Blood vessels Circulation

1. Chapter 21 Blood vessels & Circulation

2. Vessel comparison

3. 3 Main Tunics Tunica intima (tunica interna) Endothelial layer that lines the lumen of all vessels In vessels larger than 1 mm, a subendothelial connective tissue basement membrane is present Tunica media Smooth muscle and elastic fiber layer, regulated by sympathetic nervous system Controls vasoconstriction/vasodilation of vessels Tunica externa (tunica adventitia) Collagen fibers that protect and reinforce vessels Larger vessels contain vasa vasorum

4. Structure of Vessel Walls

5. Arteries Transport blood away from the heart 3 types: 1. Elastic (conducting)- near the heart (aorta & major branches) elastin in all 3 tunics (most in media) a lot of smooth mm but not very active in vasoconstriction 2. Muscular (distribution)-deliver blood to specific organs thickest tunica media of all vessels less elastic tissue & more smooth mm active in vasoconstriction elastic lamina on each side of the tunica media 3. Arterioles - smallest of the arteries large - all 3 tunics present (mostly tunica media) small - mostly smooth mm spiraling around the endothelium control the amount of blood entering capillaries vasoconstriction - blood bypasses tissues vasodilation - blood flow into local capillaries increases dramatically

6. Capillaries 3 types: continuous, fenestrated, sinusoidal Average length is 1 mm average diameter is 8-10 micrometers RBCs fit through in a single file structure is ideal for function (exchange) all tissues have capillaries except: cornea/lens (aqueous humor) cartilage/epithelia (nutrition from BVs in nearby CTs) tendons/ligament�s are poorly vascularized also

7. Continuous capillary Most common-found in abundance in skin & muscle adjacent cells joined by tight junctions may leave an intercellular cleft to allow passage of fluids and small solutes tight junctions of brain capillaries are always complete around the entire perimeter of the endothelial cells and makes up the blood brain barrier

8. Fenestrated capillary Some endothelial cells have pores (fenestra) fenestra = window found where active capillary absorption or filtration occurs small intestine-receive digested nutrients endocrine organs-H�s have rapid entry into blood kidneys-rapid filtration of plasma choroid plexus

9. Sinusoids Highly modified, leaky capillaries found only in liver, bone marrow, lymphoid tissue, & some endocrine organs Irregular shaped w/ fewer tight junctions & larger intercellular clefts to allow large molecules (proteins and blood cells) to pass between the blood and surrounding tissues Slows blood flow and allows time for it to be modified ie-liver has time to absorb nutrients from digestive organs while removing & destroying any bacteria

10. Capillary Beds A microcirculation of interwoven networks of capillaries, consisting of: Vascular shunts � (metarteriole) thoroughfare channel connecting an arteriole directly with a postcapillary venule True capillaries � 10 to 100 per capillary bed, capillaries branch off the metarteriole and return to the thoroughfare channel at the distal end of the bed

11. Blood Flow Through Capillary Beds Precapillary sphincter Cuff of smooth muscle that surrounds each true capillary Regulates blood flow into the capillary Blood flow is regulated by vasomotor nerves and local chemical conditions (vasomotion), so it can either bypass or flood the capillary bed

12. Venules Venules are formed when capillary beds unite Smallest (post-capillary) venules are only an endothelial lining w/ a few pericytes larger ones have 1-2 layers of smooth mm cells (tunica media) and a scanty tunica externa

13. Veins Have 3 distinct tunics but the walls are always thinner (w/ larger lumens) than those in arteries usually carry ~65% of total blood volume at a given time & are still not filled to capacity Called �capacitance vessels� Valves (folds of tunica intima resembling semilunar heart valves), which prevent backflow of blood varicose veins, hemorrhoids

14. Blood Distribution Heart, arteries, and capillaries: 30�35% of blood volume Venous system: 60�65% 1/3 of venous blood is in the large venous networks of the liver, bone marrow, and skin Veins (capacitance vessels) stretch more than arteries

15. Venous return 1. Muscular pump skeletal muscle contraction milks blood towards heart 2. Respiratory pump inhale->increases intra-abdominal pressure, squeezing local veins-> pressure w/in chest decreases & thoracic veins expand to speed blood entry into the RV

16. Vascular anastomoses Collaterals Multiple arteries that contribute to 1 capillary bed Allow circulation if 1 artery is blocked

