Cardiac System and Disorders: Exam One Notes
Exam One Information
The upcoming exam, Exam One, will cover all material up to this point, including the cardiac system. The exam is scheduled for October
1 providing ample time for review, including dedicated review days, worksheet sessions, and group project work. A cardiac self-assessment worksheet is provided for practice; completing it and handing it in will grant full credit, though actually doing the worksheet is recommended for exam preparation. The due date will be announced later, with potential in-class work time.
The Cardiac System: Structures and Functions
Components of the Cardiac System
The cardiac system encompasses the following key components:
The heart
Blood vessels
The lymphatic system
Blood
Layers of the Heart
The heart is a muscular organ composed of three distinct layers:
Myocardium: This is the middle layer and constitutes the primary muscle portion of the organ. Its fundamental role is contraction and relaxation.
Endocardium: The innermost epithelial layer, where all cardiac valves are situated.
Pericardium: The outermost layer that encases and protects the heart, connecting it to surrounding organs like the lungs.
Four Properties of the Heart Muscle
For effective pumping, the heart muscle exhibits four essential properties:
Automaticity: The inherent ability of cardiac cells to spontaneously generate electrical impulses, leading to contraction without external stimuli. This means the heart beats on its own, independent of conscious thought.
Excitability: The capacity of cardiac cells to respond to electrical impulses.
Conductivity: The ability of cardiac cells to transmit electrical impulses throughout the heart.
Contractility: The muscle's ability to shorten and perform work, which in the heart's context means pumping blood.
Ion Role in Heart Function
Proper heart function relies on a semi-permeable cell wall and specific ion concentrations. Sodium, potassium, and calcium ions are crucial for conducting cardiac contraction:
Sodium and Potassium: The sodium-potassium pump initiates depolarization, leading to muscle contraction.
Calcium: Essential for maintaining muscle contractility and a critical balance, especially given the heart's constant beating.
Cardiac Impulse Conduction and EKG
Cardiac impulse conduction generates electrical currents detectable by electrodes, forming an Electrocardiogram (EKG):
Depolarization: An increase in electrical charge across the cell membrane due to ion exchange, which generates cardiac muscle contraction.
Repolarization: The recovery phase of the ventricles after contraction.
EKG Waves: Each wave represents specific electrical events:
P wave: Represents atrial contraction (depolarization of the atria). It is always smaller than the QRS complex.
QRS complex: Represents ventricular contraction (depolarization of the ventricles).
T wave: Represents repolarization of the ventricles (ventricular recovery).
U wave: A small wave that may follow the T wave, not consistently present.
Abnormalities: Any arrhythmias or dysrhythmias would be visible in the repolarization phase (T wave).
Blood Pressure
Blood pressure (BP) is the force exerted by blood on the walls of blood vessels. It is recorded as a fraction:
Systolic pressure (top number): The force exerted on arteries when the heart contracts (during ventricular ejection).
Diastolic pressure (bottom number): The pressure remaining on arteries when the ventricles relax (during filling).
Variability: BP can change based on factors like stress and activity (e.g., high during running or stress, lower during rest).
Blood Flow Through the Heart: Valves and Sounds
Blood flow within the heart is precisely controlled by valves and produces characteristic sounds:
Heart Valves
Atrioventricular (AV) Valves:
Tricuspid valve: Located between the right atrium and right ventricle.
Mitral valve (Bicuspid valve): Located between the left atrium and left ventricle.
Semilunar Valves:
Pulmonic valve: On the right side, between the right ventricle and pulmonary artery.
Aortic valve: On the left side, between the left ventricle and aorta.
Heart Sounds
Normal Sounds: S1 (lub) and S2 (dub) are normal heart sounds heard during auscultation, corresponding to valve closures.
Abnormal Sounds: S3, S4, or an S2 split are abnormal and can indicate issues such as murmurs, systolic dysfunction, myocardial infarction (MI), or shock.
Pulse
The pulse is generated by the contraction of the left ventricle pushing blood out into the body.
Normal Pulse Rate: 60 to 100 beats per minute.
Tachycardia: An increased pulse rate, above 100 bpm.
Bradycardia: A decreased pulse rate, below 60 bpm.
Pulmonary Artery Pressure (PAP)
PAP is a measure of pressure in the pulmonary artery, typically measured in the right ventricle, often via echocardiogram.
Normal Range: 17 to 25 mmHg (millimeters of mercury).
