Lead II is the limb lead that will show the best view of atrial depolarization. Remember, atrial depolarization starts at the sinoatrial node in the right atrium and spreads through the right and left atria. If you were to draw this on a heart, the net vector would be down (inferiorly) and to the left side of the heart. Also remember, lead II is the lead configuration that runs from the right arm to the left leg. aVR would show a good view of atrial depolarization (although it would be a negative deflection), but it is an augmented lead, not a limb lead. aVL is also an augmented lead.

Normal blood pH is about 7.4 in most tissues (it is a bit lower in veins since they carry waste products, which are acidic). To get back to the physiological set point of pH = 7.4, we want to remove the acid from the blood. The major blood buffer system is shown in the following equation:

As we know, carbon dioxide is one of the major byproducts of respiration, and is considered waste for our bodies. Combined with water and catalyzed by carbonic anhydrase, it is converted into carbonic acid. Carbonic acid is a weak acid and will partially dissociate into hydrogen ions and bicarbonate ions. Thus, overall, carbon dioxide and water yields acid (hydrogen ions). As a result, excess carbon dioxide in the blood will lower the pH.

In order to increase the pH, we must stop this equation from proceeding in the forward direction; thus, (remember Le Chatelier's principle) we must remove carbon dioxide from the left side. This will push the reaction in the reverse direction, quenching hydrogen ions (acid) and removing them from the blood, increasing blood pH back to normal.

Since we want to get rid of excess carbon dioxide, we breathe faster. Oxygen does not have any effect on blood pH. Furthermore, the atmospheric oxygen level (21%) is plenty for our bodies to utilize, as when we exhale there is about 15% oxygen left over, meaning we only use about 25% of the oxygen we breathe (this is why CPR works!).

Pericytes, sometimes referred to as Rouget cells, are cells that wrap around the endothelial cells of capillaries. These cells play an important role in angiogenesis, the process of formation of new blood vessels. Monocytes are macrophages of the immune system, Purkinje fibers are found in the heart, and allow conductance of electricity to the cardiomyocytes, Schwann cells are glial cells in the peripheral nervous system that myelinate axons.

Arteries are strong elastic vessels that carry blood moving away from the heart. As arteries move away from the heart they become smaller (think of a tree and its branches, the trunk is larger than the branches and as each branch grows out it becomes smaller) and are referred to as arterioles, which connect to capillaries. Capillaries penetrate nearly all tissue; their walls are very thin and allow exchange of materials (oxygen, nutrients) between blood and tissues. Veins are thinner-walled and less muscular. The smallest ones are called venules and connect to capillaries.

Veins always carry blood towards the heart. The blood in veins is mostly deoxygenated, however the pulmonary vein, which goes from the lungs to the left atrium, carries newly oxygenated blood back to the heart for it to be pumped to the rest of the body.

In contrast, arteries always travel away from the heart and usually carry oxygenated blood, with the exception of the pulmonary arteries. Arteries and arterioles have a thick layer of smooth muscle that helps to regulate blood pressure. Veins may have some smooth muscle, but are not nearly as significant in helping to regulate blood flow.

The P wave represents atrial depolarization. The QRS complex represents ventricular depolarization, the T wave represents repolarization, and the U wave (if present) represents late ventricular repolarization (e.g. repolarization of bundle branches). There are no X waves on an EKG.

Lead II is the limb lead that will show the best view of atrial depolarization. Remember, atrial depolarization starts at the sinoatrial node in the right atrium and spreads through the right and left atria. If you were to draw this on a heart, the net vector would be down (inferiorly) and to the left side of the heart. Also remember, lead II is the lead configuration that runs from the right arm to the left leg. aVR would show a good view of atrial depolarization (although it would be a negative deflection), but it is an augmented lead, not a limb lead. aVL is also an augmented lead.

The heart has four chambers. During diastole the atria contract to push blood into the ventricles, which are relaxed, but during systole the atria relax to fill with blood while the ventricles contract. This alternating contraction moves blood through the heart, the pulmonary circulatory path, and eventually out of the heart.

When the atria contract during diastole, blood is moving into the right ventricle and also into the left ventricle. During systole when the ventricles contract, blood is moving from the right ventricle towards the pulmonary circuit and from the left ventricle to the rest of the body.

Pericytes, sometimes referred to as Rouget cells, are cells that wrap around the endothelial cells of capillaries. These cells play an important role in angiogenesis, the process of formation of new blood vessels. Monocytes are macrophages of the immune system, Purkinje fibers are found in the heart, and allow conductance of electricity to the cardiomyocytes, Schwann cells are glial cells in the peripheral nervous system that myelinate axons.

Veins always carry blood towards the heart. The blood in veins is mostly deoxygenated, however the pulmonary vein, which goes from the lungs to the left atrium, carries newly oxygenated blood back to the heart for it to be pumped to the rest of the body.

In contrast, arteries always travel away from the heart and usually carry oxygenated blood, with the exception of the pulmonary arteries. Arteries and arterioles have a thick layer of smooth muscle that helps to regulate blood pressure. Veins may have some smooth muscle, but are not nearly as significant in helping to regulate blood flow.