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Table of Contents
- Course Home
- Objectives
- Matching Exercise
- Introduction
- Anatomy: The Heart
- Anatomy: The Vessels
- Physiology: Introduction
- Physiology: Review of Circulation
- Physiology: The Cardiac Cycle
- Physiology: Innervation of the Heart
- Cardiac Electrophysiology: Introduction
- Cardiac Electrophysiology: Electrolytes
- Cardiac Electrophysiology: Cardiac Automaticity
- Cardiac Electrophysiology: The Cardiac Conduction System
- Cardiac Electrophysiology: Myocardial Contraction
- Sample Cases
- Case 1: Cardiac Tamponade
- Figure 1: Assessment
- Figure 2: Management
- Case 2: Cocaine Overdose
- Figure 3: Assessment
- Figure 4: Management
- Case 3: Hemorrhagic Shock
- Figure 5: Assessment
- Figure 6: Management
- Conclusion
- References
- Bibliography
Physiology: Introduction
The most important function of the cardiovascular system is to supply the tissues of the body with oxygen and nutrients. Any disruption of this process can result in tissue dysfunction. Therefore, it is important that you have a solid working knowledge of normal cardiovascular physiology, so that you can better understand how alterations in circulatory function may affect a patient.
The following are some basic definitions pertaining to cardiovascular physiology that you may hear being used around the ED, as well as by your patients.
Basic Definitions Pertaining to Cardiovascular Physiology:
-Ejection Fraction: Is the ratio of the amount of blood ejected from the left ventricle to the amount of blood contained in the left ventricle immediately prior to contraction.
-A healthy heart ejects around two thirds of the blood that is present in the ventricles at the end of filling (known as diastole).
-Stroke Volume: Is the volume of blood ejected by the heart with one contraction.
-There are three factors that affect the heart’s stroke volume – preload, contractility, and afterload.
-Preload: Refers to the amount of blood that fills the ventricles during diastole.
-The more blood that returns to the heart, the more that must be pumped out. The venous system volume can be contracted or expanded to return more or less blood to the heart in order to meet the demands of the body.
-Contractility: Refers to the contractile strength of the heart.
-Contractility is influenced by preload and nervous system stimulation. As the preload increases, the ventricular muscle becomes increasingly stretched, which leads to the generation of a stronger ventricular contraction. This phenomenon is known as Starling’s Law of the heart. However, this stretch-induced increase in contractility has its limits. If preload volume becomes too great, the contractility of the heart will decrease as muscle fibers are stretched to their limits. In addition to preload, nervous system stimulation can increase or decrease the contractile force of the heart.
-Afterload: Refers to the resistance against which the ventricles must pump.
-Afterload is essentially determined by the peripheral vascular resistance. The pressure generated by the contractile force of the heart must exceed the resistance of the peripheral vasculature in order for the heart to pump blood out into the periphery. An increase in the peripheral vascular resistance will decrease the stroke volume. Conversely, a decrease in the peripheral vascular resistance will increase the stroke volume.
-Cardiac Output: Is the amount of blood that can be pumped by the heart in one minute.
-The cardiac output is calculated by the stroke volume multiplied by the heart rate. This equation highlights that heart rate, preload, afterload, and contractility can affect the cardiac output.
-Blood Pressure: Is the tension the blood exerts on the walls of the arteries.
-Blood pressure is determined by cardiac output and peripheral vascular resistance. Peripheral vascular resistance increases when vessels constrict and decreases when vessels dilate.