TOPIC 4 Circulatory System – Blood Flow, Blood Pressure & Capillary Dynamics

1. Biology 221 Anatomy & Physiology II TOPIC 4Circulatory System –Blood Flow, Blood Pressure & Capillary Dynamics Chapter 20 pp. 727-747 E. Lathrop-Davis / E. Gorski / S. Kabrhel

2. Blood Flow • “volume of blood flowing through a vessel, an organ, or the entire circulation in a given period” • Measured in ml/min • To entire system: • blood flow (BF) = cardiac output (CO) • relatively constant at rest • To specific organ or tissue, flow varies with demand

3. Blood Flow: Controlling factors BF = P / R • directly proportional to blood pressure gradient (P) between two points • inversely proportional to peripheral resistance (R) • resistance is more important in controlling local flow

4. Resistance to Blood Flow • “a measure of the amount of friction blood encounters as it passes through vessels” • Peripheral resistance (PR) is resistance in peripheral vessels; accounts for most resistance in system • Resistance is inversely proportional to flow • Think About It: If something is directly related to resistance, what affect will it have on blood flow? What about things that are inversely related to resistance?

5. Resistance to Blood Flow • Sources of resistance include: • blood viscosity – “thickness” of blood • directly proportional to resistance • affected by number of blood cells • blood volume – directly proportional • dehydration  decreases volume  decreases resistance • water retention  increases volume  increases resistance • total blood vessel length – directly proportional • angiogenesis - formation of new blood vessels • fat and tumors lead to angiogenesis

6. Resistance to Blood Flow • blood vessel diameter – inversely related to resistance; main source of resistance • inversely proportional to resistance • increased diameter  decreased resistance • varies as inverse of radius to 4th power (1/r4) • i.e., double radius  resistance decreases to 1/16 original PR • controlled at small arterioles in response to neural and chemical controls • controlled overall by sympathetic vasomotor tone • sudden decrease in size of lumen (e.g., due to partial blockage) creates turbulence  increases resistance

7. Blood Pressure (BP) • “force per unit area exerted on the wall of a blood vessel by its contained blood” • In common usage, “blood pressure” usually refers to blood pressure in systemic arteries near heart • Pressure gradient keeps blood flowing • Measured in mm Hg (millimeters of mercury)

8. Blood Pressure (con’t) Varies through vascular system • Highest and most variable in aorta and other elastic arteries - Why? • Decreases through arterioles and capillaries • Lowest in venae cavae Fig. 20.5, p. 729

9. Arterial Blood Pressure Varies with: • age • gender • weight • stress level • mood • posture • physical activity

10. Arterial Blood Pressure • Depends on: • compliance (distensibility) of elastic arteries • stroke volume • Rises during ventricular systole, decreases during diastole • systolic pressure (Ps) - pressure in arteries during ventricular systole • diastolic pressure (PD) -pressure in arteries during ventricular diastole

11. Systolic and Diastolic Pressures • Systolic pressure (Ps) ~ 110-120 mm Hg • semilunar valves open and blood is ejected • compliance of elastic arteries decreases pressure needed to eject blood into arteries • increased stroke volume (amount ejected)  increased pressure • Diastolic pressure (PD) ~ 70-80 mm Hg • occurs when the semilunar valves are closed because the heart is in diastole • elastic recoil of arteries contributes to continued pressure  movement of blood

12. Pulse Pressure (PP) • difference between systolic (PS) and diastolic (PD) pressures: PP = PS – PD • increased by increased stroke volume (SV) during exertion • increased by arteriosclerosis (loss of elasticity requires much more pressure to force blood into vessels during systole)

13. Mean Arterial Pressure (MAP) • average pressure in main arteries • heart spends more time in diastole • therefore, MAP = diastolic pressure (PD) + (pulse pressure [PP] divided by 3) • MAP = PD + (PP /3)

14. Measuring Pulse • palpation of pulse points (pressure points) • pulse can be felt at major arteries • count number of pulsations felt • pulse decreases strength away from heart Think About It: Where would you expect pulse to be strongest? Fig. 20.11, p. 737

15. Measuring Blood Pressure Auscultatory method • sphygmomanometer • brachial artery usually used • Korotkoff sounds – sounds heard as blood moves through partially blocked artery • normally, ~ 120/80 (for a healthy, young male) • varies with age, sex, physical condition, gender, weight, stress, mood, posture

