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Aims • Regulation of vascular tone. • Capillaries • Readings; Sherwood, Chapter 10 Vascular Tone • The state of arteriole resistance that is established by the arteriolar smooth muscle. – Normally under a slightly constricted state • Smooth muscle cells undergo self-induced contractile activity due to ___________________ potentials. Sherwood’s Human Physiology 10-10 Vasoconstriction • Increased contraction of circular smooth muscle cells in arteriolar wall. • Results in: – Increased resistance. – __________________________ flow. • Due to: – – – – – – Increased myogenic activity. Increased oxygen. Decreased carbon dioxide. Increased sympathetic stimulation. Vasopressin, Angiotensin II Cold Sherwood’s Human Physiology 10-10 Vasodilation • Decreased contraction of circular smooth muscle cells in arteriolar wall. • Results in: – Decreased resistance. – _________________ flow. • Due to: – – – – – – Decreased myogenic activity. Decreased oxygen. Increased carbon dioxide. Decreased sympathetic stimulation. Histamine release Heat Sherwood’s Human Physiology 10-10 Regulation of Vascular Tone • Intrinsic (local) Controls – Important in distribution of cardiac output • Chemical influences • Physical influences • Extrinsic Controls – Important in blood pressure regulation • Neural influences • Hormonal Influences Regulation of Vascular Tone • Intrinsic (local) Controls • Chemical Influences • • • • • • • Oxygen Carbon Dioxide Blood pH Blood Osmolarity Adenosine Prostaglandins Histamine Regulation of Vascular Tone • Intrinsic (local) Controls • Physical Influences – Heat or Cold application – Myogenic response to stretch Regulation of Vascular Tone • Extrinsic Controls • _____________________ Nervous System – Mainly vasoconstrictor nerve fibers (norepinephrine). • Most potent in kidneys, gut, spleen, and skin. • Less potent in skeletal muscle and brain. – Very few vasodilator fibers (epinephrine). • Unlikely to play an important role in normal circulation. • Fainting due to emotional distress. Regulation of Vascular Tone • Extrinsic Controls • Hormonal System – Norepinephrine and epinephrine from the adrenal medulla • Support sympathetic nervous stimulation. – Vasopressin and Angiotensin II • Potent vasoconstrictors • Play important role in maintaining blood pressure during hemorrhage. Regulation of Vascular Tone • Extrinsic Controls • Adrenergic Receptors – 1 (vasoconstriction) – 2 (vasodilation) Sherwood’s Human Physiology Table 10-4 Response to Exercise and Fright • Stimulate sympathetic system and inhibit parasympathetic system. – Almost all arterioles are constricted. (increase peripheral resistance) • Increases arterial pressure. – Veins are constricted. • Displaces blood from large peripheral blood vessels toward the heart. • Result in heart beating stronger. (increasing stroke volume) – Autonomic nervous system stimulates heart rate. Response to Exercise and Fright • Exercise causes local O2 depletion. – Resulting in local vasodilation • Elevated BP and local vasodilation = great supply for skeletal muscle. Sherwood’s Human Physiology 10-12 Summary of Regulation Sherwood’s Human Physiology 10-14 Veins are Volume Reservoirs • Veins serve as ________ resistance passageways to return blood to the heart. • Additionally, they serve as a blood reservoir. – Thinner walls and less smooth muscle than arteries. Sherwood’s Human Physiology 10-28 (10-27 6th Edition) Veins Have Valves • Valves prevent backward blood movement. • When valves no longer function properly you get varicose veins. – Allows for the pooling of blood and swelling of the vein. Sherwood’s Human Physiology 10-33 (10-32 6th Edition) Venous Pump • Gravity forces blood to pool in the lower extremities. – Thus pressure in the lower veins is greater than that of the higher veins. • The venous pump or muscle pump pushes blood up from the lower extremities. – Utilizing muscle contraction and valves to ratchet blood upward. • Thus, when walking the blood pressure in leg is 25mm instead of 90mm Guyton’s Textbook of Medical