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بسم هللا الرحمن الرحيم ﴿و ما أوتيتم من العلم إال قليال﴾ صدق هللا العظيم االسراء اية 58 dr abdelaziz Hussein, Mansoura Faculty of Medicine By Dr. Abdel Aziz M. Hussein Lecturer of Medical Physiology Member of American Society of Physiology dr abdelaziz Hussein, Mansoura Faculty of Medicine dr abdelaziz Hussein, Mansoura Faculty of Medicine dr abdelaziz Hussein, Mansoura Faculty of Medicine • They are extremely thin vessels to allow exchange of nutrients and cellular excreta ( ) the tissues and the circulating blood occur No • About 10 billion capillaries Surface area • Total surface area is about 500 to 700 m2. dr abdelaziz Hussein, Mansoura Faculty of Medicine dr abdelaziz Hussein, Mansoura Faculty of Medicine dr abdelaziz Hussein, Mansoura Faculty of Medicine Thickness 0.5 um Endothelial cell Diameter 4-9 um Basement membrane dr abdelaziz Hussein, Mansoura Faculty of Medicine dr abdelaziz Hussein, Mansoura Faculty of Medicine •Blood in capillary is not continuous, but it is intermittent Cause: • 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 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 Regulation of capillary Vasomotion: Low O2 tension dr abdelaziz Hussein, Mansoura Faculty of Medicine Regulation of capillary Vasomotion: High O2 tension dr abdelaziz Hussein, Mansoura Faculty of Medicine • 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 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. Arterial End 30-35 mmHg Mean Cap. P. 25 mmHg dr abdelaziz Hussein, Mansoura Faculty of Medicine Venous End 10-15 mmHg Extracapillary Passive Factors 1. Arteriolar diameter Active Capillary Contraction 1. Chemical factors 2. Venous Pressure 2. Nervous factors 3.Gravity 3. Physical factors dr abdelaziz Hussein, Mansoura Faculty of Medicine 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 Arteriolar dilatation Arteriolar Constriction dr abdelaziz Hussein, Mansoura Faculty of Medicine ↑ capillary B.P. ↓ capillary B.P. ↑ capillary B.P. ↓ capillary B.P. dr abdelaziz Hussein, Mansoura Faculty of Medicine ↑ venous B.P. ↓ Venous B.P. dr abdelaziz Hussein, Mansoura Faculty of Medicine dr abdelaziz Hussein, Mansoura Faculty of Medicine Swelling of endothelial cells Contraction of metarterioles Contraction of precapillary sphincter dr abdelaziz Hussein, Mansoura Faculty of Medicine 1. Vasoconstrictor substances 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. dr abdelaziz Hussein, Mansoura Faculty of Medicine 2. Vasodilator substances 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. dr abdelaziz Hussein, Mansoura Faculty of Medicine • 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 • Warming causes capillary VD because heat relaxes the smooth ms fibers of the precapillary sphincter. • Cooling 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 ◊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. 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 T=Pxr dr abdelaziz Hussein, Mansoura Faculty of Medicine 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. ◊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). dr abdelaziz Hussein, Mansoura Faculty of Medicine dr abdelaziz Hussein, Mansoura Faculty of Medicine THANKS dr abdelaziz Hussein, Mansoura Faculty of Medicine