Cardiac Physiology

The Cardiovascular System

• The heart is a dual pump with two atria and two ventricles, pumping blood from low-pressure veins to high-pressure arteries.
• The right ventricle pumps blood into the pulmonary artery, while the left ventricle pumps blood into the aorta.
• The output of the heart is controlled by intrinsic mechanisms and can be regulated by autonomic nerves and hormones.
• Blood pressure in the pulmonary circuit is approximately 28/8 mmHg, while in the systemic circuit it is approximately 120/80 mmHg.

Cardiac Cycle

• The cardiac cycle includes a complete relaxation and contraction of both atria and ventricles.
• The cycle consists of:
  • Diastole (relaxation phase)
  • Systole (contraction phase)
• Pressure changes occur within the heart during the cardiac cycle, directing blood flow through different chambers and the body.
• Valves within the heart regulate blood flow.

Electrical Activity of the Heart

• The heart has distinct electrical characteristics.
• Intercalated discs allow action potentials to pass to adjacent cells, allowing the heart muscle to function as a syncytium.
• Myocardial cells can spontaneously depolarize, generating a pacemaker potential.

Action Potential of the Heart

• Phases of the cardiac action potential:
  1. Rapid depolarization
  2. Early rapid repolarization
  3. Plateau phase
  4. Final rapid repolarization
• Ion channels regulate the movement of ions across the cell membrane, controlling the action potential.

Pacemaker Potentials

• Pacemaker potentials are generated by the sinoatrial (SA) node, which depolarizes over time, triggering an action potential.
• The SA node can be influenced by sympathetic and parasympathetic nerves to increase or decrease heart rate.

Excitation-Contraction Coupling

• Excitation-contraction coupling is the process by which an electrical action potential leads to contraction of cardiac muscle cells.
• Calcium is the crucial mediator that couples electrical excitation to physical contraction.

Electrocardiography (ECG)

• The ECG is an electrical trace resulting from action potentials in all heart muscle fibers.
• The ECG varies depending on the direction of travel, whether cells are depolarizing or repolarizing, and the size of the change in potential.

Events of the Cardiac Cycle

• The cardiac cycle can be divided into:
  • P wave (atrial depolarization)
  • QRS complex (ventricular depolarization)
  • PR interval (atrial muscle depolarization to ventricular depolarization)
  • PR segment (atrial muscle repolarization)

Cardiac Output

• Cardiac output is the product of stroke volume and heart rate.
• Regulation of stroke volume:
  • Preload (extent of stretch of the heart muscle)
  • Afterload (pressure against which the heart needs to pump)
  • Contractility (ability of the muscle to produce a force)
• The Frank-Starling mechanism: an increase in blood returning to the heart increases end-diastolic volume, leading to increased stretching and increased force of contraction.

Neural Control of the Heart Rate

• Neural control of the heart is regulated by sympathetic and parasympathetic divisions of the autonomic nervous system.
• The autonomic nervous system regulates heart rate, conduction velocity, force of contraction, and coronary blood flow.
• Sympathetic and parasympathetic stimulation have opposite effects on the sinoatrial (SA) node action potential, influencing heart rate.

Chronotropic Effects

• Positive chronotropic effects: increases in heart rate, mediated by the sympathetic nervous system.
• Negative chronotropic effects: decreases in heart rate, mediated by the parasympathetic nervous system.### Neural Factors Affecting the Heart
• The sympathetic nervous system (SNS) increases heart rate and contractility
• The parasympathetic nervous system (PNS) decreases heart rate but has little effect on contractility
• Resting heart rate is approximately 70bpm, which is reduced from 100bpm by 'vagal tone'

Effects of Autonomic Nervous System on Pacemaker Cells

• Parasympathetic nervous system:
  • Decreases rate of Na+ influx through funny channels
  • Decreases Ca2+ influx
  • Increases time taken for pacemaker potential to reach threshold for action potential
• Sympathetic nervous system:
  • Increases rate of Na+ and Ca2+ influx
  • Increases pacemaker potential development, leading to cardiac action potentials

Baroreceptors and Chemoreceptors

• Baroreceptors (carotid artery):
  • Stimulated by increased blood pressure
  • Decreases heart rate and stroke volume
  • Increases parasympathetic activity and decreases sympathetic activity
• Chemoreceptors (carotid body):
  • Stimulated by increased PCO2 and/or decreased PO2
  • Increases heart rate and stroke volume
  • Increases sympathetic activity and decreases parasympathetic activity

Systemic Hypertension

• Defined as the pressure of blood moving through the arterial system
• Blood pressure (BP) = cardiac output (CO) x systemic vascular resistance (SVR)
• Systolic BP is largely determined by characteristics of stroke volume (SV) being ejected by the heart and the ability of the aorta to stretch to accommodate SV

Elastic Tissue in Blood Vessels

• When blood is pushed into vessels, elastic tissue is stretched
• Elasticity 'absorbs' pressure, preventing a sharp rise in pressure
• Between heart beats, elastic tissue recoils, maintaining continuous blood flow

Regulation of Blood Pressure

• Regulated by autonomic and hormonal mechanisms
• Baroreceptor reflex:
  • Primary mechanism for blood pressure regulation in acute settings
  • Acts as a buffer for changes in posture and acute changes in blood volume
• Renin-angiotensin-aldosterone mechanism:
  • Maintains long-term blood pressure
  • Can reset or downregulate the baroreceptor reflex to a higher setpoint

Blood Pressure and Age

• Arterial pressure rises with advancing age
• Increased stiffness of large arteries due to arteriosclerotic lesions and calcification
• Decreased baroreceptor sensitivity
• Increased responsiveness to sympathetic nervous system activity
• Alteration in renin-angiotensin-aldosterone system relationships

Hypertension

• Age-related changes:
  • Increased stiffness of large arteries
  • Decreased baroreceptor sensitivity
  • Increased responsiveness to sympathetic nervous system activity
• Primary hypertension:
  • 95% of all cases
  • Has no identifiable cause
• Secondary hypertension:
  • 5-10% of all cases
  • Has an identifiable cause (e.g. kidney disease, adrenal cortical disorders)

Effects of Hypertension on the Body

• Stroke due to brain hemorrhage
• Damage to capillaries in the eye, leading to eyesight damage
• Edema
• Left ventricular hypertrophy, leading to heart failure
• Damage to kidney blood vessels, leading to renal failure
• Injury to artery walls, precipitating atherosclerosis

Risk Factors for Hypertension

• Essential/primary hypertension:
  • Excess salt consumption
  • Overweight/obesity
  • Alcohol consumption
  • Inactivity
  • Smoking
  • Age
  • Race
  • Genetic factors
• Secondary hypertension:
  • Renal disease
  • Excess aldosterone
  • Pheochromocytoma
  • Other disorders

Learn about the structure and function of the heart, including its dual pump system, blood pressure, and circulation pathways. Understand how the heart pumps blood from low pressure veins to high pressure arteries and how output is controlled.