They are highly specialized cell type and cover the luminal surface of the tunica intima. Endothelial cells have a number of critical functions. During a resting phase, endothelial cells have important anti-clotting functions.
They have anticoagulant effects expression of heparin like molecules, production of thrombomodulin, tissue factor pathway inhibitor , inhibit platelet aggregation ADPase, prostacyclin , and promote fibrinolysis synthesis of tissue type plasminogen activator. Other functions include:. Endothelial cells can be identified by their location. They are the flattened, fusiform cells lining the lumen of vessels. Other ultrastructural features include cytoplasmic vesicles that reflect phagocytic and pinocytotic activity.
Highly specialized endothelial modifications can be present depending on the organ and location see discussion on capillary structure and function. Supporting the vascular endothelium, and forming the remainder of the tunica intima, is a basement membrane and a subendothelial layer.
Diapedesis, the exit of leukocytes from the vasculature, occurs also at this level of the microvasculature. You will study this process in more detail in Pathology. Veins of small and medium size are characterized by a thin media containing only a few layers of smooth muscle cells. These vessels have a much thicker adventitia composed of collagen and occasionally some longitudinal smooth muscle fibers.
In general, veins are larger in diameter and have thinner walls than arteries. The tunica adventitia makes up the greater part of the venous wall of large veins and is usually considerably thicker than the tunica media. Other variations in the structure of blood vessels occur in certain organs in response to special functional and anatomical conditions.
Some examples of special vessels include:. Pre-Lab Quiz List, in order, the specific segmental variations of blood vessels that a red blood cell would travel through on a continuous circuit beginning in the left ventricle and ending in the right atrium of the heart. Answer: Elastic artery, muscular artery, arteriole, capillary, small venule, large venule, vein. Answer: The blood-brain barrier has a continuous endothelium, which can also be found in the muscles, lung, and skin.
Gases diffuse across the endothelium, but most other substances must be transported. Answer: You would expect to see a decrease in the amount of elastic tissue and an increase in the amount of smooth muscle cells. Answer: The intima remains the same, the media decreases in size, and the adventitia increases in size as you move from the arteries to the veins. Slides Please select whether to view the slides in study mode or quiz mode.
In study mode, the images will contain labels and a description. In quiz mode, labels and description will be hidden. Study Mode. Virtual Microscope Slides Aorta The aorta is a large elastic artery. Begin at low magnification by identifying the three layers of the artery wall and their major cellular and structural components.
Begin by identifying the major components of the neurovascular bundle: the nerves, artery, and vein. Structure of the Artery Wall : This diagram of the artery wall indicates the smooth muscle, external elastic membrane, endothelium, internal elastic membrane, tunica externa, tunica media, and tunica intima. A major structural difference between arteries and veins is the presence of valves.
In arteries, the blood is pumped under pressure from the heart, so backflow cannot occur. However, passing through the capillary network results in a decrease in blood pressure, meaning that backflow of blood is possible in veins. To counteract this, veins contain numerous one-direction valves that prevent backflow. Blood plays many critical roles within the body: delivering nutrients and chemicals to tissues, removing waste products, and maintaining homeostasis and health.
The circulatory system is transports blood through the body to perform these actions, facilitated by the extensive network of blood vessels.
The circulatory system can be split into two sections, systemic and pulmonary. Upon reaching the capillary networks, gas exchange between tissue and blood can occur, facilitated by the narrow walls of the capillaries.
Oxygen is released from the blood into the tissues and carbon dioxide, a waste product of respiration, is absorbed. The much smaller pulmonary system reoxygenates the blood and facilitates the removal of carbon dioxide. After leaving the heart through the right ventricle, the blood passes through the pulmonary artery, the only artery in the body that contains deoxygenated blood, and into the capillary network within the lungs.
The close association of the thin-walled alveolae with the equally thin-walled capillaries allows for rapid release of carbon dioxide and uptake of oxygen. After leaving the lungs through the pulmonary vein, the only vein which carries oxygenated blood, the blood enters the left atrium.
This completes the pulmonary circulatory system. The Circulatory System : This simplified diagram of the human circulatory system anterior view shows arteries in red and veins in blue.
Blood vessels also facilitate the rapid distribution and efficient transport of factors such as glucose, amino acids, or lipids into the tissues and the removal of waste products for processing elsewhere, such as lactic acid to the liver or urea to the kidneys. Additionally, blood vessels provide the ideal network for immune system surveillance and distribution.
Numerous white blood cells circulate around the body, sensing for infection or injury. Once an injury is detected, they rapidly leave the circulatory system by passing through gaps in vessel walls to reach the affected area while signalling for a larger targeted immune response.
Mechanically the blood vessels, especially those near the skin, play a key role in thermoregulation.
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