Understanding the Cardiovascular System: Heart Anatomy, Function, and Disorders

Posted on Feb 6, 2025 in Biology

The Cardiovascular System: An Overview

The cardiovascular system is responsible for several critical functions:

• Transportation: The cardiovascular system transports nutrients, oxygen, and hormones to the cells and metabolic wastes like carbon dioxide away from the cells.

• Protection: Circulating throughout the blood are leukocytes (white blood cells) and other defenses against antigens.

• Regulation: Homeostasis is maintained in the body through regulation of pH, temperature, and water content of cells.

To determine the values for these calculations, data like blood pressure and heart rate must be measured with a sphygmomanometer. When this is being used, a doctor puts a cuff around the arm and pumps it up, exerting pressure and cutting off blood flow. As the pressure is being released, the blood starts to flow again, and doctors listen to these sounds to measure the systolic pressure and diastolic pressure. Heart rate can be measured by counting the number of beats felt in a minute and doesn’t require any machinery.

Key Cardiovascular Measurements

• Heart Rate (HR): Number of heartbeats per minute

• Stroke Volume (SV): Amount of blood pumped out of the heart in one beat: Stroke Volume = End Diastolic Volume – End Systolic Volume (SV = EDV – ESV)

• Pulse Pressure (PP): The difference between systolic pressure (SP) and diastolic pressure (DP) – Pulse Pressure = Systolic Pressure – Diastolic Pressure (PP = SP – DP)

• Cardiac Output (CO): Amount of blood pumped out of the heart in one minute – Cardiac Output = Heart Rate x Stroke Volume (CO = HR x SV)

• Mean Arterial Pressure (MAP): The average pressure in the arteries during one cardiac cycle – Mean Arterial Pressure = (2/3) Diastolic Pressure + (1/3) Systolic Pressure (MAP = (2/3) DP + (1/3) SP) OR Mean Arterial Pressure = Diastolic Pressure + (1/3) Pulse Pressure (MAP = DP + (1/3) PP)

The heart is the main organ of the cardiovascular system. It is a muscular organ about the size of one’s fist and weighs between 7-15 ounces. It acts as a pump and propels blood through the blood vessels. The average heart beats about 100,000 times a day, circulating about 2,000 gallons of blood through about 60,000 miles of blood vessels. An adult has, on average, 5 to 6 quarts of blood. The average adult resting heart rate is 72 bpm (beats per minute).

In general, the left side of the heart (which is located on the right side in a diagram) is the pump for the systemic circuit, which takes blood to and from the body. The right side of the heart (which is located on the left in a diagram), is the pump for the pulmonary circuit, which takes blood to and from the lungs.

The sound of the heartbeat can be described as “lubb-dupp”. The first sound, or the “lubb”, comes from the closing of the atrioventricular valves, which are the tricuspid and mitral (bicuspid) valves. The second sound, which is the “dupp”, comes from the closing of the other two valves (semilunar valves), which are the aortic and pulmonary valves. After the dupp should be a small pause before the lubb comes again.

Inside the heart are four chambers, two superior atria (left and right) and two inferior ventricles (left and right). The atria are areas under low pressure while the ventricles are areas under high pressure. The left side of the heart is separated from the right side of the heart by a wall of muscle known as the septum. As a general rule, ventricles are more muscular than atria because they have to pump blood to another location outside of the heart. The left ventricle is the most muscular chamber of the heart. For the right ventricle, this location is the lungs while for the left ventricle, this location is the rest of the body via the aorta. There are also four valves inside the heart: the tricuspid valve, the pulmonary valve, the mitral (bicuspid) valve, and the aortic valve. The purpose of the valves is to prevent backflow of blood back into the chamber that it came from.

Main Parts of the Heart in Order of Blood Flow

(also includes vessels leading in and out of the heart):

Superior Vena Cava/Inferior Vena Cava, right atrium, tricuspid valve, right ventricle, pulmonary valve, pulmonary artery, pulmonary capillary bed (lungs), pulmonary veins, left atrium, bicuspid (mitral valve), left ventricle, aortic valve, aorta, arteries, arterioles, capillaries, venules, veins, Superior vena cava/inferior vena cava.

• Superior Vena Cava: The superior vena cava is a tube located superior to the right atrium. It brings deoxygenated blood from the upper parts of the body to the heart.

• Inferior Vena Cava: The inferior vena cava is a tube located inferior to the right atrium. It brings deoxygenated blood from the lower parts of the body to the heart.

