# The Mammalian Heart ## Heart Structure Overview - The human heart weighs approximately 300g and is roughly the size of a clenched fist - Location: within the chest cavity - Protected by the **pericardium**, a tough and fibrous sac that surrounds the heart

The heart is a hollow, muscular organ that pumps blood around the body in a double circulatory system

## The Four Chambers - The heart is divided into **four chambers**: two upper chambers and two lower chambers - **Atria** (singular: atrium) - the two upper chambers - **Ventricles** - the two lower chambers ### The Septum - A wall of muscular tissue called the **septum** separates the left and right sides of the heart - **Interatrial septum** - separates the left and right atria - **Interventricular septum** - separates the left and right ventricles

The septum prevents mixing of oxygenated and deoxygenated blood between the two sides of the heart

### Left vs Right Side - **Right side of the heart**: contains deoxygenated blood - Pumps blood to the lungs - **Left side of the heart**: contains oxygenated blood - Pumps blood to the rest of the body

On diagrams, the right side of the heart appears on the left of the page as if the heart were in your own body and flipped around

## Valves in the Heart - The heart contains four main valves that control blood flow direction - Valves ensure blood flows in one direction only, preventing backflow ### How Valves Work - Valves **open** when the pressure of blood behind them is greater than the pressure in front of them - Valves **close** when the pressure of blood in front of them is greater than the pressure behind them - This pressure-based mechanism maintains correct blood flow direction and pressure in the heart chambers ### The Four Main Valves | Valve name | Alternative name | Location | Function | |------------|-----------------|----------|----------| | Tricuspid valve | Atrioventricular valve (right) | Between right atrium and right ventricle | Controls blood flow from right atrium to right ventricle | | Pulmonary valve | Semilunar valve (right) | Between right ventricle and pulmonary artery | Controls blood flow from right ventricle to pulmonary artery | | Bicuspid valve | Mitral valve | Between left atrium and left ventricle | Controls blood flow from left atrium to left ventricle | | Aortic valve | Semilunar valve (left) | Between left ventricle and aorta | Controls blood flow from left ventricle to aorta |

Atrioventricular valves separate the atria from the ventricles; semilunar valves separate the ventricles from the arteries

### Tendons - Atrioventricular valves (tricuspid and bicuspid) are attached to the heart wall by **tendons** - These tendons prevent the valves from inverting when pressure builds up in the ventricles during contraction - Think of them like guy-ropes on a tent - they anchor the valve flaps in place ## Blood Vessels Connected to the Heart ### Vessels Bringing Blood TO the Heart - **Vena cava** - brings deoxygenated blood from the body to the right atrium - Superior vena cava (from upper body) - Inferior vena cava (from lower body) - **Pulmonary vein** - brings oxygenated blood from the lungs to the left atrium ### Vessels Taking Blood AWAY FROM the Heart - **Pulmonary artery** - takes deoxygenated blood from the right ventricle to the lungs - **Aorta** - takes oxygenated blood from the left ventricle to the body

The pulmonary artery is the only artery in the body that carries deoxygenated blood; the pulmonary vein is the only vein carrying oxygenated blood

## Coronary Arteries - The heart muscle itself requires oxygen and glucose for aerobic respiration - The heart cannot absorb these directly from the blood inside its chambers - Instead, the heart has its own dedicated blood supply via **coronary arteries** - These arteries branch over the surface of the heart - They deliver oxygenated blood to cardiac muscle cells ### Clinical Importance - It's critical that coronary arteries remain clear and unobstructed - Blockages caused by plaques (fatty deposits) can lead to serious conditions: - **Angina** - chest pain due to reduced blood flow to heart muscle - **Myocardial infarction** (heart attack) - death of heart muscle tissue when blood supply is completely blocked

Coronary artery disease occurs when plaques build up in coronary arteries, restricting blood flow to the heart muscle

### Why This Matters - Just like any other organ, heart muscle cells need: - Oxygen for aerobic respiration - Glucose as a respiratory substrate - Removal of carbon dioxide and other metabolic waste - Without adequate coronary blood flow, cardiac muscle cannot function properly - This can lead to reduced pumping efficiency or cardiac arrest ## Blood Flow Through the Heart - A Summary 1. **Deoxygenated blood** enters the right atrium via the vena cava 2. Blood flows through the **tricuspid valve** into the right ventricle 3. Right ventricle contracts, forcing blood through the **pulmonary valve** into the pulmonary artery 4. Blood travels to the **lungs** where gas exchange occurs 5. **Oxygenated blood** returns to the left atrium via the pulmonary vein 6. Blood flows through the **bicuspid valve** into the left ventricle 7. Left ventricle contracts, forcing blood through the **aortic valve** into the aorta 8. Blood is pumped around the **body** to deliver oxygen to tissues

The left ventricle has thicker muscular walls than the right ventricle because it needs to generate higher pressure to pump blood all around the body, whereas the right ventricle only pumps to the nearby lungs

## Key Structural Features Summary | Feature | Description | Function | |---------|-------------|----------| | Four chambers | Two atria (upper), two ventricles (lower) | Separate oxygenated and deoxygenated blood; atria receive blood, ventricles pump it out | | Septum | Muscular wall dividing left and right sides | Prevents mixing of oxygenated and deoxygenated blood | | Valves | One-way doors between chambers and vessels | Ensure unidirectional blood flow; prevent backflow | | Pericardium | Tough fibrous sac | Protects and anchors the heart in the chest cavity | | Coronary arteries | Blood vessels on heart surface | Supply heart muscle with oxygen and nutrients | | Muscular walls | Cardiac muscle tissue | Contract to pump blood; left ventricle wall thicker than right |