The document summarizes cardiac physiology, including: 1. The cardiac cycle involves repetitive contraction (systole) and relaxation (diastole) of the heart chambers. Blood moves through the circulatory system based on pressure differences. 2. Key factors that influence cardiac output are preload, contractility, and afterload. Cardiac output is calculated as stroke volume multiplied by heart rate. 3. The autonomic nervous system and electrolytes like sodium, potassium, and calcium play important roles in regulating heart rate and contractility. The conduction system allows for coordinated contraction of the heart chambers.

2. Cardiac Physiology Review

3. Cardiac Cycle
• The heart is 2 pumps that work together
– right (pulmonary) and left (systemic) half
• Repetitive, sequential contraction
(systole) and relaxation (diastole) of
heart chambers

4. Cardiac Cycle
• Blood moves through circulatory system
from areas of higher  lower pressure
– Contraction of heart produces the pressure

5. Cardiac Physiology
The Cardiac Cycle
– Diastole
Relaxation/filling phase
Coronary arteries fill
– Systole
Ejection fraction
Normally, 2/3 of ventricular volume is ejected

6. Bledsoe/Porter/Cherry, Essentials of Paramedic Care, Second
Edition Update
Diastole

7. Bledsoe/Porter/Cherry, Essentials of Paramedic Care, Second
Edition Update
Systole

8. Cardiac Physiology
• Stroke volume
Amount of blood ejected in 1 cycle
Dependent on
Preload
Cardiac contractility
Afterload

9. Preload
• Affected by venous blood pressure
and the rate of venous return
• Related to the ventricular end-
diastolic volume (EDV)
– a higher EDV implies a higher preload
• Amount of stretch to RV/LV due to end
diastolic pressures

10. Contractility
• Intrinsic ability of the heart to
contract independent of preload
and afterload
• Contractility is synonymous with inotropy

11. Afterload
• Maximum tension of the myocardium mass
at end of systole
– Tension or stress developed in the wall of the left
ventricle during ejection (systole)
• Dilated LV has a higher afterload
• Conversely, a hypertrophied LV has a lower
afterload

12. Starling’s Law
The more the myocardial muscle is
stretched, the greater its force of contraction
will be
The more diastolic volume, the greater the
cardiac output.

14. Cardiac Output
Volume of blood that the heart pumps in 1
minute
Stroke volume (mL) x heart rate (bpm) =
cardiac output (mL/min)
SV x HR = CO

17. Endocrine Role
Gail Walraven, Basic Arrhythmias, Seventh Edition
©2011 by Pearson Education, Inc., Upper Saddle River, NJ

18. The Heart is an Endocrine Organ
• Hormones are secreted by the heart in
response to hemodynamic stress
• Effects
Diuresis (loss of water), natriuresis (loss of
sodium), and vasodilation
• These hormones are referred to as
natriuretic peptides:
Atrial natriuretic peptide (ANP)
Brain natriuretic peptide (BNP)

19. Natriuretic peptides
• Atrial natriuretic peptide (ANP)
Released by atrial muscle cells in response to
atrial distension and sympathetic stimulation
Counters renin-angiotensin-aldosterone system
• Brain natriuretic peptide (BNP)
Secreted principally by the ventricles of the heart
in response to excessive stretching of myocytes
Results in decreased central venous pressure
(CVP), cardiac output, and blood pressure
BNP levels are elevated in congestive heart failure

20. Nervous System Control
Gail Walraven, Basic Arrhythmias, Seventh Edition
©2011 by Pearson Education, Inc., Upper Saddle River, NJ

21. Nervous System Control
Electrolytes
Sympathetic
Parasympathetic
Autonomic Control of
the Heart
– Chronotropy
• Rate
– Inotropy
• Contractility
– Dromotropy
• Conductivity

22. Autonomic Nervous System

23. Cardiac Plexus
• Formed by the cardiac nerves derived from
the cervical ganglia of the sympathetic trunk
& the cardiac branches of the vagus &
laryngeal nerves

24. Function of the Heart & Control of Heartbeat
•Contracts spontaneously
• does not need nervous stimulation to contract
• Motor nerves that supply the human heart
modulate HR
• Sympathetic motor impulses ↑ HR
• T3-T4
•GO UP TO THE NECK, AND COME BACK
DOWN TO THE HEART
•Parasympathetic motor impulses ↓ HR
•VAGUS NERVE (X)

25. Electrolytes!

26. Sodium & Potassium
Sodium (Na+)
Plays a major role in depolarizing
Greater concentration outside cell
Must be actively pumped in
Sodium-Potassium Pump = active transport
**requires ENERGY**
Triggered by depolarization propagation
Potassium (K+)
Influences repolarization
Greater concentration inside cells

27. The Sodium Potassium Pump

30. • Depolarization
caused Ca++ to
enter cell and
enables transmitter
molecule to be
released
– Which excites
neighboring cells

31. Electrolytes
Chloride (Cl–)
Transmission of impulses (wave of
depolarization)

32. Magnesium (Mg2+)
Intracellular magnesium is correlated
with intracellular K+
essential for nucleic acids, enzyme
function (esp. synthesis of ATP)

33. Cardiac Muscle
3

34. Characteristics of Cardiac
Muscle
1. Cardiac muscle = intermediate
• between skeletal & smooth
muscle
• Excitatory and conductive
fibers
2. Cardiac muscle = uninucleate

35. Intercalated discs
When one cell becomes excited, the
action potential spreads rapidly across
the entire group of cells
Syncytium
Work together

36. Synctia
The heart has two syncytia:
– Atrial syncytium
Contracts from superior to inferior
– Ventricular syncytium
Contracts from inferior to superior
The only way an impulse can be
conducted from the atria to the ventricles
is through the atrioventricular (AV)
bundle.

38. Cardiac Muscle 1000X
intercalated disc
striations
short branching cells; intercalated discs at cell junctions
nucleus

39. Initiation of Electrical Flow
• Polarization
– “Ready” state
• Depolarization
– “Discharge” state
• Repolarization
– “Recovery” state

41. Depolarization of
Cardiocytes
• Resting Potential
Inside of the cell is more negatively charged than
the outside
• Action Potential
Influx of sodium changes the membrane polarity

44. Repolarization
• Cell membrane remains permeable to sodium for
only a fraction of a second
• Sodium actively pumped outside the cell
• Returns to polarized state

46. Properties of Conduction System
1. Excitability
2. Conductivity
• Dromotropy
3. Automaticity
4. Contractility
• Inotropy

47. Bledsoe et al., Paramedic Care: Principles & Practice, Volume 3: Medical Emergencies, 3rd Ed.
© 2009 by Pearson Education, Inc. Upper Saddle River, NJ

48. 3 A-V bundle
path shown
with blue
1
2 3 A-V bundle
path shown
with blue
arrows

49. Conduction System
Each component of the conductive system
has its own intrinsic rate of self-excitation
– SA node = 60–100 bpm
– AV node = 40–60 bpm
– Purkinje system (Ventricles)
= 20–40 bpm
• What happens if SA node stops
firing???

50. Pacemaker site with the
fastest rate will
generally control the
heart

51. Irritability
• A site along the conduction pathway
becomes irritable and speeds up
• Overrides higher pacemaking sites for
control of the heart

52. Escape Mechanism
• The normal pacemaker slows down or
fails
• Lower pacing site assumes
pacemaking responsibility
– This is how you get escape
beats, junctional and ventricular rhythms