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.

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 voltage-gated channels along the axonal plasma membrane open and close in response to changes in voltage, and may exist in three distinct states: deactivated, activated, and inactivated. While the axon is at rest, these channels are said to be deactivated; they are impermeable to sodium ions since their activation gates are closed. Once the neuron gets depolarized to the threshold of the voltage-gated sodium channels, the activation gates open, allowing the influx of sodium down its concentration gradient into the cell. During this time the channels are in their activated state. At the peak of the action potential the activation gates are still open, but the inactivation gates close, stopping the flow of sodium through the channels. The channels are in the inactivated state due to the cell becoming depolarized. Once the membrane potential drops back down towards resting, the inactivation gates open, and the activation gates close, thereby deactivating the channels again, until another action potential depolarizes the membrane.

The autonomic nervous system (ANS) consists of a set of pathways to and from the central nervous system (CNS) that innervate and regulate smooth muscle, cardiac muscle, and glands. The ANS is distinct from the somatic nervous system, which innervates skeletal muscle. The ANS has three divisions the sympathetic, parasympathetic, and enteric nervous systems.

The signal for muscle contraction causes the release of calcium from the sarcoplasmic reticulum. This calcium floods the cell and is necessary for causing muscle contraction. Parvalbumin, a protein in the cytoplasm, binds to calcium and acts as a slow-releaser of calcium. This binding reaction of calcium with parvalbumin causes the release of heat, which is termed as 'unexplained heat.' The 'unexplained heat' is also known as 'labile heat.'

The adrenal medulla produces epinephrine and norepinephrine, which cause the fight or flight response. The thyroid gland produces triiodothryonine and thyroxine, which are used for metabolism, growth, and development. The ovaries and placenta produce estrogen and progesterone, which are used for fetal/maternal development and egg production. Lastly, the stomach and small intestines produce gastrin and secretin, which are used to assist digestion and nutrient absorption.

The endosteum is the layer that is found deep to all layers. It is found in the inner lining of the bone. Meanwhile, the periosteum is the most superficial layer of the bone which compromises the outer covering of bones.

Preganglionic neurons of the parasympathetic nervous system originate in the nuclei of the craniosacral region (cranial nerves and in the spinal cord segments of S2-S4). Those of the sympathetic nervous system originate in the thoracolumbar region (spinal cord segments of T1-L3).

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.

The pupillary sphincter muscles contain muscarinic cholinergic receptors that help the eyes to respond to parasympathetic tone. When this parasympathetic signal is blocked (antagonized) via atropine, you will observe a "sympathetic response" at the level of the pupil. In this scenario, you would observe the sympathetic response of pupil dilation. Hint: "aTROP'ine" and "seeing a TROPical setting" both cause your pupils to dilate nice and wide!