16. Capillary Blood Pressures • Pressure in capillaries • Pressure drops from ~ 35-40 mm Hg (at arterial end) to ~ 17-20 mm Hg (at venous end) • Lower pressure helps prevent breakage of capillary walls & decreases fluid loss to tissues Fig. 20.5, p. 729

17. Venous Blood Pressures • Low, steady pressure • Venous return supported by: • valves – prevent backflow • varicose veins (see Topic 3: Blood Vessels) • respiratory pump – changes in thoracic and abdominal pressures during breathing • during inspiration: thoracic pressure decreases and abdominal pressure increases, thus pushing blood from the abdominal vessels (inferior vena cava inferior to the diaphragm) into the thoracic part of the inferior vena cava

18. Venous Blood Pressures (con’t) • muscular pump • “milking” by skeleltal muscle promotes return • prolonged inactivity or prolonged contraction causes blood to pool (may allow clots to form)

19. Maintaining Blood Pressure Blood pressure (BP) varies directly with: • Cardiac output (CO; see Topic 2) • controlled by cardiac centers in medulla oblongata • cardioacceleratory center (CAC)  sympathetic outflow • cardioinhibitory center (CIC) parasympathetic outflow Think About It: What is the relationship between HR and BP? Fig. 20.7, 20.8

20. Maintaining Blood Pressure • Peripheral resistance (PR) - pressure varies directly with resistance Think About It: What is the relationship between blood vessel diameter and BP? • Blood volume (BV) - BP varies directly with BV Think About It: What is the relationship between CO and BV? Fig. 20.7, 20.8

21. Short-Term Control of Resistance • Based on controlling blood vessel diameter • Mechanisms include neural and chemical controls • Goals: • alter distribution to meet demands of various organs/tissues • maintain overall MAP through vasomotor tone Fig. 20.8, p. 733

22. Neural Control of Resistance Vasomotor center (VMC) controls vasomotor tone • located in medulla oblongata (as part of cardiovascular center) • maintains vasomotor tone in all vessels • vasomotor fibers are part of the sympathetic division of the ANS (for the most part)

23. Neural Control of Resistance • Vasomotor fibers: • most use norepinephrine (NE) as their • increased activity  vasoconstriction  increased BP • fibers to vessels serving skeletal muscle use ACh • increased sympathetic activity  vasodilation  increased flow to skeletal muscle (generally little importance to overall BP) • Reflexes initiated by baroreceptors or chemoreceptors integrated in medulla oblongata (reticular formation; See A&P I: Unit 6 - Brain)

24. Baroreceptor-initiated Reflexes • Baroreceptors (pressoreceptors) present in carotid sinus*, aortic arch*, most other elastic arteries of neck and thorax • Increased BP stimulates baroreceptors  increased afferent impulses inhibit vasomotor center  decreased sympathetic outflow vasodilation • Afferent impulses from baroreceptors also go to CIC in medulla and increase parasympathetic outflow to heart; also inhibit CAC, thus decreasing sympathetic outflow • Prolonged hypertension causes baroreceptors to “reset” to higher pressure

25. Chemoreceptor-initiated Reflexes • Chemoreceptors located in aortic arch and large arteries of neck • Connected to CAC and vasomotor center by afferent fibers • Respond to oxygen (O2), pH (hydrogen ion), carbon dioxide (CO2) levels • decreased O2 or pH, or increased CO2 increases impulses to CAC and vasomotor center  increased sympathetic outflow • increased heart rate and vasoconstriction  increased BP  helps move blood through system faster  gets blood to lungs faster

26. Influence of Higher Brain Centers on Vasomotor Tone • Cerebral cortex and hypothalamus connected to cardiovascular center (cardiac centers [CAC and CIC] and vasomotor center) in medulla oblongata • Hypothalamus: threats initiate “fight-or-flight” response mediated by hypothalamus  increased sympathetic outflow (CAC and vasomotor center) • hypothalamus directs changes in flow during activity and to control body temperature • Cerebral cortex: bio-feedback - person learns to relax, increasing parasympathetic outflow and decreasing sympathetic, resulting in decreased blood pressure

