Blood Flow

1. Blood Flow • The purpose of cardiovascular regulation is to maintain adequate blood flow through the capillaries to the tissues • Actual volume of blood flowing through a vessel, an organ, or the entire circulation in a given period: • Is measured in ml/min. • Is equivalent to cardiac output (CO), considering the entire vascular system • Is relatively constant when at rest • Varies widely through individual organs

2. Blood Flow Through Tissues • Blood flow, or tissue perfusion, is involved in: • Delivery of oxygen and nutrients to, and removal of wastes from, tissue cells • Gas exchange in the lungs • Absorption of nutrients from the digestive tract • Urine formation by the kidneys • Blood flow is precisely the right amount to provide proper tissue function

3. Brain Heart Skeletal muscles Skin Kidney Abdomen Other Total blood flow at rest 5800 ml/min Total blood flow during strenuous exercise 17,500 ml/min Figure 19.13

4. 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 • Three types include: • Hypovolemic shock – results from large-scale blood loss or dehydration • Vascular shock – normal blood volume but too much accumulate in the limbs (long period of standing/sitting) • Cardiogenic shock – the heart cannot sustain adequate circulation

5. Blood flow • Capillary blood flow is determined by the interplay between: • Pressure • Resistance • The heart must generate sufficient pressure to overcome resistance

6. Physiology of Circulation: Blood Pressure (BP) • Force per unit area exerted on the wall of a blood vessel by its contained blood • Expressed in millimeters of mercury (mm Hg) • Measured in reference to systemic arterial BP in large arteries near the heart • The differences in BP within the vascular system provide the driving force that keeps blood moving from higher to lower pressure areas • Absolute pressure is less important than pressure gradient

7. Physiology of Circulation: Resistance • Opposition to flow • Measure of the amount of friction blood encounters • Generally encountered in the peripheral systemic circulation • Because the resistance of the venous system is very low (why?) usually the focus is on the resistance of the arterial system: • Peripheral resistance (PR) is the resistance of the arterial system

8. Physiology of Circulation: Resistance • Three important sources of resistance • Blood viscosity • Total blood vessel length • Blood vessel diameter

9. Resistance – constant factors • Blood viscosity • The “stickiness” of the blood due to formed elements and plasma proteins • Blood vessel length • The longer the vessel, the greater the resistance encountered

10. Resistance – frequently changed factors • Changes in vessel diameter are frequent and significantly alter peripheral resistance • Small-diameter arterioles are the major determinants of peripheral resistance • Frequent changes alter peripheral resistance • Fatty plaques from atherosclerosis: • Cause turbulent blood flow • Dramatically increase resistance due to turbulence

11. Systemic Blood Pressure • The pumping action of the heart generates blood flow through the vessels along a pressure gradient, always moving from higher- to lower-pressure areas • Pressure results when flow is opposed by resistance • Systemic pressure: • Is highest in the aorta • Declines throughout the length of the pathway • Is ~0 mm Hg in the right atrium • The steepest change in blood pressure occurs between arteries to arterioles

12. Arterial Blood Pressure • Arterial pressure is important because it maintains blood flow through capillaries • Blood pressure near the heart is pulsatile • Systolic pressure: pressure exerted during ventricular contraction • Diastolic pressure: lowest level of arterial pressure

13. Arterial Blood Pressure • A pulse is rhythmic pressure oscillation that accompanies every heartbeat • Pulse pressure = difference between systolic and diastolic pressure • Mean arterial pressure (MAP): pressure that propels the blood to the tissues MAP = diastolic pressure + 1/3 pulse pressure • Pulse pressure and MAP both decline with increasing distance from the heart • Efficiency of the circulation can be assessed by taking pulse and blood pressure measurements

14. Measuring BP • Systemic arterial BP is measured indirectly • The first sound heard is recorded as the systolic pressure • The pressure when sound disappears is recorded as the diastolic pressure • http://www.youtube.com/watch?v=bNApnYMR16w

15. 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

16. Capillary Blood Pressure • Ranges from 15 to 35 mm Hg • Low capillary pressure is desirable • High BP would rupture fragile, thin-walled capillaries • Low BP is sufficient to force filtrate out into interstitial space and distribute nutrients, gases, and hormones between blood and tissues • Capillary blood pressure is one of the driving forces for transport through the capillary wall (filtration)

17. Passage through capillary - diffusion • Water, ions, and small molecules such as glucose • Diffuse between adjacent endothelial cells • Or through fenestrated capillaries • Some ions (Na, K, Ca2, Cl) • Diffuse through channels in plasma membranes

