Heart location, coverings, chambers and great vessels

The mediastinum is the central part of the thoracic cavity, between the two pleural cavities. The heart, the large vessels, the trachea and the esophagus all run through it.

• Correct: The mediastinum:
• The abdominopelvic cavity:
• The dorsal body cavity:

The parietal layer lines a cavity wall and the visceral layer covers an organ. Between them is a potential space holding a thin film of serous fluid, which lets the layers slide with little friction.

• A thick pad of fat:
• Correct: A thin film of serous fluid:
• A layer of skeletal muscle:

Dense connective tissue is packed with collagen fibers, which resist pulling. Simple squamous epithelium is a single thin layer of flat cells, and cardiac muscle tissue contracts rather than resisting stretch.

• Simple squamous epithelium:
• Correct: Dense connective tissue:
• Cardiac muscle tissue:

The heart sits in the mediastinum, the central compartment of the thoracic cavity. It lies between the lungs, behind the sternum, in front of the esophagus and vertebral column, and on the diaphragm.

• The left pleural cavity: The left pleural cavity surrounds the left lung. The heart leans to the left, but it stays in the mediastinum, medial to that cavity.
• Correct: The mediastinum: Correct. The mediastinum lies between the two pleural cavities and holds the heart.
• The abdominopelvic cavity: The abdominopelvic cavity lies below the diaphragm. The heart rests on top of the diaphragm, so it is above this cavity.
• The dorsal body cavity: The dorsal body cavity holds the brain and spinal cord. The heart is in the ventral body cavity, in the thorax.

The epicardium lies on the outer surface of the myocardium, just inside the pericardial cavity. It is also called the visceral pericardium.

• Parietal pericardium: The parietal pericardium is on the outer side of the pericardial cavity, fused to the fibrous pericardium. The pinned layer is on the inner side.
• Correct: Epicardium: Correct. The layer on the heart muscle, facing the pericardial cavity, is the epicardium (visceral pericardium).
• Endocardium: The endocardium lines the inside of the heart, on the inner surface of the myocardium, far from the pericardial cavity.
• Myocardium: The myocardium is the thick muscle layer itself, not the thin covering on it.

Outside to inside: the tough fibrous pericardium, the parietal serous layer fused to it, the fluid film of the pericardial cavity, the epicardium on the heart surface, the thick myocardium, and finally the endocardium lining the inside.

• Correct order: 1. Fibrous pericardium 2. Parietal pericardium 3. Pericardial cavity 4. Epicardium (visceral pericardium) 5. Myocardium 6. Endocardium

The fibrous pericardium is dense collagen and cannot expand fast. Blood collecting in the pericardial cavity therefore raises the pressure in the sac, which squeezes the heart and keeps it from filling fully between beats. That is cardiac tamponade.

• The endocardium tears, so blood inside the heart leaks out through its wall: A tear in the endocardium alone would not put blood around the heart, and the signs here come from compression from outside, not from a leak inside.
• The epicardium thickens within minutes and stiffens the heart muscle below it: The epicardium does not thicken within minutes, and a stiffer wall is not what compresses the heart in this scenario.
• Correct: The fibrous pericardium cannot stretch quickly, so pressure in the sac rises steeply: Correct. The unyielding fibrous pericardium turns a small added volume into a large rise in pressure.
• The myocardium relaxes fully, letting the heart balloon outward into the sac: If the heart could balloon outward it would fill more, not less. The problem is that the heart is squeezed and cannot expand.

Collagen in the fibrous pericardium cannot stretch fast, so a small, rapid volume causes a steep pressure rise. Over weeks the tissue remodels and the sac enlarges, so a large, slow volume can fit with little rise in pressure. Rate matters more than volume.

• Patient B's myocardium is weaker, so it is less affected by pressure: A weaker muscle would make the heart more vulnerable to outside compression, not less, and nothing in the scenario says the muscle is weak.
• Serous fluid is thinner than blood, so it exerts no pressure: Any fluid in a closed sac exerts pressure. What matters is how much the sac's pressure rises, which depends on how fast it can enlarge.
• Patient B's parietal and visceral layers have fused, closing the cavity: If the layers had fused there would be no cavity for 900 mL of fluid to fill, so this cannot explain Patient B.
• Correct: Over weeks the fibrous pericardium enlarges, so pressure stays low: Correct. Slow accumulation gives the fibrous pericardium time to enlarge; rapid accumulation does not.

The pulmonary trunk is the short, wide vessel leaving the top of the right ventricle at the front of the base. It splits into the right and left pulmonary arteries, one to each lung.

• Superior vena cava: The superior vena cava comes down into the right atrium on the heart's right side (the left of the drawing). It does not leave the front of the ventricles.
• Arch of aorta: The arch of the aorta curves up, back and to the left above the heart, over the top of the pinned vessel.
• Left auricle: The left auricle is the ear-shaped pouch of the left atrium, lying just beside the pinned vessel. It is part of a chamber, not a vessel.
• Correct: Pulmonary trunk: Correct. This is the pulmonary trunk, leaving the right ventricle.

Blood low in oxygen returns from the body's tissues through the two venae cavae into the right atrium. The right ventricle sends it out through the pulmonary trunk and pulmonary arteries to the lungs. Blood that has picked up oxygen in the lungs returns through the pulmonary veins to the left atrium and leaves the left ventricle through the aorta.

• Correct: Superior vena cava: Yes. The superior vena cava returns blood from the head, neck, arms and upper chest after the tissues have taken oxygen from it.
• Pulmonary veins: No. The pulmonary veins carry blood that has just picked up oxygen in the lungs back to the left atrium.
• Correct: Pulmonary trunk: Yes. The pulmonary trunk carries blood from the right ventricle toward the lungs, before it picks up oxygen.
• Aorta: No. The aorta carries blood rich in oxygen from the left ventricle out to the body.
• Correct: Inferior vena cava: Yes. The inferior vena cava returns blood from below the diaphragm after the tissues have taken oxygen from it.
• Correct: Pulmonary arteries: Yes. The pulmonary arteries are the two branches of the pulmonary trunk, carrying blood low in oxygen to the lungs.

An artery carries blood away from the heart and a vein carries blood toward it. Oxygen content plays no part in the definition. The pulmonary veins carry blood toward the heart, so they are veins even though that blood has just picked up oxygen in the lungs.

• The pulmonary veins really carry blood low in oxygen, like other veins: The pulmonary veins carry blood that has just passed through the lungs, so it is rich in oxygen. The student has the oxygen right; the definition is what is wrong.
• They drain into the left atrium, which makes them part of the heart: Draining into a chamber does not make a vessel part of the heart. The venae cavae drain into the right atrium and are still veins.
• Correct: A vessel's name depends on its direction relative to the heart: Correct. Direction relative to the heart, not oxygen content, decides whether a vessel is an artery or a vein.
• Vessels inside the chest follow a separate naming system from the body: The same rule applies everywhere: the pulmonary trunk and aorta both carry blood away from the heart and are both arteries.