Levick's Introduction to Cardiovascular Physiology
Highlights
- Darcy’s law relates flow to pressure difference (View Highlight)
- both Darcy’s law and Bernoulli’s law apply to steady-state flow, that is, flow that is not varying with time. If the flow is pulsatile, as in arteries, the laws still apply to the mean flow, but they do not tell us how the flow varies instant by instant. (View Highlight)
- All blood pressures are conventionally expressed as pressure above atmospheric pressure, and since CVP is close to atmospheric pressure, CVP ≅ 0. Equation 8.1b therefore simplifies to
CO≅ PTPR. (View Highlight)
- mechanical energy is the sum of the pressure energy, potential energy and kinetic energy (View Highlight)
- At the entrance to a tube, where there has been insufficient time for the parabolic profile to develop, the velocity profile is almost flat (Figure 8.3a, left). This is the situation in the ascending aorta, where the flat profile simplifies the estimation of aortic flow by Doppler ultrasound (View Highlight)
- The marginal plasma layer is important because it greatly facilitates the flow of blood through narrow resistance vessels (View Highlight)
- If the pressure driving fluid through a tube is progressively increased, a point is reached where flow no longer increases linearly with pressure, but increases as the square root of pressure (Figure 8.4). This marks the transition from laminar flow to a disorganized, turbulent flow (View Highlight)
- An innocent ejection murmur may develop during exercise (because ejection velocity increases) and during pregnancy/anaemia (because blood viscosity falls). (View Highlight)
- A capillary, diameter 5–6 μm, is narrower than a human red cell (8 μm). Consequently, red cells must bend into a folded, parachute-like configuration to pass through capillaries (View Highlight)
- Around 67%–80% of the stroke volume is temporarily stored in the elastic arteries during systole, while 20%–33% runs off through the peripheral resistance. The mechanical energy stored in the stretched elastin serves to maintain the blood pressure during diastole (known as the Windkessel effect). (View Highlight)
- Increased ejection velocity also raises pulse pressure because the viscoelastic artery wall has less time to undergo viscous relaxation. This contributes to the increase in pulse pressure during exercise. (View Highlight)
- When ejection ceases, a slight backflow closes the aortic valve. Valve closure produces a notch, the incisura, in the descending limb of the arterial pressure trace, followed by a brief, high-frequency wavelet due to vibration of the tensed aortic valve cusps. In more peripheral arteries, such as the brachial artery, there is a pronounced notch, the dicrotic notch (Greek dikrotos = beating twice) resulting from a diastolic reflected wave (View Highlight)
- transmission velocity increases with wall stiffness. (View Highlight)
- The same stroke volume ejected at a higher mean pressure (filled circles) causes a bigger pulse pressure, because distension increases the arterial stiffness (elastance). (View Highlight)
- The pulse transmission velocity, 4–15 m/s, is much faster
than the blood velocity, which averages ~0.2 m/s in the ascending aorta. (View Highlight)
- The therapeutic benefit of GTN, a vasodilator used to treat angina (cardiac pain due to O2
demand
exceeding supply), is partly because it relaxes large arteries and veins, and to a lesser degree, resistance vessels. Large artery relaxation increases arterial compliance and reduces systolic augmentation by wave reflection. The resulting fall in systolic pressure reduces afterload and cardiac O2
demand, and so contributes to the relief of angina (View Highlight)
- Whether the central pulse has a diastolic wave or a systolic inflection depends on: (1) the degree of reflection by peripheral vessels, which is increased by vasoconstriction and reduced by vasodilatation; and (2) pulse transmission velocity, which increases with ageing and mean blood pressure. (View Highlight)
- Four features of the pressure wave change: • The systolic pressure wave grows taller, rather like a sea wave approaching the beach. This pressure amplification can be as much as 60% in the femoral artery of young humans. Pressure amplification is less marked in middle-aged humans and is absent in older individuals (Figure 8.11).
• The incisura is damped out and disappears. • The mean pressure falls only slightly, by ~2 mmHg between the ascending aorta and radial artery, showing that large arteries account for only ~2% of TPR.
• A new pressure wave appears in late diastole, the dicrotic wave, preceded by a pronounced dicrotic notch (View Highlight)
- The pulse pressure continues to increase as far as third- or fourth-generation arteries, such as the radial artery. Beyond this, the pulse becomes progressively damped out by the viscous properties of the vessel wall and blood (Figure 1.10, top). The pressure oscillations dwindle and the flow becomes more continuous as the blood enters the resistance vessels. (View Highlight)
- Viscosity decreases in microvessels (the Fåhræus–Lindqvist radius effect) (View Highlight)
- Several mechanisms underlie the Fåhræus–Lindqvist
effect. In capillaries, bolus flow reduces the effective viscosity (Section 8.2). In arterioles, the viscosity is reduced by the peripheral plasma stream generated by axial flow (Figure 8.3). Since shear rates are highest close to the vessel wall, a reduction in friction close to the wall has a marked, beneficial effect on net viscosity. (View Highlight)
- The Fåhræus–Lindqvist effect is not noticeable in tubes wider than 1 mm because the thickness of the marginal plasma layer, 2–4 μm, becomes negligible relative to the tube width. (View Highlight)
- Haematocrit decreases in microvessels (Fåhræus effect) (View Highlight)
- Blood viscosity begins to fall in tubes <1 mm wide (small arteries), and falls to ~2.5 in tubes of 30–40 μm arteriolar width. In tubes of capillary width, ~6 μm, blood viscosity reaches a minimum, almost as low as plasma viscosity. This ‘Fåhræus–Lindqvist effect’ accounts for the surprisingly low effective viscosity of blood in the circulation (Figure 8.19). The Fåhræus–Lindqvist effect is important because it greatly reduces microvascular resistance, and therefore the arterial pressure needed to perfuse the microcirculation. Without this effect, arterial pressure and cardiac work would be much higher. (View Highlight)
- The dynamic haematocrit is particularly low in capillaries,
because the glycocalyx makes the effective radius of the tube even narrower than its apparent radius. (View Highlight)