2 Cardiovascular System
2.1 Overview and Basic Wall Structure
A Overview of the cardiovascular system
The cardiovascular system is a closed system of vessels through which the blood is transported. This circulation is necessary to supply the organs with oxygen, nutrients, and hormones and to carry carbon dioxide and other metabolic waste products away to the excretory organs. Additionally, cells and proteins of the immune system travel through the bloodstream. Using blood as a transport medium, they “patrol” the body by constantly looking out for pathogens. The blood can also transport heat, so that circulation helps to regulate body temperature. In addition to these functions, the blood also helps to seal off leaks. It contains clotting factors that are activated when vessels gets damaged. The circulation is powered by the heart which functions as a pressure pump.
The circulatory system can be divided into two main circuits:
• the systemic circulation (high-pressure system, average blood pressure of 100mmHg in the major arteries) and
• the pulmonary circulation (low-pressure system, average blood pressure of 12mmHg; the difference in pressure from the systemic circulation is almost a factor of 10).
Regarding the vessels and pump, both circulatory systems can be divided into four parts:
• arteries and arterioles: they lead away from the heart and distribute blood to the organs
• capillaries: they connect arterioles to venules and enable the exchange of substances in organs
• venules and veins: receive blood from the capillaries and carry it back to the heart
• heart: functions as a circulation pump and transports the blood back to the arteries
The lymphatic system is an additional vascular system that carries fluid away from the organs. It begins with lymphatic capillaries in the organs and transports lymph back to the venous system.
Note: Whether to refer to a vessel as an artery or vein depends on the direction of blood flow, not on blood oxygen level. Arteries carry blood away from the heart, and veins carry blood toward the heart. Hence, in the diagram, the pulmonary artery contains oxygen-low blood (blue), while the pulmonary vein contains oxygen-rich blood (red).
B Basic wall structure of large blood vessels
a The major blood vessels (arteries and veins) generally consist of three layers:
• The tunica intima (intima): an endothelium consisting of a single layer of squamous epithelial cells, with the cells elongated in the direction of blood flow, and a thin a layer of subendothelial connective tissue
• The tunica media (media): consisting of a circular arrangement of smooth muscle cells, and elastic fibers of the internal elastic membrane (which separates the intima from the media) and external elastic membrane (which separates the media from the adventitia)
• The tunica adventitia (adventitia): consisting mainly of loose connective tissue, which integrates the blood vessel into its surroundings and allows for movement of vessels with organ movements. It can contain blood and lymphatic vessels as well as nerves.
b While veins have a similar three-layered structure as arteries, they have fewer and less dense layers of smooth muscle cells, giving the media of veins a looser structure. These structural characteristics are the result of lower venous blood pressure compared to arterial blood pressure. The peripheral veins in limbs contain valves to help direct blood flow back to the heart. The small exchange vessels, the capillaries, have no muscle tissue and consist only of endothelium and basement membrane.
C Blood pressure in different regions of the cardiovascular system
The function and structure of the cardiovascular system are closely interconnected, as higher blood pressure leads to the thickening of blood vessel walls and lower blood pressure allows walls to thin. Thus, knowledge of blood pressures is important when interpreting morphology. In the heart and major arteries closest to the heart, blood pressure fluctuates substantially with each cardiac cycle. While blood pressure in the left ventricle reaches 120 mmHg during systole, during diastole it drops to 0 mmHg. Due to the vessel wall properties of the arteries close to the heart, blood pressure fluctuations in them during the cardiac cycle are less extreme. Resistance vessels further help regulation so that capillary pressure remains constant. Pressure is lowest in the central veins closest to the heart. Because of their thin walls, they can expand and store blood.
Note: The different regions of the vascular system are assigned specific functions, which are described in the illustration above.
2.2 Terminal Vessels and Overview of the Major Blood Vessels
A Terminal vessels
a The primary function of the arteries and veins is to transport blood. Terminal vessels are concerned with the exchange of substances between blood and tissue. This is often called the microcirculation. The terminal vascular bed consists of
• Arterioles
• Capillaries
• Venules
b It is important to point out that capillary perfusion can vary within organs. Precapillary sphincters, which consist of smooth muscle cells, help to regulate perfusion in one capillary. Terminal vessel perfusion within a specific organ is related to the organ’s function and varies from organ to organ.
c Additionally, arteriovenous anastomoses help regulate the circulation in a group of neighboring capillaries that have formed one functional unit. Thus, entire capillary beds can be shut down.
Disruption of the fine regulation of the microcirculation is a major problem when patients go into shock because blood can pool in capillaries.
B Vascular relationships
In addition to the above mentioned descriptions of typical organ circulation: artery – capillary – vein, there are additional blood flow patterns in some organs.
a Flow of arterial blood through two serially connected capillary beds: Two serially connected capillary beds are found in the kidney where arterial blood initially flows through the renal corpuscles (glomeruli) and then into the capillaries of the renal medulla.
b Flow through two venous circuits (portal venous system): Venous blood flowing through two serially connected capillary beds is known as a portal venous system. For clarification, the blood in the first capillary bed is colored purple because it is not yet completely deoxygenated. Such a portal venous system exists in the digestive tract, where the portal vein collects the venous blood from the unpaired abdominal organs (stomach, intestines, spleen). From there it flows to the capillaries of the liver.