17. Vascular aneurysms

18. Dynamics of Blood Circulation Blood flow (ml/min) - actual vol of blood flowing thru a vessel, organ, or entire circulation in a given period; CO if considering entire vascular system Blood Pressure (BP) - force exerted on a vessel by its contained blood; expressed in mmHg; referring to systemic arterial blood unless otherwise stated; different than pressure gradient Pressure gradient is the difference between pressure at the heart and pressure at peripheral capillary beds

19. Circulation terms Resistance - opposition to blood flow (friction); usually refer to peripheral resistance (PR) since this is where most friction is encountered 3 sources of resistance: Blood viscocity - related to thickness or stickiness of blood; more viscous�slower flow (4x water) Total BV length - longer vessel=more resistance ~ 1# fat�1 mile of vasculature to supply it BV diameter - smaller=more resistance turbulence from erratic BV walls, heart chambers, and plaque�more resistance

20. Resistance Factors: Blood Vessel Diameter Changes in vessel diameter are frequent and significantly alter peripheral resistance Resistance varies inversely with the fourth power of vessel radius (one-half the diameter) For example, if the radius is doubled, the resistance is 1/16 as much

21. Relationship b/t flow, pressure, & resistance Blood flow (F) is directly proportional to the difference in blood pressure (?P) between two points in the circulation If ?P increases, blood flow speeds up; if ?P decreases, blood flow declines Blood flow is inversely proportional to resistance (R) If R increases, blood flow decreases Resistance is much more of a factor than systemic pressure think dilation of a vessel & resistance

22. Systemic blood pressure BP = CO x PR systolic pressure - w/ contraction of LV diastolic pressure - w/ relaxation of LV pulse pressure - difference b/t systolic & diastolic pressure mean arterial pressure (m.a.p.) � pressure that propels the blood to the tissues m.a.p. = diastolic p + 1/3 pulse pressure

23. Arterial Blood Pressure Arterial BP reflects two factors of the arteries close to the heart Their elasticity (compliance or distensibility) The amount of blood forced into them at any given time Blood pressure in elastic arteries near the heart is pulsatile (BP rises and falls)

24. Capillary Exchange of Respiratory Gases and Nutrients Oxygen, carbon dioxide, nutrients, and metabolic wastes diffuse b/t the blood and interstitial fluid along concentration gradients Oxygen and nutrients pass from the blood to tissues Carbon dioxide and metabolic wastes pass from tissues to the blood Water-soluble solutes pass through clefts and fenestrations Lipid-soluble molecules diffuse directly through endothelial membranes

25. Direction and amount of fluid flow depends upon the difference between: Capillary hydrostatic pressure (CHP) Capillary colloid osmotic pressure (BCOP) CHP � pressure of blood against the capillary walls: Tends to force fluids out of the capillary walls Is greater at the arterial end of a bed than at the venule end BCOP� created by nondiffusible plasma proteins, which draw water toward themselves (into the capillary) Capillary Exchange: Fluid Movements

26. Net Filtration Pressure (NFP) NFP � considers all the forces acting on a capillary bed NFP = (CHP � IHP) � (BCOP � ICOP) At the arterial end of a bed, hydrostatic forces dominate (fluids flow out) At the venous end of a bed, osmotic forces dominate (fluids flow in) More fluids enter the tissue beds than return blood, and the excess fluid is returned to the blood via the lymphatic system

27. Net Filtration Pressure (NFP)

28. Capillary Dynamics Hemorrhaging: reduces CHP and NFP increases reabsorption of interstitial fluid (recall of fluids) Dehydration: increases BCOP accelerates reabsorption Increase in CHP or BCOP: fluid moves out of blood builds up in peripheral tissues (edema)

29. Tissue Perfusion Blood flow through the tissues Carries O2 and nutrients to tissues and organs Carries CO2 and wastes away Is affected by: cardiac output peripheral resistance blood pressure

30. 3 factors affecting blood pressure 1. Cardiac output (depends on blood volume) 2. Peripheral resistance 3. Blood volume BLOOD PRESSURE=C.O. X P.R.