Physiology 15-10 Capillaries Capillaries make up most of the total cross-sectional area of blood vessels (600 m2). Every cell is within 0.01 cm of a capillary. Contain only about 5% of total blood volume at any given time. Very slow velocity of blood flow (distance/time). Sherwood’s Human Physiology 10-16 Blood Flow through Capillaries Same volume flows from A1-B1 as flows from A2-B2 in 1 minute (= flow rate), but the distance from A1-B1 > A2-B2 (velocity of flow). Sherwood’s Human Physiology 10-17 Capillary lumen size • Capillaries have lumens about 7 mm but can be as small as 3.5mm. • Barely big enough for erythrocytes (about 8 mm) to squeeze through. Sherwood’s Human Physiology 10-15 Capillary Regulation of Blood Flow • Capillaries branch off of metarterioles. • At this location the capillary is circled by a smooth muscle cell forming the precapillary sphincter. • Regulates whether the capillary is open or closed. • O2 Dependent. Sherwood’s Human Physiology 10-19 Capillaries are the site of fluid and nutrient exchange • Capillaries prime function is _____________ exchange • • • • Intercellular clefts. Plasmalemmal vesicles. Vesicular channels Fenestrations Sherwood’s Human Physiology 10-18 Diffusion Through Membranes • Lipid soluble substances pass right through the endothelial cell. – O2 & CO2 Sherwood’s Human Physiology 10-18 Intercellular Clefts • Intercellular cleft – Water filled pore. – 6-7 nm wide • Small water-soluble substances pass through these pores. – Ions Na+ and K+ – Glucose – Amino acids Sherwood’s Human Physiology 10-18 Plasmalemmal Vesicles • Lipid insoluble proteins are moved across by vesicular transport. – Non-plasma proteins. – Plasma proteins are usually unable to leave the circulation. Sherwood’s Human Physiology 10-18 Vesicular Channels • Vesicles coalesce to form channels that go completely through the endothelial cell. Adopted from Ganong’s Review of Medical Physiology Similar to Guyton’s Textbook of Medical Physiology 16-2 Specialized Capillaries • Fenestrations – Numerous small oval windows which pass through the endothelial cell. – Found in the kidney. – Responsible for filtration in the glomeruli. Adopted from Ganong’s Review of Medical Physiology Similar to Guyton’s Textbook of Medical Physiology 16-2 Specialized Capillaries • In the liver sinusoids the intercellular clefts are very large. – Resulting in almost all dissolved substances including plasma proteins can pass from the blood to the liver tissue. • In the _________________ the intercellular clefts do not exist due to tight junctions between endothelial cells. – Resulting in only small molecules such as water, CO2 and O2 can pass from the blood to this tissue. Interstitial Fluid • Makes up 80% of the ECF while the plasma makes up the remaining 20%. • Gel of _________________ and Proteoglycans. • Collagen fiber bundles. • Free fluid vesicles. • Free fluid. Sherwood’s Human Physiology 10-21 5th Edition only & Guyton’s Textbook of Medical Physiology 16-4 Exchange of Solutes • Solutes will diffuse __________________ their concentration gradients during capillary-cell exchange. • Also leave capillaries via bulk flow which is most important in maintaining plasma volume. Sherwood’s Human Physiology 10-22 (10-21 6th Edition) Forces Influencing Bulk Flow • Capillary hydrostatic pressure – Forces fluid out of the capillaries. • Interstitial fluid hydrostatic pressure – Usually forces fluid into the capillaries. • Plasma colloid osmotic pressure – Causes osmosis driving fluid into the capillary. • Interstitial fluid colloid osmotic pressure – Causes osmosis driving fluid out of the capillaries. Guyton’s Textbook of Medical Physiology 16-5 Exchange of Solutes Sherwood’s Human Physiology 10-24 (10-23 6th Edition) Next Time • Lymphatic system. • Blood composition – Erythrocyte production • Reading; Sherwood, Chapter 10 Objectives 1. Describe the function and properties of blood vessels and their regulation. 1. Distensibility and Compliance 2. Pulsatility 3. Vascular Tone (vasoconstriction & vasodilation) 2. Describe the structure and function of veins.