❤ The Engine of Life: Comprehensive Study Notes on the Mammalian Heart (9700 Biology)

Welcome! The heart is arguably the most vital organ in the body. It’s a remarkable, tireless pump that ensures every single cell receives the oxygen and nutrients it needs. Understanding its structure and the perfect, rhythmic timing of its beat is key to mastering the Transport in Mammals topic. Don't worry if the names of the structures seem confusing—we’ll break them down step-by-step!

Key Takeaway from the Introduction: The heart is the muscular pump responsible for maintaining the closed double circulatory system in mammals.

1. External and Internal Structure of the Mammalian Heart (LO 8.3.1)

The mammalian heart is essentially four distinct, muscular chambers designed to keep oxygenated and deoxygenated blood strictly separate.

A. The Four Chambers

• Atria (Singular: Atrium): The two upper, thinner-walled chambers. They are the receiving chambers, taking blood from the veins.

• Ventricles: The two lower, thicker-walled chambers. They are the pumping chambers, pushing blood out into the arteries.

B. The Major Blood Vessels

The main vessels entering and leaving the heart are critical:

1. Vena Cava (Superior and Inferior): Brings deoxygenated blood from the body tissues into the Right Atrium.

2. Pulmonary Artery: Carries deoxygenated blood from the Right Ventricle to the lungs.

3. Pulmonary Vein: Brings oxygenated blood from the lungs into the Left Atrium.

4. Aorta: Carries oxygenated blood from the Left Ventricle to the rest of the body.

Memory Aid: Remember the heart pumps blood *away* via Arteries (A for Away) and takes blood *in* via Veins.

C. The Valves

Valves are essential to ensure one-way flow of blood, preventing backflow (regurgitation).

• Atrioventricular (AV) Valves: Located between the atria and ventricles.

  - Tricuspid Valve: On the right side (has three flaps).

  - Bicuspid (Mitral) Valve: On the left side (has two flaps).

• Semilunar Valves: Located between the ventricles and the major arteries.

  - Pulmonary Valve: Between the Right Ventricle and the Pulmonary Artery.

  - Aortic Valve: Between the Left Ventricle and the Aorta.

Quick Review: The right side handles deoxygenated blood (pulmonary circuit), and the left side handles oxygenated blood (systemic circuit).

2. Relating Structure to Function: Differences in Wall Thickness (LO 8.3.2)

The thickness of the muscular walls (myocardium) reflects the amount of force (pressure) required to push the blood to its destination.

A. Atria vs. Ventricles

The walls of the atria are much thinner than those of the ventricles.

• Why? Atria only need to contract strongly enough to push blood a short distance—downwards into the adjacent ventricle. This requires very low pressure.

The walls of the ventricles are much thicker.

• Why? Ventricles must generate high pressure to pump blood out of the heart and around the body circuits.

B. Right Ventricle vs. Left Ventricle

The left ventricular wall is significantly thicker than the right ventricular wall (it is roughly 2 to 3 times thicker).

• Right Ventricle Function: Pumps blood to the lungs (pulmonary circuit). This circuit is short and requires low pressure to avoid damaging the delicate capillaries in the alveoli.

• Left Ventricle Function: Pumps blood to the rest of the body (systemic circuit). This circuit is long and requires a massive amount of force and high pressure to overcome resistance in the vast network of arteries and arterioles.

Analogy: Think of the Right Ventricle as needing to toss a ball across a small room, while the Left Ventricle needs to throw that ball across a football field!

3. The Cardiac Cycle: A Rhythmic Pumping Sequence (LO 8.3.3)

The cardiac cycle describes the sequence of events in one complete heartbeat (the process of filling and emptying the chambers).

The cycle alternates between systole (contraction/pumping phase) and diastole (relaxation/filling phase).

Step 1: Diastole (Relaxation and Filling)

• The heart muscle relaxes completely.

• Blood flows into both atria and passively into the ventricles (since all valves are initially closed, but then AV valves open due to pressure).

• Pressure Changes: Arterial pressure is high (maintaining flow). Atrial and ventricular pressures are low.

• Valve Action: The high pressure in the arteries causes the semilunar valves to snap shut. This closing makes the second heart sound, "Dupp".

Step 2: Atrial Systole (Atrial Contraction)

• The atria contract simultaneously.

• This pushes the remaining blood (about 20%) forcibly into the already filled ventricles.

• Pressure Changes: A slight, temporary increase in atrial pressure.

Step 3: Ventricular Systole (Ventricle Contraction)

This is the main pumping phase and happens in two parts:

Part A: Isovolumetric Contraction (Closing the AV Valves)
• The ventricles begin to contract.

• As pressure inside the ventricles rises sharply, it quickly exceeds the pressure in the atria.

• Valve Action: This forces the atrioventricular (AV) valves shut, preventing backflow into the atria. This closing creates the first heart sound, "Lubb".

Part B: Ejection (Opening the Semilunar Valves)
• Ventricular pressure continues to rise until it exceeds the pressure in the aorta and pulmonary artery.

• Valve Action: The high pressure forces the semilunar valves open, and blood is ejected rapidly into the arteries.

Common Mistake to Avoid: Ventricular systole does NOT immediately begin ejecting blood. The pressure must first build up enough to close the AV valves and then enough to open the semilunar valves.

Key Takeaway Summary: Pressure and Valves

When ventricular pressure > atrial pressure → AV valves close (Lubb)
When ventricular pressure > arterial pressure → Semilunar valves open
When arterial pressure > ventricular pressure (during diastole) → Semilunar valves close (Dupp)

4. Coordination of the Heartbeat (LO 8.3.4)

The mammalian heart is myogenic, meaning the heartbeat originates from within the cardiac muscle tissue itself, specifically from a group of specialised cells.

The Electrical Pathway (The Conduction System)

The cardiac cycle is coordinated by electrical impulses generated and spread by specific nodes and tissues. (Note: We focus only on the intrinsic electrical system, not the nervous/hormonal inputs.)

1. Sinoatrial Node (SAN)

• Location: Wall of the Right Atrium.

• Role: Acts as the heart's natural pacemaker. It generates electrical impulses that spread across the walls of both atria, causing atrial systole.

2. Atrioventricular Node (AVN)

• Location: Base of the Right Atrium, near the septum.

• Role: Receives the impulse from the SAN. The AVN introduces a brief delay (about 0.1 second). This delay is crucial because it allows time for the ventricles to fully fill with blood before they start to contract.

3. Purkyne Tissue (Bundle Branches)

• Role: The impulse leaves the AVN and travels rapidly down specialised fibres (known as the Bundle of His, leading into the Purkyne tissue) which run down the septum (the wall dividing the ventricles).

• The impulse then spreads quickly upwards through the muscular walls of the ventricles via the extensive network of Purkyne tissue.

The Result: The ventricles contract from the base upwards (from the apex towards the arteries), efficiently forcing the blood out into the aorta and pulmonary artery.

Did You Know?

If the SAN fails, the AVN can take over as a secondary pacemaker, but the heart rate will be much slower (a condition often requiring an artificial pacemaker).

Key Takeaway: The myogenic system (SAN, AVN, Purkyne tissue) ensures a synchronised and efficient contraction, starting with the atria and followed closely by the ventricles.