31. Cardiac Output (CO) Cardiac output is determined by venous return and neural and hormonal controls Resting heart rate is controlled by the cardioinhibitory center via the vagus nerves Stroke volume is controlled by venous return (EDV) Under stress, the cardioacceleratory center increases heart rate and stroke volume The end systolic volume (ESV) decreases and MAP increases

32. Cardiovascular Regulation Changes blood flow to a specific area: at an appropriate time in the right area without changing blood flow to vital organs

33. 3 Regulatory Mechanisms Control cardiac output and blood pressure: 1. Autoregulation: causes immediate, localized homeostatic adjustments 2. Neural mechanisms: respond quickly to changes at specific sites 3. Endocrine mechanisms: direct long-term changes

34. Neural control mechanism Vasomotor center - SNS neurons in the medulla overseeing changes in vessel diameter teamed up w/ cardioacceleratory centers make up cardiovascular center which affect BP thru changes in C.O. & BV diameter sends SNS efferents out cord levels T1-L2 to innervate smooth mm of BVs (especially arterioles) arterioles in a state of vasomotor tone increased SNS-> vasoconstriction decreased SNS-> vasodilation

35. Modifiers of vasomotor tone Baroreceptors carotid sinus, aortic sinus, right atrium stretching of vessel (increase in Pressure) causes (+) of PsNS & (-) of vasomotor center�increases vessel diameter, decreases HR, CO, PR, & BP resulting decrease in MAP causes reflex vasoconstriction, increased CO & BP so that the resulting BP changes are minimized fnx w/ rapid changes in pressure (sitting to standing) change �set point� w/ chronic hypertension�not effective with sustained stretching

36. Baroreceptor-Initiated Reflexes Declining BP stimulates the cardioacceleratory center to: Increase cardiac output and peripheral resistance Low BP also stimulates the vasomotor center to constrict b.v.�s

37. Modifiers of vasomotor tone Chemoreceptors - Blood pressure is regulated by chemoreceptor reflexes sensitive to oxygen and carbon dioxide drop in pH--> (+) of cardioacceleratory center & vasomotor center to speed the return of the blood to the heart & lungs to blow off CO2 Prominent chemoreceptors are the carotid and aortic bodies Reflexes that regulate blood pressure are integrated in the medulla Higher brain centers (cortex and hypothalamus) can modify BP via relays to medullary centers hypothalamus helps redirect blood w/ changes from exercise and changes in body temperature

38. Chemoreceptor Reflexes

39. Local vasoconstrictors Adrenal medulla hormones � norepinephrine and epinephrine increase blood pressure Antidiuretic hormone (ADH) � causes intense vasoconstriction in cases of extremely low BP Angiotensin II � kidney release of renin generates angiotensin II, which causes intense vasoconstriction Endothelium-derived factors � endothelin and prostaglandin-derived growth factor (PDGF) are both vasoconstrictors

40. Local vasodilators Atrial natriuretic peptide (ANP) � causes blood volume and pressure to decline Nitric oxide (NO) � has brief but potent vasodilator effects Inflammatory chemicals � histamine, prostacyclin, and kinins are potent vasodilators Alcohol � causes BP to drop by inhibiting ADH Low oxygen/high carbon dioxide levels Low pH

41. The Endocrine System Hormones have short-term and long-term effects on cardiovascular regulation e.g., E and NE, hormones produced by adrenal medullae

42. Other Hormones Antidiuretic hormone (ADH)-from low blood volume or high plasma [ ] Reduces water loss at kidneys Angiotensin II-from drop in renal BP Angiotensinogen from liver Powerful vasoconstrictor Stimulates ADH & aldosterone production Erythropoietin (EPO)-from low oxygen in kidneys Natriuretic peptides (ANP, BNP) Atrial natriuretic peptide (cells in right atrium) Brain natriuretic peptide (ventricular muscle cells)

43. Monitoring Circulatory Efficiency Efficiency of the circulation can be assessed by taking pulse and blood pressure measurements Vital signs � pulse and blood pressure, along with respiratory rate and body temperature Pulse � pressure wave caused by the expansion and recoil of elastic arteries Radial pulse (taken on the radial artery at the wrist) is routinely used Varies with health, body position, and activity

44. Measuring Blood Pressure Systemic arterial BP is measured indirectly with the auscultatory method A sphygmomanometer is placed on the arm superior to the elbow Pressure is increased in the cuff until it is greater than systolic pressure in the brachial artery Pressure is released slowly and the examiner listens with a stethoscope�called Korotkoff sounds The first sound heard is recorded as the systolic pressure The pressure when sound disappears is recorded as the diastolic pressure

45. Alterations in Blood Pressure Hypotension � low BP in which systolic pressure is below 100 mm Hg Hypertension � condition of sustained elevated arterial pressure of 140/90 or higher Transient elevations are normal and can be caused by fever, physical exertion, and emotional upset Chronic elevation is a major cause of heart failure, vascular disease, renal failure, and stroke

46. Hypotension Orthostatic hypotension � temporary low BP and dizziness when suddenly rising from a sitting or reclining position Chronic hypotension � hint of poor nutrition and warning sign for Addison�s disease Acute hypotension � important sign of circulatory shock Threat to patients undergoing surgery and those in intensive care units

