*need oxygen supply…O2 diffuses to cells….hence distance from O2 source becomes imp….average distance from O2 source is about 100 micrometer…this optimal distance is cuz of CVS
*Fluid cannot move through a system unless some energy is applied to it. In fluid dynamics, this energy is in the form of a difference in pressure, or pressure gradient, between two points in the system. Pressure is expressed as units of force, or weight, per unit area. A familiar example of this is in the pounds per square inch (psi) recommendation stamped on the side of tires. The psi indicates the pressure to which a tire should be inflated with air above atmospheric pressure. Inflating a tire to 32 psi signifies that 32 more pounds press against every square inch of the inner tire surface than against the outside of the tire.The pressure exerted at any level within a column of fluid reflects the collective weight of all the fluid above that level as it is pulled down by the acceleration of gravity. It is defined aswhere P = pressure, Ï = the density of the fluid, g = the acceleration of gravity, and h = the height of the column of fluid above the layer where pressure is being measured. The force represented by pressure in a fluid system is often described as the force that is able to push a column of fluid in a tube straight up against gravity. In this way the magnitude of the force resulting from fluid pressure can be measured by how high the column of fluid rises in the tube In physiological systems, this manner of expressing pressure is designated as centimeters H2O, or the more convenient mm Hg, because mercury is much denser than water and therefore will not be pushed upward as far by typical pressures seen within the cardiovascular system.Without going into mechanistic details at this time, arterial pressure peaks shortly after the heart contracts and pumps blood into the aorta and falls to a lower value when the heart relaxes between beats and is therefore not pumping blood into the aorta. The peak pressure during contraction of the heart is called the systolic pressure and is typically about 120 mm Hg in humans, whereas the minimum arterial pressure value during relaxation of the heart is called the diastolic pressure and is about 80 mm Hg. Thus, if one end of a tube were to be inserted into the aorta with the other end connected to a column of mercury sitting perpendicular to the ground at the level of the heart, that column of mercury would rise 120 mm during systole and fall to 80 mm during diastole. In clinical practice, human arterial pressure is reported as systolic over diastolic pressure or, in this example, 120/80. (Our mean arterial pressure is not the arithmetic mean of systolic and diastolic pressure but is instead about 93 mm Hg, because the time the heart spends relaxing is longer than the time it spends contracting and ejecting blood into the aorta.)
Compliance, therefore, is related to the ease by which a given change in pressure causes a change in volume.In biological tissues, the relationship between DV and DP is not linear. As shown in Figure 1, compliance (which is the slope of the line relating volume and pressure) de-creases at higher volumes and pressures.Another way to view this is that the “stiffness” of a cardiac chamber or vessel wall increases at higher volumes and pressures.
*Systolic & Diastolic P are peak/lowest arterial pressuresIn reality arterial pressures vary around average values from heartbeat to heart beat & minute to minuteFor complex reasons, compliance (TPR) does not significantly influence MAP. So e.g. In arteriosclerosis, pulse P raises a lot, not MAP!!
*means increases or decreases in CO have proportionate effects on MAP
Cardiovascular System<br />Multicellular organisms need CVS*<br />
General Principles<br />Output of the Right and Left Heart Are Interdependent Because Their Chambers Are Connected in Series<br />2-bucket example<br />
General Principles<br />Blood Flow to Individual Organs Can Be Controlled Primarily Independently Because Circulations to Individual Organs Are Arranged in parallel<br />Liver is an exception – own arterial supply + splanchnic circulation<br />Hence tissue need dictates its own blood flow<br />
General Principles<br />Lumen diameter of all Arteries & Veins can be actively changed by contraction or relaxation of the circular layers of SM within their walls<br />Scores of normal physiological, pathological, and pharmacological agents that can alter vessel lumen<br />
General Principles<br />Cardiac output – controlled – sum of all local tissue flows<br />Arterial pressure – controlled – independently<br />
HemodynamicsBlood flow, Pressure & Resistance<br />Blood flow through a vessel:<br />∆P*<br />Resistance <br />F= ∆P/R (Ohm’s Law)<br />Laminar Vs turbulent flow<br />Eddie currents – more turbulence<br />Tendency for turbulence – measured by Reynold’s number (Re) <br />(Re= v.d.p/ŋ)<br />Re >2000 – significant turbulence<br />Blood pressure is the ‘force exerted by blood against any unit area of vessel wall’<br />
