# Study Notes: The Heart The heart is a vital muscular organ responsible for pumping blood throughout the body. It is located in the **mediastinum**, the central compartment of the thorax, between the lungs. Below is a detailed breakdown of its structure, function, and associated systems. --- ## **1. Anatomy of the Heart** ### **1.1 Location and Orientation** - **Position**: The heart is situated in the mediastinum, covered by a tough membrane called the **pericardium**, which separates it from other thoracic structures. - **Shape**: - In 3D: Resembles an upside-down, tipped-over pyramid. - In 2D: Appears trapezoidal. - **Borders**: - **Superior border**: Formed by the right and left atria. - **Inferior border**: Mainly the right ventricle and part of the left ventricle. - **Right border**: Right atrium. - **Left border**: Left ventricle and part of the left auricle. - **Apex**: Tip of the left ventricle, directed anteriorly and slightly to the left. The apex of the heart is clinically significant as it is the point of maximal impulse (PMI) felt during a physical exam. --- ### **1.2 Chambers of the Heart** The heart is divided into **four chambers**: 1. **Right Atrium**: Receives deoxygenated blood from the body. 2. **Right Ventricle**: Pumps deoxygenated blood to the lungs. 3. **Left Atrium**: Receives oxygenated blood from the lungs. 4. **Left Ventricle**: Pumps oxygenated blood to the entire body. #### **Functional Division** - **Right Heart**: Right atrium and right ventricle (pulmonary circulation). - **Left Heart**: Left atrium and left ventricle (systemic circulation). --- ## **2. Blood Flow Through the Heart** ### **2.1 Pulmonary Circulation** 1. **Deoxygenated blood** enters the **right atrium** via: - **Superior vena cava** (from upper body). - **Inferior vena cava** (from lower body). 2. Blood flows through the **tricuspid valve** into the **right ventricle**. 3. The right ventricle pumps blood through the **pulmonary semilunar valve** into the **pulmonary trunk**, which branches into the **right and left pulmonary arteries**. 4. Blood is oxygenated in the lungs. ### **2.2 Systemic Circulation** 1. **Oxygenated blood** returns to the **left atrium** via the **four pulmonary veins**. 2. Blood flows through the **mitral valve** into the **left ventricle**. 3. The left ventricle pumps blood through the **aortic semilunar valve** into the **aorta**, which distributes it to the body. For example, during exercise, the left ventricle works harder to pump oxygen-rich blood to meet the increased demand of the muscles. --- ## **3. Valves of the Heart** ### **3.1 Atrioventricular (AV) Valves** - **Tricuspid Valve**: Between the right atrium and right ventricle; has three cusps. - **Mitral Valve**: Between the left atrium and left ventricle; has two cusps (bicuspid). #### **Mechanism** - Cusps are anchored to **papillary muscles** via **chordae tendineae**, preventing backflow during ventricular contraction (systole). Students often confuse the tricuspid and mitral valves. Remember: "Tri" is on the "Right" side. ### **3.2 Semilunar Valves** - **Pulmonary Valve**: Between the right ventricle and pulmonary trunk. - **Aortic Valve**: Between the left ventricle and aorta. #### **Mechanism** - These valves have **three semilunar cusps** that snap shut during ventricular relaxation (diastole) to prevent backflow. To remember the semilunar valves, think of them as "exit doors" for blood leaving the heart. --- ## **4. Conducting System of the Heart** The heart's conducting system ensures coordinated contractions for efficient blood flow. ### **4.1 Key Components** 1. **Sinoatrial (SA) Node**: The pacemaker of the heart, located at the junction of the superior vena cava and right atrium. 2. **Atrioventricular (AV) Node**: Located in the interatrial septum near the coronary sinus. 3. **AV Bundle (Bundle of His)**: Divides into right and left branches within the interventricular septum. 4. **Purkinje Fibers**: Spread impulses to the ventricular walls. ### **4.2 Sequence of Impulse Transmission** 1. SA node generates an impulse → Atria contract. 2. Impulse reaches AV node → Delayed briefly. 3. AV bundle transmits impulse to Purkinje fibers → Ventricles contract. The delay at the AV node allows the atria to empty completely before the ventricles contract. --- ## **5. Coronary Circulation** The heart is nourished by its own blood supply via the **coronary arteries**. ### **5.1 Coronary Arteries** - **Right Coronary Artery (RCA)**: Supplies the right atrium, right ventricle, and part of the left ventricle. - **Left Coronary Artery (LCA)**: Divides into: - **Left Anterior Descending (LAD)**: Supplies the anterior interventricular septum and left ventricle. - **Circumflex Artery**: Supplies the left atrium and lateral wall of the left ventricle. ### **5.2 Coronary Veins** - Blood is drained by the **coronary sinus**, which empties into the right atrium. Blockage of the LAD artery is often referred to as the "widowmaker" due to its critical role in supplying the heart. --- ## **6. Nervous Control of the Heart** ### **6.1 Sympathetic Innervation** - Increases heart rate and force of contraction. - Originates from thoracic spinal segments (T1-T5). ### **6.2 Parasympathetic Innervation** - Decreases heart rate. - Derived from the **vagus nerve**. Sympathetic = "Fight or flight"; Parasympathetic = "Rest and digest." --- ## **7. Key Takeaways** - The heart is a four-chambered organ responsible for systemic and pulmonary circulation. - Blood flow is unidirectional, regulated by valves. - The conducting system ensures synchronized contractions. - Coronary circulation nourishes the heart muscle. - Nervous control modulates heart rate and force based on physiological needs. --- ## **8. Practice Questions** 1. What are the main differences between the tricuspid and mitral valves? 2. Trace the path of blood through the heart, starting from the superior vena cava. 3. What is the role of the SA node in heart function? 4. Which coronary artery supplies the interventricular septum? Review these questions to solidify your understanding of the heart's anatomy and physiology.