47. Hypertension Hypertension maybe transient or persistent Primary or essential hypertension � risk factors in primary hypertension include diet, obesity, age, race, heredity, stress, and smoking Secondary hypertension � due to identifiable disorders, including excessive renin secretion, arteriosclerosis, and endocrine disorders

48. Blood flow thru body tissues Tissue perfusion 1. Delivery of O2 & nutrients/removal of wastes 2. Gas exchange in lungs 3. Absorption of nutrients from digestive tract 4. Urine formation by kidneys Blood flow to each organ/tissue is in exact proportion to its needs

49. Body at rest Brain- 13% of total blood flow heart- 4% � � kidneys - 20% � � abdominal organs - 24% � � skeletal mm (~1/2 body tiss) - 20% � �

50. Velocity of blood flow Blood velocity: Changes as it travels through the systemic circulation Is inversely proportional to the cross-sectional area of the vessel Slow capillary flow allows adequate time for exchange between blood and tissues

51. Velocity of Blood Flow

52. Vessel Diameter comparison

53. Autoregulation Short-term metabolic - CO2, ions, acids (ie.lactic), inflammatory chemicals (ie. histamine) myogenic - stretch of vessel responds w/increased tone (vasoconstriction) while a reduced stretch ? vasodilation to bring increased blood flow to the tissue Long-term angiogenesis- when long term nutritional needs are not met (ie. High altitude/coronary vessel occlusion) the # of BVs in a region increases and those already present enlarge

54. Special Circulation Through organs with separate mechanisms to control blood flow: brain heart lungs

55. Blood Flow: Brain Blood flow to the brain is constant, as neurons are intolerant of ischemia Metabolic controls � brain tissue is extremely sensitive to declines in pH, and increased carbon dioxide causes marked vasodilation Myogenic controls protect the brain from damaging changes in blood pressure Decreases in MAP cause cerebral vessels to dilate to ensure adequate perfusion MAP below 60mm Hg can cause syncope (fainting) Increases in MAP cause cerebral vessels to constrict MAP above 160 can result in cerebral edema

56. Blood Flow: Heart Small vessel coronary circulation is influenced by: Aortic pressure The pumping activity of the ventricles During ventricular systole: Coronary vessels compress Myocardial blood flow ceases Stored myoglobin supplies sufficient oxygen During ventricular diastole, oxygen and nutrients are carried to the heart

57. Blood Flow: Lungs Blood flow in the pulmonary circulation is unusual in that: The pathway is short Arteries/arterioles have a much lower arterial pressure (24/8 mm Hg versus 120/80 mm Hg) The autoregulatory mechanism is exactly opposite of that in most tissues Low oxygen levels cause vasoconstriction; high levels promote vasodilation This allows for proper oxygen loading in the lungs

58. Blood Distribution during Exercise

59. 3 Effects of Light Exercise Extensive vasodilation occurs: increasing circulation Venous return increases: with muscle contractions Cardiac output rises: due to rise in venous return (Frank�Starling principle) and atrial stretching

60. 5 Effects of Heavy Exercise Activates sympathetic nervous system Cardiac output increases to maximum: about 4 times resting level Restricts blood flow to �nonessential� organs (e.g., digestive system) Redirects blood flow to skeletal muscles, lungs, and heart Blood supply to brain is unaffected

61. Training and Cardiovascular Performance

62. Circulatory shock Circulatory shock � any condition in which blood vessels are inadequately filled and blood cannot circulate normally Results in inadequate blood flow to meet tissue needs

63. Hypovolemic shock Excessive blood loss/ diarrhea/ vomiting/ excessive burns initial vasoconstriction must replace volume fast

64. Responses to Blood Loss

65. 3 Short-Term Responses to Hemorrhage 1. Carotid and aortic reflexes: increase cardiac output (increasing heart rate) cause peripheral vasoconstriction 2. Sympathetic nervous system: triggers hypothalamus further constricts arterioles venoconstriction improves venous return 3. Hormonal effects: increase cardiac output increase peripheral vasoconstriction (E, NE, ADH, angiotensin II)

66. Vascular shock Blood volume is normal & constant poor circulation ->extreme vasodilation-> big drop in PR common w/anaphylaxis due to widespread histamine release from an allergy, septicemia (from toxins causing vasodilation), and a failure of ANS regulation (neurogenic shock) From pump failure heart cannot sustain adequate circulation often from M.I.