27. Short-Term Chemical Control of Resistance Chemicals that act on vessels, heart or blood volume • Norepinephrine (NE; from adrenal medulla)  vasoconstriction • Epinephrine (epi; from adrenal medulla): • vasoconstriction, except in skeletal and cardiac muscle • nicotine (in tobacco) – stimulates sympathetic ganglionic neurons and adrenal medulla • also increases heart rate and strength of contraction

28. Short-Term Chemical Controls (con’t) • Antidiuretic hormone (ADH; a.k.a., vasopressin; released from neurohypophysis; see A&P I: Unit 11- Endocrine System) • stimulates water reabsorption • at high levels, causes vasoconstriction • Angiotensin II (see A&P I: Unit 11 - Endocrine; and A&P II Topic 10 - Urinary System) • produced from angiotensinogen in response to renin from kidney • causes intense vasoconstriction • stimulates secretion of ADH and aldosterone (long term control)

29. Short-Term Chemical Controls (con’t) • Atrial natriuretic peptide (ANP; from atria of heart) – antagonizes aldosterone and causes general vasodilation • Alcohol • inhibits ADH secretion • depresses vasomotor center • Inflammatory chemicals – cause vasodilation • histamine, prostacyclins, kinins and others • released during inflammatory response

30. Short-Term Chemical Controls (con’t) • Endothelium-derived factors – affect vascular smooth muscle • endothelin – potent vasoconstrictor, released in response to low blood flow • nitric oxide (NO) –vasodilator released in response to high blood flow; causes systemic and local vasodilation

31. Long-Term Control of Resistance: Renal Regulation • Regulates blood volume (BV) • Blood volume important to: venous pressure, venous return, EDV, SV, CO • Control: • direct renal control • indirect renal control

32. Direct Renal Control • Increased BP  increased filtration  increased water loss  decreased BV • Decreased BP  decreased filtration  decreased water loss  increased BV Fig. 20.9, p. 735

33. Indirect Renal Control • Renin-angiotensin pathway • Decreased BP juxtaglomerular cells of kidney tubules secrete renin enzymatic cascade  converts angiotensinogen to angiotensin I angiotensin II • Kidney also releases renin in response to sympathetic impulses • Angiotensin II • stimulates aldosterone secretion • stimulates ADH secretion • causes vasoconstriction Fig. 20.9, p. 735

34. Blood Pressure Disorders: Hypotension Systemic systolic BP < 100 mm Hg • Orthostatic hypotension – drop in BP on rising from sitting or laying; common in the elderly • Chronic hypotension – long-term depression in BP • possible causes: poor nutrition, Addison’s disease, hypothyroidism • Acute hypotension – rapid drop in BP • most often due to hemorrhage • sign of circulatory shock

35. Blood Pressure Disorders: Hypertension Long-term elevation of arterial pressure > 140/90 • Results in damage to heart, kidneys, brain (stroke), blood vessels overall • Primary hypertension • 90% of all cases • no clearly identifiable cause • Secondary hypertension • ~ 10% of cases • cause is identified

36. BP Disorders: Primary Hypertension • possible causes: • diet high in Na+, saturated fat, cholesterol; low in K+, Ca2+, Mg2+ • obesity, heredity, age • stress, smoking • treatment: • changes in diet, weight loss, exercise, stress management • antihypertensive drugs: diuretics, beta-blockers, calcium-channel blockers

37. BP Disorders: Secondary Hypertension • causes: • excess renin secretion • arteriosclerosis • hyperthyroidism • Cushing’s disease • treatment aimed at cause

38. Changes in Blood Distribution During Exercise • Total increases from ~ 5,800 ml/min at rest to ~ 17,500 ml/min during exercise • Brain – flow remains relatively steady (~750 ml/min) • Skeletal muscle, heart – flow increases dramatically to supply oxygen and nutrients and remove wastes • Skin – flow increases for heat loss (thermoregulation) • Kidney – flow decreases (decreases urine output) • Abdominal organs – flow decreases (redirected to skeletal muscle & heart) • Other – flow decreases (redirected to skeletal muscle & heart) Fig. 20.12, p. 738

