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Chapter 21. Blood Vessels and Hemodynamics. Vessel Structure and Function. The blood vessels of the body should not be thought of as mere “pipes” carrying blood – they are dynamic , interactive, essential components of the cardiovascular system.

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Chapter 21

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    1. Chapter 21 Blood Vessels and Hemodynamics

    2. Vessel Structure and Function • The blood vessels of the body should not be thought of as mere “pipes” carrying blood – they are dynamic, interactive, essential components of the cardiovascular system. Basic components of the CV organ system

    3. Vessel Structure and Function • Blood Vessel Types • Arteries – carry blood away from the heart • Large elastic arteries; medium muscular arteries; arterioles • Capillaries – site of nutrient and gas exchange • Venules are small veins • Veins – carry blood towards the heart

    4. Vessel Structure and Function • Blood vessels in the body share components of 3 basic layers or “tunics” which comprise the vessel wall: • Tunica interna (intima) • Tunica media • Tunica externa

    5. Vessel Structure and Function • The tunica intima (interna) is the inner lining in direct contact with blood. It consists of: • The epithelium or the endothelium with underlying basement membrane • Internal elastic lamina • The tunica media is chiefly composed of smooth muscle that regulates the diameter of the vessel lumen, by vasoconstriction ( VC) & vasodilation ( VD) • It is seperated from the tunica externa by external elastic lamina. • The tunica externa is connective tissue- contains vasa vasorum , nerves & anchors vessel to surrounding tissue

    6. Elastic arteries • The largest arteries are the elastic arteries, the aorta and its major branches • Also called conducting arteries • Their tunica media is mainly made up of elastic fibers. • Elastic arteries are stretched as blood is ejected from the heart during systole accomodating the surge of blood (pressure reservoirs) • They recoil during diastole – help propel the blood

    7. Pressure reservoir function of elastic arteries

    8. Muscular arteries • Medium sized muscular (distributing) arteries have more smooth muscle in their tunica media. • Capable of VC and VD • Muscle layer remains in a state of partial contraction- vasomotor tone to ensure efficient blood flow • Examples- axillary artery, brachial artery in the arm and radial artery in the forearm.

    9. Anastomosis • An anastomosis is a union of vessels supplying blood to the same body tissue. Should a blood vessel become occluded, a vascular anastomosis provides collateral circulation (an alternative route) for blood to reach a tissue. • The shaded area here shows overlapping blood supply to the ascending colon. Arteries that do not anastomose are end arteries

    10. Arterioles • Arteriolesdeliver blood to capillaries and have the greatest influence on local blood flow and blood pressure. • Sympathetic nerve supply & chemicals can alter the diameter of the arterioles & change the resistance to flow • VC- decreases the vessel diameter increases the resistance & decreases blood flow to capillaries • VD- increases the vessel diameter decreases the resistance & increases blood flow • Called resistance vessels

    11. Arterioles • The terminal end of an arteriole tapers toward the capillary junction to form a metarteriole. • At the metarteriole-capillary junction, muscle cells forms the precapillary sphincter which monitors and regulates blood flow into the capillary bed.

    12. Capillaries • Capillaries are the only sites in the entire vasculature where gases, water, nutrients & wastes are exchanged (exchange vessels) Capillaries function as capillary beds ( of 10-100 capillaries) • Postcapillary venules are formed when capillaries unite

    13. The Microcirculation: flow of blood from a metarteriole, through capillaries & into post capillary venules • Pre-capillary sphincters • control blood flow through capillaries • When arterioles dilate and sphincters are open- blood flows in entire capillary bed • Distal part of metarteriole is a thoroughfare channel- direct route of blood from an arteriole to venule

    14. Capillaries • Capillaries are different from other vascular structures in that they are made of endothelial lining & a basement membrane – they lack a tunica media & externa • This allows capillaries to be permeable to many substances (gases, fluids, and small ionic molecules).

    15. Capillaries • The body contains three types of capillaries: • Continuous capillaries are the most common with endothelial cells forming a continuous tube, interrupted only by small intercellular clefts. E.g. in skin, blood brain barrier of nervous system • Fenestrated capillaries (fenestra = windows), found in the kidneys, villi of small intestines, and endocrine glands are much more porous. • Sinusoids form very porous channels (large intercellular clefts) through which even blood cells can pass, e.g., in liver, spleen, bone marrow.

    16. 3 Types of capillaries in the body

    17. Veins • Veins have thinner walls, and larger lumen compared to arteries • The largest part of blood volume ( 64%) is in the systemic veins- and are called blood reservoirs • Because intravenous pressures are low, veins have valves to keep blood flowing in only one direction. • When exposed to higher than normal pressures, veins can become incompetent (varicose veins).

    18. Veins

    19. Venous Reserve • Because systemic veins and venules contain a large percentage of the blood volume (about 64% at rest), they function as blood reservoirs from which blood can be diverted quickly if needed. • With a drop in BP, stimulation of the sympathetic NS will cause venoconstriction, allowing a greater volume of blood to flow to tissues where it is needed more.

