Venules are the postcapillary blood vessels that connect directly to the capillary bed, where the exchange of substances between blood and surrounding tissues takes place. They are already visible to the naked eye and represent the beginning of the venous vascular system that transports blood back to the heart. Unlike the larger veins into which the venules flow, they are not equipped with venous valves.
What is the venule?
Blood pumped from the heart to the target tissue in the great circulation (systemic circulation) and the small circulation (pulmonary circulation) flows in the ever-branching arteries. In the target tissue, the blood passes through the narrow capillary system, where the exchange of substances with the surrounding tissue cells takes place. Directly “behind” the capillary system begins the venous vascular system. The venules, with a diameter of 10 to 100 micrometers, immediately adjoin the capillaries and are already visible to the naked eye. As they progress, venules coalesce and form veins, which in turn flow into larger veins – roughly comparable to a river that takes in tributaries. Postcapillary venules differ from veins not only in their smaller diameter, but they also lack the venous valves that ensure that blood in veins is transported exclusively in one direction, toward the heart. The walls of the venules immediately adjacent to the capillaries, with a diameter of 10 to 30 micrometers, do not yet have a distinct layer of smooth muscle cells (tunica media). The characteristic layers of smooth muscle cells are found only in the thicker collecting venules and in muscular venules.
Anatomy and structure
Venules can be divided into three categories: postcapillary venules (10 to 30 microns), collecting venules (30 to 50 microns), and muscular venules (50 to 100 microns), each with a slightly different structure. The walls of the thin postcapillary venules are partially permeable, similar to the walls of capillaries. They still provide the ability to exchange substances with the tissue, as a downstream “last chance”, so to speak. In lymphatic tissue (lymph nodes, tonsils), the postcapillary venules are formed as so-called high-endothelial venules. Their inner walls (endothelium) consist of specially shaped cells, which allow the large leukocytes to escape into the surrounding tissue in the event of a necessary immune response. The reverse process, the entry of leukocytes formed in lymphoid follicles, is also possible. Both processes are referred to as lympho- or leukodiapedesis. That part of the venules whose epithelium contains no or few smooth muscle cells cannot actively contract or relax. Therefore, they are enclosed by extensions of pericytes. These are connective tissue cells whose extensions have the ability to contract and relax. The missing active part of the venules for contracting and relaxing is largely taken over by the pericytes.
Function and Tasks
The main function of the venules is to receive blood after it has passed through the capillaries and to drain it into the veins. In the case of the great circulation, venous blood is deoxygenated and enriched with degradation products from body metabolism. The metabolic products are mainly excreted or further metabolized in the liver and kidneys. In the case of the small body or pulmonary circulation, the blood in the capillaries is enriched with oxygen from the alveoli and the carbon dioxide content is reduced. The carbon dioxide excreted into the alveoli is exhaled with the breath. In addition to the main task of initiating the return transport of blood to the heart, the venules immediately adjacent to the capillaries also perform part of the exchange of substances with the surrounding tissue. The additional function of the venules thus slightly overlaps with the function of the capillaries. In specialized lymphoid tissue such as lymph nodes and pharyngeal tonsils (tonsils), the postcapillary venules perform a special function. Their epithelium is designed to take up leukocytes formed in nearby lymphoid follicles, for example, into their lumen when needed, or to release leukocytes into the tissue. In certain tissues, such as the nasal mucosa, venules form an interconnected network.If the downstream veins constrict and the blood flow is slowed as a result, a regular blood congestion can occur in the network of venules. The nasal mucosa can then swell so much that the nose “closes up” and breathing through the nose is no longer possible.
Diseases
The exchange of substances between tissue and blood, which takes place in the capillaries and postcapillary venules, is of enormous importance in supplying cells with the necessary energy and with required substances. Equally important is disposal, the movement of breakdown products into the bloodstream so that the “waste products” can be disposed of into the environment or further metabolized in specific organs. Diseases and ailments related to a restricted exchange of substances are usually due to a change in the walls of the microvessels (arterioles, capillaries, venules). Due to pre-existing conditions such as diabetes, high blood pressure and chronic stress, as well as lack of exercise and smoking, deposits can form in the walls of the microvessels, impairing the circulation of the blood and impeding the exchange of substances. As a result, premature aging processes of the cells occur. Complaints and symptoms such as memory and concentration problems, tinnitus or the well-known “shop window disease” in heavy smokers are typical accompanying symptoms. The extent to which high cholesterol levels, especially a high proportion of LDLs in the total cholesterol fraction, can be causative for plaques in the blood vessels has been critically questioned by experts for some years.