C Dual organ circulation
The liver receives its blood supply from the hepatic artery and the hepatic portal vein (a). The vessel responsible for suppling oxygenated blood to the liver tissue is the hepatic artery. The vessel that contains the blood with the substances to be metabolized in the liver is the portal vein. The lungs also have a dual arterial supply (b). Here, the pulmonary arteries contain deoxygenated blood and the bronchial arteries contain oxygenated blood. Another pattern of multiple blood supply can be found in the brain. Four arteries form a closed ring (the circle of Willis) from which other vessels supply the brain (c). All three forms of blood supply through multiple vessels allow for a certain degree of compensation in case one of the supplying vessels fails.
D Major blood vessels
This overview depicts the major arteries (a) and veins (b) in the human body. In the following organ descriptions, knowledge of the major vascular trunks is assumed, and the smaller organ-supplying vessels will be discussed with the respective organs.
2.3 Cardiogenic Area, Development of the Heart Tube
Characteristics
In many respects, the cardiovascular system is extraordinary. It is the first system to function in the human embryo; it is already functional by the end of the third week (first contractions of the primitive heart tube). Additionally, the cardiac loop (see below) is the body’s first asymmetrical structure. Since the human embryo is poorly supplied with yolk, which ensures nutrition by diffusion for a limited time only, it depends on extraembryonic circulation from a very early stage. While the yolk sac circulation appears earlier, it is the placental circulation that ultimately provides nutrients and removes waste over the course of embryonic and fetal development (see D).
A Origins of the cardiac tissue (cardiogenic area)
Dorsal view of the embryonic disc from the amniotic cavity. During the third week of development (presomite stage), the cardiogenic mesoderm, from which the heart develops, forms a horseshoe-shaped area (cardiogenic area) that consists of a thickened layer of mesenchymal cells. It lies anterolateral to the neural plate. At this stage in development, the mesenchyme is still located under the similarly horseshoe-shaped intraembryonic coelomic cavity. The cardiogenic area is composed of splanchnopleure (the layer of lateral plate mesoderm facing the viscera) and it borders the future pericardial cavity (see Be). During craniocaudal and lateral embryonic folding, the cardiogenic area, which originally lies in the anterolateral portion of the embryonic disc, moves ventrally under the developing foregut along with the adjacent coelomic cleft (see Bc).
B Formation of the heart
a–d Sagittal sections; e–h Cross-sections (21–23 days / 4–12 somites); Lateral (a–d) and rostral (e–h) views; For location of the respective plane of section see A.
As a result of craniocaudal folding (a–d) the heart primordium and the adjacent pericardial cavity rotate 180 degrees and move under the foregut (descent of the heart). The prechordal plate (the future site of the oral cavity), which previously was located caudally, is now rostral to the developing heart. The septum transversum (future central tendon of the diaphragm) also moves caudally under the heart and pericardial cavity. During the slightly delayed process of lateral folding (e–h) the initially paired heart primordia fuse to form the unpaired heart tube (h).
During this fusion, endothelial-lined embryonic vessels (endocardial tubes) that developed from angioblasts in the cardiogenic area fuse to form a single cavity in the heart tube. After fusing with the opposite side, the adjoining splanchnopleure thickens and develops into cardiac muscle (myocardium). Between the endocardial and myocardial layers develops a basement membrane-like structure consisting of a gelatinous extracellular matrix (cardiac jelly). Thus, the fused embryonic heart tube consists of three layers—from inside to outside: endocardium, cardiac jelly, and myocardium. The visceral layer of the pericardium, the epicardium, develops from progenitor cells in the area around the sinus venosus, which then overgrow the myocardium.
C Formation of the cardiac loop
a Left lateral view; b–d Anterior view (with the pericardial cavity opened).
During cranial embryonic folding, the developing heart and pericardial cavity shift in a ventral and caudal direction. With the start of the fourth week, the heart tube elongates and curves to form the cardiac loop, which at this stage is attached by a dorsal mesocardium to the posterior wall of the pericardial cavity. Over the course of development, this connection regresses (allowing formation of the transverse pericardial sinus), so that only the venous inflow and arterial outflow tracts attach the heart tube to the pericardium (see c). During formation of the cardiac loop, the cranial portion of the heart tube shifts ventrocaudally and to the right, while the caudal portion moves dorsocranially and to the left (d). Thus, the venous inflow tract lies dorsal and the arterial outflow tract ventral. At the same time, the cardiac loop subdivides into multiple portions as a result of constriction and expansion, forming the following regions:
• truncus arteriosus
• conus cordis
• primitive ventricle
• primitive atrium
• sinus venosus
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