Chapter 1: The Cardiovascular System
Welcome to your study notes for the Cardiovascular System, part of AS 2: Human Body Systems. The cardiovascular system is the body's ultimate delivery network. It continuously pumps blood to transport oxygen, glucose, and nutrients to every living cell while removing waste products such as carbon dioxide.
Don't worry if this topic feels detailed at first! We will break down the anatomy, electrical wiring, calculations, and clinical measurements into clear, manageable steps to help you master your CCEA AS examinations.
1. Heart Anatomy and Blood Vessels
Gross Anatomy of the Mammalian Heart
The human heart is a muscular double pump divided into four distinct chambers:
• Right Atrium: Receives deoxygenated blood from the body tissues via the superior vena cava (upper body) and inferior vena cava (lower body).
• Right Ventricle: Pumps deoxygenated blood to the lungs through the pulmonary artery.
• Left Atrium: Receives oxygenated blood returning from the lungs through the pulmonary veins.
• Left Ventricle: Pumps oxygenated blood under high pressure to the entire body via the aorta.
Why is the Left Ventricle wall much thicker than the Right Ventricle wall?
This is a classic exam question! The right ventricle only pumps blood a short distance to the delicate capillary beds of the lungs (pulmonary circulation), which operates at relatively low pressure. In contrast, the left ventricle must pump blood through the entire body (systemic circulation). The systemic circuit has a much higher peripheral resistance, requiring the left ventricle myocardium (muscle) to generate significantly higher hydrostatic pressures.
Coronary Circulation:
The heart muscle itself requires a continuous supply of oxygen and glucose to respire aerobically. Blood inside the chambers cannot diffuse fast enough through the thick muscle walls. Instead, the coronary arteries branch directly from the aorta to supply the heart muscle (myocardium), while coronary veins carry deoxygenated blood back into the right atrium.
Heart Valves and How They Function
Valves ensure that blood flows in one direction only, preventing backflow during pumping.
• Atrioventricular (AV) Valves: Located between the atria and ventricles.
- Tricuspid valve: Located on the right side (has three cusps/flaps).
- Bicuspid (Mitral) valve: Located on the left side (has two cusps/flaps).
Function: Prevent backflow of blood from the ventricles into the atria during ventricular contraction (systole).
• Semilunar (SL) Valves: Located at the base of the two major exit arteries.
- Pulmonary valve: At the base of the pulmonary artery.
- Aortic valve: At the base of the aorta.
Function: Prevent backflow of blood from the aorta and pulmonary artery into the ventricles during ventricular relaxation (diastole).
Examiner Warning: Valves do not actively contract or open on their own using muscles. They are purely passive flaps that open and close in response to differences in hydrostatic pressure across them!
Structure and Adaptations of Blood Vessels
The cardiovascular system uses three main types of blood vessels, each adapted for a specific physiological role:
1. Arteries:
• Function: Carry blood away from the heart at high hydrostatic pressure.
• Structural Adaptations: Thick wall composed of a prominent tunica media containing abundant smooth muscle and elastic tissue. Elastic fibres stretch under systolic pressure and recoil during diastole to maintain a continuous, smooth blood flow. They have a relatively narrow lumen to maintain high pressure.
2. Veins:
• Function: Return blood to the heart at low hydrostatic pressure.
• Structural Adaptations: Thin muscular wall and a large, wide lumen to minimise resistance to flow. They contain internal pocket valves to prevent the retrograde (backward) flow of blood. Blood flow through veins is aided by the contraction of surrounding skeletal muscles, which squeeze the veins and propel blood upwards towards the heart.
3. Capillaries:
• Function: Site of metabolic exchange (oxygen, carbon dioxide, glucose, and urea) between blood and body tissues.
• Structural Adaptations: Microscopic vessels with walls consisting of a single-cell thick endothelium. This creates an extremely short diffusion distance, allowing rapid and efficient exchange of substances.
Key Takeaway: Arteries withstand and smooth out high pressure (thick elastic/muscle wall, narrow lumen); veins return low-pressure blood (valves, wide lumen); capillaries facilitate rapid exchange (single-cell thick endothelium).
