Aims

1. Aims • Regulation of vascular tone. • Capillaries • Readings; Sherwood, Chapter 10

2. 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

3. 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

4. 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

5. 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

6. Regulation of Vascular Tone • Intrinsic (local) Controls • Chemical Influences • Oxygen • Carbon Dioxide • Blood pH • Blood Osmolarity • Adenosine • Prostaglandins • Histamine

7. Regulation of Vascular Tone • Intrinsic (local) Controls • Physical Influences • Heat or Cold application • Myogenic response to stretch

8. 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.

9. 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.

10. Regulation of Vascular Tone • Extrinsic Controls • Adrenergic Receptors • 1 (vasoconstriction) • 2 (vasodilation) Sherwood’s Human Physiology Table 10-4

11. 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.

12. 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

13. Summary of Regulation Sherwood’s Human Physiology 10-14

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)

15. 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)

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. Diffusion Through Membranes • Lipid soluble substances pass right through the endothelial cell. • O2 & CO2 Sherwood’s Human Physiology 10-18

23. 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

24. 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

25. 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

26. 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

27. 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.

28. 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

29. 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)

30. 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

31. Exchange of Solutes Sherwood’s Human Physiology 10-24 (10-23 6th Edition)

32. Next Time • Lymphatic system. • Blood composition • Erythrocyte production • Reading; Sherwood, Chapter 10

33. Objectives • Describe the function and properties of blood vessels and their regulation. • Distensibility and Compliance • Pulsatility • Vascular Tone (vasoconstriction & vasodilation) • Describe the structure and function of veins.