In the capillaries, a two-directional exchange of fluid occurs between the blood and tissues. The fluid, called blood filtrate, carries oxygen and metabolites and passes through the capillary wall. In the tissues, these molecules are exchanged for carbon dioxide and waste products. Most of the fluid reenters the distal or venous end of the blood capillaries. The remaining fluid enters lymphatic capillaries as lymph and is ultimately returned to the bloodstream through a system of lymphatic vessels that join the blood system at the junction of the internal jugular veins with the subclavian veins. Normally, many of the white blood cells conveyed in the blood leave the blood vessels to enter the tissues. This occurs at the level of the postcapillary venules. When pathologic changes occur in the body, as in the inﬂammatory reaction, large numbers of white blood cells emigrate from these venules.
Arteries are the vessels that deliver blood to the capillaries. The smallest arteries, called arterioles, are functionally associated with networks of capillaries into which they deliver blood. The arterioles regulate the amount of blood that enters these capillary networks. Together, the arterioles, associated capillary network, and postcapillary venules form a functional unit called the microcirculatory or microvascular bed of that tissue. Veins, beginning with the postcapillary venule, collect blood from the microvascular bed and carry it away.
FIGURE 13.1 • Photograph of the human heart. This specimen was sectioned in the oblique plane to visualize all of the chambers of the heart. The posterior part of the heart is on the left; the anterior part has been removed and is shown on the right. Note the thickness of the ventricular walls and the intermuscular septum. The interatrial septum, which separates the atria, is also visible.
Two circuits distribute blood in the body: the systemic and the pulmonary circulations.
Two pathways of circulation are formed by the blood vessels and the heart:  Pulmonary circulation conveys blood from the heart to the lungs and from the lungs to the heart (Fig. 13.2).
arch of aorta  Systemic circulation conveys blood from the heart to other tissues of the body and from other tissues of the body to the heart.
Although the general arrangement of blood vessels in both circulations is from arteries to capillaries to veins, in some parts of the systemic circulation it is modified so that a vein or an arteriole is interposed between two capillary networks; these vessels constitute a portal system. Venous portal systems occur in vessels carrying blood to the liver, namely, the hepatic portal system (portal vein), and in vessels leading to the pituitary, the hypothalamic–hypophyseal portal system.
FIGURE 13.2 • Diagram depicting circulation of blood through the heart. Blood returns from the tissues of the body via the superior vena cava and inferior vena cava. These two major venous vessels carry the blood to the right atrium. Blood then passes into the right ventricle and is pumped into the pulmonary trunk before flowing into the pulmonary arteries, which convey the blood to the lungs. The blood is oxygenated in the lungs and is then returned to the left atrium via the pulmonary veins. Blood then passes to the left ventricle and is pumped into the aorta, which conveys the blood to the tissues of the body. From the heart to the lungs and from the lungs to the heart constitutes the pulmonary circulation; from the heart to the tissues and from the tissues to the heart constitutes the systemic circulation.
The heart lies obliquely, about two-thirds on the left side of the thoracic cavity, in the middle mediastinum—the space enclosed between sternum, vertebral column, diaphragm, and lungs. It is surrounded by a tough fibrous sac, the pericardium, which also contains the beginnings and ends of the great vessels entering and leaving the heart. Through the pericardium, the heart is strongly attached to the diaphragm and neighboring organs that lie in the thoracic cavity.