2. The Cardiac Cycle and Electrical Conduction
Stages of the Cardiac Cycle
The cardiac cycle is the sequence of mechanical events that takes place during a single heartbeat. It lasts approximately \(0.8\text{ s}\) at rest and consists of three distinct phases:
1. Atrial Systole (Atrial Contraction):
• The atria contract, raising the pressure inside them.
• The atrioventricular (AV) valves are forced open.
• Blood is forced from the atria into the relaxed ventricles.
2. Ventricular Systole (Ventricular Contraction):
• After filling, the ventricles contract from the apex (bottom) upwards.
• Pressure inside the ventricles rises sharply and exceeds atrial pressure.
• This pressure difference forces the AV valves to snap shut, preventing backflow into the atria. This closing produces the first heart sound, "lub".
• Ventricular pressure rises above the pressure in the aorta and pulmonary artery, forcing the semilunar (SL) valves open.
• Blood is ejected forcefully into the aorta and pulmonary artery.
3. Diastole (Complete Relaxation):
• Both the atria and ventricles relax.
• As the ventricles relax, ventricular pressure falls below the pressure in the aorta and pulmonary artery.
• Blood attempts to flow backward, causing the semilunar (SL) valves to snap shut. This produces the second heart sound, "dub".
• The atria begin to fill passively with blood returning via the vena cava and pulmonary veins.
Intrinsic Electrical Conduction System
Cardiac muscle is myogenic, meaning it can initiate its own electrical contractions without receiving nervous stimulation from the brain.
The Conduction Pathway Step-by-Step:
Step 1: Sinoatrial Node (SAN)
Located in the upper wall of the right atrium. The SAN is the primary natural pacemaker of the heart. It spontaneously generates a wave of electrical depolarisation (excitation) that spreads across the walls of both atria, stimulating atrial systole.
Step 2: Atrioventricular Node (AVN)
Located near the bottom of the right atrium near the septum. A layer of non-conducting collagen tissue prevents the impulse from passing directly into the ventricles. Instead, the electrical wave is collected by the AVN. The AVN introduces a crucial delay of approximately \(0.1\text{ s}\). This delay ensures that the atria have completely emptied and the ventricles are fully filled with blood before ventricular contraction starts.
Step 3: Bundle of His and Purkinje Fibres
The AVN releases the electrical impulse down specialised conducting fibres called the Bundle of His, which runs down the central septum. At the base (apex) of the heart, the impulse divides and travels upwards through the ventricular walls via the Purkinje fibres. This causes the ventricles to contract efficiently from the apex upwards, squeezing blood upward into the exit arteries.
Key Takeaway: Sequence of conduction = \(\text{SAN} \rightarrow \text{Atrial wall} \rightarrow \text{AVN (0.1 s delay)} \rightarrow \text{Bundle of His} \rightarrow \text{Purkinje fibres}\).
3. Physiological Calculations and Formulae
Cardiac Output Formula
Cardiac Output (\(CO\)) is the total volume of blood pumped by one ventricle of the heart in one minute.
It is calculated using the following equation:
\(CO = HR \times SV\)
Where:
• \(CO\) (Cardiac Output): measured in \(\text{cm}^3\,\text{min}^{-1}\) or \(\text{L}\,\text{min}^{-1}\)
• \(HR\) (Heart Rate): the number of beats per minute, measured in \(\text{bpm}\)
• \(SV\) (Stroke Volume): the volume of blood pumped out by a ventricle during each single beat, measured in \(\text{cm}^3\) or \(\text{mL}\)
Worked Example:
Question: A student has a resting heart rate (\(HR\)) of \(72\text{ bpm}\) and a resting stroke volume (\(SV\)) of \(70\text{ cm}^3\). Calculate their cardiac output in \(\text{L}\,\text{min}^{-1}\).
Step 1: Calculate \(CO\) in \(\text{cm}^3\,\text{min}^{-1}\):
\(CO = 72\text{ bpm} \times 70\text{ cm}^3 = 5040\text{ cm}^3\,\text{min}^{-1}\)
Step 2: Convert \(\text{cm}^3\) to \(\text{L}\) (divide by \(1000\)):
\(5040 \div 1000 = 5.04\text{ L}\,\text{min}^{-1}\)
Key Takeaway: Always check your units! If the question asks for \(\text{L}\,\text{min}^{-1}\), ensure you divide \(\text{cm}^3\) by \(1000\).
