Chapter 17 Cardiovascular System Physiology

Pacemaker Cells

spontaneously depolarize and create an action potential; responsible for sending the message to contract

make up about 1% of the cardiac cells

Contractile Cells

muscle cells stimulated by action potential, causing a physical contraction; contain large amounts of myoglobin(O2), mitochondria(ATP), intercalated discs(gap junctions) between adjacent contractile cells

make up about 99% of the cardiac cells

Intercalated Disc

contain tunnels leading from one cell to another, allowing stimulants/chemicals/others to travel directly from one place to another; increases the speed of an action potentials transmission

this is what allows an entire chamber to contract in unison

HCN Channel

general channel that opens and allows positive ions to move into a cell

Pacemaker Cell Action Potentials

1. During the initial depolarization phase, more cations leak in than out via HCN channels in the plasma membrane, resulting in the membrane to depolarize to threshold
   

2. During the Full Depolarization phase, voltage gated calcium channels will open at threshold, allowing calcium to enter the cell, then causing the membrane to fully depolarize
   

3. Repolarization phase begins, calcium channels close and potassium channels open
   

4. Potassium channels remain open and the membrane hyperpolarizes. This causes the HCN channels to open and the cycle to repeat.
   
   the last event of a cycle is then going to trigger the first event of the next cycle; there is never a pause in action potential, as our heart never “stops” beating

Sinoatrial (SA) Node

where the pacemaker cells are located; depolarizes approximately 60-70 times a minute

Atrioventricular (AV) Node

another group of pacemaker cells, located in the bottom left area of the right atrium; depolarizes around 40-50 times a minute

penetrates into the Interatrial Septum, branching down the septum. these branches then deviate left/right and branch further into the Purkinje Fibers; this allows the ventricles to contract

Purkinje Fiber

further branching of the bundle branching of the AV Bundle; forms around the ventricular walls and makes contact with the contractile cells.

Cardiac Conducting System

1. SA Node generates action potential, spreading to the atrial cells and the AV (Atrioventricular) Node
   

2. After delayed notice to the AV Node, the action potential is conducted to the AV Bundle and then to the left and right bundle branches
   

3. Action potential spreads from bundle branches along the Purkinje fibers to the contractile cells of the ventricles

Contractile Cells Action Potential

1. Rapid Depolarization Phase; voltage gated sodium channels activate and sodium enters, rapidly depolarizing the membrane
   

2. Initial Repolarization Phase; sodium channels close and some potassium channels open, causing a small initial repolarization
   

3. Plateau Phase; Calcium channels open and calcium enters the cell as potassium enters, prolonging the depolarization.
   

4. Repolarization Phase; sodium and calcium channels close as potassium continues to exit, causing repolarization

Baseline mV for Cardiac Muscles

~-85mV

Effective Refractory Period

refractory potential of cardiac contractile cells is notably longer than that of skeletal muscle fibers; this slows down the pace of our heart, allowing for effective contractions and pumping of the blood that is lost with speed

occurs during the Plateau Phase

Electrocardiogram(ECG/EKG)

P Wave: when the atria depolarizes(getting ready to contract)

QRS Complex: Ventricles depolarize(contraction)

T Wave: repolarization of the ventricle

R-R Interval: full duration of a cycle

P-R Interval: full atrial depolarization/contraction

Q-T Interval: ventricular depolarization/contraction

S-T Interval: plateau phase(contracting but stalled in contraction momentarily)

Pressure, Flow, and Valve Function While Contracted

Event: Left Ventricle Contracts

Mitral Valve Position: closed(no backflow to the atria)

Aortic Valve Position: open(blood flow to the body)

Area of High Pressure: Ventricle

Area of Low Pressure: Aorta

Blood Flow: Ventricle → Aorta → Body

Pressure, Flow, and Valve Function While Relaxed

Event: Left Ventricle Relaxes

Mitral Valve Position: open

Aortic Valve Position: closed

Area of High Pressure: Atria

Area of Low Pressure: Ventricle

Blood Flow: Atria → Ventricle → Lungs

Heart Sounds

Location of Sounds;

Aortic: second intercostal space, right sternal border

Lub S1

AV Valves closing

Dub

Semilunar Valves closing

Systole

contracting.emptying

Diastole

relaxing/filling

Cardiac Cycle

Phase: Isovolumetric Contraction:
Atria: Relaxed
Ventricles: Full/Contracting, High Pressure
AV Valves: Close
SL Valves: Close

Phase: Ventricular Ejection
Atria: Relaxed
Ventricles: Empty
AV Valves: Closed
SL Valves: Open
Additional Term;

Phase: Isovolumetric Relaxation
Atria: Relaxed
Ventricles: Relaxed(but not filling
AV Valves: closed
SL Valves: closed

Ventricular Filling
Atria: emptying
Ventricles: Filling
AV Valves: Open
SL Valves: Close
Additional Terms: End Diastolic

Additional Terms: End Systolic Volume(blood left in ventricle after contraction

1. Ventricular Filling; atria are emptying and ventricles filling(AV Valves open, Semilunar Valves closed)

2. Isovolumetric Contraction Phase: Ventricular Systole begins

End Diastolic Volume

maximum volume of blood that fills the ventricle