Anatomical Principles of the Circulatory System



Fig. 2.1
Layers of the heart wall. Reprinted from Davies A and Scott A. Cardiac Anatomy and Electrophysiology. In: Starting to Read ECG. Springer. 2015



This hollow organ is divided into four different chambers, two atria and two ventricles that are respectively responsible for receiving and pumping blood. Moreover, one can separate the blood flow into two circuits: pulmonary and systemic circuit that are responsible, consecutively, for the blood convey between heart and lungs, and heart and tissues [1] (Fig. 2.2).

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Fig. 2.2
Overview of pulmonary and systemic circuits

Because the left ventricle is responsible for pumping blood at much higher pressure to the systemic than to the pulmonary circuit, it usually has the greatest hypertrophied wall of the ventricles [5].



Vessels’ Wall: The Endothelium


The walls of vessels differ based on whether it is carrying blood from or towards the heart. Arteries are exposed to a much higher internal pressure and so are thicker and morphologically more complex than veins, which carry blood from tissues to the heart at a lower pressure [1].

The endothelium is the inner cellular lining of all blood vessels, which come into direct contact with circulating blood or lymph [6] (Fig. 2.3). Endocardium is the endothelium of the interior surfaces of the heart chambers. Vascular endothelial cells are those in direct contact with blood whereas those in direct contact with lymph are known as lymphatic endothelial cells . The endothelium acts also as a barrier and regulates the exchange of small and large molecules [7].

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Fig. 2.3
Structure of vessels’ wall

Endothelial cells are located on the inner layer of the blood vessels, the tunica intima. Endothelium has a substantial role in regulating the function of vasomotion by its ability to metabolize circulating vasoactive substances and responding to neurotransmitters and vasoactive factors [7, 8].

Endothelial cells are typically aligned in the direction of blood flow, overlapping immediately adjacent cells. They may be continuous (fenestrated or nonfenestrated) and discontinuous. Nonfenestrated continuous endothelium can be found in arteries, veins, and capillaries of the brain, lung, heart, or skin. In arteries and veins, they appear more continuous and thicker than those in capillaries [8]. Fenestrated endothelium occurs in locations of increased filtration or transendothelial transport, such as exocrine and endocrine glands, such as gastric and intestinal mucosa, choroid plexus, glomeruli , and renal tubules. Discontinuous endothelium exists in the liver [6]. The endothelium is indispensable for body homeostasis. It participates in both physiologic and pathologic processes including atherosclerosis, hypertension, pulmonary hypertension, sepsis, and inflammatory syndromes [7].


Coronary Irrigation


As a muscle pump, the external portion of the heart’s wall requires its own blood supply, which is provided by arteries and veins surrounding its wall. Apart from that, the inner portion of the heart’s wall, i.e., the endocardium, receives nutrients and oxygen supply directly from the chambers and so does not need vessels for its irrigation. Therefore, the atria and ventricles have both, arterial and venous supply [5].

There are two main coronary arteries composing the arterial supply of the heart, the right coronary artery (RCA) and the left coronary artery. The right coronary artery originates from the right aortic sinus of the ascending aorta and runs within the coronary sulcus between atria and ventricles (Figs. 2.4 and 2.5). It is responsible for irrigating the right atrium, sinuatrial (SA) and atrioventricular (AV) nodes as well as the interventricular septum. Accordingly, from the right coronary artery arises a sinuatrial nodal branch to supply the heart’s pacemaker, the SA node. However, in 40 % of people, the circumflex branch of the left coronary artery is that gives off the SA nodal branch. In its turn, the AV node is also supplied by a subdivision of the right coronary artery, the atrioventricular nodal branch. That arm arises from the right coronary artery in the posterior portion of the heart at the crux of the heart i.e., the meeting point between the four chambers. Moreover, a so-called right marginal branch emerges from the right coronary artery in its path within the sulcus towards the apex. The right border of the heart is supplied by the right marginal branch and does not reach its apex [5].

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Fig. 2.4
Aortic sinuses of the heart. Reprinted from Devarajan and Subramaniam. Applied Anatomy of the Aorta. In: Subramaniam K, Park KW and Subramaniam B. Anesthesia and Perioperative Care for Aortic Surgery. Springer. 2011


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Fig. 2.5
Heart supply with overview of arteries and veins

The ventricles area is supplied by the posterior interventricular branch, which also comes from the right coronary artery . The right coronary artery supplies most of the right ventricle and the diaphragmatic surface of the left ventricle.

In its turn, the left coronary artery (LCA) originates from the left aortic sinus running with the right coronary artery in the coronary sulcus on the left side of the pulmonary trunk (Fig. 2.4). Then, the left coronary artery splits into two arms: the circumflex branch and the anterior interventricular branch. As mentioned previously in this section, the SA nodal branch often emerges from the right coronary artery precisely from the circumflex branch of the left coronary artery. The circumflex branch arises from the right coronary artery and follows towards the posterior portion of the heart and to the left within the coronary sulcus [5]. It is then responsible for irrigating the left atrium and left ventricle. This comes to a great importance as anomalies of the circumflex branch have been reported. The second branch of the left coronary artery, the anterior interventricular, runs in the anterior interventricular sulcus towards the apex. It is the blood supply for the ventricles [5].