39. Tissue Perfusion • Blood flow through tissues • Varies with need of tissue • Functions: 1) delivery of oxygen & nutrients, removal of wastes 2) gas exchange in lung 3) absorption of nutrients from gut 4) urine production in kidney Fig. 20.12, p. 738

40. Velocity of Blood Flow • Inversely related to total cross-sectional area of blood vessels to be filled • Branching of arteries increases cross-sectional area • Velocity lowest in capillaries • allows time for exchange between blood and tissue • Increases as capillaries rejoin to form venules and venules join to form veins Fig. 20.13, p. 739

41. Autoregulation of Blood Flow • Local (intrinsic) regulation of blood flow • blood vessels serving tissues adjust to meet needs of tissue • if blood flow is inadequate  tissue metabolism decreases  cell death • Long-term autoregulation  increase in number and size of blood vessels = angiogenesis • Short-term autoregulation – 2 mechanisms of control: • metabolic control – response to chemical needs of tissue • myogenic control – response to stretch

42. Metabolic Control of Blood Flow • Maintains proper chemical environment for cells • Causes relaxation (vasodilation) of precapillary sphincter to increase blood flow • Important vasodilating chemicals include: • nitric oxide (NO) • attaches to hemoglobin in lungs as O2 is loaded • released at capillaries as O2 is released • inflammatory chemicals (histamine, kinins)

43. Metabolic Control (con’t) • active hyperemia - due to chemicals changes associated with hard-working tissues: • decreased oxygen and/or other nutrients • products of metabolic activity: increased K+, adenosine, lactic acid, H+ (decreased pH)

44. Myogenic Control of Blood Flow • Maintains relatively steady flow to tissues in spite of changes in overall BP • Response of vascular smooth muscle to stretch • increased stretch  vasoconstriction  decreased flow • decreased stretch  vasodilation  increased flow • Reactive hyperemia – • dramatic increase in blood flow following removal of blockage • results from: • stretching of arteriole upstream from blockage, and • accumulation of wastes in tissue

45. Capillary Dynamics Movement across capillary is based on gradients • Solute gradient (diffusion) • Water gradient (osmosis) • Pressure gradient (hydrostatic pressure) Fig. 20.14, p. 742

46. Diffusion • small water-soluble molecules pass between endothelial cells through small clefts (desmosomes are loose cell junctions) • lipids and lipid-soluble (non-polar) materials diffuse directly through the lipid bilayer of the endothelial cells • Osmosis • special form of diffusion in which solvent (water) moves across membrane (diffusion of water) • moves toward area of higher solute concentration (lower water concentration)

47. Bulk Fluid Flow Moves fluids and dissolved substances through capillary walls together using the following forces • Hydrostatic Pressure: • physical pressure exerted by a fluid in an enclosed space • fluids and dissolved substances move from areas of high to areas of low hydrostatic pressure • Osmotic Pressure: • “pull” exerted on solvent by solute in solution • solution with more solute has greater osmotic pressure

48. Forces Moving Fluid OUT Of Capillary i.e., moving fluid INTO interstitial space • Capillary hydrostatic pressure= HPc • also called capillary blood pressure (or blood hydrostatic pressure) • pushes fluid out of capillary • 35 mm Hg at arterial end of capillary (average) • 17 mm Hg at venous end of capillary (average) • Interstitial fluid osmotic pressure =OPif • proteins in interstitial fluid exert osmotic pressure on plasma • pulls fluid out of capillary into tissues • BUT, normally very little protein present in IF; average value is 1 mm Hg

49. Forces Moving Fluid INTO Capillary • Interstitial fluid hydrostatic pressure (HPif) – physical pressure pushing interstitial fluid into the capillary • ranges from slightly negative to slightly positive • 0 mm Hg is generally used in equations • fluid removed by lymphatic system • Capillary osmotic pressure (OPc) - pressure due to presence of large, nondiffusible molecules (e.g., plasma protein) that draws fluid into the capillary from the interstitial fluid • average value is 26 mm Hg • little change along capillary from arterial to venous end

50. Net Filtration Pressure • Sum of all hydrostatic and osmotic forces acting on fluids as they move through capillary walls • Can be seen as difference between forces moving fluid out of capillary and forces moving fluid into it net filtration pressure (NFP) = [sum of outward forces] – [sum of inward forces] = [HPc + OPif] - [HPif + OPc]