    20. Artery Vein

    21. Capillary Exchange • Diffusion: • Of gases, nutrients, wastes, occurs down the concentration gradient • Water-soluble substances such as glucose and amino acids diffuse across capillaries through intercellular clefts or fenestrations • Lipid-soluble materials, such as O2, CO2, and steroid hormones, may diffuse across endothelial cell plasma membrane

    22. Capillary Exchange • Transcytosis: • Substances in blood plasma become enclosed within tiny pinocytic vesicles • enter endothelial cells by endocytosis • move across the cell and exit on the other side of cell by exocytosis. • Important for large, lipid-insoluble molecules for example, the hormone insulin ( a protein)

    23. Capillary Exchange • Bulk flow • Bulk flow is important for regulation of blood & interstitial fluid volume • Bulk flow is the fluid exchange with a large number of ions and particles from an area of high to low pressure • Filtration is the pressure driven movement of fluid through the walls of the capillary into the interstitial fluid • Reabsorption is the pressure driven movement of fluid from the interstitial fluid back into the capillary • As blood flows to the tissues of the body, hydrostatic and osmotic forces at the capillaries determine how much fluid leaves the arterial end of the capillary and how much is then reabsorbed at the venous end. These are called Starling Forces.

    24. Fluid Exchange & Starling Forces • Two pressures promote filtration: • Blood hydrostatic pressure (BHP) the capillary blood pressure - decreases from 35 to 16 from the arterial to the venous end of the capillary- the pushing force • Interstitial fluid osmotic pressure (IFOP), which is about 1 mmHg ( due to tiny amount of proteins in IF)

    25. Fluid Exchange - Starling Forces • Two pressures promote reabsorption: • Blood colloid osmotic pressure (BCOP) is due to the presence of plasma proteins too large to cross the capillary – averages 26 mmHg on both ends- the pulling force • Interstitial fluid hydrostatic pressure (IFHP) is normally zero

    26. Fluid Exchange - Starling Forces

    27. Fluid Exchange - Starling Forces • Normally there is nearly as much fluid reabsorbed as there is filtered. • At the arterial end, net filtration pressure is outward at 10 mmHg and fluid leaves the capillary (filtration). • At the venous end, net filtration pressure is inward at –9 mmHg (reabsorption). • On average, about 85% of fluid filtered is reabsorbed • Fluid that is not reabsorbed (about 3L/ day for the entire body) enters the lymphatic vessels to be eventually returned to the blood.

    28. Clinical connection- Edema • Edema is an abnormal increase in interstitial fluid, due to either excess filtration or inadequate reabasorption • Causes of edema: • Increased capillary hydrostatic pressure causes more fluid to be filtered from capillaries. e.g. in congestive heart failure • Increased permeability of capillaries – due to chemical released in inflammation; more fluid to be filtered • Blocked lymphatics- tissue fluid not drained away • Decreased blood colloid osmotic pressure due to reduced concentration of plasma proteins • Inadequate dietary intake in malnutrition • decreased plasma protein synthesis with liver disease • Loss of plasma proteins in burns

    29. Hemodynamics: Factors Affecting Blood Flow • Blood flow is the volume of blood that flows through any tissue/ organ in a given time period in ml/min • Total blood flow is cardiac output (CO), the volume of blood that circulates through systemic (or pulmonary) blood vessels each minute • Blood Pressure (BP) • Force exerted on the vessel wall by the contained blood- mmHg • Blood moves from higher to lower pressure areas • Resistance • Resistance is opposition to flow- the friction blood encounters as it passes through BVs

    30. Pressure, Flow, and Resistance • Blood flow (F) is directly proportional to the difference in blood pressure • The greater the difference in blood pressure between two points, the more the blood flow • Blood flow is inversely proportional to resistance (R) • If resistance increases, blood flow decreases • In an effort to meet physiological demands, we can increase blood flow by: • Increasing BP • Decreasing systemic vascular resistance in the blood vessels

    31. Vascular Resistance • Peripheral resistance (PR) most resistance is encountered in the peripheral systemic circulation well away from the heart • Vascular Resistance is influenced by: • Blood viscosity ( due to RBCs)- constant • Vessel length (body size)- constant • Blood vessel diameter • Changes in vessel diameter alter peripheral resistance • Mainlythearteriolesdetermine peripheral resistance- dilate & constrict in response to neural & chemical stimuli

    32. Vascular Resistance • Resistance is inversely proportional to the diameter of the blood vessel's lumen • The smaller the diameter of the blood vessel, the greater the resistance to blood flow • Vasoconstriction narrows the lumen, and vasodilation widens it • Moment-to-moment fluctuations in blood flow through a tissue are due VC and VD of the tissue's arterioles • As arterioles dilate, resistance decreases, and blood pressure falls • As arterioles constrict, resistance increases, and blood pressure rises.

