Cardiovascular Physiology-RSK.ppsx

(1)

(2)

Cardiovascular System Function

• _{Functional components of the cardiovascular}

system:

–

_Heart

–

_{Blood Vessels}

–

_Blood

• _{General functions these provide}

–

_{Transportation}

• _{Everything transported by the blood}

–

_Regulation

• _{Of the cardiovascular system}

– _{Intrinsic v extrinsic}

–

_Protection

• _{Against blood loss}

(3)

Cardiovascular System Function

• _{To create the “pump” we have to examine}

the Functional Anatomy

–

_{Cardiac muscle}

–

_Chambers

–

_Valves

(4)

Classes of blood vessels

• _{Blood vessels fall into three major}

classes:

–

_{Arteries and arterioles carry blood away from}

the heart.

–

_{Veins and venules carry blood to the heart.}

–

_{Capillaries allow exchange of nutrients,}

(5)

Arteries

• _{Arteries are}

thick-walled, and lined

with smooth

muscle.

• _{Arteries expand}

with each heart

beat, and

contract

afterwards,

helping to move

blood.

(6)

Arterioles

• Arterioles branch off of arteries.

• Arterioles can constrict to direct and

control blood flow. They may, for example,

increase blood supply to the skin to allow

more heat to dissipate, or constrict during

stress to redirect blood to the heart and

muscles.

(7)

Capillaries

• _{Body tissues contain a}

vast network of thin

capillaries.

• _{Capillary walls are only}

one cell thick, allowing

exchange of gases,

nutrients, and wastes.

• _{Capillaries are so fine}

that RBCs must line up

single-file to go through

them.

(8)

Venules

• _{Venules are thin-walled collectors of}

blood.

• _{Low pressure in the venules allows the}

(9)

Veins

• _{Veins have thinner}

walls than arteries.

• _{Contraction of skeletal}

muscles helps move

blood up the limbs and

back to the heart.

• _{Valves in the veins}

prevents backflow of

blood.

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Functional Anatomy of the Heart

Cardiac Muscle

• _{Characteristics}

–

_Striated

–

_{Short branched cells}

–

_Uninucleate

–

_{Intercalated discs}

–

_{T-tubules larger and}

(12)

Functional Anatomy of the Heart

Chambers

• _{4 chambers}

–

_{2 Atria}

–

_{2 Ventricles}

• _{2 systems}

–

_Pulmonary

–

_Systemic

(13)

Functional Anatomy of the Heart

Valves

• _{Function is to prevent backflow}

–

_{Atrioventricular Valves}

• _{Prevent backflow to the atria}

• _{Prolapse is prevented by the chordae}

tendinae

–

_{Tensioned by the papillary muscles}

–

_{Semilunar Valves}

(14)

Lecture Outline

• _{Cardiovascular System Function}

• Functional Anatomy of the Heart

• Myocardial Physiology

–

_{Autorhythmic Cells (Pacemaker cells)}

–

_{Contractile cells}

• Cardiac Cycle

(15)

Functional Anatomy of the Heart

Intrinsic Conduction System:

Special cardiac tissues

of the heart

• _{Consists of}

“pacemaker” cells

and conduction

pathways

–

_{Coordinate the}

contraction of the

atria and

(16)

Myocardial Physiology

Autorhythmic Cells (Pacemaker Cells)

• _{Characteristics of}

Pacemaker Cells

–

_{Smaller than}

contractile cells

–

_{Don’t contain many}

myofibrils

–

_{No organized}

sarcomere structure

• _{do not contribute to}

the contractile force

of the heart

normal contractile myocardial cell

conduction myofibers

SA node cell

(17)

Myocardial Physiology

Autorhythmic Cells (Pacemaker Cells)

• _{Characteristics of Pacemaker Cells}

–

_{Unstable membrane potential}

• _{“bottoms out” at -60mV}

• _{“drifts upward” to -40mV, forming a pacemaker potential}

–

_Myogenic

• _{The upward “drift” allows the membrane to reach threshold}

potential (-40mV) by itself

• _{This is due to}

1. Slow leakage of K+_{out & faster leakage Na}+_in

» _{Causes slow depolarization}

» Occurs through I_f channels (f=funny) that open at negative

membrane potentials and start closing as membrane approaches threshold potential

2. Ca2+_{channels opening as membrane approaches threshold}

» _{At threshold additional Ca}2+_{ion channels open causing more}

rapid depolarization

» _{These deactivate shortly after and}

3. Slow K+_{channels open as membrane depolarizes causing an}

(18)

