Aorta compliance

Simple Model using Aorta Compliance and Peripheral Resistance... 

During diastolic filling (A to B), ventricular pressure is below atrial and aortic pressures, so the mitral valve (between the left atrium and the left ventricle) is open, and the aortic valve (between the left ventricle and the aorta) is closed. Blood therefore flows from the atrium into the ventricle, based on the pressure gradient and the inertances and resistances of the inlet. Note that the pressure gradient will decrease as the ventricle fills due to the passive compliance of the chamber and the drop in pressure in the atrium. 

The geometrical parameters of the canine systemic and pulmonary circulations are summarized in Table 56.1. Vessel diameters vary from a maximum of 19 mm in the proximal aorta to 0.008 mm (8 m) in the capillaries. Because of the multiple branching, the total cross-sectional area increases from 2.8 cm in the proximal aorta to 1357 cm in the capillaries. Of the total blood volume, approximately 83% is in the systemic circulation, 12% is in the pulmonary circulation, and the remaining 5% is in the heart. Most of the systemic blood is in the venous circulation, where changes in compliance are used to control mean circulatory blood pressure. This chapter will be concerned with flow in the larger arteries, classes 1 to 5 in the systemic circulation and 1 to 3 in the pulmonary circulation in Table 56.1. Flow in the microcirculation is discussed in Chapter 59, and venous hemodynamics is covered in Chapter 60. 

The circulatory fluid is ejected by an electropneumatically driven ventricular pump. Downstream of the pump, an aortic valve assembly is located two different models have been built in order to offer lateral or frontal view of the prosthesis movements. Suitable stent adapters allow to test prostheses of different type and size. The aorta is a variable compliance rubber tube. Through a rigid conduit the fluid is conveyed to the laminar flow assembly which controls peripheral resistances. Aortic compliance and peripheral resistances are hydropneumatically controlled. The fluid, passing through a venous reservoir open to atmospheric pressure, reaches the left atrium. This is a rigid wall chamber in which a hydropneumatic system relates cardiac output to venous return, reproducing Frank--Starling s Law. Between atrium and ventricle there is another valve test assembly which allows to test mitral valves. 

The simplest possible model of the cardiovascular system takes into account the compliance of the aorta and the arterial system. The resistance tMt the blood flow encounters is assumed to be represented by one so-called peripheral resistance. This situation is shown in Fig. 18.5. 

Once ventricular pressure exceeds the pressure in the aorta, the aortic valve opens and systolic ejection commences (C to D). Blood flows out from the ventricle into the aorta based on the pressure gradient, the inertances, and the resistances of the outlet. The muscle continues to contract until the cardiac action potentials have run their course. As the muscle begins to relax, pressure will drop until it falls below the aortic pressure. At this time, the aortic valve closes, and again blood flow ceases. Meanwhile, the ventricle continues to relax, decreasing the ventricular pressure. This period (D to A) is termed isovolumetric relaxation. Here, muscle relaxation can be thought of as increasing the compliance of the... 

Stress-strain measurements were made using an Instron Model TM-M Tensile Tester (500 g load cell, cross-head speed was 50 mm/min) with sample holder designed for testing tubular specimens. Test specimens were tubular sections 0.5 cm wide. All measurements were done using wet samples. The copolyurethane samples are shown in Fig. 1 along with the stress-strain curves of samples cut from fresh sections (less than 30 min. old) of the canine thoracic aorta and the inferior vena cava. The samples were maintained in buffered saline from their removal until being measured. Compliancies were calculated as the force (in dynes/test section) required to produce a displacement to 20% increase in cross-sectional area of the lumen. 

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