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Lecture 8

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Daryl Schwenke

Aorta to Capillaries Objectives:  Understand the design and function of the aorta, large and small arteries, and the arterioles. Aorta and arteries:  Thick walled, smooth muscular structure- makes them extremely tough. Need these characteristics because they work to maintain a constant through the system.  Pressure reservoir: reduce fluctuations in pressure and flow i.e. maintains steady flow even during diastole.  The ability to do this is largely due to the compliance of a vessel:  Arterial compliance:  During systole, the generated energy promotes blood flow through the arteries, and is stored in the elastic elements of the vessel was (potential energy). The large arteries act as a ‘reservoir’ for most of the SV during systole.  During diastole, the potential energy is released due to the elastic recoil of the vessel wall. Thus, flow is more or less steady into the smaller arteries through the entire cardiac cycle.  Conduit vessels- must do their function in an optimal way. Small arterioles:  These vessels are the main determinants of total peripheral resistance and local resistance.  Provides the necessary resistance for maintaining high pressure during diastole.  Arterial system is extremely efficient at reducing pulsatile fluctuations- by the time we reach small arterioles, flow is relatively non-pulsatile. Arterial Blood Pressure:  The maximul arterial pressure reached during peak ventricular ejection = systolic pressure (SP)  Minimum pressure just before ventricle contraction = diastolic pressure (DP)  Can estimate mean arterial pressure: ABP = DP + 1/3 (SP-DP)  Pulse pressure: P = SP-DP Determinants of blood pressure:  120 over 80mmHg- absolutely critical that we maintain this.  Can lead to adverse consequences- e.g. development of hypertension.  Mean ABP is ‘determined’ by:  Cardiac output (i.e. blood flow)  Total peripheral resistance = sum of global resistance from all vascular beds (i.e. blood outflow)  I.e. MABP = Q x TPR  Arterial pressure depends on both Q and TPR NB: under normal conditions, blood inflow = blood outflow. Will never change, except in case of heart failure.  Arterial pressure can be adjusted by altering either Q or TPR  E.g. a decrease in MABP causes a reflex increase in TPR (through vasoconstriction) restoring a ‘normal’ MABP (which is a critical homeostatic variable) Blood Flow Distribution:  APB is regulated to a constant value. Thus a change in regional blood flow is achieved by altering regional resistance (radius of the arterioles). Remember: Q is proportional to r^4  Thus the regional blood flow of various vascular beds are adjusted to maintain a stable ABP by adjusting resistance.  Without any change in CO, if blood flow to one vascular bed is up regulated, blood flow to other vascular bed must be down regulated in order to maintain total peripheral resistance  At a constant pressure, flow is entirely determined by radii (since length and viscosity do not change). Resistance Control Mechanisms:  Neural controls:  Vasoconstrictors: Sympathetic nerves: releases noradrenaline which binds alpha1- adrenergic receptors (unlike beta receptors in the heart). Increase in SNA = vaso
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