9.2 The Heart Revision Notes

1. Overview

The heart is a muscular pump that provides the pressure required to circulate blood through the blood vessels of the body. It ensures that oxygen and nutrients are delivered to respiring cells while carbon dioxide and waste products are removed, making it essential for maintaining life in multicellular organisms.

Key Definitions

Heart: A hollow, muscular organ that pumps blood throughout the circulatory system.
Atria: The two upper chambers of the heart that receive blood from the veins.
Ventricles: The two lower chambers of the heart that pump blood out into the arteries.
Septum: A thick wall of muscle that separates the left and right sides of the heart.
Coronary Arteries: Small arteries on the surface of the heart that supply the heart muscle itself with oxygenated blood.
Pulse Rate: The number of times the heart beats per minute, typically measured at the wrist or neck.

Core Content

Heart Structure

The mammalian heart is a "double pump" consisting of four chambers.

Muscular Wall: Made of cardiac muscle which can contract without tiring.
Septum: Prevents the mixing of oxygenated (left side) and deoxygenated (right side) blood.
Chambers: The right and left atria (top) and right and left ventricles (bottom).
Valves: One-way valves ensure blood flows in the correct direction and prevents backflow.
Coronary Arteries: These supply the heart muscle with glucose and oxygen for aerobic respiration.

📊A cross-section of the heart. The right side (left on page) shows the Vena Cava entering the Right Atrium, flowing through a valve to the Right Ventricle, then out the Pulmonary Artery. The left side (right on page) shows the Pulmonary Vein entering the Left Atrium, through a valve to the thick-walled Left Ventricle, then out the Aorta. The septum is the middle wall.

Blood Flow Direction

Arteries: Carry blood Away from the heart (A for Away).
Veins: Carry blood back into the heart.

Monitoring Heart Activity

The activity of the heart can be monitored using:

ECG (Electrocardiogram): Records the electrical activity of the heart.
Pulse Rate: Counting the "surge" of blood in an artery per minute.
Listening to Valve Sounds: Using a stethoscope to hear the "lub-dub" sound caused by valves closing.

Physical Activity and Heart Rate

During exercise, the heart rate increases to supply muscles with more oxygen and glucose for increased aerobic respiration.
Investigation: Measure resting pulse for 1 minute, perform 2 minutes of intense exercise, and then measure the pulse immediately after and every minute until it returns to resting rate.

Coronary Heart Disease (CHD)

CHD occurs when the coronary arteries become blocked by the buildup of fatty deposits (cholesterol/plaques). This reduces blood flow to the heart muscle, potentially leading to a heart attack.

Risk Factors:
- Diet: High levels of saturated fat/cholesterol.
- Lifestyle: Smoking, lack of exercise, high stress.
- Uncontrollable: Genetic predisposition, increasing age, and sex (males are statistically at higher risk).
Prevention: Eating a balanced diet (low in saturated fats) and regular exercise (strengthens the heart muscle and lowers blood pressure).

Extended Content (Extended Only)

Valves of the Heart

Atrioventricular (AV) Valves: Located between atria and ventricles. They prevent blood from flowing back into the atria when the ventricles contract. (Tricuspid on the right, Bicuspid on the left).
Semilunar Valves: Located at the base of the Aorta and Pulmonary Artery. They prevent blood from flowing back into the ventricles when the heart relaxes.

Relative Thickness of Muscle Walls

Left vs. Right Ventricle: The wall of the left ventricle is much thicker than the right. The right ventricle only pumps blood to the lungs (short distance, low pressure required). The left ventricle must pump blood to the entire body (long distance, high pressure required).
Atria vs. Ventricles: Atria have much thinner walls because they only pump blood a short distance into the ventricles. Ventricles must pump blood out of the heart.

Function of the Septum

The septum is vital because it separates oxygenated blood (left side) from deoxygenated blood (right side). This ensures that the blood sent to the body has the highest possible concentration of oxygen, making transport efficient.

Functioning of the Heart (The Cardiac Cycle)

Atrial Contraction: Atria contract, pushing blood through the AV valves into the ventricles.
Ventricular Contraction: Ventricles contract. The pressure closes the AV valves (the "lub" sound) and opens the semilunar valves, forcing blood into the arteries.
Relaxation (Diastole): The heart relaxes. The high pressure in the arteries closes the semilunar valves (the "dub" sound), and blood flows from the veins into the atria.

Explaining the Effect of Exercise

During exercise, cells respire faster and produce more Carbon Dioxide ($CO_2$).

Increased $CO_2$ lowers the pH of the blood (becomes more acidic).
Sensors in the brain (medulla) and arteries detect this change.
The brain sends nerve impulses to the heart to increase the heart rate.
This speeds up oxygen delivery and $CO_2$ removal.

Key Equations

Heart Rate (bpm) = $\frac{\text{Number of beats}}{\text{Time in minutes}}$
Stroke Volume = The volume of blood pumped per beat (measured in $cm^3$).
Cardiac Output = $\text{Heart Rate} \times \text{Stroke Volume}$ (measured in $cm^3/min$).

Common Mistakes to Avoid

❌ Wrong: Thinking the right side of the heart contains oxygenated blood.
✓ Right: Remember LORD (Left Oxygenated, Right Deoxygenated).
❌ Wrong: Labeling the heart's left and right sides as they appear to you on the paper.
✓ Right: Always label from the perspective of the person whose heart it is (the side on your right-hand side is the Heart's Left).
❌ Wrong: Saying arteries "carry oxygenated blood" and veins "carry deoxygenated blood."
✓ Right: Use the heart-centered definition. Arteries go Away, Veins go In. (Note: The Pulmonary Artery is an exception as it carries deoxygenated blood).

Exam Tips

Command Words: If a question says "State," give a short fact. If it says "Explain," you must use the word "because" to link structure to function (e.g., "The left ventricle is thicker because it must pump blood at high pressure to the whole body").
Sports Context: Questions often use athletes' heart rates. Remember that a "fitter" person usually has a lower resting heart rate and their heart rate returns to normal faster after exercise.
Typical Values: A resting heart rate is usually around 60–80 bpm. During exercise, it can rise to 150–200 bpm.
Diagrams: Practice drawing the heart and labeling the four main blood vessels: Vena Cava, Pulmonary Artery, Pulmonary Vein, and Aorta.