Chapter 9 Cardiac Physiology
• Basics of cardiovascular system and heart
- function of CV system is transport
1. heart is the pump
a. establishes a pressure gradient (flow occurs from greater lesser pressure)
2. blood vessels are passageways
3. blood is the transport medium
- heart is a double pump
1. pulmonary circuit
a. heart lungs and back (at lungs, picks up O , r2leases CO ) 2
b. right side is pump
2. systemic circuit
a. heart body tissues and back (at tissues, picks up CO , re2eases O ) 2
b. left side is pump
- valves prevent backflow of blood
1. AV valves attached by chordae tendinae to papillary muscles so that they open in one
direction only
2. semilunar valves are shaped like cups, this structure prevents them from opening backward
- fibrous skeleton
1. provides attachment for valves and muscle
2. separates atria from ventricles (important so they contract at different times)
- heart wall is 3-layered
1. endocardium (epithelial lining)
2. myocardium (muscle)
3. epicardium (thin fibrous connective tissue layer)
- pericardial sac
1. fibrous covering continuous with epicardium
a. anchors heart
b. filled with fluid to prevent friction • Electrical activity of heart
- autorhythmic cells
1. specialized cells that initiate and conduct APs
2. display pacemaker activity (no resting potential, changes in potential due to voltage-gated
Na , K and Ca 2+channels)
a. decreased flow of K out and constant flow of Na in results in slow depolarization
b. near threshold, voltage-gated transient Ca 2+ channels open and bring membrane to
threshold (Ca 2+flows in)
c. at threshold voltage-gated long-lasting Ca 2+channels open, more Ca 2+ flows in (this
is the AP)
d. at peak of depolarization, voltage-gated K channels open and repolarization occurs
3. locations
a. SA node (sinoatrial)
(1) right atrium
(2) fastest rate of autorhythmicity, it's the pacemaker of the heart
(under usual conditions initiates APs)
(3) interatrial pathway extends to left atrium (spreads AP, atria contract)
(4) internodal pathway extends to next node
b. AV node (atrioventricular)
(1) electrical connection between atria and ventricles
(2) signal slightly delayed (.1 sec) to allow atria to finish contracting - more
efficient blood pumping
c. AV bundle (bundle of His)
(1) branch into ventricles
d. Purkinje fibers
(1) branch throughout ventricular myocardium (ventricles contract)
- contractile cells
1. APs spread from cell to cell
a. adjacent cells joined by intercalated discs
(1) desmosomes resist mechanical stress
(2) gap junctions allow spread of electrical signals
2. cardiac muscle APs
a. cell depolarized by autorhythmic activity
+ +
b. voltage-gated Na channels open, Na in, cell depolarized to +30 mV c. at the peak of depolarization, Na channels close, voltage-gated slow Ca 2+ channels
2+
open (Ca flows in - results in plateau, the channels are a “slow” version of the long-
2+
lasting Ca channels in autorhythmic cells)
2+ +
d. at the end of the plateau, Ca channels close, voltage-gated K channels open
(K out, repolarization)
3. excitation-contraction coupling
a. AP travels down T tubules, voltage-gated Ca 2+channels open,
Ca 2+ in from ECF
2+ 2+
b. Ca influx triggers further release of Ca from sarcoplasmic reticulum
2+
c. Ca binds with troponin-tropomyosin complex, cross bridge cycling occurs
2+
d. Ca actively pumped back to ECF and SR
2+
e. extent of cross bridge cycling depends on amount of Ca that enters cytosol (unlike in
2+
skeletal muscle, where enough Ca for maximum contraction is always released)
4. long contractions due to lots of Ca 2+from ECF and SR
5. long refractory period
a. due to inactivation of Na channels during plateau
b. tetanus does not occur (no summation)
c. keeps cardiac functioning efficient
• Cardiac cycle
- systole (contraction) and diastole (relaxation) occur in atria and ventricles - most often refers to
ventricles
1. diastole
a. AV valves open
b. blood fills ventricles (amount at end of diastole is called end-diastolic volume, EDV is
about 135 ml)
c. atria contract
2. systole
a. ventricles contract, closing AV valves
b. semilunar valves open
c. blood ejected (amount of blood still in ventricles is end-systolic volume, ESV is about
65 ml)
• Cardiac output (CO)
- CO is the volume of blo
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