The circulatory system consists of the cardiovascular system and lymphatic system. The cardiovascular system conducts the blood and is composed of arteries, veins, capillary nets, and the heart. The lymphatic system conducts the lymph and is constituted by more heterogeneous structures like lymphatic vessels, lymphatic nodes, nodules, and organs like the spleen and thymus.
The cardiovascular system is the main system that communicates the organs and internal regions of the body of animals. It pumps and conducts the blood to irrigate every body region. Blood is necessary for carrying nutrients, waste products, oxygen, carbon dioxide, hormones, immune cells, and to maintain pH balance. It also has other functions, like regulating the body temperature.
The cardiovascular system is a double circuit, one irrigating the lungs and the other irrigating the rest of the body (Figure 1). Both begin and end in the heart, which is the organ in charge of keeping the the continuous flow of blood. The pattern of blood vessels is the same in the two circuits: heart, arteries, arterioles, capillary network, small veins, veins, and back to the heart. Sometimes, arterioles or small veins may be found connecting two capillary networks. This is a portal system, like the hepatic portal system, which includes the intestine and liver.
The stalling of blood circulation leads to the death of tissues, mainly due to a lack of oxygen. Some tissues are more vulnerable to damage caused by oxygen depletion. For instance, while the hands and kidneys may remain viable for 1 hour without oxygen, the cornea can survive for several hours out of the body. However, the heart and the brain develop damage after a few minutes without blood circulation. Under some circumstances, there are animals that can change their normal blood flow to adap to low oxygen concentrations. For instance, during diving, the flow of blood that irrigates the vital organs is increased, while the less-vital organs receive lower blood flux. This is a reflex mediated by the brain. When seals are diving (not breathing), their heartbeat dorps about 10 times. Curiously, this reflex is present in fish taken out of the water, and it is also observed in hibernating animals. Although the blood pressure is maintained, the body temperature, metabolism, and heart rhythm are much lower.
The main blood vessels are arteries, veins, and capillaries. Arteries and veins are made up of three layers, or tunics: tunica intima, tunica media, and tunica adventitia (Figure 2). The tunica intima is the innermost layer, in contact with the blood, and it consists of a squamous simple epithelium (endothelium), a basal lamina, and a layer of loose connective tissue. The tunica media is mainly composed of smooth muscle cells. The tunica adventitia is the outermost layer, and it is connective tissue. Arteries and arterioles show thicker walls than veins and small veins, respectively, to resist stronger blood pressures because they are closer to the heart. Arteries usually have a smaller diameter than veins and, together with their thicker walls, make them more rounded in histological sections, whereas veins show more irregular shapes.
Arteries and large veins contain small blood vessels irrigating their walls. This network of small blood vessels is known as "vasa vasorum" (vessels of the vessels), and it is found closer to the tunica adventitia in arteries, while it is closer to the lumen in veins. Arteries are more easily injured than veins during pathological situations because the part of the tunica media (mostly smooth muscle) close to the lumen receives less blood supply. Within the walls of arteries and veins, there are nerve endings that modulate the contraction strength of the smooth muscle, thus affecting the diameter of the lumen.
1. Arteries
Arteries are blood vessels that conduct the blood from the heart to other parts of the body. The walls of the arteries are thick to withstand the blood pressure caused by the heart beating. According to their diameter, arteries are classified into large (or elastic arteries), medium (or muscular arteries), and small (or arterioles).
Elastic arteries
Blood leaves the heart through the aorta and pulmonary arteries. Both branch near the heart. These two main arteries and their first branches are classified as elastic arteries. Their walls contain a large amount of the elastic fibers that make them recover to a normal diameter after every expansion produced by the heartbeats. Elastic arteries show a thick tunica media (Figure 3). The endothelium is made up of cells with tight junctions and desmosomes that keep them tightly attached to each other. The major axis of the endothelial cells is aligned parallel to the direction of the blood flux. The subendothelial layer is connective tissue with collagen, elastic fibers, and some smooth muscle cells. The elastic lamina, which separates the tunica intima from the tunica media, is thin and sometimes difficult to observe. The tunica media is very thick, mostly made up of elastic and collagen fibers, and contains many smooth muscle cells. The tunica adventitia is a layer of connective tissue with fibroblasts. Here, there are no smooth muscle fibers. It is worth noting that smooth muscle fibers of elastic arteries, besides being responsible for the artery diameter by cell contraction, synthesize and release elastic and collagen fibers, thus taking on the role of producing extracellular matrix.
Muscular arteries
Muscular arteries are medium-size arteries with variable diameters. Their histological organization is transitional between elastic arteries and arterioles. The diameter of muscular arteries ranges from 0.1 to 10 mm. Thus, larger muscular arteries resemble elastic arteries, and the smaller ones are more like arterioles. Proportionally, muscular arteries contain fewer elastic fibers and more smooth muscle cells than elastic arteries.
Small arteries and arterioles
The diameter of small arteries and arterioles is variable, and they are identified by the number of layers of smooth muscle cells. As a consensus, small arteries show from 2 to 8 layers of smooth muscle. Arterioles have 1 to 2 layers with a diameter of about 30μm. Arterioles are major players in controlling the flow of blood that enters the capillary network by contracting their smooth muscle cells. In fact, these muscle cells are usually slightly contracted, so they can precisely regulate the flow of blood. Actually, they are the main ones responsible for blood pressure. Both small arteries and arterioles have the same three tunics that are found in other arteries. Both small arteries and arterioles have the same three tunics that are found in other arteries.
