Biology (0610) | Heart

Introduction: Meet Your Heart, The Ultimate Pump!

Welcome to the study of the heart! This small, fist-sized organ is the most vital pump in your body, relentlessly working every second of your life to deliver oxygen and nutrients to trillions of cells. Understanding the heart’s structure and function is essential for understanding how the entire **transport system in animals** works.

Don't worry if the names sound complicated—we will use analogies and simple language to break down this amazing biological machine!

9.2 The Mammalian Heart: Core Structure and Function

An Overview of the Heart’s Structures (Core & Supplement)

The mammalian heart has four chambers and a tough, specialized muscular wall. When you look at a diagram of the heart, remember that what is shown on the right side of the diagram is the heart’s anatomical left side (and vice versa).

Key Structures to Identify:

• Muscular Wall: The thick muscle layer responsible for contracting and pumping blood.
• Septum: The thick wall of muscle that completely divides the left side of the heart from the right side.
• Left Atrium & Right Atrium: The upper chambers of the heart. These are the receiving chambers, gathering blood returning from the body or lungs.
• Left Ventricle & Right Ventricle: The lower, more muscular chambers. These are the pumping chambers, pushing blood out of the heart and around the body.
• One-Way Valves (Atrioventricular and Semilunar): Flaps of tissue that ensure blood flows in the correct direction and prevents backflow.
• Coronary Arteries: Crucial blood vessels running across the surface of the heart, supplying the heart muscle itself with oxygen and nutrients.

Memory Aid: The Atria are where blood Arrives. The Ventricles are where blood Ventures out (is pumped away).

The Importance of the Septum

The human body uses a **double circulation system**, meaning blood passes through the heart twice for every complete circuit around the body.

The **septum** acts as a vital barrier.

• Function: To completely separate the oxygenated blood (on the left side) from the deoxygenated blood (on the right side).
• Why it matters (Supplement): This separation is essential for maintaining high efficiency. Oxygenated blood can be pumped around the body at high pressure without mixing with oxygen-poor blood. This is a major advantage of the mammalian double circulation system.

Understanding Heart Wall Thickness (Supplement)

If you were to feel the heart muscle, you would notice different thicknesses in different areas. This reflects the different jobs each chamber has:

• Atria walls vs. Ventricle walls:
  • Atria have relatively thin walls because they only need to push blood a short distance—downwards into the ventricles.
  • Ventricles have much thicker walls because they must contract with huge force to pump blood out of the heart (into the arteries).
• Right Ventricle vs. Left Ventricle:
  • The **Right Ventricle** pumps blood only to the lungs (pulmonary circuit). This requires relatively low pressure to avoid damaging the delicate capillaries in the lungs. Therefore, its wall is moderately thick.
  • The **Left Ventricle** has the thickest muscular wall of all. It must generate immense pressure to pump blood around the *entire* rest of the body (systemic circuit).

Analogy: Think of the heart as a two-storey house. The atria (upstairs) just drop the blood downstairs (easy task, thin walls). The ventricles (downstairs) have to pump the blood back up to the roof (the lungs) or across the whole garden (the body), requiring much more powerful engines (thick walls).

How the Heart Beats (The Cardiac Cycle) (Supplement)

The heart functions by coordinated contractions of the atrial and ventricular muscles (the cardiac cycle), driven by electrical impulses.

Step-by-Step Functioning (S10):

1. Atrial Contraction (Systole): The muscle walls of the atria contract, pushing blood through the atrioventricular (AV) valves into the relaxed ventricles.
2. Ventricular Contraction (Systole): The muscle walls of the ventricles contract forcefully. This high pressure forces the AV valves shut (preventing backflow into the atria) and pushes the blood through the semilunar valves into the main arteries (Aorta and Pulmonary Artery).
3. Relaxation (Diastole): Both atria and ventricles relax. The pressure drops, causing the **semilunar valves** to snap shut (producing the 'dup' sound of the heartbeat, preventing backflow from the arteries). The atria begin to fill again, restarting the cycle.

The **valves** (both AV and Semilunar) are crucial because they ensure the blood flows strictly in one direction only.

Monitoring Heart Activity and Coronary Heart Disease

Monitoring Heart Activity (Core)

We can easily monitor how well the heart is working using three main methods:

1. Pulse Rate: Counting the beats per minute by feeling the pulse in an artery (like the wrist or neck).
2. Electrocardiogram (ECG): A machine that measures the electrical activity of the heart muscle during the cardiac cycle.
3. Listening to Valve Sounds: Using a stethoscope to hear the characteristic 'lub-dup' sounds, caused by the closing of the valves. Abnormal sounds can indicate faulty valves.

The Effect of Physical Activity on Heart Rate (Core & Supplement)

You know your heart speeds up when you exercise—but why?

• Observation (Core): Physical activity causes the heart rate (pulse rate) to increase.
• Explanation (Supplement):
  • When you exercise, your muscle cells respire faster to get energy (ATP).
  • This aerobic respiration produces more **carbon dioxide**.
  • The increased concentration of CO₂ in the blood is detected by receptors (in the main arteries and brain).
  • The brain signals the heart to beat faster and harder, and breathing becomes deeper and quicker.
  • This action ensures rapid delivery of oxygen to the muscles and the rapid removal of carbon dioxide, keeping up with the body's increased metabolic demands.

Coronary Heart Disease (CHD) (Core)

The heart muscle itself needs a constant supply of oxygen and glucose, provided by the **coronary arteries**. If these arteries become blocked, it results in Coronary Heart Disease (CHD).

• Cause: Blockage of the coronary arteries, usually by a fatty material deposit (atherosclerosis). This restricts blood flow, starving the heart muscle of oxygen.
• Effect: If the blockage is severe, parts of the heart muscle die due to lack of oxygen, causing a heart attack.

Possible Risk Factors for CHD:

It is important to know the factors that increase a person's risk of developing CHD:

1. Diet: A diet high in saturated fats and cholesterol.
2. Lack of Exercise: Leading a sedentary lifestyle.
3. Smoking: Chemicals in smoke damage the artery walls.
4. Stress: Prolonged stress can increase blood pressure.
5. Genetic Predisposition: A family history of heart disease.
6. Age: Risk naturally increases as you get older.
7. Sex: Males typically have a higher risk than females before menopause.

Reducing the Risk of CHD (Core)

Fortunately, lifestyle choices can significantly lower the risk:

• Diet: Eating a balanced diet low in saturated fats and salt, and high in fibre, fruits, and vegetables.
• Exercise: Regular physical activity helps control weight, lowers blood pressure, and strengthens the heart muscle, making it more efficient.

Did you know? The heart beats about 100,000 times a day, pumping around 7,500 litres of blood!