkinje - 30 → 30

Define chronotropic effects

Factors that affect heart rate

What 3 ion channels affect pacemaker potential (sympathetic)?

↑ Funny Na current, ↑ T-type Ca current, same K current

Gets potential to threshold sooner

What 3 ion channels affect pacemaker potential (parasympathetic)?

↑ v-g K current, ↓ funny Na current, ↓ T-type Ca current

Slows down potential

Define dromotropic effects

Change in conduction velocity, specifically, the upstroke of the AP curve

What ion channel affects conduction velocity in nodal cells

L-type Ca current. ↓ in parasympathetic, ↑ in sympathetic

Stroke volume equation

End-diastolic volume - end-systolic volume

Explain the Frank-Starling mechanism / “preload”

If ventricle fills to a larger volume, improving overlap of actin and myosin crossbridging → next beat ejects more blood

An intrinsic mechanism of regulating SV by changing ventricular EDV (↑ EDV → ↑ length of cells → ↑ tension → ↑ SV)

Implications of Frank-Starling mechanism

Prevents rise in ESV, preventing clotting

Prevents rise in venous pressure, forcing out blood back ups that can cause edema

Explain sympathetic regulation of SV

Sympathetic nervous system releases NE + E, which can bind to a b1 receptor → G proteins → adenylyl cyclase —> activates protein kinase A —>

5 targets of phosphorylation for…

• Faster and more Ca release (DHPR/L-type + RyR)

• Stronger, briefer contraction (thin and thick filaments)

• Faster Ca removal (SERCA pump)

Effects of sympathetic regulation of SV

Increased contractility by increasing Ca release at any given EDV, makes new Frank-Starling curve

Increased L-Ca current, more Ca in cytosol, more troponin saturation

Faster reuptake

Increased ejection fraction

Is an ionotropic effect

Ionotropic effect

Changing contractility that is resulting from any given EDV

Define ejection fraction + formula

Clinical measurement of contractility

EF = SV/EDV

Where are parasympathetic neuronal signals sent from

The vagus nerve

Explain parasympathetic ganglion anatomy

Sends signals through long pre-ganglionic axons, short post-ganglionic

Ganglion is near where heart is

Where are sympathetic neuronal signals sent from

Thoracic spinal nerves

Explain sympathetic ganglion anatomy

Sends signals through short pre-ganglionic axons, long post-ganglionic axons

What organ secretes epinephrine

Adrenal medulla