1. صدق الله العظيم الاسراء اية 58 dr abdelaziz Hussein, Mansoura Faculty of Medicine

2. By Dr. Abdel Aziz M. Hussein Lecturer of Medical Physiology Member of American Society of Physiology dr abdelaziz Hussein, Mansoura Faculty of Medicine

5. They are extremely thin vessels to allow exchange of nutrients and cellular excreta ( ) the tissues and the circulating blood occurNo About 10 billion capillaries Surface area Total surface area is about 500 to 700 m 2. dr abdelaziz Hussein, Mansoura Faculty of Medicine

10. Blood in capillary is not continuous, but it is intermittent Cause: ( Vasomotion)It is due to intermittent contraction of the metarterioles and precapillary sphincters ( Vasomotion) There is a cycle of constriction and relaxation for 5 to 10 times per min, so only about 10-20% of the capillaries are opened at a time. dr abdelaziz Hussein, Mansoura Faculty of Medicine

11. Regulation of capillary Vasomotion:Regulation of capillary Vasomotion: Vasomotion is controlled by the O2 concentration in the tissues. When the O2 concentration is very low→ the intermittent periods of blood flow occur more often and the duration of each period of flow lasts for a long time, thereby allowing the blood to carry ↑ed quantities of O2 to the tissues. dr abdelaziz Hussein, Mansoura Faculty of Medicine

12. Regulation of capillary Vasomotion:Regulation of capillary Vasomotion: dr abdelaziz Hussein, Mansoura Faculty of Medicine

13. Regulation of capillary Vasomotion:Regulation of capillary Vasomotion: dr abdelaziz Hussein, Mansoura Faculty of Medicine

14. It is about 0.5-1 mm/second→ slow blood flow→ suitable for exchange of materials between the plasma and interstitial fluid. dr abdelaziz Hussein, Mansoura Faculty of Medicine

15. Value: At the arterial end, it is about 30-35 mmHg. At the venous end, it is about 10-15 mmHg. The mean capillary blood pressure is 25 mmHg. dr abdelaziz Hussein, Mansoura Faculty of Medicine

16. Extracapillary Passive Factors 1. Arteriolar diameter2. Venous Pressure3.Gravity Active Capillary Contraction 1. Chemical factors2. Nervous factors3. Physical factors dr abdelaziz Hussein, Mansoura Faculty of Medicine

17. a. Diameter of arterioles: Arteriolar VD →↑es the capillary blood flow and the capillary pressure. VC of the arterioles has the reverse effect. b. Venous pressure: -↑ed venous pressure → ↑capillary BP and ↓capillary blood flow. -↑ed venous pressure occurs in; Congestive heart failure. Local obstruction of the vein by thrombosis or tumour. c. Gravity: It ↑es the capillary blood pressure in parts below the level of the heart by antagonizing the VR. It ↓es the capillary BP in parts above the level of the heart. dr abdelaziz Hussein, Mansoura Faculty of Medicine

23. a.Adrenaline: VC of cutaneous and splanchnic capillaries. b.Noradrenaline →VC of all the capillaries except the coronaries. c.ADH → generalized VC of the capillaries d.Angiotensin II → generalized VC of the capillaries. 1. Vasoconstrictor substances dr abdelaziz Hussein, Mansoura Faculty of Medicine

24. a.Adrenaline: VD of Sk. Ms and coronary capillaries. b.Acetyl choline →VD of all the capillaries including coronaries c.Histamine → generalized VD of the capillaries d.Gases → CO2 excess, ↑ed H and O2 lack produce capillary dilatation. 2. Vasodilator substances dr abdelaziz Hussein, Mansoura Faculty of Medicine

25. True capillaries receive no nerve supply, so it is not affected by nervous factors. Capillary dilatation or constriction is dependent on the tone of the precapillary sphincter which is influenced by the local regulatory mechanisms. dr abdelaziz Hussein, Mansoura Faculty of Medicine

26. Warming Warming causes capillary VD because heat relaxes the smooth ms fibers of the precapillary sphincter. CoolingCooling of the skin for a short time produces VC, while prolonged cooling causes VD due to accumulation of metabolites. Skin colour and temperature The colour of the skin depends on the state of capillaries The temperature of the skin depends on the state of the arterioles dr abdelaziz Hussein, Mansoura Faculty of Medicine

27. ◊Def, It is the liability of the capillary wall to rupture on raising the capillary pressure. The capillaries do not rupture easily under normal conditions because of their very small diameter→ according to the law of Laplace. This law states that the tension developed in the wall of a hollow organ (T) is equal to the distending pressure (P) x the radius (r) of the organ Law of Laplace T = P x r dr abdelaziz Hussein, Mansoura Faculty of Medicine

28. Law of Laplace As the pressure in the aorta is 4 times the pressure in the capillary and its radius is 4000 times as much as the radius of the capillary, so the tension in the aortic wall is 16000 times the tension in the capillary wall. This very low tension in the capillary wall (1/16000 that in the aortic wall) explains the little liability of the capillary wall to rupture. dr abdelaziz Hussein, Mansoura Faculty of Medicine ◊Causes of ↑ed capillary fragility: a.Defects in their wall (e.g. vitamin C deficiency, certain toxins, allergic conditions, old age). b.Defect in blood (e.g. thrombocytopenic purpura).

29. dr abdelaziz Hussein, Mansoura Faculty of Medicine

30. THANKS dr abdelaziz Hussein, Mansoura Faculty of Medicine