Blood Pressure

Hypertension - is very common (25% above 140/80), usually symptomless, with the potential of devastating effects. It is a major risk factor for coronary heart disease and stroke; it may lead to heart failure and to renal damage. Hypertension can be divided into 2 groups, primary and secondary. Primary hypertension is the commonest - 95% of patients with raised BP come in this category, for which there is no known cause, we only know the effects. Of the remaining 5% who have secondary hypertension, most are as a result of renal disease, the remainder are endocrine, vascular or neurogenic causes. Most hypertension is known as benign, in other words stable and is compatible with a long life, whilst 5% show a rapid rise in BP with a risk of death within a year or two - this is not surprisingly known as malignant hypertension.

Blood pressure control  is dependent on cardiac output and the total peripheral resistance. The resistance is mainly dependent on the constriction of arterioles and the volume of blood. This is regulated by a 'feed back mechanism' of nerves and hormones that monitor the volume of blood and diameter of the blood vessels and force of the heartbeat.

Imagine that the cardiovascular system is like a balloon filled with water, the blood vessels are the walls of the balloon and the blood is like the water. So the pressure of the water inside depends on the amount of water and the elasticity of the balloon. Blood Pressure will depend on the strength of the pump, diameter of the vessels and the volume of the blood.

Anatomy and physiology recap of heart

The myocardium of the heart is made up of smooth muscle, arranged in layers, encircling the heart like a squeezing fist. The myocardium contains fibres that form a conducting system that initiates and spreads the heartbeat. The standard rate of the heart beat is 100, but it receives a nerve supply from both the sympathetic and parasympathetic system, the parasympathetic slows the rate of the heart down, whilst the sympathetic (vagus) speeds it up. So the pre-set rate is normally in 'slow down mode' to about 60 beats a minute. Contraction of the heart is triggered by depolarisation of the plasma membrane of the muscle cells, starting at the sinoatrial node in the Rt atrium to the atrioventricular node, down the bundle of His and along its two branches (Rt and Lt bundle branch) to the purkinje fibres. This stimulates the atria to contract first followed by the ventricles. (Sodium and Potassium involvement in the depolarisation, mediating the opening of calcium channels that allow Calcium to flow into the cells to constrict muscles).

The myocardium contains cells that secrete the hormone atrial natriuretic hormone (ANH) that has some control over reducing the re-absorption of sodium from the kidneys, it also inhibits the secretion of renin and aldosterone.

Baroreceptors

But major sensors are called barorecptors, they lie in the carotid sinus and aortic arch. These are stretch receptors that respond to altered pressure within the walls of vessels and send a message back to the pituitary. There are also other baroreceptors in the large veins, pulmonary vessels and walls of the heart. The feed back message acts on the sympathetic and parasympathetic nervous system to the muscles of the heart, arterioles and veins to alter their degree of constriction. These receptors can trigger the release of anti diuretic hormone (ADH), also known as vasopressin, which alters the excretion of salt and water from the kidneys and therefore affecting the total plasma volume. The baroreceptors also alter the generation of angiotensin II . These receptors can monitor change but at the same time can be 'reset' at a higher pressure within a couple of days and so get used a higher BP as the norm.

Like the muscular walls of the heart, the walls of the arteries and arterioles are also made up of smooth muscle and capable of relaxation and contraction, thus altering the diameter of the blood vessels. As they dilate, so the pressure drops.

Cardiac Output

The cardiac output is made up of the amount of blood returning to the heart, it is pumped into the heart the skeletal pump (such as the muscles of the legs), and the pumping action of when we breath in, by creating pressure on the abdominal veins, and as the diaphragm drops down, decreasing the pressure in the thorax, enhancing venous return to the Rt atrium of the heart.

However the rate of venous blood returning can be altered. The veins have thinner walls than arteries, but also contain smooth muscle innervated by the sympathetic nervous system therefore capable of some contraction. Venous return can also be increased by the skeletal pump - running and by the respiratory pump - breathing faster.

The volume of blood pumped out of the heart will then depend on the strength and rate of the heart pumping, which is controlled by the nervous and hormonal feed back system, which determines the output from the heart. However, the pressure in the arteries will also be determined by the resistance it meets in the arterioles.

So the formula for determining the mean arterial pressure is taken by multiplying the total peripheral resistance by the cardiac output.

Other factors

There are other factors influencing blood pressure, there are various local controls.

Types of hypertension