18. Passage through capillary - diffusion • Large, water-soluble compounds • Pass through fenestrated capillaries • Lipids and lipid-soluble materials such as O2 and CO2 • Diffuse through endothelial plasma membranes • Plasma proteins • Cross endothelial lining in sinusoids

19. Passage through capillary - filtration • Driven by hydrostatic pressure • Water and small solutes forced through capillary wall • Leaves larger solutes in bloodstream

20. Passage through capillary - reabsorption • The result of osmotic pressure (OP) • Blood colloid osmotic pressure (BCOP) • Equals pressure required to prevent osmosis • Caused by blood proteins that are too large to cross capillary walls

21. Hydrostatic Pressures • Capillary hydrostatic pressure (HPc) (capillary blood pressure) • Tends to force fluids through the capillary walls • Is greater at the arterial end (35 mm Hg) of a bed than at the venule end (17 mm Hg) • Interstitial fluid hydrostatic pressure (HPif) • Usually assumed to be zero because of lymphatic vessels

22. Colloid Osmotic Pressures • Capillary colloid osmotic pressure (oncotic pressure) (OPc) • Created by non-diffusible plasma proteins, which draw water toward themselves • ~26 mm Hg • Interstitial fluid osmotic pressure (OPif) • Low (~1 mm Hg) due to low protein content

23. Net Filtration Pressure (NFP) • NFP—comprises all the forces acting on a capillary bed • NFP = (HPc—HPif)—(OPc—OPif) • At the arterial end of a bed, hydrostatic forces dominate • At the venous end, osmotic forces dominate • Excess fluid is returned to the blood via the lymphatic system

24. HP =hydrostatic pressure • Due to fluid pressing against a wall • “Pushes” • In capillary (HPc) • Pushes fluid out of capillary • 35 mm Hg at arterial end and 17 mmHg at venous end of capillaryin this example •In interstitial fluid (HPif) • Pushes fluid into capillary • 0 mm Hg in this example Arteriole Venule Interstitial fluid Capillary Net HP—Net OP (17—0)—(26—1) Net HP—Net OP (35—0)—(26—1) OP =osmotic pressure • Due to presence of nondiffusible solutes (e.g., plasma proteins) • “Sucks” •In capillary (OPc) • Pulls fluid into capillary • 26 mm Hg in this example •In interstitial fluid (OPif) • Pulls fluid out of capillary • 1 mm Hg in this example Net HP 35 mm Net OP 25 mm Net OP 25 mm Net HP 17 mm NFP (net filtration pressure) is 10 mm Hg; fluid moves out NFP is ~8 mm Hg; fluid moves in CHP – 35-17 mmHg IHP – 0 mmHg BCOP – 25 mmHg ICOP – 0 mmHg Figure 19.17

25. Fluid Exchange at a Capillary • Hydrostatic pressure and osmotic pressure regulate bulk flow Figure 15-19a

26. Venous Blood Pressure • Changes little during the cardiac cycle • Small pressure gradient, about 15 mm Hg

27. Factors Aiding Venous Return • Venous BP alone is too low to promote adequate blood return and is aided by the: • Respiratory “pump” – pressure changes created during breathing suck blood toward the heart by squeezing local veins • Muscular “pump” – contraction of skeletal muscles “milk” blood toward the heart • Valves prevent backflow during venous return

28. Controls of Blood Pressure • Short-term controls: • Counteract moment-to-moment fluctuations in blood pressure by altering peripheral resistance • Long-term controls regulate blood volume

29. Central Regulation Central regulation involves neuroendocrine mechanisms that control the total systemic circulation. This regulation involves both the cardiovascular centers and the vasomotor centers. Short-term elevation of blood pressure bysympatheticstimulation of theheart and peripheralvasoconstriction Stimulation ofreceptors sensitiveto changes insystemic bloodpressure orchemistry Activation ofcardiovascularcenters Neuralmechanisms Stimulationof endocrineresponse Long-term increasein blood volumeand blood pressure Endocrine mechanisms If autoregulation is ineffective HOMEOSTASISRESTORED

30. Factors that Influence Mean Arterial Pressure Figure 15-10

31. Controls of Blood Pressure • Autoregulation • Causes immediate, localized homeostatic adjustments • Neural mechanisms • Respond quickly to changes at specific sites • Endocrine mechanisms • Direct long-term changes

32. Autoregulation of blood flow within tissues • Autoregulation – automatic adjustment of blood flow to each tissue in proportion to its requirements at any given point in time • Local vasodilators accelerate blood flow in response to: • Decreased tissue O2 levels or increased CO2 levels • Generation of lactic acid • Rising K+ or H+ concentrations in interstitial fluid • Local inflammation • Elevated temperature • Vasoconstrictors: • Injured vessels constrict strongly (why?) • Drop in tissue temperature (why?)

