Circulatory System and Heart

The Circulatory System

• The circulatory system consists of two parts: the cardiovascular system and the lymphatic vascular system.

Heart

• The heart is composed of four chambers: the atria and ventricles, with three major layers: the endocardium, myocardium, and epicardium.
• The endocardium is the inner layer, consisting of endothelium, a middle layer of connective tissue and smooth muscle, and a subendocardial layer of loose connective tissue.
• The myocardium is the middle layer, containing cardiac muscle with fibers arranged spirally around each heart chamber.
• The epicardium is the outer layer, consisting of a simple squamous mesothelium supported by a layer of loose connective tissue containing blood vessels and nerves.

Impulse Conducting System

• The impulse conducting system consists of the sinoatrial (SA) node, the atrioventricular (AV) node, the AV bundle (of His), and Purkinje fibers.
• The Purkinje fibers are pale-staining, larger than adjacent contractile muscle fibers, with sparse, peripheral myofibrils and much glycogen.

Tissues of the Vascular Wall

• The walls of all blood vessels except capillaries contain smooth muscle and connective tissue in addition to the endothelial lining.
• The endothelium acts as a semipermeable barrier between blood plasma and interstitial tissue fluid.
• Endothelial cells have several functions, including:
  • Presenting a nonthrombogenic surface
  • Regulating local vascular tone and blood flow
  • Participating in inflammation and local immune responses
  • Secreting growth factors

Blood Vessel Walls

• All blood vessels except capillaries have walls with three concentric layers or tunics: the tunica intima, tunica media, and tunica externa.
• The tunica intima consists of the endothelium and a thin subendothelial layer of loose connective tissue.
• The tunica media consists of concentric layers of helically arranged smooth muscle cells, elastic fibers, and reticular fibers.
• The tunica externa consists of type I collagen and elastic fibers.

Elastic Arteries

• Elastic arteries are conducting arteries, including the aorta, pulmonary artery, and their largest branches.
• The intima is well-developed, with many smooth muscle cells in the subendothelial connective tissue.
• The media is thick, with elastic laminae alternating with layers of smooth muscle fibers.

Muscular Arteries

• Muscular arteries distribute blood to organs and help regulate blood pressure by contracting or relaxing smooth muscle in the media.
• The intima has a very thin subendothelial layer and a prominent internal elastic lamina.
• The media may contain up to 40 layers of large smooth muscle cells interspersed with elastic laminae.

Arterioles

• Arterioles are the smallest arteries, branching from muscular arteries.
• The subendothelial layer is very thin, and elastic laminae are absent.
• The media consists of circularly arranged smooth muscle cells.

Capillaries

• Capillaries are composed of a single layer of endothelial cells rolled up as a tube.
• They permit and regulate metabolic exchange between blood and surrounding tissues.
• There are three histologic types of capillaries: continuous, fenestrated, and discontinuous (sinusoidal) capillaries.

Postcapillary Venules and Venules

• Postcapillary venules converge into larger collecting venules.
• The tunica intima of these vessels is composed of endothelium and a very thin subendothelial layer.
• Venules become surrounded by a recognizable tunica media with two or three smooth muscle layers.

Veins

• Veins carry blood back to the heart from microvasculature all over the body.
• Blood entering veins is under very low pressure and moves toward the heart by contraction of smooth muscle fibers in the media and by external compressions from surrounding muscles and other organs.