Blood Pressure



Blood Pressure

Introduction

The blood pressure is the force that causes blood to flow through the arteries, capillaries, and finally veins back to the heart. It is closely regulated via several physiological mechanisms to ensure an adequate tissue blood flow. Both systolic and diastolic pressures increase with age (figure 1).

The blood pressure is determined by the rate of blood flow produced by the heart (cardiac output), and the resistance of the blood vessels to blood flow. This resistance is produced mainly in the arterioles and is known as the systemic vascular resistance (SVR) or the peripheral vascular resistance (PVR). The interactions between blood flow, pressure and SVR are shown in the equation in the following equation:

Blood pressure = cardiac output x SVR

Using the above equation we can see that the blood pressure can be raised either by increasing cardiac output or SVR. Conversely the blood pressure is reduced by a fall in cardiac output or SVR.

The autonomic nervous system (ANS) plays an important role in the maintenance of homeostatic processes in the body. The cardiovascular system (CVS) which includes the heart and the circulatory system is regulated by the ANS. Any changes in heart rate (HR) and blood pressure (BP) are homeostatically adjusted by the ANS via sympathetic or parasympathetic nervous system.

The primary cardiac control centre (cardioacceleratory, cardioinhibitory) and vasomotor centres (large group of neurons for vasoconstriction and small group for vasodilation) are located in the medulla oblongata. The cardioacceleratory centre controls the sympathetic nervous system which responds by increasing HR and force of contraction. The cardioinhibitory centre controls the heart via parasympathetic neurons which in turn reduces HR. Vasoconstriction is brought about by the SNS and vasodilation by PNS. These centres are influenced by reflex pathways and via input from higher centres especially from the cerebral cortex, bulbar reticular area, upper spinal cord and hypothalamus (Williams et al, 2008). Ex: Reduction in partial pressure of oxygen (PO2) in the blood is detected by chemoreceptors which stimulate the cardioacceleratory centre to increase cardiac activity and stimulate vasodilation.

The sympathetic (SNS) and parasympathetic (PNS) nervous system controls the heart by means of the cardiac plexus and both innervate the Sinoatrial Node (SAN), Atrioventricular Node (AVN) and the myocardium. The ventricular myocardium predominantly has SNS fibres compared to PNS fibres (Martini and Nath, 2008).

Procedure

Participants were told to access a program called 'The virtual cat v2.5.6' which is a computer simulation program of an anesthetised cat preparation, using the computers available in the physiology suite. It was accessed using the following pathway:

Access My Computer displayed as 45000000(student number) on SCI 000000 (PC number). Then use the following pathway G:\Strathclyde University\Anaesthetized Cat

They were initially told to access and read the 'contents' section using the 'Help' tab. Then they were told to click the 'Options' tab and uncheck nictitaing membrane and skeletal muscle so that only heart rate (HR) and blood pressure (BP) showed up on screen. They were also told to uncheck the 3 boxes at the bottom of the screen which included 'stimulate vagus nerve', ...