• Right Atrium: The right atrium takes in deoxygenated blood from the superior and inferior vena cavae. It then pumps the blood through the tricuspid valve and into the right ventricle. The Sinoatrial (SA) node is located on the roof of the right atrium.

• Tricuspid Valve: The tricuspid valve, located between the right atrium and the right ventricle, prevents blood flow backward from the ventricle to the atrium. It is called the tricuspid valve because it has three leaflets.

• Right Ventricle: The right ventricle takes deoxygenated blood that has traveled through the tricuspid valve from the right atrium. It then pumps that blood through the pulmonary valve and into the pulmonary trunk.

• Pulmonary Valve: The pulmonary valve is one of the four valves of the heart, located between the right ventricle and the pulmonary trunk. Like all valves, it prevents blood from flowing backwards. The pulmonary valve is a semilunar valve, which has three cusps.

• Pulmonary Trunk: The pulmonary trunk takes in deoxygenated blood from the right ventricle and takes it to the pulmonary arteries.

• Pulmonary Arteries: The pulmonary arteries are the last part of the pulmonary circuit of the heart. They take deoxygenated blood from the pulmonary trunk and the right ventricle and take it to the lungs so that gas exchange can occur. There are two pulmonary arteries, the left and the right, which take the deoxygenated blood to the left and right lungs, respectively.

• Pulmonary Capillary Beds: The pulmonary capillary beds are located in the alveoli of the lungs. They are responsible for exchanging the carbon dioxide in the venous blood for oxygen.

• Pulmonary Veins: After gas exchange occurs at the capillary beds of the lungs, oxygenated blood goes back to the heart via the left and right pulmonary veins. They then take the blood to the left atrium.

• Left Atrium: The left atrium receives oxygenated blood from the left and right pulmonary veins. It then pumps the blood through the mitral valve and into the left ventricle.

• Mitral Valve: The mitral valve is another of the four valves of the heart. It allows blood to pass from the left atrium to the left ventricle and prevents backflow back into the atrium. It is also known as the bicuspid valve because it has two leaflets.

• Left Ventricle: The left ventricle takes in oxygenated blood from the left atrium and pumps through the aortic valve and into the aorta and then the whole body. It is the most muscular chamber of the heart due to the fact that it has to pump blood to the rest of the body.

• Aortic Valve: The aortic valve is the last of the four valves of the heart. It allows blood to pass from the left ventricle and into the aorta. It is a semilunar valve and has three cusps.

• Aorta: The aorta receives oxygenated blood from the left ventricle to take to the whole body. It is the largest artery of the body, about two centimeters in width. Three smaller arteries branch off of the aortic arch, which are the brachiocephalic artery, the left common carotid artery, and the left subclavian artery, which all supply oxygenated blood to the head and arms.

Layers of the Heart Wall

• Endocardium: The endocardium is the most interior layer of the heart wall. It is simple squamous epithelium and lines the inside of the heart’s chambers. It is very smooth and is responsible for keeping the blood from sticking to the wall and potentially forming blood clots.

• Myocardium: The myocardium is the next most interior layer of the heart wall. It is the middle and muscular layer, which contains the cardiac muscle tissue. It is the thickest part of the wall and makes up most of the mass of the heart wall. The function of the myocardium is to contract and pump blood throughout the heart.

• Epicardium: The epicardium is the outermost layer. It is another name for the visceral layer of the pericardium, which is the sac that contains the heart. The epicardium is a layer of serious membrane that lubricates and protects the outside of the heart.

• Pericardium: The pericardium is the sac that the heart is inside of. Pericardium is the name for the walls and lining of the pericardial cavity. The pericardial cavity is a fluid-filled cavity with two walls on each side. The outer layer of the pericardium is the parietal layer (made of dense, fibrous connective tissue) while the inside layer is the visceral layer. The pericardium is a serous membrane that produces serous fluid to lubricate the heart. Another purpose of the pericardium is holding the heart in position and maintaining a hollow space (pericardial cavity) for the heart to expand into.

The heart contains a complex electrical system in which electricity moves to the different parts of the heart to allow for muscle contraction.

The Heart’s Electrical System

Parts of Electrical Conduction in the Heart

The image on the right shows the heart’s electrical system with numbers in the approximate order that electricity travels (electricity travels through some parts at the same time). The labels for the numbers are as followed:

1. Sinoatrial Node: Also known as the SA node, the sinoatrial node is located on the roof of the right atrium. It is the natural pacemaker of the heart.