4. Clinical Measurements and Diagnostic Monitoring
Blood Pressure Measurement
Blood pressure is the hydrostatic force exerted by circulating blood against the walls of systemic arteries. It is measured using a sphygmomanometer (either digital or manual) and a stethoscope in units of millimetres of mercury (\(\text{mmHg}\)).
Recording Format: \(\text{Systolic Pressure} / \text{Diastolic Pressure}\)
• Systolic Pressure: The maximum pressure recorded in the artery during ventricular contraction (ventricular systole).
• Diastolic Pressure: The minimum baseline pressure recorded in the artery while the ventricles are relaxing (diastole).
• Standard baseline reference: approximately \(120/80\text{ mmHg}\).
Manual Measurement via Korotkoff Sounds:
1. An inflatable cuff is wrapped around the upper arm and inflated above systolic pressure to temporarily stop arterial blood flow.
2. As the cuff slowly deflates, blood begins to spurt turbulently through the partially opened artery. These tapping sounds heard through a stethoscope are known as Korotkoff sounds.
3. The pressure at which the first sound is heard corresponds to systolic pressure.
4. The pressure at which the sounds completely disappear (continuous laminar flow restored) corresponds to diastolic pressure.
Experimental Design in Blood Pressure Investigations:
When designing or analysing experiments investigating blood pressure and heart rate during exercise, remember these key scientific controls:
• Resting Baseline: Always record a baseline measurement at rest before physical activity begins.
• Standardisation: Standardise the type, intensity, and duration of exercise across all participants.
• Repeats & Averages: Take repeated readings to calculate reliable class means/averages.
• Category Separation: Group data clearly when comparing variables (e.g., separating results into male vs. female cohorts or trained vs. untrained groups).
Electrocardiogram (ECG) Traces
An ECG records the electrical activity of the heart over time using electrodes placed on the skin.
A standard healthy ECG cycle displays three main components:
• P wave: Represents atrial depolarisation (the electrical wave spreading across the atria, which triggers atrial systole).
• QRS complex: Represents ventricular depolarisation (the electrical wave spreading through the ventricles, triggering ventricular systole). It is large because the ventricle muscle mass is much greater than the atria. Note: Atrial repolarisation occurs at the same time but is masked by the large QRS complex.
• T wave: Represents ventricular repolarisation (the recovery and electrical resetting of the ventricular muscle cells during diastole).
Examiner Warning: Do not say the P wave is atrial contraction! The P wave is the electrical depolarisation that triggers the mechanical contraction.
Analysing Abnormal Heart Rhythms
• Tachycardia: An abnormally fast resting heart rate exceeding \(100\text{ bpm}\) (\(>100\text{ bpm}\)). Waves appear closer together.
• Bradycardia: An abnormally slow resting heart rate below \(60\text{ bpm}\) (\(<60\text{ bpm}\)). Waves are spaced far apart.
• Arrhythmia: An irregular heart rhythm where the spacing between QRS complexes varies randomly, indicating uncoordinated electrical conduction.
Key Takeaway: P wave = atrial depolarisation; QRS complex = ventricular depolarisation; T wave = ventricular repolarisation. Normal resting heart rate is between \(60\text{ bpm}\) and \(100\text{ bpm}\).
Chapter Summary Review
• Heart chambers: 4 chambers; the left ventricle wall is thicker due to high systemic resistance.
• Vessels: Arteries (high pressure, elastic/muscle walls), Veins (low pressure, wide lumen, valves), Capillaries (exchange, 1-cell thick).
• Cardiac Cycle: Atrial systole \(\rightarrow\) Ventricular systole \(\rightarrow\) Diastole. Valves operate passively via pressure changes.
• Conduction: \(\text{SAN} \rightarrow \text{Atria} \rightarrow \text{AVN (delay)} \rightarrow \text{Bundle of His} \rightarrow \text{Purkinje fibres}\).
• Calculation: \(CO = HR \times SV\).
• Diagnostics: Blood pressure is measured in \(\text{mmHg}\) (Systolic/Diastolic \(\approx 120/80\text{ mmHg}\)); ECG: P (atrial depolarisation), QRS (ventricular depolarisation), T (ventricular repolarisation).