Capillaries
Capillaries are the smallest blood vessels, sometimes having a diameter smaller than a red blood cell. The exchange of gases and substances between the blood and tissues occurs mainly across the capillary walls. Capillary walls are made up of a thin endothelium and a basal lamina, so molecules and gases can cross them easily. Capillaries are organized in large and widely distributed plexuses that irrigate all the organs of the body. This type of irrigation is known as perfusion.
The endothelium can be continuous, fenestrated, or discontinuous, according to the cellular features and organization (Figure 4). The continuous endothelium is the more abundant type. The endothelial cells seal the intercellular spaces so tightly that only the smallest molecules can cross the endothelium through the intercellular spaces. The endothelial cells show many cytoplasmic vesicles that indicate strong endocytosis and exocytosis processes. The fenestrated endothelium shows endothelial cells with channels or passages directly connecting the blood with the basal lamina. This type of endothelium is found in the endocrine glands and intestine, places where the flux of substances into the blood is very intense. The discontinuous, or sinusoidal, endothelium is less frequent. Here, endothelial cells do not completely seal the intercellular spaces, allowing substances and even cells to freely cross the endothelial layer. The discontinuous endothelium is found in the liver, bone marrow, and spleen.
Veins
Veins show the same histological organization as arteries. However, veins usually have larger diameters, and the tunica media is not so developed (Figure 5). In addition, many veins, particularly those in the limbs, contain valves within the lumen (Figure 6), preventing blood from going backward, which may be caused by gravity or mechanical pressures. Veins are classified according to their size into large, medium, and small veins, or venules.
Large veins
Large veins are larger than 10 mm in diameter. Their tunica intima is formed of endothelium, scarce subendothelial tissue, and few smooth muscle cells. The differences between tunica intima and tunica media are not easily distinguished. The tunica media is thin, with smooth muscle fibers arranged perpendicular to the long axis of the vessel. The tunica adventitia, the thicker layer of the large veins, is made up of connective tissue and muscle fibers oriented longitudinally.
Middle-sized veins
These veins are smaller (about 10 mm in diameter) than large veins and account for most of the veins of the organism. The shape of the medium-sized veins is more irregular than that of the arteries. The three tunicae are clearly observed. The tunica intima shows endothelium, basal lamina, and a thin layer of connective tissue containing a few smooth muscle cells. Sometimes, an internal elastic membrane can be distinguished. The tunica media, while thinner than in the larger arteries, show several layers of smooth muscle cells interspersed with connective tissue. The tunica adventitia is formed of connective tissue and is thicker than the tunica media.
Venules
There are two types of venules: postcapillary and muscular. Postcapillary veins obtain the blood directly from capillaries. They have a very small diameter (about 0.1 mm). The adhesion properties of the endothelial cells change by molecular signals, which allow lymphocytes and blood serum to easily cross the endothelial layer. These venules do not show a "true" tunica media (figure 5). Muscular venules, about 1 mm in diameter, are located after the postcapillary venules. They have a tunica media with one or two layers of smooth muscle cells and also show a thin tunica adventitia.
Heart
The heart is the organ that pumps the blood in the circulatory system, helped by body movements. It is mostly made up of cardiac muscle cells, a cell type unique to this organ.
The mammalian heart consists of four chambers: two ventricles that pump the blood and two auricles, one that receives the pulmonary blood and the other that collects the blood from the rest of the body. The two auricles are separated by the interventricular septum, and the two ventricles by the ventricular septum (Figure 7). At each of the auricle entrances, there is a valve that prevents the blood from flowing backward.
The walls of both ventricles and auricles are made up of three layers: epicardium, myocardium, and endocardium. The epicardium is the outer layer composed of mesothelium outward and connective tissue inward. Blood vessels and nerves that enter the cardiac muscle run through the epicardium. Many adipose cells are also found in this layer. The myocardium is mostly cardiac muscular cells, intermingled with scarce connective tissue. In the ventricles, the myocardium is thicker than in the auricles and shows an inner and an outer layer. The muscles of the outer layer are arranged in spirals and swirls, whereas the inner layer wraps around the ventricles and auricles. The endocardium is formed of an endothelium in contact with the blood and connective tissue containing some smooth muscle cells. The connective tissue in the endocardium, in contact with the myocardium, contains blood vessels and nerves.
The interventricular septum (or ventricular septum) is composed of cardiac muscle lined by endothelium. The interauricular septum (or atrial septum) is thinner and shows the same tissular organization as the interventricular septum, although it is a fibrous structure in some parts.
Cardiac valves consist of connective tissue covered by endothelium. From the inner to the outer part, each valve is made up of three layers: fibrosa, spongiosa, and ventricularis. They comprise different types of connective tissue: dense, loose, and dense, respectively.
In humans, there is a specialized structure known as the atrial appendix, which is found in the pericardium, next to the free wall of the left auricle. It regulates the vascular blood volume. The atrial appendix is 46 mm in size and may contain 9 ml of blood. When cardiomiocytes are stretched, the atrial appendix releases hormones like atrial natriuretic peptide and brain natriuretic peptide into the coronary sinus. These peptides enter the bloodstream and regulate blood pressure.
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Bibliography ↷
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Bibliography
CNX Anatomy and Physiology. Structure and Function of Blood Vessels
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Integument