1, right auricle; 2, left auricle; 3, right ventricle; 4, left ventricle; 5, vena cava superior; 6, vena cava inferior; 7, pulmonary arteries; 8, lungs; 9, pulmonary veins; 10, aorta; 11, alimentary canal; 12, liver; 13, hepatic artery; 14, portal vein; 15, hepatic vein.

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All the veins of the body, except those from the lungs and the heart itself, unite into two large veins, as already described, which pour their contents into the right auricle of the heart, and thus the grand round of circulation is continually maintained.  This is called the systemic circulation.  The whole circuit of the blood is thus divided into two portions, very distinct from each other.

191.  The Portal Circulation.  A certain part of the systemic or greater circulation is often called the portal circulation, which consists of the flow of the blood from the abdominal viscera through the portal vein and liver to the hepatic vein.  The blood brought to the capillaries of the stomach, intestines, spleen, and pancreas is gathered into veins which unite into a single trunk called the portal vein.  The blood, thus laden with certain products of digestion, is carried to the liver by the portal vein, mingling with that supplied to the capillaries of the same organ by the hepatic artery.  From these capillaries the blood is carried by small veins which unite into a large trunk, the hepatic vein, which opens into the inferior vena cava.  The portal circulation is thus not an independent system, but forms a kind of loop on the systemic circulation.

The lymph-current is in a sense a slow and stagnant side stream of the blood circulation; for substances are constantly passing from the blood-vessels into the lymph spaces, and returning, although after a comparatively long interval, into the blood by the great lymphatic trunks.

Experiment 90. To illustrate the action of the heart, and how it pumps the blood in only one direction.  Take a Davidson or Household rubber syringe.  Sink the suction end into water, and press the bulb.  As you let the bulb expand, it fills with water; as you press it again, a valve prevents the water from flowing back, and it is driven out in a jet along the other pipe.  The suction pipe represents the veins; the bulb, the heart; and the tube end, out of which the water flows, the arteries.

[NOTE.  The heart is not nourished by the blood which passes through it.  The muscular substance of the heart itself is supplied with nourishment by two little arteries called the coronary arteries, which start from the aorta just above two of the semilunar valves.  The blood is returned to the right auricle (not to either of the venae cavae) by the coronary vein.]

The longest route a portion of blood may take from the moment it leaves the left ventricle to the moment it returns to it, is through the portal circulation.  The shortest possible route is through the substance of the heart itself.  The mean time which the blood requires to make a complete circuit is about 23 seconds.