2. Intra-atrial Pathway: The intra-atrial pathway carries electricity from the sinoatrial node on the roof of the right atrium to the left atrium. This pathway leads to the Bachmann’s bundle, which is a band of cardiac muscle in the left atrium which controls contraction of the left atrium.

3. Internodal Pathway: The internodal pathway carries electricity from the sinoatrial node on the roof of the right atrium to the atrioventricular (AV) node located between the atria and the ventricles.

4. Atrioventricular Node: Also known as the AV node, the atrioventricular node is the backup pacemaker. It is located in the interatrial septum. Its function is to slow conduction of electricity between the atria and the ventricles so blood can fill up in the ventricles and so that they do not contract at the same time.

5. Bundle of His: The bundle of His is the last part of atrial conduction. It is made up of myocardial cells and is located in the ventricular wall (interventricular septum) and allows the electricity to move from the AV node and into the bundle branches in the ventricles.

6. Right Bundle Branch: The right bundle branch comes off of the Bundle of His. It is one of the two branches that come off of it, with the other branch being the left bundle branch. The right bundle branch carries electricity to the right ventricle.

7. Purkinje Fibers: The Purkinje fibers are located in the subendocardium layer of the heart tissue, which is the innermost layer. They distribute electrical energy to the myocardium, which is the layer of muscular tissue.

8. Left Bundle Branch: The left bundle branch comes off of the Bundle of His and is the second of the two branches that come off of it. This bundle branch carries electricity to the left ventricle.

Abbreviated EKG or ECG, an electrocardiogram is a depiction of the electrical conduction of the heart. It is taken by a machine known as an electrocardiograph. When an electrocardiogram is being taken with a traditional 12 lead electrocardiograph, 10 electrodes are placed on the limbs and chest of the person. This allows for the electrical conduction of the heart to be measured from 12 different perspectives. The machine is usually run for about 10 seconds. An EKG can help cardiologists diagnose cardiovascular disorders by detecting irregular heartbeat patterns (arrhythmias) and finding any possible areas of heart muscle damage. It may also be useful in monitoring the effects of certain medications.

Understanding the EKG Waveforms

• P Wave: The P wave, shown by the first small bump in the electrocardiogram, marks the beginning of the heartbeat. When the electrical impulse is first sent from the sinoatrial node in the right atrium, it travels to the atrioventricular node and atrial muscle. The P wave is then generated by the activation of the muscle in the atria (atrial depolarization). Its duration should not be longer than .11 seconds.

• QRS Complex: The QRS complex is made up of the Q, R, and S waves, and is shown by a small peak below the baseline (Q wave), a large peak upward (R wave), and a peak slightly below the baseline again (S wave). After the electrical impulse reaches the AV node, it travels through the bundle of His, then the bundle branches, and then the Purkinje fibers to finally get to the ventricular muscle. When the septum is activated, the Q wave is generated. Next the ventricular free walls (both left and right) get activated, generating the R wave. Lastly, a few small areas which didn’t get the electrical impulse before finally get it, generating the S wave. All in all, the QRS complex shows ventricular depolarization, and shouldn’t last longer than .1 seconds.

• T Wave: The T wave is the last bump for one beat on an ECG. It should be larger than the bump that is the P wave. It indicates ventricular repolarization, or relaxation of the ventricles.

• ST Segment: The ST segment is the line between the S wave and the T wave. In a normal EKG, this segment should be mostly a straight line along the baseline. If the line goes up, or is elevated, more than 1mm, it indicates early ventricular repolarization, which is irregular and can lead to other cardiovascular problems.

• PR Interval: The PR interval is the time between the P wave and the R wave. It indicates the conduction time of the atrioventricular node, or the length of time it takes for the impulse to go through the AV node to the bundle of His. The normal duration time is .12 to .20 seconds.

• QT Interval: The QT Interval is the measure between the Q wave and the T wave. It measures the total time between ventricular depolarization and repolarization. The time for this measure is dependent on the heart rate, but the normal duration is .32-.4 seconds for a heart rate between 65-90bpm.

The cardiac cycle is the sequence of events that occur in a single heartbeat and consists of two main parts: systole (ventricular contraction) and diastole (ventricular relaxation) of the atria and the ventricles.

Common Cardiovascular System Disorders