The Human Heart

The human heart is located in the middle of the chest, in a hollow space between the lungs known as the mediastinum.
The heart is protected by the surrounding ribs and sternum.

The primary function of the heart is to serve as a pump that generates pressure to circulate blood throughout the body.

The heart is encased in the pericardium, which is composed of two inelastic membranes:
- One membrane is connected directly to the heart.
- The other membrane is not attached, allowing for movement.
Between these membranes, fluid is secreted, facilitating smooth movement over one another.

Coronary arteries are visible on the exterior of the heart.
Their role is to supply the heart muscle with oxygen and nutrients.
A blockage of these arteries can lead to the death of heart muscle tissue due to a lack of oxygen, resulting in a heart attack, also known as myocardial infarction or coronary thrombosis.

The left ventricle has exceptionally thick walls to pump blood throughout the entire body.
The right ventricle has thinner walls as it only pumps blood to the lungs for oxygenation.

The myocardium is a layer of heart wall composed of cardiac muscle tissue.
It is unique because it has the ability to contract rhythmically without fatigue, enabling continuous blood circulation.

The septum is the structure that divides the heart into the right and left sides.
A condition referred to as a 'hole in the heart' occurs when there is a defect in the septum, resulting in the mixing of oxygenated and deoxygenated blood.
- Surgical intervention can correct this condition.

The heart is lined internally by a smooth membrane known as the endocardium, which is composed of endothelial tissue.

The ventricles feature bumps called papillary muscles, from which tendinous cords extend to the atrioventricular valves.

The heart contains two main sets of valves to regulate blood flow, ensuring it moves in one direction:
- Atrioventricular Valves:
- Positioned between the atria and ventricles.
- Semi-lunar Valves:
- Located at the base of the blood vessels exiting the heart, specifically the pulmonary artery and aorta.

Blood returning to the heart fills the atria, applying pressure on the atrioventricular valves, causing them to open.
As the ventricles fill, the valves hang limply into the ventricles.
When the atria contract, additional blood is forced into the ventricles.
- (a) Atrioventricular valves opened; pressure in atria greater than in ventricles.
- Visual: Cusps of valves open, chordae tendineae attached to papillary muscles.

When the ventricles contract, blood is pushed against the cusps of the atrioventricular valves, causing them to close.
Simultaneously, papillary muscles contract, tightening the chordae tendineae, which prevents the valves from inverting into the atria.
- (b) Atrioventricular valves closed; pressure in ventricles greater than in atria.
- Visual: Cusps of the valves are closed with blood in the ventricle.

When the ventricles contract, the intraventricular pressure rises, and blood is pushed up against the semi-lunar valves, causing them to open.
- (a) Semi-lunar valves opened.
When the ventricles relax, the pressure falls and blood flows back from the arteries, filling the cusps of the semi-lunar valves, which forces them to close.
- (b) Semi-lunar valves closed.
The closure of the semi-lunar valves generates the second heart sound, with the aortic sound occurring slightly before the pulmonary sound.

Please complete learning activity 2 on page 82, which pertains to the topics covered in the lesson.
A video is provided for further illustration of the discussed concepts.