The human cardiovascular system carries blood from the heart to all organs and tissues of the body, delivering nutrients and oxygen to them, removing carbon dioxide and other waste products. This is the most important system that has a complex structure. However, to understand the processes occurring in the body, it is necessary to understand at least in general terms how it works. We will talk about this.
Development of the cardiovascular system
Due to the need to organize the transport of nutrients and blood within the embryo, the organs of the vascular system begin to develop among the first. Before others, they also reach some functional maturity.
Around the 18th day of pregnancy, the cells of the embryo begin to cluster between the outer shell (ectoderm) and the inner shell (endoderm). Soon they are rearranged so that the cells lying on the periphery unite, forming a continuous flattened layer. It seems to contain more centrally located cells; the latter remain suspended in the liquid medium. The tubes then expand and combine to form a network; thus primitive blood vessels appear.
So, by the month of pregnancy, the blood vessels consist of a closed system of tubes through which blood is transported to all parts of the body of the embryo and back to its heart. The structure and function of blood vessels are very closely related to the heart.
Elements of the circulatory system
There are several types of blood vessels of different sizes and with different functionality. So, the first type among them is an artery. Its wall consists of three layers:
- Intima, the innermost layer, is a smooth endothelium;
- Media is the middle shell, which is the thickest in arteries, especially in large ones. It consists of smooth muscle cells intertwined with elastic fibers;
- Adventitia is the outer, most durable layer, consisting of collagen and elastic fibers. The adventitia provides a restrictive barrier, protecting the vessel from overexpansion.
The outer layer is characterized by the presence of small blood vessels, called “vasa vasorum”, supplying the walls of large vessels. In contrast, the inner and middle layers are nourished by diffusion from the blood as it is transported. The thicker and more elastic wall of the arteries allows them to expand with the passage of a pulse wave and then restore their original size.
The next caliber of blood vessels are arterioles. The transition from artery to arteriole is gradual, marked by progressive thinning of the vessel wall and a decrease in the size of its lumen. Their middle shell no longer contains elastic fibers and consists of only one layer of circular or spiral smooth muscle fibers. The adventitial sheath consists of connective tissue elements.
Small arteries and arterioles act as control valves through which blood is ejected into capillaries, the smallest vessels. A strong muscular wall is able to close the passage or allow it to expand to several times its normal size, thereby greatly altering the blood flow to the capillaries and therefore to the tissues. With the help of this feature, more blood is directed to those organs and tissues that need it most.
The next group of vessels is venous. The main function of venules is to collect blood from a network of capillaries. These vessels consist of an endothelial tube supported by a small amount of collagen tissue. Larger venules may contain some smooth muscle fibers. As the venules continue to grow in size, they begin to exhibit the wall structure of arteries, although they are much thinner.
In veins, which serve to carry blood from peripheral tissues to the heart, the endothelial lining is surrounded by a media, which contains less muscle and elastic tissue than the arterial wall. The outer layer, the adventitia, is made up primarily of connective tissue.
The blood pressure in these vessels is extremely low compared to the pressure in the arterial system. This creates the need for a special mechanism to keep the blood moving as it returns to the heart.
To do this, many veins have a unique valve system. They, formed by the semilunar folds of the intima, are arranged in pairs and serve to direct blood flow to the heart, especially in an upward direction. When blood flows to the heart, the valve leaflets press against the wall of the vein. On the contrary, at the moment when the pressure of the blood and surrounding tissues fills the pocket of the valve, it seems to “protrude” into the lumen of the vessel, blocking it. Most of these valves are in the veins of the lower extremities, since it is most difficult to raise blood from them to the heart.
Veins are more distensible than arteries and their walls are designed to allow them to expand or contract. Veins tend to run parallel to arteries, but are present in greater numbers. Their channels are larger than those of the arteries, and the walls are thinner.
Anatomy of the arterial network
The pulmonary trunk runs diagonally upward to the left along the course of the aorta. Then it divides into two branches – the right and left pulmonary arteries, which flow into the lungs. Once in the lungs, the vessels branch into smaller ones – up to the capillaries. Surrounding the alveoli (air sacs) of the lungs, these small vessels take in oxygen and release carbon dioxide. Capillaries carrying oxygenated, i.e. oxygenated blood join larger and larger vessels until they reach the pulmonary veins, which carry oxygenated blood from the lungs to the left atrium of the heart.
The aorta is the largest vessel in the systemic circuit, originating from the left ventricle. It is divided into three sections: the ascending aorta, the aortic arch and the descending aorta; the latter can be further subdivided into the thoracic and abdominal aorta. The right and left coronary arteries depart from the ascending aorta and supply the heart with oxygenated blood. Three large arteries depart from the aortic arch, named in order of origin from the heart brachiocephalic, left common carotid and left subclavian. These three branches supply blood to the head, neck and arms.
Two vertebral arteries, one of which is a branch of the brachiocephalic and the other of the left subclavian artery, join at the base of the brain to form the basilar artery. The blood supply to the brain is carried out mainly from the vessels of the circle of Willis – two vertebral and two internal carotid arteries and connecting vessels between them.