Coronary Vessels
Coronary vessels, specifically coronary arteries, are the primary blood supply to the heart muscle, providing oxygen and nutrients.
Function: The right coronary artery supplies the right side of the heart, and the left coronary artery supplies the left side.
Venous System: The cardiac venous system drains deoxygenated blood from the myocardium.
Filling: Coronary vessels fill during diastole and originate above the aortic valve, requiring pressure from the left ventricle to fill.
Drainage: They empty into the coronary sinus, located on the right side of the heart.
Collaterals: The vessels can form collateral circulation.
Diagnostics: Issues can be identified using diagnostic tools:
Cardiac Catheterization: A procedure where dye is injected into the heart (via wrist or groin) to visualize blood flow and identify blockages or abnormal pathways.
EKG: Can indicate abnormalities in P waves or QRS complexes, signaling underlying issues which catheterization can then pinpoint.
Blood Flow Through the Heart (Detailed Pathway)
Blood circulates through the body and heart in a specific sequence, always starting from the right side and ending with the left:
Capillary bed: Deoxygenated blood from systemic capillaries collect into venules.
Venules to Veins: Blood travels from venules to larger veins.
Vena Cava to Right Atrium: Superior and inferior vena cava deliver deoxygenated blood to the right atrium.
Tricuspid Valve to Right Ventricle: Blood passes through the tricuspid valve into the right ventricle.
Pulmonic Valve to Pulmonary Arteries: From the right ventricle, blood is pumped through the pulmonic valve into the pulmonary arteries.
Pulmonary Arterioles to Pulmonary Capillary Bed: Blood flows into smaller pulmonary arterioles and then into the pulmonary capillary bed surrounding the alveoli.
Gas Exchange: In the pulmonary capillary bed, gas exchange occurs: carbon dioxide is released, and oxygen is absorbed.
Pulmonary Venules to Pulmonary Veins: Oxygenated blood from the pulmonary capillary bed collects into pulmonary venules and then into pulmonary veins.
Left Atrium: Pulmonary veins deliver oxygenated blood to the left atrium.
Bicuspid Valve to Left Ventricle: Blood passes through the bicuspid (mitral) valve into the left ventricle.
Aortic Valve to Aorta: From the left ventricle, blood is pumped through the aortic valve into the aorta.
Arteries to Arterioles to Capillaries: The aorta branches into systemic arteries, then arterioles, which lead back to the systemic capillary beds, completing the cycle.
Factors Affecting Blood Flow
The flow of blood depends on several factors:
Pressure Difference: Blood flow is driven by the pressure difference between the ends of vessels; adequate pressure prevents vessel collapse.
Vessel Resistance: Vessels resist fluid flow. Resistance increases with smaller diameter vessels and longer vessels. Conversely, larger diameter and shorter vessels offer less resistance.
Heart's Pumping Action: The heart creates the necessary pressure differences to propel blood.
Vessel Constriction: When arterioles constrict, resistance increases.
Hematocrit Levels: Increased hematocrit in the blood also increases resistance.
Cardiac Output (CO) and Stroke Volume (SV)
These are critical measures of heart function:
Cardiac Output (CO): The total amount of blood the heart pumps in one minute. The average normal resting CO is 5 to 6 liters per minute. It is calculated as: {CO = HR imes SV}.
Stroke Volume (SV): The amount of blood ejected from the heart (specifically the left ventricle) with each contraction.
Example: If heart rate (HR) is 70 bpm and stroke volume (SV) is 70 mL/beat, then {CO = 70 imes 70 = 4900 ext{ mL/min} = 4.9 ext{ L/min}}. If HR is 80 bpm and SV is 80 mL/beat, then {CO = 80 imes 80 = 6400 ext{ mL/min} = 6.4 ext{ L/min}}.
Symptoms of Inadequate Cardiac Output
When cardiac output is insufficient (less than the normal 5-6 L/min), patients may exhibit:
Chest discomfort
Shortness of breath (SOB)
Dizziness on exertion (DOE)
Syncope (fainting or lightheadedness)
Change in level of consciousness (LOC)
Hypotension (low blood pressure)
Factors Affecting Cardiac Output
Several factors directly influence cardiac output:
Peripheral Vascular Resistance (PVR): The force opposing blood flow in the peripheral circulation. PVR increases as the diameter of blood vessels decreases.
