Cardiovascular system

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Cardiovascular system:

The cardiovascular system is a continuous, completely closed system of endothelial tubes. The general purpose of the cardiovascular system is the perfusion of capillary beds permeating all organs with fresh blood over a narrow range of hydrostatic pressures. The circulation is divided into the systemic and pulmonary circulation.

Arteries transport blood under high pressure and their muscular walls are thick. The veins are conduits for transport of the blood from tissues back to the heart.


the cardiac wall consists of three layers:

1-Endocardium:consisting of an endothelial lining and subendothelial connective tissue. It is homologous to the tunica intima of blood vessels.

The subendothelial connective tissue layer consists of collagen and elastic fibers synthesized by fibroblasts.

2-Myocardium: a functional syncytium of striated cardiac fibers. It is continuous with the tunica media of blood vessels.

Individual cardiac muscle cells have a central nucleus and are linked to each other by intercalated disks. The presence of gap junction in the longitudinal segment of the intercalated disks between connected cardiac muscle cells allow free diffusion of ions and the rapid spread of the action potential from cell to cell.

Purkinje fibers lie beneath the endocardium lining the two sides of the interventricular septum. They are bundles of impulse-conducting cardiac fibers. Like cardiac muscle fibers, Purkinje fibers are striated and are linked to each other by atypical intercalated disks. Purkinje fibers can be distinguished from regular cardiocytes(cardiac muscle fibers) by their location, their large size, and lighter cytoplasmic staining due to high glycogen content.

The heart has two specialized conductive systems:

-The sinus node, or sinoatrial (S-A) node, which generates impulses to cause rhythmic contractions of the cardiac muscle.

-A specialized conductive system (A-V) node, consisting of the internodal pathway, which conduct the impulse from the S-A node to the atrioventricular (A-V) node, in which the atrial impulse is delayed before reaching the ventricles (the atrioventricular bundle)which conducts the impulse from the atria to the ventricles and the left and right bundles of Purkinje fibers, which conduct the impulse to all parts of the ventricles.

3-Pericardium: the visceral layer of the pericardius is epicardium, ia a low-friction surface lined by a mesothelium in contact with the parietal pericardial space.


Arteries are organized in three major tunics or layers:

1-The tunica intima is the innermost coat, It consists of

- an endothelial lining, continuous with the endocardium .

-an intermediate layer of loose connective tissue, the subendothelium.

-Internal elastic lamina, is an external layer of elastic fibers.

2-The tunica media, is the middle coat. It consists mainly of smooth muscle cells surrounded by a variable number of collagen fibers, extracellular matrix, and elastic sheath with irregular gaps.

Collagen fibers provide a supporting framework for smooth muscle cells and limit the distensibility of the wall of the vessel. Veins have a higher content of collagen than arteries.

3-The tunica adventitia, is the outer coat and consists mainly of connective tissue. An external elastic lamina can be seen separating the tunica media from the adventitia. The adventitia of large vessels (arteries and veins) contains small vessels (vasa vasorum) that penetrate the outer portion of the tunica media to supply oxygen and nutrients.

From the heat to capillaries, the arteries can be classified into three major groups

1-large elastic arteries 2-medium-sized muscular arteries 3-small arteries and arterioles.

Large elastic arteries are conducting vessels:

The aorta and its branches are elastic and they are conduct blood from the heart to the medium-sized distributing arteries.

The tunica intima of the elastic arteries consists of the endothelium and te subendothelial connective tissue

Large amounts of fenestrated elastic sheaths are found in the tunica media with bundles of smooth muscle cells permeating the narrow gaps between the elastic lamellae. Collagen fibers are present in all tunics, but especially in the adventitia. Blood vessels (vasa vasorum), nerves (nervi vasorum), and lymphatics can be recognized in the tunica adventitia.

Medium-sized muscular arteries are distributing vessels:

They allow a selective distribution of blood to different organs in response to functional needs Examples, radial, tibial, splenic, mesentric and intercostal arteries.

The tunica intima consists of three layers: (1)the endothelium, (2)the subendothelium, and (3) the internal elastic lamina.

The internal elastic lamina is a fenestrated band of elastic fibers that often show folds in sections of fixed tissue owing to contraction of the smooth muscle cell layer (tunica media).

The tunica media shows increase in smoothe muscle fibers and significant reduction in elastic fibers.

A fenestrated external elastic lamina can bee seen at te junction of the tunica media and the adventitia.

Arterioles are resistance vessels:

Arterioles are final branches of the arterial system. They regulate te distribution of blood to different capillary beds by vasoconstriction and vasodilation in localized regions.

The diameter of arterioles and small arteries renge from 20-130 µm. Because the lumen is small, these vessels can be closed down to generate high resistance to blood flow.

The tunica imtima has an endothelium, subendothelium, and internal elastic lamina.

The tunica media consists of two to five layers of smooth muscle cells.

The tunica adventitia contains slight collagenous tissue, binding with its surroundings.

The segment beyond the arteriole proper is the metarteriole, the terminal branch of the arterial system. It consists of one layer of muscle cells, often discontinuous, and represents an important local regulator of blood flow.

Capillaries are exchange vessels:

Capillaries are extremely thin tubes formed by single layer of highly permeable endothelial cells surrounded by a basal lamina. The diameter range of a capillary is about 5 to 10 µm. The microvascular bed is the site of the microcirculation, is composed of the terminal arteriole(metarteriole), the capillary bed, and the postcapillary venules.

Three morphologic types of capillaries are recognized: continuous, fenestrated, and discontinuous(sinusoids).

Continuous capillaries:

are lined by a complete simple squamous endothelim and a basal lamina Pericytes can occur between the endothelium and the basal lamina. Pericytes are undifferentiated cells that resemble smooth muscle cells. Endothelial cells are linked by tight junctions and transport fluids and solutes by pinocytotic vesicles. Continuous capillaries occur in the brain, muscle, skin, thymus, and lungs.

Fenestrated capillaries:

have pores, with or without diaphragms. They founf in intestines, endocrine glands, around renal tubules.

Discontinuous capillaries:

are characterized by an incomplete endothelial lining and basal lamina, with gaps or holes between and within endothelial cells. The are found in the liver and spleen.


The venous system starts at the end of the capillary bed with a postcapillary venules that structurally resembles continuous capillaries but with a wide lumen. Postcapillary venules is the preffered site of migration of blood cells into tissues by a mechanism called diapedesis.

Postcapillary venules converge to form muscular venules which converge into collecting venules, leading to a series of veins of progressively larger diameter.

Veins have a relatively thin wall in comparison with arteries of the same size.

Similar to arteries, veins consist of tunics.owever, the distiction of a tunica media from a tunica adventitia is often not clear. The lumen is lined by an endothelium and a subjacent basal lamina A distinct internal elastic lamiba is not seen.

The muscular tunica media is thinner than in arteries, and smooth muscle cells have an irregular orientation, approximately circular.

The tunica adventitia consists of collagen fibers and fibroblasts with few nerve fibers. A typical characteristic of veins is the presence of valves to prevent reflux of blood. A valve is a projection into the lumen of the tunica intima, covered by endothelial cells and strengthened by elastic and collagen fibers.

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