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BIOC33H3 Lecture Notes - Magnetic Resonance Angiography, Myoglobin, Systolic Geometry

Biological Sciences
Course Code
Stephen Reid

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Chapter 32: Cardiovascular System
The heart is a four-chambered organ that lies in the mediastinal space in the thorax.
The heart is divided by the septum, forming the right and left atrium and the right and left
Valves separate the chambers of the heart:
o Mitral valve separates the left atrium and the left ventricle.
o Aortic valve separates the left ventricle and the aorta.
o Tricuspid valve separates the right atrium and the right ventricle.
o Pulmonic valve separates the right ventricle and the pulmonary artery.
The heart is:
o Composed of three layers: endocardium, myocardium, and epicardium.
o Surrounded by a fibroserous sac called the pericardium.
The right side of the heart receives blood from the body (via the vena cava) and pumps it to the
lungs where it is oxygenated. Blood returns to the left side of the heart (via the pulmonary
arteries) and is pumped to the body via the aorta.
The coronary circulation provides blood to the myocardium. The right and left coronary arteries
are the first branches of the aorta.
The conduction system consists of specialized cells that create and transport electrical impulses.
These electrical impulses initiate depolarization (contraction) of the myocardium and ultimately
a cardiac contraction.
Each electrical impulse starts at the SA node (located in the right atrium), travels to the AV node
(located at the atrioventricular junction), through the bundle of His, down the right and left
bundle branches (located in the ventricular septum), terminating in the Purkinje fibers.
The electrical activity of the heart is recorded on the electrocardiogram (ECG).
Systole, contraction of the myocardium, results in ejection of blood from the ventricles.
Relaxation of the myocardium, or diastole, allows for filling of the ventricles.

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Cardiac output (CO) is the amount of blood pumped by each ventricle in 1 minute. It is
calculated by multiplying the amount of blood ejected from the ventricle with each heartbeat,
the stroke volume (SV), by the heart rate (HR) per minute: CO = SV HR.
Factors affecting SV are preload, afterload, and contractility. Preload is the volume of blood in
the ventricles at the end of diastole, and afterload represents the peripheral resistance against
which the left ventricle must pump.
Cardiac reserve refers to the heart’s ability to alter the CO in response to an increase in demand
(e.g., exercise, hypovolemia).
Stimulation of the sympathetic nervous system increases HR, speed of conduction through the
AV node, and force of atrial and ventricular contractions, whereas stimulation of the
parasympathetic nervous system decreases HR.
Baroreceptors, located in the aortic arch and carotid sinus, respond to stretch or pressure within
the arterial system. Stimulation of these receptors results in temporary inhibition of the
sympathetic nervous system and an increase in parasympathetic influence.
Chemoreceptors, located in the aortic arch and carotid body, can initiate changes in HR and
arterial pressure in response to decreased arterial O2 pressure, increased arterial CO2 pressure,
and decreased plasma pH.
Arterial blood pressure (BP) measures the pressure exerted by blood against the walls of the
arterial system.
The systolic blood pressure (SBP) is the peak pressure exerted against the arteries when the
heart contracts. The diastolic blood pressure (DBP) is the residual pressure of the arterial
system during ventricular relaxation (or filling). Normal blood pressure is systolic BP less than
120 mm Hg and diastolic BP less than 80 mm Hg.
The two main factors influencing BP are cardiac output (CO) and systemic vascular resistance
(SVR), which is the force opposing the movement of blood.
BP can be measured by invasive (catheter inserted in an artery) and noninvasive techniques
(using a sphygmomanometer and a stethoscope).
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