From July 23rd to August 3rd, I was at Stanford from the Cardiothoracic Surgical Skills Summer Internship. It was a fascinating experience, combining surgical lab work with porcine hearts with informative anatomy lessons and lectures about cardiac surgery from various doctors. In regards to the anatomy lessons, we learned about the entire thoracic cavity, which includes both the heart and the lungs. The anatomy lessons were led by informative medical school students who taught me, as well as all of the other interns, a lot about medicine.

To understand anatomy, understanding the lexicon is paramount. There are three slices of the body—coronal, transverse, and sagittal. Coronal slices go vertically from the front to the back. Transverse slices go horizontally from top to bottom. Sagittal slices go vertically from left to right. There are ways of describing location is medicine as well. Superior means towards the head while inferior means towards the feet. Anterior (or ventral) means towards the front while posterior (or dorsal) means towards the back. Additionally, when referring to one structure, proximal, meaning near the body, and distal, meaning further away from the body, can be used. Lastly, what is closer to the patient’s midline is medial while what is farther from the midline is lateral.

The lungs of our body are protected by various different bones. The ribs have 12 sections: 9 are true ribs with their own costal cartilage attached to the sternum; 3 are false ribs, joining with rib 7’s cartilage; and 2 are floating, not connecting with the sternum. The sternum, or the breastbone, has three parts—the manubrium, body, and xiphoid process. Other important bones in the thoracic cavity include the clavicle (collarbone), scapula (shoulder bone), and the 12 thoracic vertebrae. Each of the vertebrae articulates with each other, giving strength and flexibility to the spinal column. While not being bone, the costal cartilage mentioned before also plays a role in protecting the lungs while also allowing them to expand.

There are also a few important muscles related to the thoracic cavity. The pectoralis major, pectoralis minor, serratus anterior, and scalene muscles are four examples, which are all involved in the movement of the shoulder joint. The intercostal wall is made up of the external intercostal, internal intercostal, and the innermost intercostal muscles with the first being important for inspiration (expanding of thoracic cavity) and the other two being important for expiration (contraction of thoracic cavity). Between the internal and innermost intercostal muscles runs the neurovascular bundle of the intercostal vein, intercostal artery, and intercostal nerve in that order with the acronym VAN. They run in the costal groove, directly underneath the rib, while the collateral branches run right above the next inferior rib.

Underneath the innermost intercostal muscles and the endothoracic fascia lies the pleura. The pleura includes both the lining of the chest wall (parietal pleura) and the lining of the lungs (visceral pleura). The pleural cavity between them is filled with 10-20 CCs of fluid that serve as a lubricant, allowing the two layers to move freely with each other, and permits the lung to expand along with the thoracic cavity. If this space fills up with more fluid than needed, the patient has pleural effusion, and the fluid must be drained via a chest tube. If the fluid is bloody, the cause is likely trauma/cancer; if the fluid is cloudy, the cause is likely bacterial infection; if the fluid is milky, the cause is likely a lymphatic system issue; and if the fluid is clear, the cause is likely heart failure.

The pleura protects the lungs themselves, which are not symmetrically identical. The left lung has two lobes (superior left lobe and inferior left lobe) while the right lung has three lobes (superior right lobe, middle right lobe, and inferior right lobe), each of which is linked by fissures. There is also a cardiac notch in the left lung, allowing space for the heart. As the diaphragm, a muscle that separates the thorax from the abdomen, constricts, a person inhales, and when the diaphragm expands, the person exhales. Air enters the lungs via the trachea where rings of cartilage provide structure and support. The trachea branches into the left and right main bronchus, reducing in diameter but increasing in surface area, as it becomes the lobar (secondary) bronchus and segmental (tertiary) bronchus. Foreign bodies tend to enter the right lung because the bronchus is straighter and steeper there, causing aspiration, a large issue for toddlers who like to put foreign bodies in their mouths.

The pulmonary artery, becoming smaller capillaries, carries deoxygenated blood to the alveoli, small sacs that allow for gas exchange. The thin-walled capillaries meet with the alveolar walls where the protein hemoglobin works to move the oxygen from the alveoli into the blood. This now oxygenated blood is carried to the heart via the pulmonary veins before it is pumped to the rest of the body.

 

The mediastinum is the area between the two lungs and can be split up into the superior and inferior mediastinum, distinguished by being above and below the Angle of Louis (between the manubrium and the body of the sternum) respectively. The superior mediastinum includes the thymus, which is important for the development of T lymphocytes but begins atrophy in adults, along with important transport tubes like the trachea, esophagus, and the great vessels. The inferior mediastinum can be split into three sections, the anterior mediastinum, the middle mediastinum, and the posterior mediastinum. The anterior mediastinum has mostly just fat and the inferior portion of the thymus while the middle mediastinum has the heart and pericardium. The posterior mediastinum has the transport tubes like the esophagus and thoracic aorta along with the azygos system of veins and lymphatic ducts. It also includes the thoracic sympathetic trunk of nerves that runs along the vertebrae, and the vagus nerve, which controls many of the gut movements. The second anatomy lecture concerned itself primarily with the middle mediastinum.

