Blood vessel networks and pressure regulation

Haemodynamic in blood vessel networks:<img alt="Haemodynamics in blood vessel networks arteriole capillary artery Vessels in series: Rtotal = R 1 + R 2 + R 3 + R 4 + R5 R smallest vessel Vessels in parallel: 1 1 1 Increasing the number of vessels lowers resistance 1 1 total 1 1 Rtotal = 1 Rtota, < R of the vessel with the lowest resistance Rtota, is low in vascular networks arranged in parallel e high resistance capillaries contribute little to Rtota, in a parallel network changing resistance in a few vessels Of a large parallel network has little effect on Rrotal " src="https://knowt-user-attachments.s3.amazonaws.com/ac7fdada-8d75-4bb1-bf11-54bdbbb83e1b.png" /><ul><li>Networks of blood vessels can be arranged in series or in parallel.</li><li> For vessels arranged in series, the total resistance to flow is equal to the sum of the resistances in all sections. This approximates to the resistance of the smallest vessel, which gives the highest resistance in the circuit.</li><li>Blood vessels are mostly organised in parallel. In this situation, the inverse of the total resistance is equal to the sum of the inverse of the resistances in each parallel vessel. So, the total resistance to flow is 1 over the sum of the inverse resistances. From this equation, you can see that the total resistance is less than that of the vessel with the lowest resistance.<ul><li>the total resistance is lower in networks with vessels arranged in parallel.</li><li>capillaries with a high resistance contribute little to the total resistance in the network.</li><li>changing the resistance of a few vessels in a large parallel network has little effect on the total resistance to blood flow.</li></ul></li><li> Importantly, resistance can be kept low by increasing the number of parallel vessels.</li></ul><img alt="Branching network aorta 100 mmHg small arteries arterioles capillaries venules small veins vena cava P— O mmHg (above atmospheric) e Flow is the same through each section pressure drops across each section Tiny vessels = big pressure drop Many vessels in parallel = less pressure drop " src="https://knowt-user-attachments.s3.amazonaws.com/4707c053-89df-4d1c-b940-d8dfec4eb5ae.png" /><ul><li>This figure illustrates the increase in vessel numbers in the circulation as vessels get smaller, from the aorta through to the capillaries. As a result, the resistance to blood flow falls at each branch.</li></ul><img alt="ßueuouqnd salnuan Å•euowlnd sapell!de• ,oeuouulnd salovaue ÅJeuouqnd sawaue Å'euou.qnd ueaq euaA su!aA aalel supa news salnuan savell!deo R 8 salouaue savaue news sauaue euoe (SH utu) aunssaud " src="https://knowt-user-attachments.s3.amazonaws.com/f582640f-c8f0-47f6-a8de-904019f45428.png" /><ul><li>This figure shows how pressure in the blood vessels changes throughout the circulation. Pressure is high in the arterial system and low in the venous system.</li><li>The largest drop in pressure occurs across the arterioles. Oscillations in the trace from large arteries are due to the pulsatile nature of the heartbeat.</li><li>The pressure is higher during systole as blood is actively pumped and lower during diastole.</li><li>As the vessels get smaller, the pressure becomes more constant. That is because the elasticity in the arteries can damp down the pressure changes, so that by the time blood reaches the capillaries, it flows smoothly under constant resistance and pressure</li></ul> Total peripheral resistance:The total peripheral resistance (TPR) is the term given to the total resistance in the systemic vasculature. It is derived from the equation R = P/F, where P is the total pressure drop across the systemic circulation and F is cardiac output (CO). Thus: <img alt="М АР- RAP " src="https://knowt-user-attachments.s3.amazonaws.com/7632a165-35d4-4542-90e8-2e8c375abf83.gif" />where MAP is mean arterial pressure and RAP is right atrial pressure.TPR is measured in peripheral resistance units or PRUs, where 1 PRU = 100 mm.Hg / 100 ml.s-1.The peripheral (systemic) circulation is a high resistance, high pressure system with TPR in a healthy person at rest around 90 mm.Hg / 83 ml.s-1 = 1.08 PRU.Total pulmonary resistance The total pulmonary resistance is a measure of the total resistance to flow across the circulation of the lungs. It can be calculated in the same way as TPR, except that the total pressure is the mean pulmonary artery pressure minus the left atrial pressure.The pulmonary circulation is a low resistance, low pressure system with total pulmonary resistance in a healthy person at rest around 12 mm.Hg / 83 ml.s-1 = 0.14 PRU. This is important as it protects the thin gas exchanging surfaces in the lung from damage.Blood pressure regulation:<ul><li>MAP = CO X TPR</li><li>Mean arterial pressure = cardiac output x total peripheral pressure</li></ul>