In most humans, the heart is located slightly to the left of the chest’s midline. The hollow muscle is divided into four parts, with the first division moving right to left. A thick muscle called the septum separates the sides of the heart. The muscles contain bundles of nerves that conduct impulses, and tissue that acts as a pacemaker regulating the contraction of the heart. Each half is also divided into top and bottom sections. To further confuse you, there are three layers of tissue that make up the heart. Each chamber has a thin lining called the endocardium, the muscle is called the myocardium, and the sac surrounding the heart is called the pericardium.

The Heart in Action

Blood enters the relaxed top chambers (atrium) of the heart from the superior and inferior vena cava veins for the right side, and from the pulmonary vein for the left side. This is called the diastole phase. When the chamber fills, the sinoatrial (SA) node located at the top of the right atrium sends an impulse for the upper parts of the heart to start to contract. The lower part of the heart—the ventricle—is relaxed. On the right side, blood is forced through the tricuspid valve into the right ventricle and on the left side through the mitral valve into the left ventricle. At this point, the atrioventricular (AV) node, located at the bottom of the right atrium, sends its signal down the bundle of His—a slender bundle of modified cardiac muscle that passes from the AV node in the right atrium to the right and left ventricles by way of the septum and that maintains the normal sequence of the heartbeat. The signal continues into the Purkinje fibers, which causes the ventricles to contract. This is called the systole phase. The tricuspid and mitral valves close, preventing backflow into the atrium; the pulmonary and aortic valves open, which allows the blood from the right ventricle to move to the lungs and from the left ventricle to the rest of the body. At the end of the contraction, the ventricular valves close and diastole starts again. This is called a normal sinus rhythm. Any contraction that does not follow this sequence is called an arrhythmia. We will cover more information on the electrical activity of the heart in a future “ICC Prep” article in 24×7.

An ectopic beat occurs when the SA node is not functioning or some other part of the heart tissue assumes the role of the pacemaker. A multifocal beat is when both the SA and AV nodes are not properly working and contractions originate elsewhere in the heart.

Structural faults in the heart can cause oxygenated and non- oxygenated blood to mix. These defects are generally holes in the septum that did not close after birth. Many babies will have this type of defect, and it self-corrects after a few days. Sometimes, these holes can be patched using special catheters without the patient going through an open-heart procedure. It is not unusual for patients to live most of their lives with the defect and not know it.

Review Questions

1) The two upper chambers of the heart are called?

  1. Alveolus
  2. Epiglottis
  3. Aorta
  4. Atrium

2) Backflow between lower and upper chambers is sometimes called?

  1. Regurgitation
  2. Prolapse
  3. Stenosis
  4. Angina

3) Which vein in the body carries oxygenated (arterial) blood?

  1. The vena cava
  2. The pulmonary vein
  3. The Saphenous vein
  4. Cephalic vein

4) The right and left ventricles are separated by the

  1. Septum
  2. Sternum
  3. Carotid
  4. Pericardium

See the answers

Basic Hemodynamics in the Heart

The blood pressure in the right atrium is the lowest of all the chambers and is generally the same as the central venous pressure (CVP) of the patient. Two to 10 mmHg is the common range—some sources say 0 to 8—and this is a mean pressure. Mean pressure is 0.707 of the highest pressure—think of it like an RMS voltage. The right ventricle pressures during contraction (systolic) ranges between 15 and 30. At rest, or diastolic, it is the same as the right atrium. The left atrium pressure ranges between 5 and 12 mmHg but is almost never directly measured in a clinical setting. The left ventricle systolic pressures range between 90 and 140, while the end diastolic is 5 to 12, all in mmHg.

While not part of the heart, the pressure in the pulmonary artery is of clinical importance and is often monitored. Typical pressures in the pulmonary artery are systolic 15 to 30, diastolic 4 to 12, and mean 9 to 16, all in mmHg. Also measured is the pulmonary capillary wedge pressure, which is a mean pressure and ranges from 1 to 10 mmHg. When monitoring the pulmonary artery pressures it is possible to get a negative blood pressure if the catheter is not correctly placed, meaning it is too far down the artery. In this case, as the patient inspires he or she creates a vacuum at the catheter tip, which will show up as a negative pressure on some monitors. The key troubleshooting point on this is that the negative blood pressure only shows up at the same frequency that the patient is breathing.

Some monitors will not show a negative pressure, but will flatline at the zero pressure point. It is extremely rare that the computed reading (numeric display of systolic, diastolic, and mean pressures) will show a negative number, because it is averaged over 3 to 5 contractions. The clinical staff may complain about low numbers, but the key is to look at the waveform for negative going or cut-off at the zero line on the screen. If this is present, the catheter is too far into the pulmonary artery. Also, remember that the pulmonary artery contains venous blood while the pulmonary vein has oxygenated or arterial blood.

Problems with valves in the heart are generally found by listening with a stethoscope or by using ultrasound scans. If the valves do not close properly, blood will regurgitate between chambers. The cardiac valves can thicken from various illnesses, and some drugs, both legal and otherwise, can affect them. The common term for thickening or narrowing of valves is “stenosis.” Sometimes the term “prolapse” is used for valves not completely closing properly.

The ideal pressure is 120 mmHg systolic and 60 diastolic, although it used to be 120 over 80. Systolic pressures above 140 or diastolic pressures above 90 are of clinical concern. The difference between the systolic and diastolic pressures is called the pulse pressure. This generally runs between 40 and 60 mmHg. The mean pressure can be computed by multiplying the systolic pressure by 0.707. An estimated mean pressure can be obtained by adding one third of the pulse pressure to the diastolic pressure.

Pressure ranges are as follows:

  • Arterial: 30 to 250;
  • Venous: 2 to 50;
  • Central venous: 1 to 20 (CVP);
  • Pulmonary artery: 4 to 30 (PAP); and
  • Wedge: 2 to 15.

David Harrington, PhD, is director of staff development and training at Technology in Medicine (TiM), Holliston, Mass, and is a member of 24×7‘s editorial advisory board. For more information, contact .