Blood Pressure|High blood pressure|Low blood pressure| Medical world

BLOOD PRESSURE

INTRODUCTION

Blood pressure is the force or pressure that the blood exerts on the walls of blood vessels. Systemic arterial blood pressure maintains the essential flow of blood into and out of the organs of the body. Keeping blood pressure within normal limits is very important. If it becomes too high, blood vessels can be damaged, causing clots or bleeding from sites of blood vessels rapture. If it falls too low, then blood flow through tissue beds may be inadequate. This is particularly dangerous for essential organs such as the heart, brain or kidneys.

The systemic arterial blood pressure, usually called simply arterial blood pressure, is the result of the discharge of blood from the left ventricle into the already full aorta. Blood pressure varies according to the time of day, the posture, gender and age of the individual. Blood pressure falls at rest and during sleep. It increase with age and is usually higher in women than in men

SYSTOLIC DIASTOLIC PRESSURE

 
When the left ventricle contracts and pushes blood into the aorta, the pressure produced within the arterial system is called the systolic blood pressure. In adults it is about 120 mmHg or 16Kpa. 
In complete cardiac diastole when the heart is resting following the ejection of blood, the pressure within the arteries is much lower and is called diastolic blood pressure. In an adult this is about 80mmHg or 11Kpa. The difference between systolic and diastolic blood pressure is the pulse pressure. 
Arterial blood pressure is measured with a sphygmomanometer and is usually expressed with the systolic pressure within above the diastolic pressure. 

BP= 120/80 mmHg

 

Elasticity of arterial walls

There is a considerable amount of elastic tissue in the arterial walls, especially in large arteries. Therefore, when the left ventricle ejects blood into the already full aorta, the aorta expands to accommodate it, and then recoils because of the elastic tissue in the wall. This pushes the blood forwards, into the systemic circulation. This distention and recoil occurs throughout the arterial system. During cardiac diastole the elastic recoil of the arteries maintains the diastolic pressure.

FACTORS DETERMINING BLOOD PRESSURE

Blood pressure is determined by cardiac output and peripheral resistance. Change in either of these perimeters tends to alter systemic blood pressure, although the body's compensatory mechanism usually adjust for any significant change.

Blood pressure = cardiac output × peripheral resistance

Cardiac output

Cardiac output is determined by the stroke volume and the heart rate. Factors that affect the heart rate and stroke volume and they may increase or decrease cardiac output and, in turn, blood pressure. An increase in cardiac output raises both systolic and diastolic pressures. An increase in stroke volume increases systolic pressure more than diastolic pressure.

Peripheral or arteriolar resistance

Arterioles, the smallest arteries, have a tunica media composed almost entirely of smooth muscles, which responds to nerve and chemical stimulation. Construction and dilation of the arterioles are the main determinants or peripheral resistance. Vasoconstriction causes blood pressure rise and vasodilation causes it to fall.

Auto regulation

Systemic blood pressure continually rises and falls, according to levels of activity, body position, etc. however, the body organs are capable of adjusting blood flow and blood pressure in their own local vessels independently of systemic blood pressure. The property is called auto regulation.

Control of blood pressure

Blood pressure is controlled in two ways-

short-term control

long-term control

SHORT TERM CONTROL

The cardiovascular centre (CVC) is a collection of interconnected neurones in the medulla and pons of the brain stem. The CVC sends autonomic nerves to the heart and blood vessels. It controls BP by slowing down or speeding up there heart rate and by dilating or constructing blood vessels.

Baroreceptor

Within the walls of the aorta and carotid sinuses are baroreceptors , nerve endings sensitive to stretch, which are the body's principal moment-to-moment regulatory mechanism for controlling blood pressure. A rise in blood pressure in these arteries stimulates the baroreceptors, increasing their input to the CVC. The CVC responds by increasing parasympathetic nerve activity to the heart; this slows the heart down. At the same time, sympathetic stimulation to the blood vessels is inhibited, causing vasodilation . The net result is a fall in systemic blood pressure.

Chemoreceptor

These are nerve endings situated in the carotid and aortic bodies, and are primarily involved in control of respiration. They are sensitive to changes in the levels of carbon dioxide, oxygen and the acidity of the blood. Rising blood CO2, falling blood O2 levels and or fallings arterial blood PH all indicate failing tissue perfusion. When these changes are detected by the chemoreceptors, they send signals to the CVC, which then increases sympathetic drive to the heart and blood vessels, pushing blood pressure up to improve tissue blood supply. Because respiratory efforts is also stimulated, blood oxygen levels rise as well.

Chemoreceptors input to the CVC influence its output only when severe disruption of respiratory function occurs or when arterial BP falls to less than 80mmHg. Similar chemoreceptors are found on the brain surface in the medulla oblongata, and they measure carbon dioxide/oxygen levels and PH of the surrounding cerebrospinal fluid. Changes from normal activity responses .

Higher centres in the brain

Input to the CVC from the higher centres is influenced by emotional states such as fear, anxiety, pain and anger that may stimulate changes in blood pressure. The hypothalamus in the brain control body temperature and influences the CVC, which responds by adjusting the diameter of blood vessels in the skin. This important mechanism regulates conservation remains in the normal range.

LONG TERM CONTROL

Slower, longer lasting changes in blood pressure are effected by the renin-angiotensin-aldosterone system and the action of antidiuretic hormone. Both of these systems regulate blood volume, thus influencing blood pressure. In addition, by the heart itself, causes sodium and water loss from the kidney and reduces blood pressure, opening the activities of both ADH and RAAS.

Pressure in the pulmonary circulation

Pulmonary blood pressure is much lower than in the systemic circulation. This is because although the lungs receive the same amount of blood from the right ventricle, there are so many capillaries in the lungs that pressure is kept low. If pulmonary capillary pressure exceeds 25mmHg, fluid is forced out of the bloodstream and into the airsacs, with very serious consequences. Auto regulation in the pulmonary circulation makes sure that blood flow through the vast network of capillaries is directed through well-oxygenated airsacs.