Revise The heart for Cambridge 9700 (8.3)

Four Chambers: The heart has two upper chambers, the atria (singular: atrium), which receive blood, and two lower chambers, the ventricles, which pump blood out.

Septum: A muscular wall called the septum separates the left and right sides of the heart, preventing the mixing of oxygenated and deoxygenated blood.

Valves: Four valves ensure unidirectional blood flow:

• Atrioventricular (AV) Valves: Located between atria and ventricles. The tricuspid valve is on the right, and the bicuspid (or mitral) valve is on the left.
• Semilunar (SL) Valves: Located at the exit of the ventricles. The pulmonary valve is at the start of the pulmonary artery, and the aortic valve is at the start of the aorta.

Major Blood Vessels:

• Vena Cava (Superior & Inferior): Brings deoxygenated blood from the body to the right atrium.
• Pulmonary Artery: Carries deoxygenated blood from the right ventricle to the lungs.
• Pulmonary Veins: Bring oxygenated blood from the lungs to the left atrium.
• Aorta: Carries oxygenated blood from the left ventricle to the rest of the body.

Coronary Arteries: These are the heart's own blood supply, branching off the aorta to deliver oxygen and nutrients to the cardiac muscle itself.

Pulmonary Circuit (Right Side): Deoxygenated blood enters the right atrium via the vena cava.

The right atrium contracts, pushing blood through the tricuspid valve into the right ventricle.

The right ventricle contracts, pumping blood through the pulmonary valve into the pulmonary artery, which leads to the lungs.

Systemic Circuit (Left Side): Oxygenated blood returns from the lungs via the pulmonary veins into the left atrium.

The left atrium contracts, pushing blood through the bicuspid (mitral) valve into the left ventricle.

The powerful left ventricle contracts, pumping blood through the aortic valve into the aorta, which distributes it to the entire body.

Atrial Systole: The atria contract, pushing the remaining blood into the ventricles. During this phase, the ventricles are relaxed (in diastole), the AV valves are open, and the semi-lunar valves remain closed. Pressure in the atria momentarily exceeds ventricular pressure.

Ventricular Systole: The ventricles contract. The rapid increase in ventricular pressure initially forces the AV valves closed (producing the 'lub' sound). As ventricular pressure surpasses the pressure in the aorta and pulmonary artery, the semi-lunar valves are forced open, ejecting blood into these major vessels. The atria are simultaneously relaxing and refilling.

Diastole (General Relaxation): Both atria and ventricles relax. As ventricular pressure drops below arterial pressure, the semi-lunar valves snap shut (producing the 'dub' sound), preventing backflow into the ventricles. Blood from the vena cava and pulmonary veins flows into the atria and passively into the ventricles, preparing for the next cycle.

Crucial Principle: All valve action is passive, driven entirely by pressure gradients. A valve opens when the pressure behind it is higher than the pressure in front of it, and it closes when the pressure in front exceeds the pressure behind.

Sinoatrial Node (SAN): Known as the pacemaker, the SAN is a small patch of specialised cells in the wall of the right atrium. It spontaneously generates waves of electrical excitation at regular intervals.

Atrial Contraction: The wave of excitation spreads from the SAN across the walls of both atria, causing them to contract simultaneously (atrial systole).

Non-conducting Tissue: A layer of non-conducting tissue prevents the impulse from passing directly from the atria to the ventricles. This ensures the ventricles only contract after receiving the signal via the correct pathway.

Atrioventricular Node (AVN): The impulse is channelled to the AVN, located in the septum between the atria. The AVN introduces a crucial delay of about 0.1 seconds.

Importance of Delay: This delay ensures that the atria have finished contracting and have completely emptied their blood into the ventricles before the ventricles begin their contraction.

Ventricular Contraction: From the AVN, the impulse travels down a specialised bundle of conducting fibres called the Bundle of His, which runs down the septum and branches into Purkyne fibres that spread throughout the ventricle walls. This causes the ventricles to contract forcefully from the apex (base) upwards, efficiently ejecting blood into the aorta and pulmonary artery.

Arteries (e.g., Aorta):

• Function: Transport blood at high pressure away from the heart.
• Structure:
  • Thick, muscular, and elastic wall: The tunica media is the thickest layer, rich in smooth muscle and elastic fibres. This allows the wall to withstand high pressure, stretch as blood surges through (systole), and recoil (diastole), which smooths blood flow and maintains pressure.
  • Narrow lumen (relative to wall thickness): Helps maintain high blood pressure.
  • No valves (except semilunar valves at the heart): High pressure prevents backflow.

Veins (e.g., Vena Cava):

• Function: Transport blood at low pressure towards the heart.
• Structure:
  • Thin wall: The tunica media is much thinner with less muscle and elastic tissue.
  • Wide lumen: Reduces resistance to flow for low-pressure blood.
  • Valves: Present in many veins (especially in limbs) to prevent backflow of blood, particularly against gravity.
  • Thickest layer: The tunica externa, composed of collagen fibres, is often the thickest layer, providing structural support.

Capillaries:

• Function: The site of exchange of substances between blood and tissues.
• Structure:
  • Extremely thin wall: Consists of only a single layer of endothelial cells (tunica intima), providing a very short diffusion path.
  • Very narrow lumen: Just wide enough for red blood cells to pass in single file. This slows blood flow, increasing time for exchange, and maximises the surface area-to-volume ratio.
  • No muscle or elastic tissue: Structure is purely for exchange, not pressure maintenance.