Cardiovascular System

What is unusual about the membrane potential of an autorhythmic (pacemaker) cell?

they have an unstable membrane potential, it never rests at a constant value, because it keeps drifting upward toward threshold. This slow drift is called the pacemaker potential.

Which channels open at about -60 mV and produce the pacemaker potential?

If channels, open at about -60 mV, at the most negative point of the cycle. They produce the slow depolarization of the pacemaker potential.

What determines heart rate in an autorhythmic cell?

How fast the pacemaker potential depolarizes to threshold

Which channels cause the rapid depolarizing phase of the autorhythmic action potential?

Voltage-gated Ca++ channels (T and L type)

What are If channels permeable to, and what is the net result?

Both K+ and Na+, with more Na+ than K+, so net Na+ comes in

What happens at the peak of the action potential for authorythmic cells?

Ca++ channels close and K+ channels open, so K+ goes out

Which ion movement causes repolarization in the autorhythmic cell?

K+ leaving the cell

What happens at the lowest point (about -60 mV) after repolarization? in autorhythmic cells

What happens at the lowest point (about -60 mV) after repolarization?

What happens to If channels as the cell reaches threshold?

If channels close and some Ca++ channels open

How is the autorhythmic cell action potential different from a skeletal muscle action potential?

The rapid rise is due to Ca++ in, not Na+ in, and there is no stable resting potential

What is the membrane potential of each muscle type?

Skeletal -70, contractile -90, autorhythmic unstable and starts at -60

What brings a skeletal muscle fiber to threshold?

Na+ entry through ACh-operated channels

What brings a contractile myocardial cell to threshold?

Depolarization entering through gap junctions

What brings an autorhythmic cell to threshold?

Na+ entry through If channels, reinforced by Ca++ entry

What causes the rising phase of the action potential in each type?

Na+ in skeletal and contractile; Ca++ in autorhythmic

How does repolarization differ among the three types?

Skeletal: rapid K+ efflux; contractile: plateau (Ca++ entry) then rapid K+ efflux; autorhythmic: rapid K+ efflux

Which type has an extended plateau caused by Ca++ entry?

Contractile myocardium

Which type shows an after-hyperpolarization from excessive K+ efflux?

Skeletal muscle

Why does contractile myocardium show no after-hyperpolarization?

Its resting potential (-90 mV) is the equilibrium potential for K+

What happens at the end of repolarization in an autorhythmic cell?

Normally none; the If channels open at -60 mV, and ACh can hyperpolarize the cell

rank the duration of action potentials for skeletal msucle, contractile myocardium and autorhythmic myocardium from largest to smallest

contractile myocardium, autorhythmic myocardium, skeletal muscle

Which type has a long refractory period?

Contractile myocardium

Why is the refractory period long in contractile myocardium?

Resetting of Na+ channel gates is delayed until the end of the action potential

what does the sinoatrial (SA) node set?

the pace

what causes the AV valves to close?

isovolumic contraction

what causes the SL valves to close?

early ventricular diastole

what event results in ventricular systole?

QRS complex

in which types of action potential is the depolarizing phase due to flow of sodium?

contractile myocardium and skeletal muscle

ventricular conducting system

AV bundle (bundle of His) —→ bundle branches —→ Purkinje fibers

conduction pathway

Autorhythmic cells transmit electrical activity

what does an ECG represent?

the summed electrical activity of ALL heart cells, NOT a single action potential

contractile action potential

recorded from a SINGLE CELL using an intracellular electrode

P wave

atrial depolarization

PQ or PR segment

atrial contraction occurs, conduction through AV node and AV bundle

QRS complex

ventricular depolarization

S-T segment

ventricular contraction occurs

T wave

represent ventricular repolarization

What is happening during late diastole?

both the atria and the ventricles are relaxed, and the ventricles fill passively as blood flows in from the atria through the open AV valves.

What happens to atrial pressure during atrial systole?

Atrial pressure rises when the atria contract. That pressure pushes blood across the open AV valves into the ventricles.

What is end-diastolic volume (EDV)?

The volume in the ventricle at the end of diastole, after atrial systole

How does a larger EDV affect the next ventricular contraction?

A stronger contraction, because the ventricular fibers are more stretched

What happens during isovolumic ventricular contraction

The ventricles contract and push the AV valves closed, but pressure is not yet enough to open the semilunar valves

Why is the ventricular volume constant during isovolumic contraction?

Because both the AV and semilunar valves are closed, so no blood moves in or out

What causes the semilunar (SL) valves to open at the start of ventricular ejection

Ventricular pressure exceeds the pressure in the arteries

What is the state of the valves, and what are the atria doing, during ventricular ejection?

SL valves open and AV valves remain closed; atria continue to fill

What is end-systolic volume (ESV)?

The volume left in the ventricle after ejection

When is ESV reached in the cardiac cycle?

At the end of ventricular ejection

where are the heart sounds S1 and S2?

S1 at the start of ventricular systole; S2 at the end of ventricular systole

what causes the S2 heart sound

the vibrations due to closing of the SL valves

Cardiac output (CO) =

heart rate (HR) * stroke volume (SV)

cardiac output is…

the volume of blood pumped by one ventricle per minute

stroke volume is…

mL of blood pumped by a ventricle per contraction

heart rate is…

the number of heart beats per minute

stroke volume =

EDV - ESV

what 3 factors influence venous return?

ske;etal muscle pump, respiratory pump, venoconstriction

MAP =

diastolic + 1/3 (systolic – diastolic )

pulse pressure =

systolic - diastolic

flow =

pressure gradient/resistance

R (resistance) =

1/r^4

artery role

elasticity, pressure reservoir

arteriole role

control blood flow distribution

capillary role

exchange nutrients and wastes

venule

collect blood from capillaries

vein

compliance (less elastic, expant easily) volume reservoir, venous return

where is the velocity of blood flwo the lowest?

capillaries

transcytosis

combination of endocytosis, vesicular transport and exocytosis

hydrostatic pressure

blood pressure (MAP), pushes water and solutes out of capillary

colloid osmotic pressure

due to plasma proteins, pushes water and solutues into capillary

net filtration pressure =

hydrostatic pressure - colloid osmotic pressure

where is blood pressure highest and lowest

hifhest in arteries, lowest in veins

blood pressure reflect…

pressure exerted by blood on walls of blood vessels

MAP reflects…

average driving pressure for blood flow to tissues, indicates whether there’s enough pressure to diffuse all organs

total peripheral resistance (TPR)

resistance to flow offered by arterioles

stroke volume is only influenced by…

the sympathetic nervous system

role of histamine

cause vasodilation in a specific location

what 2 areas do baroreceptors monitor blood pressure?

carotid arteries and aortic arch

what are the 3 steps of hemostasis?

1. vascular

2. platelet activation

   1. coagulation cascade

vascular phase of hemostasis

vasoconstriciton

platelet activation phase of hemostasis

platelets adhere to exposed collagen and become activated, they release platelet factors which forms the platelet plug

coagulation cascade of hemostasis

exposed collagen initiate formation of fibrin, platelet plug turns into clot

fibrinolysis

fibrin is dissolved by plasmin and the clot is dissolved