Afterload: The pressure that the left ventricle must overcome to eject blood into the aorta. Higher afterload increases the workload on the heart and decreases stroke volume. PVR significantly affects afterload.
Preload: The amount of blood returning to the heart that the ventricles must manage and pump. It reflects the stretch of the ventricles before contraction.
Blood Pressure: Both preload and afterload can influence blood pressure; an increase in either generally increases BP, and a decrease decreases BP.
Ejection Fraction (EF)
Ejection fraction measures the percentage of blood pumped out of the left ventricle with each contraction.
Normal Range: 50-70 ext{%}.
Meaning: An EF of 60 ext{%} means 60 ext{%} of the total blood in the left ventricle is ejected during each beat.
Diagnostic Utility: EF measurement helps diagnose and track conditions affecting cardiac output, often evaluated with cardiac catheterization or echocardiograms.
Renin-Angiotensin Cycle (RAS)
This cycle, initiated in the renal system, plays a critical role in blood pressure regulation.
Initiation: The cycle is activated when renal blood flow decreases.
Steps:
Renin Release: Decreased renal blood flow triggers the kidneys to release renin.
Angiotensin I Activation: Renin activates angiotensin I.
Angiotensin II Conversion: Angiotensin I is converted to angiotensin II.
Vasodilation and Aldosterone: Angiotensin II normally acts as a vasodilator and stimulates the production of aldosterone.
Aldosterone Action: Aldosterone increases blood volume by enhancing sodium reabsorption in the kidneys.
Chronic Disease Deviation (e.g., Hypertension): In chronic conditions like hypertension, the RAS can be improperly activated. Instead of vasodilation, it causes vasoconstriction, leading to a significant increase in blood pressure, exacerbating the hypertensive state.
Vessel Wall Patency: Layers and Aging Effects
The walls of blood vessels consist of three layers, whose composition varies with vessel type.
Tunica Intima: The innermost, smooth, and thin layer of the blood vessel.
Tunica Media: The middle layer, composed of elastic and smooth muscle tissue, allowing vessels to change diameter and expand.
Tunica Adventitia: The outermost layer, made of elastic and fibrous connective tissue, designed to accommodate sudden rushes of blood with each cardiac contraction, supporting higher pressures.
Aging Effects: With aging, cells experience decreased elasticity in fibrous tissues. This reduction in tunica adventitia elasticity can lead to conditions such as varicose (or spider) veins, where blood flow is impaired and surgery may be required to facilitate proper circulation.
Cardiac Disorders
Pericarditis
Definition: Inflammation of the pericardium (the outermost protective layer of the heart).
Causes: Typically triggered by a viral infection or an autoimmune condition (where the body attacks itself).
Pathophysiology: Fluid moves from capillaries into the space between the pericardium and the heart. The inflamed tissue rubs against swollen cardiac tissue.
Symptoms/Signs: Auscultation may reveal a pericardial rub (also known as a pleural friction rub), indicative of the rubbing inflamed tissues.
Complication: Rapid fluid accumulation can lead to cardiac tamponade, where excessive fluid compresses the heart, preventing it from filling adequately. This manifests as jugular vein distension, edema, and heart failure.
Treatment: Targets the underlying cause, whether viral or autoimmune.
Myocarditis
Definition: Inflammation of the myocardium (the heart muscle).
Causes: Occurs when toxins enter the myocardium, leading to dysfunction. This condition is uncommon.
Prognosis: In most cases, it is benign and resolves spontaneously. However, it can sometimes progress to heart failure and dysrhythmias.
Symptoms: Patients often present with symptoms similar to a heart attack (e.g., chest tightness, shortness of breath, arm pain) due to arterial occlusion.
Management: Identify and treat the causative toxin. Patients are typically placed on bed rest and advised to limit activity to reduce cardiac workload.
Endocarditis
Definition: Infection of the endocardium (the innermost layer of the heart).
Prerequisites: Two conditions must be present for endocarditis to occur:
An abnormal endocardium (some type of deformity).
A microorganism in the bloodstream (most commonly Staphylococcus or Streptococcus species).
Demographics: Can occur at any age but is more prevalent in men.
Location: The infection is usually confined to the left side of the heart.
Progression: Can develop slowly or suddenly.
Severity: If left untreated, endocarditis is usually fatal.
Treatment: Requires long-term antibiotic therapy to eradicate the bacterial infection.