33. Myogenic (myo =muscle; gen=origin) Controls • Inadequate blood perfusion (tissue might die) or excessively high arterial pressure (rupture of vessels) may interfere with the function of the tissue • Vascular smooth muscle can prevent these problems by responding directly to passive stretch (increased intravascular pressure) • The muscle response is by resist to the stretch and that results in vasoconstriction • The opposite happens when there is a reduced stretch • The myogenic mechanisms keeps the tissue perforation relatively constant.

34. Intrinsic mechanisms (autoregulation) Extrinsic mechanisms • Maintain mean arterial pressure (MAP) • Redistribute blood during exercise and thermoregulation • Distribute blood flow to individual organs and tissues as needed Amounts of: Nerves Sympathetic pH O2 a Receptors Epinephrine, norepinephrine Metabolic controls b Receptors Amounts of: CO2 Angiotensin II K+ Hormones Prostaglandins Antidiuretic hormone (ADH) Adenosine Nitric oxide Atrial natriuretic peptide (ANP) Endothelins Myogenic controls Stretch Dilates Constricts Figure 19.15

35. Short-Term Mechanisms: Neural Controls • Neural controls of peripheral resistance: • Alter blood distribution in response to demands • Maintain MAP by altering blood vessel diameter • Neural controls operate via reflex arcs involving: • Vasomotor centers and vasomotor fibers • Baroreceptors • Vascular smooth muscle

36. Short-Term Mechanisms: Vasomotor Center • Vasomotor center – a cluster of sympathetic neurons in the medulla that oversees changes in blood vessel diameter • Maintains blood vessel tone by innervating smooth muscles of blood vessels, especially arterioles • Vasomotor activity is modified by: • Baroreceptors (pressure-sensitive) • chemoreceptors (O2, CO2, and H+ sensitive) • bloodborne chemicals • hormones

37. Short-Term Mechanisms: Baroreceptor-Initiated Reflexes • Baroreceptors are located in • Carotid sinuses • Aortic arch • Walls of large arteries of the neck and thorax http://archive.student.bmj.com/back_issues/0599/graphics/0599ed2_1.gif

38. Short-Term Mechanisms: Baroreceptor-Initiated Reflexes • Increased blood pressure stimulates the cardioinhibitory center to: • Increase vessel diameter • Decrease heart rate, cardiac output, peripheral resistance, and blood pressure • Declining blood pressure stimulates the cardioacceleratory center to: • Increase cardiac output and peripheral resistance • Low blood pressure also stimulates the vasomotor center to constrict blood vessels

39. Blood Pressure KEY Stimulus Sensory receptor Medullarycardiovascularcontrol center Integrating center Efferent path Effector Change inbloodpressure Parasympatheticneurons Carotid and aorticbaroreceptors Sympatheticneurons SA node Ventricles Veins Arterioles Figure 15-22 (10 of 10)

40. Short-Term Mechanisms: Chemoreceptor-Initiated Reflexes • Chemoreceptors are located in the • Carotid sinus • Aortic arch • Large arteries of the neck • Chemoreceptors respond to rise in CO2, drop in pH or O2 • Increase blood pressure via the vasomotor center and the cardioacceleratory center

41. Chemicals that Increase Blood Pressure • 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 vasoconstriction • Endothelium-derived factors – endothelin and prostaglandin-derived growth factor (PDGF) are both vasoconstrictors

42. Chemicals that Decrease Blood Pressure • Atrial natriuretic peptide (ANP) – causes blood volume and pressure to decline • Nitric oxide (NO) – is a brief but potent vasodilator • Inflammatory chemicals – histamine, prostacyclin, and kinins are potent vasodilators • Alcohol – causes BP to drop by inhibiting ADH

43. Table 19.2

44. Long-Term Mechanisms: Renal Regulation • Long-term mechanisms control BP by altering blood volume • Increased BP stimulates the kidneys to eliminate water, thus reducing BP • Decreased BP stimulates the kidneys to increase blood volume and BP • Kidneys act directly and indirectly to maintain long-term blood pressure • Direct renal mechanism alters blood volume (changes in urine volume) • Indirect renal mechanism involves the renin-angiotensin mechanism

45. Indirect Mechanism: renin-angiotensin • The renin-angiotensin mechanism •  Arterial blood pressure  release of renin • Renin production of angiotensin II • Angiotensin II is a potent vasoconstrictor • Angiotensin II aldosterone secretion • Aldosterone  renal reabsorption of Na+ and  urine formation • Angiotensin II stimulates ADH release