At any given moment, the majority of blood in the human body can be found in the venous network. Veins contain 64% of the body's blood, making it the largest reservoir of blood in the body. Capillary networks contain the largest surface area of blood, however, not the largest percentage of blood volume.

Epinephrine is a hormone secreted in order to facilitate "fight or flight" reactions by the body. Secretion of epinephrine from the adrenal medulla is initiated by sympathetic stimulation. Epinephrine will increase heart rate, blood pressure, and contraction force; however, the skeletal muscle arterioles will be dilated so that more blood is able to reach the muscles. The sympathetic nervous system is designed to direct blood toward skeletal muscle and the heart, and away from the digestive tract and skin.

A lipid soluble hormone is likely to make its way to the nucleus because it can easily pass through the hydrophobic membranes of cells. A water soluble protein is likely to attach to the outside of a cell and activate a signaling pathway since it cannot readily pass through the phospholipid bilayer (cell membrane). Hydrophilic hormones are the same as water soluble hormones. Fat soluble hormones must be carried by proteins through the blood.

Hormones secreted into the blood stream have the ability to travel anywhere in the body. Such hormones are endocrine hormones and they can act on organs far from their source. Substances that are secreted outside the body (or within a body cavity) are not hormones, since all hormones are released by endocrine glands into the blood. Rather, these substances are released by exocrine glands (i.e. sweat, digestive enzymes).

Norepinephrine and epinephrine (on about an equal basis) on beta-1 receptors in the heart increase heart rate and contractility (force of contraction). These hormones are released from the adrenals in response to sympathetic stimulation.

Moving the heart rate between 60 and 100 bpm involves only the addition and removal of parasympathetic tone. Remember, parasympathetic: rest and digest; so, to raise the heart rate, parasympathetic tone would have to be withdrawn. Sympathetic tone would not be added until the heart rate exceeds 100 bpm. 

In addition to transporting steroids and some fats, albumin is also responsible for maintaining the osmotic pressure of the blood. It is the most abundant protein in the plasma and helps to draw water back into the lumen of the blood vessels.

Fibrinogen is the zymogen of fibrin, an essential clotting protein. Immunoglobulins, also called antibodies, help detect pathogens and tag them for destruction. Hemoglobin is found within erythrocytes and serves to transport oxygen. Vasopressin is not a blood protein; it is a peptide hormone secreted by the posterior pituitary gland. Vasopressin acts on the kidney to increase the hydrostatic pressure of the blood by retaining water, but does not affect the osmotic pressure of the blood in a significant way.

Oxygen-poor blood returns from the body through the superior and inferior venae cavae, which load blood into the right atrium. Blood then flows into the right ventricle, and oxygen-poor blood is pumped through the pulmonary arteries into the lungs, where blood becomes oxygenated. From the lungs, blood returns to the left side of the heart through the pulmonary veins into the left atrium. Remember, regardless of whether blood is oxygen-rich or oxygen-poor, all arteries carry blood away from the heart, and veins deliver blood back to the heart. 

The systemic circulation is the part of the cardiovascular system that pumps oxygenated blood from the left ventricle out to the rest of the body to all the tissues that need blood, including the brain, kidney, heart, and lungs. Though the lungs are part of the pulmonary circuit, involved in gas exchange, the cells of the lungs also need blood. There are alveolar cells, macrophages, and connective tissue cells that need blood for metabolism. Also, the myocardium, needs blood (and oxygen) too since it is continuously using lots of ATP to generate contractile force.

During ventricular contraction, both atrioventricular valves (the mitral and tricuspid valve) should be closed in order to prevent backflow of blood into the atria. The papillary muscles and chordae tendinae must both be tense in order to keep this valve closed.