2 Heart and Vascular Anatomy

Heart chambers and circulation

The heart has four chambers divided into right and left sides by the septum. The two atria receive blood, while the two ventricles pump blood out. The right side sends blood to the lungs through pulmonary circulation; the left side sends blood to the body through systemic circulation. The left ventricle has a thicker muscular wall because it must generate enough pressure to pump blood throughout the body.

The route of blood through the heart

Blood returning from the body enters the right atrium through the superior and inferior venae cavae. It then passes through the tricuspid valve into the right ventricle. The right ventricle pumps it through the pulmonary valve and pulmonary arteries to the lungs.

Oxygenated blood returns from the lungs through the pulmonary veins to the left atrium. It passes through the mitral (bicuspid) valve into the left ventricle, which pumps it through the aortic valve into the aorta and onward to the body.

Arteries carry blood away from the heart, and veins carry blood toward it. This distinction is based on direction of flow, not oxygen content: pulmonary arteries carry oxygen-poor blood, while pulmonary veins carry oxygen-rich blood. This is the opposite of the usual oxygen pattern in systemic arteries and veins.

Heart valves

The four valves keep blood moving in one direction by opening to let blood pass and closing to limit backward flow. The tricuspid and mitral valves lie between the atria and ventricles. The pulmonary and aortic valves lie at the exits of the ventricles. Their opening and closing are coordinated with the chambers’ pumping and filling.

Blood vessels and their functions

Blood travels through a continuous vessel network in this order:

Arteries → arterioles → capillaries → venules → veins

Most arteries and veins have three wall layers, or tunics: the inner tunica intima, the middle tunica media, and the outer tunica externa.

• Arteries have thick, elastic, muscular walls and a relatively smaller lumen. Their structure helps them withstand higher pressure. They carry blood away from the heart and branch into smaller arteries, then arterioles.

• Arterioles are small vessels with smooth muscle that can narrow or widen. They regulate how much blood enters capillary beds and help control resistance to blood flow.

• Capillaries are microscopic vessels with walls only one cell layer thick. Their thin walls support the exchange of gases, nutrients, and wastes between blood and surrounding tissues.

• Venules are small, thin-walled vessels that receive blood from capillaries. They collect blood and join to form veins.

• Veins generally have thinner walls and larger lumens than comparable arteries. Many contain valves, especially in the limbs; these valves help prevent backward flow where pressure is low. Veins return blood toward the heart.

After blood passes through capillary beds, venules collect it and carry it into progressively larger veins. In summary, arteries and arterioles carry blood away from the heart, capillaries are the principal exchange vessels, and venules and veins return blood to the heart. Vessel walls reflect their roles: arteries are adapted for higher pressure, capillaries have thin exchange surfaces, and veins return blood at lower pressure.

• mitral valve