The arms are supplied by the subclavian artery on the left and the continuation of the brachiocephalic artery on the right. Approximately at the border of the first rib, both of these vessels become the axillary artery, then the brachial. Further, approximately at the level of the elbow, the vessel divides into two terminal branches: the radial and ulnar arteries. Anastomoses (in other words, relationships) between them with branches at the level of the palm supply the hand and wrist.
The thoracic part of the descending aorta gives off branches that supply blood to the internal organs (visceral branches) and the walls surrounding the chest cavity (parietal branches). As the aorta descends below the diaphragm, the abdominal aorta begins. She again gives both visceral and parietal branches.
The abdominal aorta divides into two common iliac arteries, each of which descends and gives rise to external and internal branches. The right and left external iliac arteries are a direct continuation of the common iliac arteries and are called the femoral arteries. After passing through the inguinal region, they give off branches that supply blood to the structures of the abdomen, pelvis and lower extremities.
At a point just above the knee, the femoral artery continues like the popliteal artery. The posterior and anterior tibial arteries depart from it. The posterior tibial artery is a direct continuation of the popliteal artery, descending down the lower leg to supply blood to the structures of the posterior leg and foot. At a short distance below the knee, the peroneal artery departs from the posterior tibial artery; branches depart from it, which nourish the muscles of the lower leg and fibula and end in the foot. The anterior tibial artery runs down the leg to the ankle, where it becomes the dorsal artery of the foot.
The human venous system
Venules collect blood from capillaries and blood channels, known as sinusoids, and combine to form ever larger veins. The latter end in large veins, and those are hollow.
There are superficial and deep veins in the limbs. Superficial veins lie directly under the skin and collect blood from skin capillaries and superficial fascia (layers of fibrous tissue), while deep veins accompany the main arteries of the limbs and have similar names to them. Often there are connections between superficial and deep veins.
Superior and inferior vena cava
The veins of the head and neck, arms and part of the chest unite to form the superior vena cava. The internal jugular vein is a continuation of the venous system of the brain and goes down through the neck. It receives blood from parts of the face, neck, and brain. All veins of the arm are tributaries of the subclavian vein of this side:
- The radial and ulnar veins converge at the elbow joint, forming the brachial vein;
- Brachial vein, in turn, connects with the main vein at the level of the shoulder, forming the axillary;
- On the outer border of the first rib, the axillary vein becomes the subclavian, the end point of the venous system, characteristic of the upper limb.
The subclavian, external jugular, and internal jugular veins merge to form the brachiocephalic vein. From two sides they flow into the superior vena cava, which flows into the upper-posterior part of the right atrium. In addition to the brachiocephalic veins, the superior vena cava receives blood from the azygos vein and small veins from the mediastinum.
The inferior vena cava is a large, valveless venous trunk that receives blood from the legs, back, walls, and contents of the abdominal cavity and pelvis. At the level of the inguinal ligament, located at the anterior diagonal border between the trunk and thigh, the femoral vein becomes the external iliac vein. Connecting with the internal iliac vein, they form the common iliac vein. The two common iliac veins then join at a level above the coccyx to become the inferior vena cava. Heading up through the abdominal cavity, the inferior vena cava receives blood from the common iliac and lumbar, renal, adrenal and hepatic veins, and then flows into the right atrium.
An extensive network of approximately 10,000,000,000 microscopic capillaries ensures the exchange of fluids, nutrients, and waste products between the blood and tissues. The diameter of the largest capillary is approximately 0.2 millimeters (about the width of the tip of a pin).
These vessels are tubular extensions of cells in the inner lining of larger vessels. The capillary wall is extremely thin and acts as a semi-permeable membrane, allowing substances containing small molecules such as oxygen, carbon dioxide, water, fatty acids, glucose, and ketones to pass through.
Thus, the structure of the human vascular system is complex and extensive. Each large vessel is subdivided into many small ones, arranged in their own order and supplying blood to their area or taking blood from it.
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No not always. In any case, blood flows from the heart through the arteries, and through the veins to the heart, but venous, oxygen-poor blood flows in the pulmonary arteries. It comes there from the right parts of the heart, collected from the whole body.
Arterial blood, rich in oxygen, flows from the heart through the arteries to organs and tissues. In small capillaries, gas exchange occurs, and oxygen enters the destination. Venous vessels take the depleted blood and carry it back to the heart. In addition, blood flows from the heart to the lungs and then back to the pumping organ.
It is absolutely impossible to calculate this figure. Each person has an individual structure of the vascular network. Of course, the main large vessels are unchanged in most cases, but smaller ones – capillaries and even arterioles with venules – are variable in number and structure. We can only say that the total length of all the vessels of the body of an adult is about 100,000 km.
Each person has several types of blood vessels. First, they are divided by caliber: large, medium, small. In addition, they are divided according to what kind of blood – saturated or poor in oxygen – they carry. Thus, large arterial vessels include arteries, large venous vessels – veins. Similarly, arterioles are medium arterial vessels, arterial capillaries are small; medium venous – venules, small – venous capillaries.