Most of the muscle cells in the heart are contractile cells. The autorhythmic cells are located in these areas:

• _{Sinoatrial (SA), or sinus, node} • _{Atrioventricular (AV) node}

• _{Atrioventricular (AV) bundle (also sometimes called the bundle of His)} • _{Right & left bundle branches}

• _{Purkinje fibers}

Various automatic cells have different 'rhythms':

SA node - 60 - 100 per minute (usually 70 - 80 per minute)

AV node & AV bundle - 40 - 60 per minute

Bundle branches & Purkinje fibers - 20 - 40 per minute

SA node = has the highest or fastest rhythm &, therefore, sets the pace or rate of contraction for the entire heart. As a result, the SA node is commonly referred to as the PACEMAKER.

Spread of cardiac excitation

Begins at the SA node & quickly spreads through both atria

• _{Also travels through the heart's 'conducting system' (AV node > AV bundle> bundle branches >}

Purkinje fibers) through the ventricles

• _{For efficient pumping:}

• _{The atria should contract (& finish contracting) before the ventricles contract.}_This

occurs because of AV nodal delay (that is, the impulse travels rather slowly through the AV node & this permits the atria to complete contraction before the ventricles begin contraction).

• _{The atria should contract as a unit, & the ventricles should contract as a unit.}_{This occurs}

because the impulse spreads so rapidly that all myocardial cells in the atria and ventricles, respectively, contract at about the same time. The impulse spreads rapidly through the ventricles because of the conducting system.

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Myocardial Physiology

Contractile Cells

• _{Special aspects}

–

_{Intercalated discs}

• _{Highly convoluted and interdigitated}

junctions

– _{Joint adjacent cells with}

» _{Desmosomes & fascia adherens} – _{Allow for synticial activity}

» _{With gap junctions}

–

_{More mitochondria than skeletal muscle}

–

_{Less sarcoplasmic reticulum}

• _Ca2+_{also influxes from ECF reducing}

storage need

–

_{Larger t-tubules}

• _{Internally branching}

(22)

Myocardial Physiology

Contractile Cells

• _{Special aspects}

–

_{The action potential of a contractile cell}

• _Ca2+_{plays a major role again}

• _{Action potential is longer in duration than a “normal” action}

potential due to Ca2+_entry

• _Phases

4 – resting membrane potential @ -90mV 0 – depolarization

» _{Due to gap junctions or conduction fiber action} » _{Voltage gated Na}+_{channels open… close at 20mV}

1 – temporary repolarization

» _{Open K}+_{channels allow some K}+_{to leave the cell}

2 – plateau phase

» _{Voltage gated Ca}2+_{channels are fully open (started during initial}

depolarization) 3 – repolarization

» _{Ca2+ channels close and K+ permeability increases as slower}

activated K+ channels open, causing a quick repolarization

(23)

Myocardial Physiology

Autorhythmic Cells (Pacemaker Cells)

(24)

Myocardial Physiology

Autorhythmic Cells (Pacemaker Cells)

• _{Altering Activity of Pacemaker Cells}

–

_{Sympathetic activity}

• NE and E increase I

_f

channel activity

– Binds to β₁ adrenergic receptors which activate cAMP and

increase I_f channel open time

– _{Causes more rapid pacemaker potential and faster rate of}

action potentials

Sympathetic Activity Summary: increased chronotropic effects

heart rate

increased dromotropic effects

conduction of APs increased inotropic effects

contractility

Sympathetic Activity Summary: increased chronotropic effects

heart rate

increased dromotropic effects conduction of APs

increased inotropic effects contractility

(25)

Myocardial Physiology

Autorhythmic Cells (Pacemaker Cells)