It is common that one’s heart develops a collateral circulation should any closure in any main coronary artery occur (e.g., atherosclerosis). It occurs in the so-called functional end arteries [5].

When it comes to the venous supply, one needs to consider that the heart is greatly drained through veins that combine into a greater vessel, the coronary sinus. The second main drain course is through small veins that return blood to the right atrium [5]. Refer to Table 2.1 for details on venous supply of the heart.


Table 2.1
Main cardiac venous system [5, 10]







































Vein

Origin

Path

Fuses into the

Drain

Great cardiac vein

Apex of heart ascending with the anterior interventricular branch of the left coronary artery

First: turns left at the coronary sulcus

Coronary sinus (opens into the right atrium)

Areas supplied by the left coronary artery

Second: surrounds the left side of the heart together with the circumflex branch of the left coronary artery
 

Middle cardiac vein (or posterior interventricular vein)

Cardiac apex

Ascends to the coronary sinus

Areas supplied by right coronary artery

Small cardiac veins (or Thebesius’ vein)a

Between right atrium and ventricle

Follows the right marginal branch of the right coronary artery. Often merges with the right marginal vein

Oblique veins of the left atriumb

Left atrium

Runs obliquely on the posterior wall of left atrium. Later fusing into the great cardiac vein
 


Adapted from: “Moore K, Dalley A, Agur A. Thorax. In: Moore K, Dalley A, Agur A, editors. Clinically oriented anatomy. 7th ed. Philadelphia, PA: Wolters Kluwer/Lippincott Williams & Wilkins Health; 2014. p. 71-180.” and “Standring S. Heart and great vessels. In: Standring S, editor. Gray’s anatomy: the anatomical basis of clinical practice. 40th ed. Edinburgh: Churchill Livingstone/Elsevier; 2008. p. 959–88.”

aSmall cardiac vein: it may be absent

bOblique veins of the left atrium: it usually atrophies before birth, but can be present in some adults

Blood returning from coronary walls that is not drained through vessels mentioned in Table 2.1 returns to the right atrium through small veins and the anterior cardiac vein [9].


The Arterial System



Aorta and Its Branches


Authors anatomically divide the aorta into the ascending aorta, aortic arch, descending thoracic and abdominal aorta [10] (Fig. 2.6).

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Fig. 2.6
Aorta course and ramifications. Reprinted from Berdajs D and Turina MI. Surgical Anatomy of the Aorta. In: Operative Anatomy of the Heart. Springer. 2011

The ascending aorta usually lengths 5 cm long [10] and its diameter is 2.5 cm on average [5]. It rises obliquely from the left ventricle, curving forward and to the right, at the level of the lower edge of the third left costal cartilage and it gives origin to the coronary arteries [10].

The transverse part of the aorta is named “ aortic arch ” as it bends to become the descending aorta. The arch begins to the second right sternocostal joint next to the sternal angle and leans to the left, following inferiorly. Three branches emerge from its upper edge: the brachiocephalic trunk, the left common carotid artery, and the left subclavian artery [10].

The brachiocephalic trunk , the largest ramification of the aortic arch, is 4–5 cm long [10] and emerges from the curvature of the aortic arch posteriorly. The trunk surges behind the manubrium and anterior to the trachea, it then goes up diagonally until it positions to the right of the trachea and to the sternum-clavicular joint. Then, it divides into right common carotid and into the right subclavian arteries [5] (Fig. 2.6).

The second branch of the aortic arch, the left common carotid artery (Fig. 2.6), arises posterior to the manubrium, posterior and to the left of the brachiocephalic trunk. Subsequently, it rises above the left subclavian artery and enters the neck passing posteriorly to the left subcostal joint [10].

The left subclavian artery , the third and last branch of the aortic arch, originates on the left side of the left common carotid artery and rises along the left side of the trachea [5] (Fig. 2.6).

The aortic arch has now turned into the descending thoracic aorta, and it descends approaching the medial plane by moving the esophagus to the right [10].

The thoracic aorta runs posteriorly to the root of the left lung, pericardium, and esophagus [10]. It begins on the left side of the inferior border of the T4 vertebra descending posteriorly in the mediastinum on the left sides of T5–T12 vertebrae [5]. It enters the abdomen through the aortic hiatus of the diaphragm where it changes its name to abdominal descending aorta (Fig. 2.6), starting close to the 12th thoracic vertebra and splitting at the level of the fourth lumbar vertebra [11].

The primary visceral branches of the abdominal aorta are:


  1. 1.


    The celiac trunk that supplies the foregut through its main branches: artery left gastric, artery splenic, and artery common hepatic

     

  2. 2.


    Superior mesenteric artery , which supplies the midgut through the middle colic artery, jejunal-ileal, ileocolic, and right colic

     

  3. 3.


    The inferior mesenteric artery, which supplies the hindgut, with the branches: left colic, sigmoid arteries, and superior rectal arteries

     

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Sep 30, 2017 | Posted by in CARDIOLOGY | Comments Off on Anatomical Principles of the Circulatory System
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