    33. Factors Affecting Vascular Resistance

    34. Blood Pressure • BP= CO X PR • BP is determined by 3 factors : • cardiac output, vascular resistance, blood volume • Increase in any of these factors increases BP • BP is: highest in the aorta, declines throughout the length of the pathway

    35. Blood Pressure • Systolic pressure– pressure exerted on arterial walls during ventricular systole- 110mmHg • Diastolic pressure– lowest level of arterial pressure during ventricular diastole- 70mmHg • Pulse pressure is the difference between systolic & diastolic pressure- felt as a pulse • Mean arterial pressure (MAP)– pressure that propels the blood to the tissues MAP = diastolic pressure + 1/3 pulse pressure • MAP & PP decrease with distance from the heart

    36. Blood Pressure • Capillary BP ranges from 35mmHg at the arterial end of capillaries & 16 mm Hg at the venous ends of the capillaries • BP continues to drop in the venules & then veins • BP is 0 mm Hg in the right atrium

    37. BP in various parts of CVS

    38. Venous Return • The volume of blood returning back to the heart through the systemic veins is called the venous return. • Because venous BP is low, venous return is aided by the presence of venous valves, a skeletal muscle pump, and the action of breathing.

    39. Venous Return • The skeletal muscle pump : • uses the action of muscles to milk blood in one direction (valves prevent backflow). • The respiratory pump: • intra abdominal pressure increases during inspiration- squeezing local veins forcing blood toward the heart • Pressure in chest falls- veins in chest expand to pull venous blood towards the heart.

    40. Venous Return • Although the venous circulation flows under much lower pressures than the arterial side, usually the small pressure differences (venule 16 mmHg to right atrium 0 mmHg), plus the aid of muscle and respiratory pumps is sufficient.

    41. Velocity of Blood Flow • Blood velocity (in cm/sec): • Changes as blood travels through the systemic circulation • Is inversely proportional to the cross-sectional area • Flow fastest in aorta & slow in capillaries- (allows time for exchange between blood and tissues), speeds up again increases in veins • (total cross sectional area of all the branches of a vessel is more than the cross sectional area of the original vessel)

    42. Control of Blood Pressure & Blood Flow • Several negative feedback loops control BP by changing: • CO ( by changes in HR & SV) • PR • Blood volume • Short-term controls correct moment-to-moment fluctuations in BP by altering CO &peripheral resistance • Long-term controls mainly regulate blood volume

    43. Cardiovascular center • The Cardicovascular center not only regulates HR & SV, but also control blood flow and BP • The cardiac center ( cardioinhibitory & the cardiostimulatory center )- regulate HR & SV • The vasomotor center ( VMC)- controls blood vessel diameter via sympathetic nerves • Maintains vasomotor tone • Increasing sympathetic stimulation causing vasoconstriction • Decreasing sympathetic stimulation causing vasodilation • The CV center receives input most importantly from baroreceptors • Output from CV center stimulates sympathetic & parasympathetic nerves

    44. Function of Cardiovascular center in the Medulla oblongata

    45. Neural Regulation of Blood Pressure • Baroreceptor Reflexes • Correct moment-to-moment fluctuations in BP as in moving from prone to erect position • Baroreceptors are located in the arch of the aorta and the carotid sinus.

    46. Neural Regulation of Blood Pressure • Baroreceptor Reflexes : • The carotid sinus reflex helps regulate blood pressure in the brain • The aortic sinus reflex regulates systemic blood pressure • Example: BP falls- the baroreceptors are stretched less, they send nerve impulses at a slower rate to the cardiovascular center • CV center decreases parasympathetic stimulation of the heart via vagus N & increases sympathetic stimulation to the heart • CV center increases sympathetic stimulation to blood vessels (skin, GI tract, and kidneys )- causing VC & increased PR • As cardiac output and peripheral resistance rise- BP increases

    47. Neural Regulation of Blood Pressure • Baroreceptor Reflexes : • Example: BP rises: • the baroreceptors send impulses at a faster rate to CV center • The CV center responds by increasing parasympathetic stimulation and decreasing sympathetic stimulation • HR and force of contraction decrease- cardiac output decreases • The cardiovascular center also decreases sympathetic stimulation to cause vasodilation- lowers vascular resistance. • As cardiac output and peripheral resistance fall- BP falls

    48. Regulation of BP via baroreceptor reflexes

    49. Neural Regulation of Blood Pressure • Chemoreceptor Reflexes • Chemoreceptors are found in the carotid bodies (located close to baroreceptors of carotid sinus) and aortic bodies (located in the aortic arch). • Low oxygen & pH, or raised CO2 of blood stimulates chemoreceptors • Impulses to cardiac center- increases CO • Impulses to VMC- increase PR • BP rises, speeding return of blood to heart & lungs

    50. Hormonal Regulation of Blood Pressure • The Renin-angiotensin-aldosterone (RAA) system • Renin is released by kidneys when blood volume falls or blood flow decreases. • It converts a protein in blood angiotensinogen into angiotensin I • Angiotensin I is converted to the active hormone angiotensin II • Angiotensin II raises BP by: • causing vasoconstriction – increasing PR • and by stimulating secretion of aldosterone from the adrenal glands. • Aldosterone causes Na & water retention in blood , increases blood volume and BP