Blood flow, Pressure & Resistance<br />Resistance is impediment to blood flow in a vessel <br />If ∆P = 1 mm hg <br />And if, Flow = 1 ml/sec <br />Then, R = 1PRU<br />Total peripheral resistance<br />Strong sympathetic ++ : R=4 PRU<br />
Vessel Conductance<br />Diameter – conductance relationship <br />4-fold increase in d caused 256-fold increase in flow<br />Hence, conductance of a vessel increases in proportion to the fourth power of diameter <br />conductance ∞ diameter4<br />Poiseuille’s Law – factors that change resistance of blood vessel (or conductance <br />F = π∆P.r4 / 8ŋ.l<br />
Derivations of Poiseuille’s Law<br /><ul><li>F = ∆Pπr4/ 8ŋl or
Flow is proportional to pressure difference b/w entrance and exit points of a tube
And inversely proportional to resistance </li></li></ul><li>Vascular Compliance<br />Vascular compliance <br />Increase in V/increase in P <br />Related to the ease by which a given change in pressure causes a change in volume<br />Compliance of a systemic vein is about 24 times that of its corresponding artery<br />Since it is about 8 times as distensible, and <br />Has a volume about 3 times as great <br />8 x 3 = 24<br />Delayed compliance <br />Increase in V – increase in P pressure normalizes (vasodilation)<br />Vice versa<br />
Vessel Types<br />Windkessel Vessels<br />Elastic reservoir vessels<br />Large arteries<br />Highly distensible<br />Serve to damp large pressure fluctuations<br />Pulsatile flow converted to constant blood flow<br />Resistance Vessels<br />Arterioles, metaarterioles and pre-capillary sphincters<br />Arterioles has extensive ANS innervation<br />Alpha receptors – arterioles of skin, splanchnic, renal<br />Beta receptors – arterioles of skeletal muscle<br />Exchange Vessels<br />Capillaries<br />Capacitance Vessels<br />Veins<br />Shunt Vessels<br />Present in skin and other areas<br />Temperature regulation<br />
Arterial Pressures<br />Systolic pressure <br />In vascular system is the peak pressure reached during systole<br />Diastolic pressure<br />Lowest pressure during diastole<br />Mean arterial pressure (MAP)<br />Pulse pressure<br />
Arterial Pressure Pulsations<br />Each heart beat<br />Not only moves the blood in the vessels forward but also sets up a pressure wave<br />This wave travels along the arteries<br />It expands the arterial walls as it travels, and the expansion is palpable as the pulse<br />Rate at which the wave travels is independent of and much higher than the velocity of blood flow!<br />
Pulse Pressure<br />Pulse pressure = systolic P – diastolic P<br />Depends on:<br />Stroke Volume (SV)<br />Arterial compliance<br />Examples of variance in pulses<br />Weak ("thready") in shock<br />Strong in exercise / after administration of histamine<br />Aortic insufficiency: collapsing, Corrigan, or water-hammer pulse<br />
Transmission of Arterial Pulsations<br />Arterial pressure pulsations<br />Continuous blood flow Vs pulsatile blood flow<br />Windkessel effect<br />
Mean arterial pressure (MAP)*<br />Average of the arterial pressures measured millisecond by millisecond over a period of time<br />Not an arithmetic mean, is closer to diastolic pressure than systolic<br />Diastolic P + 1/3 Pulse P<br />Depends on:<br />Mean blood volume in arterial system (C.O.)<br />Arterial compliance (TPR)<br />BP = C.O. x TPR<br />Systolic BP is mainly controlled by CO<br />Diastolic BP is mainly controlled by TPR (BP ∞ TPR, Increase TPR – increase BP)<br />
SV, HR& TPR affect MAP, Pulse P<br />MAP = CO x TPR<br />Where CO = SV x HR<br />CO influence on MAP is independent!*<br />CO influence on Pulse P depends on whether:<br />CO has increased due to change in SV or<br />CO has increased due to change in HR<br />
Scenario 1:<br />HR increases, SV decreases<br />CO = constant; MAP = constant<br />But, Pulse P decreases (since SV has decreased)<br />Systolic P decreases<br />Diastolic P increases (decreased runoff due to lower SV)<br />Scenario 2:<br />HR decreases, SV increases (atheletes @ rest)<br />CO = constant; MAP = constant<br />But, Pulse P increases (since SV increased)<br />Systolic P increases<br />Diastolic P decreases (increased runoff due to higher SV)<br />
Variations in ArterialBP<br />Physiological variations in BP<br />Diurnal: lowest – early morning; highest – afternoon<br />Gender: females tend to have < BP<br />BP rises with age, BMI and mental stress<br />BP decreases with sleep and food intake<br />Exercise: moderate – only systolic increases; severe – both rise<br />Posture: standing upright first decreases BP (decrease VR); SNS activation restores BP<br />
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