Blood enters the heart after going through the entire body via the superior and inferior vena cava. It then enters the right atrium, passes through the tricuspid valve, and goes into the right ventricle. From there it is pumped through the pulmonary (or semilunar) valve into the pulmonary arteries, which takes this deoxygenated blood to the lungs to get oxygenated. This now oxygenated blood comes into the pulmonary veins and into the left atrium. Through the mitral valve, it goes into the left ventricle from where it is pumped through the aorta to the rest of the body via the aortic valve. Since the left ventricle pumps blood to the entire body, it is a higher pressure system than the right ventricle, resulting in a thicker septum, or wall.

It is worth going into greater detail than the simple flow presented above. To start, it is important to remember that blood provides oxygen, nutrients, and hormones while also carrying out waste removal. The constant oxygenation is essential for all of our organs to get the oxygen they need to function since, without it, tissue death will occur. As mentioned before, there are 4 valves in the heart, and the two atrioventricular valves (mitral and tricuspid valves) are attached to the ventricle by the chordae tendineae. The papillary muscles open and close the valves, making them essential to prevent backward blood flow, which could cause sitting blood and low cardiac output. During diastole, the ventricles relax and fill up with blood as the mitral and tricuspid valves open up. During systole, the ventricles contract and pump blood to the body as the aortic and pulmonary valves open up. It is also worth mentioning that the heart has a blood supply of its own through the right and left coronary arteries. The left coronary artery branches off into the circumflex artery and the left anterior descending artery. A heart attack occurs when plaque cuts off circulation through either any one of these coronary vessels.

The heart relies on an electrical system for its beating, making any disruption to this system potentially lethal. The electrical signal begins in the sinoatrial (SA) node, the natural pacemaker of the heart, which causes the atria to contract. The node passes through the atrioventricular (AV) node, the gatekeeper for the electrical pulses, slowing them down and/or limiting their number. The electrical signal then goes through the Bundle of His and the bundle branches, causing the ventricles to contract. To be more specific, the atria or ventricles contract when there is depolarization, which is going from negatively to positively charged. An EKG can measure this depolarization since the P wave refers to atrial contraction (or depolarization), QRS complex refers to ventricle contraction (or depolarization), and T wave refers to the ventricle relaxation (or repolarization). To get the EKG, various electrodes are placed on the patient to make 12 leads, each of which is a vector between two electrodes. The main leads are RA to LA (lead 1), RA to LL (lead 2), and LA to LL (lead 3) where RA is right arm, LA is left arm, and LL is left leg. In the picture below, the third EKG represents a normal sinus rhythm. The first EKG represents bradycardia (slow heart rate defined as below 60 beats per minute), which is opposite to tachycardia (fast heart rate defined as above 100 beats per minute). The second EKG is ventricular fibrillation since there is no clearly defined QRS complex while the fourth EKG is atrial fibrillation since there is no clearly defined P wave. These are all arrhythmias, which can predispose one to complications like cardiac arrest, a potentially lethal condition when the heart stops beating due to an electrical malfunction.

 

Going to the Cardiothoracic Summer Surgical Skills Internship at Stanford was an especially rewarding experience because I learned so much about cardiac surgery while surrounded by peers who were as excited to learn about the subject as I was. With amazing lectures and anatomy lessons combined with lab activities involving actual surgery on porcine hearts, this experience was one to never forget.

 

References

Aurakzai, Farooq Khan. “Intercostal Spaces & Its Content.” Prime, 18 Apr. 2018, www.prime.edu.pk/infoserver//Dr_Farooq_Khan/Thorax/Intercostal%20spaces%20-I.pdf. Accessed 7 Oct. 2018.

BC Campus. opentextbc.ca/anatomyandphysiology/chapter/22-2-the-lungs/. Accessed 7 Oct. 2018.

“Electrical System of the Heart.” My Health Alberta, myhealth.alberta.ca/Health/Pages/conditions.aspx?hwid=te7147abc. Accessed 7 Oct. 2018.

The External and Internal Intercostals of the Thoracic Wall. Teach Me Anatomy, teachmeanatomy.info/thorax/muscles/thoracic-cage/. Accessed 7 Oct. 2018.

Gross Anatomy of the Lungs. BC Campus, opentextbc.ca/anatomyandphysiology/chapter/22-2-the-lungs/. Accessed 7 Oct. 2018.

Heart Diagram. Wikipedia, en.wikipedia.org/wiki/File:Heart_diagram-en.svg. Accessed 7 Oct. 2018.

Ribs. Earth’s Lab, www.earthslab.com/anatomy/the-ribs-number-typical-atypical-ribs-ossification-and-clinical-value/. Accessed 7 Oct. 2018.