From the most interior layer to the outer, these tunics are the tunica intima, the tunica media, and the tunica externa see. However, those close to the heart have the thickest walls, containing a high percentage of elastic fibers in all three of their tunics. The internal jugular veins form the major venous drainage of the head and neck and are deep veins that parallel the common carotid artery. The arterioles can also constrict or dilate, which varies their resistance, to help distribute blood flow to the tissues. These are generally all sympathetic fibers, although some trigger vasodilation and others induce vasoconstriction, depending upon the nature of the neurotransmitter and receptors located on the target cell. Generally, the tunica media is the bulkiest layer in arteries, which bear the chief responsibility for maintaining blood pressure and proper circulation. In vessels larger than 1 mm in diameter, a subendothelial layer, consisting of a basement membrane and loose connective tissue, supports the endothelium.
Although a tight junction is usually impermeable and only allows for the passage of water and ions, they are often incomplete in capillaries, leaving intercellular clefts that allow for exchange of water and other very small molecules between the blood plasma and the interstitial fluid. Pulmonary veins then return freshly oxygenated blood from the lungs to the heart to be pumped back out into systemic circulation. In this way, they help serve cellular needs. Pressure is a measure of the force that the blood exerts against the vessel walls as it moves the blood through the vessels. Substances absorbed into the blood from the exterior, either through the external or internal surfaces of the body, are thus brought round and presented to every cell, to be taken up or rejected according to the needs of the latter. If you are able to palpate some of the superficial veins on your upper limbs and try to move them, you will find that the tunica externa prevents this. Solutes also move across the capillary wall according to their concentration gradient, but overall, the concentrations should be similar and not have a significant impact on osmosis.
Physiology of Circulation Roles of Capillaries In addition to forming the connection between the arteries and veins, capillaries have a vital role in the exchange of gases, nutrients, and metabolic waste products between the blood and the tissue cells. Chapter Review Blood pumped by the heart flows through a series of vessels known as arteries, arterioles, capillaries, venules, and veins before returning to the heart. They arise from the subclavian arteries in the base of the neck near the tip of the lungs and pass upwards through the openings foramina of transverse Temporal artery Facial artery External carotid artery Common carotid artery Temporal artery Facial artery External carotid artery Common carotid artery Fig 5. As a result, blood has a higher colloidal concentration and lower water concentration than tissue fluid. Exchange of gases and other substances occurs in the capillaries between the blood and the surrounding cells and their tissue fluid interstitial fluid.
If the tunica externa did not hold the vessel in place, any movement would likely result in disruption of blood flow. Interaction of Hydrostatic and Osmotic Pressures The normal unit used to express pressures within the cardiovascular system is millimeters of mercury mm Hg. In common usage, the term blood pressure refers to arterial blood pressure, the pressure in the aorta and its branches. Parasympathetic stimulation does trigger vasodilation as well as erection during sexual arousal in the external genitalia of both sexes. Recent studies, however, have shown that it is physiologically critical to such activities as helping to regulate capillary exchange and altering blood flow. Thus, water is drawn from the tissue fluid back into the capillary, carrying dissolved molecules with it.
Comparison of Tunics in Arteries and Veins Arteries Veins General appearance Thick walls with small lumens Generally appear rounded Thin walls with large lumens Generally appear flattened Tunica intima Endothelium usually appears wavy due to constriction of smooth muscle Internal elastic membrane present in larger vessels Endothelium appears smooth Internal elastic membrane absent Tunica media Normally the thickest layer in arteries Smooth muscle cells and elastic fibers predominate the proportions of these vary with distance from the heart External elastic membrane present in larger vessels Normally thinner than the tunica externa Smooth muscle cells and collagenous fibers predominate Nervi vasorum and vasa vasorum present External elastic membrane absent Tunica externa Normally thinner than the tunica media in all but the largest arteries Collagenous and elastic fibers Nervi vasorum and vasa vasorum present Normally the thickest layer in veins Collagenous and smooth fibers predominate Some smooth muscle fibers Nervi vasorum and vasa vasorum present Tunica Intima The tunica intima also called the tunica interna is composed of epithelial and connective tissue layers. Considering all capillaries over the course of a day, this can be quite a substantial amount of fluid: Approximately 24 liters per day are filtered, whereas 20. Thus, water is drawn from the tissue fluid back into the capillary, carrying dissolved molecules with it. In reality, all blood vessels exhibit vascular tone due to the partial contraction of smooth muscle. Sinusoid capillaries are flattened, and they have extensive intercellular gaps and incomplete basement membranes, in addition to intercellular clefts and fenestrations. These pinocytotic vesicles subsequently fuse with lysosomes to hydrolyze break down the particles. A region higher in solute concentration and lower in water concentration draws water across a semipermeable membrane from a region higher in water concentration and lower in solute concentration.
Review Questions Arterioles receive blood from arteries, which are vessels with a much larger lumen. Local controls, discussed later, account for this phenomenon. The wall of a capillary consists of the endothelial layer surrounded by a basement membrane with occasional smooth muscle fibers. Continuous Capillaries The most common type of capillary, the continuous capillary, is found in almost all vascularized tissues. Veins as Blood Reservoirs In addition to their primary function of returning blood to the heart, veins may be considered blood reservoirs, since systemic veins contain approximately 64 percent of the blood volume at any given time. .
The smooth muscle layers of the tunica media are supported by a framework of collagenous fibers that also binds the tunica media to the inner and outer tunics. Substances containing albumins, such as egg white, are called albuminoids. Between the smallest arteries, or arterioles, and the capillaries are intermediate vessels called precapillaries, or metarterioles, that, unlike the capillaries, have muscle fibres that permit them to contract; thus the precapillaries are able to control the emptying and filling of the capillaries. Thus, fluid generally moves out of the capillary and into the interstitial fluid. In turn, muscular arteries branch to distribute blood to the vast network of arterioles. The diameter of a capillary lumen ranges from 5—10 micrometers; the smallest are just barely wide enough for an erythrocyte to squeeze through. In contrast, the movement of fluid from an area of higher pressure in the tissues into an area of lower pressure in the capillaries is reabsorption.
As you will note in , approximately 21 percent of the venous blood is located in venous networks within the liver, bone marrow, and integument. Although recognized by the American Medical Association, there are currently no licensing requirements for vascular technicians, and licensing is voluntary. Normally, the precapillary sphincters are closed. It can be felt where an artery is close to the surface and rests on something firm. Vascular technicians are specialists in imaging technologies that provide information on the health of the vascular system. Thus, fluid generally moves out of the capillary and into the interstitial fluid.