Chapter 14 Cardiovascular System Blood Vessels and Blood Pressure Overview of Vasculature Blood vessels are classi ed according to whether they carry blood to or away from the heart and according to size Arteries and smaller arterioles carry blood from the heart and to capillaries Venules and then larger veins return blood back to the heart Overview of Vasculature The lumen of all blood vessels is lined by a layer of epithelial cells called endothelium The walls of all blood vessels except capillaries contain various amounts of smooth muscle Within the connecBve Bssue are collagen and elasBn proteins enabling them to withstand large pressures and expand and contract Arterioles and Resistance to Blood Flow When arteriolar smooth muscle contracBon increases the radius of the arterioles decreases vasoconstricBon and resistance increases When the arteriolar smooth muscle relaxes the radius of arterioles increases vasodilaBon and resistance decreases The average arterial pressure during the cardiac cycle is the mean arterial pressure MAP When you go to a physician to have your blood pressure taken what is actually measured is an esBmate of the arterial pressure Mean Arterial Pressure MAP HR X SR X TPR Mean arterial pressure is completely determined by three factors Heart rate Stroke Volume Total peripheral resistance Neural Control of MAP Sensory receptor neurons that respond to changes in pressure in the cardiovascular system are called arterial baroreceptors Found in two locaBons AorBc arch CaroBd sinuses of the caroBd arteries Neural Control of MAP Arterial baroreceptors respond speci cally to the stretching that occurs during pressure changes in arteries They relay pressure informaBon to the CNS which exerts control over cardiovascular funcBon via the autonomic nervous system Control Center in the Medulla Oblongata Neural control of the MAP is orchestrated primarily by the medulla oblongata which relays signals to the heart by way of sympatheBc and parasympatheBc neurons Anatomy of Capillaries Capillaries are the primary site where exchange of nutrients and waste products occur between blood and Bssue They are also the smallest blood vessels measuring only about 1 mm long and 5 10 um in diameter This small and thin wall provides a small di usion distance between the blood and the surrounding uid Anatomy of Capillaries 10 40 billion capillaries providing a surface area of about 600 square meters in the human body As blood enters networks of capillaries called capillary beds the blood velocity decreases which allows for exchange between blood and Bssue Also enhancing this exchange is the leakiness of the capillary walls Anatomy of Capillaries There are two types of capillaries The rst called con nuous capillaries are more common They have endothelial cells joined together such that the spaces between them are rela vely narrow and are highly permeable to small molecules and lipophilic molecules The second are called fenestrated capillaries whose epithelial cells posses relaBvely large pores allowing small water soluble substances to cross and in some cases even proteins and small cells What would be the simplest way to regulate the exchange of materials across the capillary walls MicrocirculaBon Most Bssues contain metarterioles which act as bypass channels or shunts allowing blood to ow from the arterioles to venule bypassing the capillaries Smooth muscles of metarterioles can contract or relax regulaBng how much blood ows through the capillaries Blood ow is also regulated by precapillary sphincter muscles which constrict the capillaries increasing their resistance to blood ow Materials are exchanged across capillary walls through di erent mechanisms depending on the molecular size and lipid solubility and on the loca on of the capillary Bulk Flow Across Capillary Walls FiltraBon occurs near the whereas absorpBon occurs near the Overview of Vasculature Blood vessels are classi ed according to whether they carry blood to or away from the heart and according to size Arteries and smaller arterioles carry blood from the heart and to capillaries Venules and then larger veins return blood back to the heart Veins The Skeletal Muscle Pump When a muscle contracts it presses against the veins which raises the venous pressure and drives blood toward the heart When it is relaxed backward ow is prevented by closure of one way valves in the veins How Venous Pressure A ects MAP
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