TEAS 7 · Science · 4 min read · Open lesson
The Heart and Blood Flow
- Identify the anatomy of the heart, including its chambers, valves, and layers.
- Trace the path of blood through the pulmonary and systemic circulation.
- Describe the electrical conduction system and the cardiac cycle.
See it move: blood flow through the heart
Viewed from the front, so the heart's right side is on your left. Tap a chamber or a valve, or walk through the steps.
Anatomy of the Heart
The heart is a four-chambered muscular organ located within the thoracic cavity. To protect this vital organ, it is enclosed in a protective sac called the pericardium.
The heart wall is composed of different layers. The innermost lining of the heart chambers is the endocardium. The middle layer, which is the muscular part of the heart, is known as the myocardium.
The heart is divided into two sides by a muscular wall called the septum. This separation ensures that the left and right sides of the heart handle different types of blood. The left side of the heart handles oxygenated blood, while the right side of the heart handles deoxygenated blood.
Think of the heart like a house with different rooms. The atria receive blood, while the ventricles pump it out of the heart: the right ventricle pumps blood to the lungs, and the left ventricle pumps blood to the rest of the body.
Chambers and Valves
The heart contains two receiving chambers called atria and two pumping chambers called ventricles. The right atrium receives deoxygenated blood from the body, while the left atrium receives oxygenated blood from the lungs.
Once blood enters the atria, it must pass through valves to enter the ventricles. The tricuspid valve is located between the right atrium and right ventricle. The mitral valve is located between the left atrium and left ventricle. These valves are essential because they prevent the backflow of blood.
The ventricles then pump the blood out of the heart. The right ventricle pumps deoxygenated blood to the lungs. The left ventricle pumps oxygenated blood to the rest of the body.
To exit the heart, blood passes through specific valves. The pulmonary valve is located at the exit of the right ventricle, and the aortic valve is located at the exit of the left ventricle.
| Chamber | Side of Heart | Blood Type | Function |
|---|---|---|---|
| Right Atrium | Right | Deoxygenated | Receives blood from body |
| Right Ventricle | Right | Deoxygenated | Pumps blood to lungs |
| Left Atrium | Left | Oxygenated | Receives blood from lungs |
| Left Ventricle | Left | Oxygenated | Pumps blood to body |
The Blood Vessels and Circulation
Blood travels through a vast network of vessels. Arteries carry blood away from the heart, while veins carry blood toward the heart. As arteries move away from the heart, they branch into smaller vessels called arterioles, which lead to the capillaries.
Capillaries are the site of gas and nutrient exchange. Their walls consist of a single layer of endothelial cells supported by a basement membrane to allow for easy exchange. After the exchange occurs, small vessels called venules collect the blood from the capillaries and lead it back toward the veins.
There are two main circuits of circulation. Pulmonary circulation carries blood between the heart and the lungs. Systemic circulation carries blood between the heart and the rest of the body.
Think of the circulatory system like a delivery service. The arteries are the highways leading away from the warehouse, the capillaries are the small driveways where the goods are actually delivered, and the veins are the return roads bringing the empty trucks back.
Electrical Conduction and the Cardiac Cycle
The heart generates its own electrical impulses, though the autonomic nervous system can adjust the heart rate and contraction strength. The process begins at the sinoatrial (SA) node, which acts as the heart's natural pacemaker and initiates the electrical impulse.
The electrical impulse travels from the atria to the ventricles. As it moves, it reaches the atrioventricular (AV) node, which delays the impulse. This delay is crucial because it allows for ventricular filling before the heart contracts.
From the AV node, the impulse travels through the bundle of His, which transmits impulses to the ventricles. Finally, the Purkinje fibers distribute these electrical impulses throughout the ventricular myocardium, ensuring a coordinated contraction.
The cardiac cycle consists of two phases: systole and diastole. Systole refers to the contraction phase, while diastole refers to the relaxation phase. The heart's pumping generates arterial blood pressure, which is also influenced by factors such as vascular resistance and arterial elasticity.
Blood Pressure and Vessel Anatomy
Blood pressure is the force exerted by blood against vessel walls. When we measure blood pressure, we look at two numbers. Systolic pressure is measured during ventricular contraction, and diastolic pressure is measured during ventricular relaxation.
The color of blood changes based on its oxygen content. Deoxygenated blood is dark red in color, whereas oxygenated blood is bright red in color.
The body uses specific large vessels to move large volumes of blood. The superior vena cava drains blood from the upper body, and the inferior vena cava drains blood from the lower body. The aorta is the largest artery in the human body.
Think of blood pressure like water pressure in a garden hose. A better analogy is a pump pushing water into a hose: ventricular contraction raises arterial pressure, while pressure falls during ventricular relaxation.
Common mistakes
- Confusing the roles of the atria (receiving) and ventricles (pumping).
- Thinking that deoxygenated blood is blue (it is actually dark red).
- Mistaking the pulmonary artery for an oxygenated vessel (it carries deoxygenated blood to the lungs).
- Confusing the SA node (the pacemaker) with the AV node (the delay mechanism).
Summary
- The heart wall has three layers: endocardium, myocardium, and epicardium; the pericardium is the sac surrounding the heart.
- The right side of the heart handles deoxygenated blood, while the left side handles oxygenated blood.
- Blood flows through valves (tricuspid, mitral, pulmonary, and aortic) to prevent backflow.
- The electrical impulse is initiated by the SA node and travels through the AV node, bundle of His, and Purkinje fibers.
- The cardiac cycle consists of systole (contraction) and diastole (relaxation).
- Circulation is divided into pulmonary (heart to lungs) and systemic (heart to body) circuits.
- Arteries carry blood away from the heart, while veins carry blood toward the heart.
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