Circulatory organization and microcirculation
Terminologically, the circulatory system is often separated from the lymphatic system. The human cardiovascular system is a closed loop forming two main circuits of circulation:
- The pulmonary circuit is responsible for gas exchange: in the pulmonary capillaries, blood releases carbon dioxide and is oxygenated.
- The systemic circuit performs a trophic function, delivering nutrients and oxygen to tissues and removing metabolic waste.
The general sequence of blood flow is as follows: blood is directed from the heart through arteries to the microcirculatory bed (MCB). The classic pathway within the MCB involves the transition from arterioles to capillaries, and then to venules. However, an alternative route also exists—the shunting of blood through arteriovenous anastomoses (AVAs) bypassing the capillary network. The return of blood to the heart occurs via the venous system.
Hemodynamic parameters: pressure and velocity
Blood in the body is divided into circulating and pooled. At rest, 1.0 to 1.5 liters are excluded from active blood flow and located in blood depots (spleen, liver, skin).
Blood movement is driven by a pressure gradient that gradually drops from the aorta to the venae cavae due to blood viscosity and friction against the vessel wall:
- In the aorta, pressure drops insignificantly (from 100 to 99 mm Hg).
- A sharp drop occurs in the arterioles (a decrease of more than 40 mm Hg).
- The most intense pressure drop per unit length is observed in the capillaries due to their narrow lumen.
- In the venae cavae and the right atrium, pressure can become negative (down to –2 mm Hg). Venous return is ensured by the suction effect during atrial relaxation, rather than residual kinetic energy.
When assessing blood flow, two types of velocity are distinguished:
- Linear velocity depends on the total vascular cross-sectional area. It is maximal in the aorta (45 cm/s) and minimal in capillaries (0.5 mm/s). In veins, it increases again as vessels merge.
- Volumetric flow rate remains constant at any level of the vascular tree and is about 5 L/min.
Temporal parameters of hemodynamics
The rate of blood transit through different segments varies significantly. Blood spends about 30 seconds in the systemic circuit and approximately 10 seconds in the pulmonary circuit. The average time for a complete cycle is 40 seconds, though some blood may pass faster through shunts (in 20–25 s), while some is significantly slower.
The greatest blood delay is observed in the microcirculatory bed (about 16 s). At the same time, the maximum residence time occurs in venules (about 9.5 s). The physiological significance of this phenomenon is that it is precisely in the venules that conditions are created for leukocytes to migrate into surrounding tissues.
Embryonic vascular development
The circulatory system develops from mesenchyme. The process of primary angiogenesis begins in the wall of the yolk sac, then extends to the chorion, and only later to the body of the embryo itself.
Vessel formation goes through the stage of so-called 'blood islands'. Cell clusters form in the mesenchyme and subsequently differentiate. The peripheral cells of the island flatten, giving rise to the endothelium (the primary vessel wall), while the central cells round up and become primary blood cells.
Isolated primary vessels gradually fuse to form a single closed network. Subsequent histogenesis of the vessel wall (development of muscular and elastic layers) directly depends on local hemodynamic conditions. It is pressure, pulsation, and flow velocity that determine whether a specific vessel will turn into an artery, vein, or capillary.
Organization of the lymphatic system
The lymphatic system permeates most body organs, except for the epithelium, cartilage tissue, brain, red bone marrow, eyeball, and placenta.
Structurally, the bed is organized from the periphery to the center:
- Lymphatic capillaries begin with blind ends. Their main task is the filtration of tissue fluid and the removal of excess water, metabolites, and foreign particles.
- Lymph nodes interrupt the course of vessels. They contain lymphocytes that trap and neutralize antigens. In oncology, lymph nodes are of immense importance: if a node is unable to destroy malignant cells, a secondary tumor focus (metastasis) forms within it.
- Large trunks collect lymph and return it to the venous system. The largest is the thoracic duct (ductus thoracicus), which drains 75% of the lymph into the left venous angle. The right lymphatic duct collects the remainder and empties into the right venous angle.
Briefly: answers to topical questions
What factors ensure the venous return of blood to the heart?
Venous return to the heart is ensured by a combination of hemodynamic and mechanical factors.
These factors include:
- The suction action of the heart — the pressure drop in the right atrium during diastole.
- The suction force of the thoracic cavity — the pressure drop in the chest cavity during inspiration.
- Capillary forces — surface tension forces.
- The valve apparatus — prevents backflow in veins below the level of the heart.
- The muscle pump — skeletal muscle contraction squeezes blood upward through the valves.
- Gravity — facilitates drainage from the head and neck.
- Intestinal peristalsis — mechanical action on abdominal veins.
What types of blood capillaries exist according to their endothelial structure?
The classification of blood capillaries according to the structure of the endothelium and basement membrane includes three types (in order of increasing permeability).
The following types are distinguished:
- Somatic capillaries — characterized by a continuous endothelial lining and a continuous basement membrane.
- Fenestrated capillaries — distinguished by the presence of pores and thinned areas (fenestrae) in the cytoplasm of endothelial cells.
- Perforated capillaries (sinusoidal) — have large transmural openings in the endothelium itself, as well as in the basement membrane.
Class and exam questions
Where in the vascular bed does blood move the slowest, and why?
The minimum linear blood flow velocity (0.5 mm/s) is observed in capillaries. This is because the linear velocity is inversely proportional to the total vascular lumen, and the total cross-sectional area of all capillaries is at its maximum.
How does blood return to the heart if the pressure in the venae cavae can be negative?
Blood return occurs not due to residual kinetic energy, but thanks to the suction effect of the atria at the moment of their relaxation (diastole).
What is the difference between circulating and pooled blood?
Circulating blood actively participates in blood flow and gas exchange. Pooled blood (1.0–1.5 L) is temporarily excluded from circulation and stored in blood depots: the spleen, liver, and skin.
From which embryonic germ layer do blood vessels develop?
Blood vessels develop from mesenchyme through the formation of 'blood islands', where peripheral cells become endothelium and central cells become blood cells.
What else to study on this topic
- Structural features of arterioles, capillaries, and venules
- Arteriovenous anastomoses (AVAs): types and functions
- Histology of the aortic wall and large veins
- Mechanisms of metastasis via lymphatic pathways
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