• _{Altering Activity of Pacemaker Cells}

–

_{Parasympathetic activity}

• _{ACh binds to muscarinic receptors}

– _{Increases K}+_{permeability and decreases Ca}2+_{permeability =}

hyperpolarizing the membrane

» _{Longer time to threshold = slower rate of action}

potentials

Parasympathetic Activity Summary:

decreased chronotropic effects

heart rate

decreased dromotropic effects

 conduction of APs decreased inotropic effects

 contractility

Parasympathetic Activity Summary:

decreased chronotropic effects heart rate

decreased dromotropic effects  conduction of APs

decreased inotropic effects  contractility

(26)

Myocardial Physiology

Contractile Cells

• _{Skeletal Action Potential vs Contractile}

(27)

Myocardial Physiology

Contractile Cells

• _{Plateau phase prevents summation due}

to the elongated refractory period

• _{No summation capacity = no tetanus}

(28)

Summary of Action Potentials

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Myocardial Physiology

Contractile Cells

• _Initiation

–

_{Action potential via pacemaker cells to}

conduction fibers

• _{Excitation-Contraction Coupling}

1. Starts with CICR (Ca

2+

_{induced Ca}

2+

release)

• _{AP spreads along sarcolemma}

• _{T-tubules contain voltage gated L-type Ca}2+

channels which open upon depolarization

• _Ca2+_{entrance into myocardial cell and}

opens RyR (ryanodine receptors) Ca2+

release channels

• _{Release of Ca}2+_{from SR causes a Ca}2+

“spark”

• _{Multiple sparks form a Ca}2+_signal

(30)

Myocardial Physiology

Contractile Cells

• _{Excitation-Contraction Coupling cont…}

2. Ca

2+

_{signal (Ca}

2+

_{from SR and ECF) binds to troponin to initiate}

myosin head attachment to actin

• _Contraction

–

_{Same as skeletal muscle, but…}

–

_{Strength of contraction varies}

• _{Sarcomeres are not “all or none” as it is in skeletal muscle}

– _{The response is graded!}

» _{Low levels of cytosolic Ca}2+_{will not activate as many}

myosin/actin interactions and the opposite is true

• _{Length tension relationships exist}

– _{Strongest contraction generated}

when stretched between 80 & 100% of maximum (physiological range)

– _{What causes stretching?} » _{The filling of chambers}

(31)

Myocardial Physiology

Contractile Cells

• _Relaxation

–

_Ca

2+

is transported back

into the SR and

–

_Ca

2+

is transported out of

the cell by a facilitated

Na

+

/Ca

2+

exchanger (NCX)

–

_{As ICF Ca}

2+

levels drop,

interactions between

myosin/actin are stopped

(32)

Lecture Outline

• _{Cardiovascular System Function}

• Functional Anatomy of the Heart

• Myocardial Physiology

–

_{Autorhythmic Cells (Pacemaker cells)}

–

_{Contractile cells}

• Cardiac Cycle

(33)

Cardiac Cycle

Coordinating the activity

• _{Cardiac cycle is the sequence of events}

as blood enters the atria, leaves the

ventricles and then starts over

• Synchronizing this is the Intrinsic Electrical

Conduction System

• _{Influencing the rate (chronotropy &}

dromotropy) is done by the sympathetic

and parasympathetic divisions of the ANS

(34)

Cardiac Cycle

Coordinating the activity

• _{Electrical Conduction Pathway}

–

_{Initiated by the Sino-Atrial node (SA node) which is myogenic at}

70-80 action potentials/minute

–

_{Depolarization is spread through the atria via gap junctions and}

internodal pathways to the Atrio-Ventricular node (AV node)

• _{The fibrous connective tissue matrix of the heart prevents further}

spread of APs to the ventricles

• _{A slight delay at the AV node occurs}

– _{Due to slower formation of action potentials} – _{Allows further emptying of the atria}

–

_{Action potentials travel down the Atrioventricular bundle (Bundle}

of His) which splits into left and right atrioventricular bundles

(bundle branches) and then into the conduction myofibers

(Purkinje cells)

• _{Purkinje cells are larger in diameter & conduct impulse very rapidly}

– _{Causes the cells at the apex to contract nearly simultaneously} » _{Good for ventricular ejection}