Vessels and Pressures Fig

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Vessels and Pressures Fig

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University of Southern California
Bisc 307l - General Physiology
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BISC 307L 2nd Edition Lecture 28 Current Lecture Blood Vessels Aneurysms only occur in arteries, because only in arteries is the wall strain high enough to cause an aneurysm. Only 3 out of these 5 vessels have smooth muscle in their walls: arteries, arterioles, and veins. Capillaries and venules don’t (they consist of a single layer of endothelial cells). The vascular smooth muscle is capable of contracting or not – when contracting, it is called vasoconstriction. When not contracting, the pressure inside the vessel causes them to inflate, and this is called vasodilation. The vasoconstriction of the arteries and especially the arterioles is going to increase the resistance to blood flow. These changes in resistance to blood flow make it possible to redirect blood flow in the systemic circulation, which has many paths - so regional vasoconstriction can redirect blood flow one way or another. For example, after lunch vasodilation causes blood to flow toward the digestive organs. Microcirculation Circulation of small vessels. Can see an artery feeding into an arteriole into capillaries – flow is down from the top, across to the right, then back up. Arteries are strongly reinforced by connective tissue sheaths. Arterioles do not have this – also, they can generate force very efficiently for long periods of time. They are always contracted to some extent, 24 hours a day – their level of contraction is called vascular tone. Capillaries have no smooth muscle except at the place where the arterioles meet the capillaries, there are bands of smooth muscles called pre-capillary sphincters, and they regulate how much blood is going through the capillary network. In most tissues, every cell in the tissue is no more than a few cells away from a capillary – so there is dense vascularization of capillaries through most tissues. Under most conditions, most of the capillaries do not have blood flowing through them – the precapillary sphincters are constricted. Only 10-20% of capillaries at a time have blood flowing through them. Blood has to get from arteriole side to venous side to get back to the heart, so if the capillaries are sealed off, how does the blood get by? They get through thoroughfare channels formed by metarterioles or arteriovenous bypasses. Metarterioles have some smooth muscle around it, but not as much as a normal arteriole. These are bigger vessels and tend to be thicker walled than capillaries so they don’t contribute much to gas/nutrient exchange. Because they are bigger, leukocytes(which are larger than RBC’s) use them instead of capillaries to get around. Angiogenesis There’s a lot of research recently on angiogenesis – the formation of new blood vessels. This is important in tumor research – a tumor attracts blood vessels so that it can grow. So there has been a lot of advances on the growth factors that promote angiogenesis. Important stimulants: -vascular endothelial growth factor (VEGF) – this can happen in the retina due to vascular damage caused by diabetes that forces VEGF to be used to vascularize ...

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