Your body uses several clever mechanisms to help keep your blood pressure within safe limits. These include altering the speed and force at which your heart contracts, relaxing or constricting your arteries and veins to vary the amount of blood they hold, and regulating the volume of blood in your circulation – by altering the amount of salt and fluids filtered out by your kidneys, and by triggering feelings of thirst.
These control mechanisms are partly regulated by nerve signals from your central nervous system, and partly by hormones and related substances released from various parts of your body including your kidneys (renin), adrenal glands (eg aldosterone, adrenaline), pituitary gland (antidiuretic hormone) and even your heart (atrial natriuretic peptide).
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Why you have blood pressure
Your blood pressure exists because your heart pumps blood around a closed system of vessels. The best way to explain how your blood pressure is maintained is to compare circulating blood with water running through a hose pipe. Pressure within the hose pipe can vary from high to low, depending on a number of factors. Pressure can be raised by increasing the power of the pump (tap), adding more water, or by squeezing the pipe and reducing its diameter.
In the same way, your blood pressure is increased by increasing the power of your heart (eg exercise) or by reducing the diameter of your blood vessels (constriction or hardening and furring up of the arteries).
At any one time, the pressure in your arteries depends on:
- how hard your heart pumps (cardiac output)
- the elasticity (peripheral resistance) of your blood vessels and their ability to constrict or relax
- the volume of fluid inside your circulation
- the integrity of the blood vessels – a leak can cause blood pressure to fall.
Systolic and diastolic blood pressure
The contraction of your left ventricle, which pushes a volume of blood out into your circulation, causes a surge in your blood pressure with each heartbeat. The highest pressure reached in your arteries during this surge is known as the systolic blood pressure as it is due to contraction (systole) of your heart. As the heart rests between beats, blood pressure falls again to its baseline value. The lowest blood pressure recorded while the heart rests (diastole) is known as the diastolic pressure.
In general, a rise in cardiac output increases systolic pressure, while a rise in peripheral resistance increases diastolic pressure.
Your cardiac output is the amount of blood pumped through your heart per minute. This depends on how fast your heart beats, the volume of blood held within the heart chambers, how well the heart muscle contracts and the pressure against which the heart must push to eject blood into the circulation (which in turn depend on your arterial blood pressure and arterial elasticity).
The average cardiac output at rest varies between 4.5 and 5.5 litres per minute, and averages around 5 litres per minute (70 ml per heart beat x 70 beats/min). If cardiac output increases, more blood is pumped into your arteries per minute, which increases blood pressure. If cardiac output falls, blood is pumped through the circulation more slowly and blood pressure falls.
Many different factors can affect your heart rate and cardiac output. For example:
- Anxiety and excitement increase cardiac output by 50% – 100%
- Eating can increase cardiac output by 30%
- Exercise increases cardiac output by up to 700%
- Pregnancy – increases cardiac output by 75% or more
- Sitting or standing from lying decrease cardiac output by 20% – 30%
- Having a rapid, irregular heartbeat (atrial fibrillation) decreases cardiac output
- Having heart failure decreases cardiac output.
The system of tubes that distribute blood through the body consist of arteries, arterioles, capillaries, venules and veins.
Arteries carry blood away from your heart and have thick, elastic walls with smooth muscle fibres wrapped round them. This allows arteries to expand and contract as each pulse of blood passes through, smoothing out and regulating your blood flow. As the pulse wave passes, the artery walls stretch and then spring back to give the blood an extra push to help ensure that forward flow is continuous. This recoil is known as the Windkessel effect after the German word for an elastic reservoir.
Large arteries branch and divide into smaller and smaller arteries and eventually divide into smaller vessels known as arterioles. Arterioles are less elastic than arteries and are also more narrow. They form a natural bottleneck that provides more resistance to blood flow and are known as resistance vessels. It is this resistance – against which your heart has to pump – that helps to maintain the blood pressure within your circulation.
Factors that trigger constriction of arteries and arterioles (eg stress, emotion, smoking, taking some drugs) will increase your peripheral resistance further and cause your blood pressure to rise.
Arterioles connect up with a vast network of thin-walled blood vessels known as capillaries. Capillaries are the site where oxygen and nutrients pass from your blood stream into the tissues, while fluid and other substances – including wastes – pass from the tissues into the circulation. Capillaries within the kidneys (glomeruli) are where excess fluid and soluble wastes are filtered from the circulation to form urine. This removal of fluid is another way in which blood pressure is regulated.
Capillaries connect to small veins called venules, which in turn feed blood into larger veins which carry blood back to the heart
Veins have thinner walls than arteries as the blood pressure within them is lower. Larger veins also contain valves to stop back-flow and keep blood moving in the right direction towards the heart. Your veins are normally in a partially collapsed state and have an oval shape in cross section. They are capable of distending significantly to absorb excess fluid to even out fluctuations in blood pressure. Veins are therefore known as capacitance vessels.
As blood flows back into the chest, the action of the rib cage on breathing in and out also helps to draw venous blood towards the heart.
During exercise, contraction of skeletal muscles compresses veins in the limbs and provides a secondary pumping action that helps to keep blood flowing through them. This muscle pump is so efficient that blood flow through contracting muscles increases as much as 30%.
Pulsation of nearby arteries may also compress the veins to keep blood moving forward – back flow is only prevented by the valves scattered throughout the larger veins. If these valves give way, blood may pool in certain veins under the influence of gravity to form varicosed (tortuous, distended, swollen) veins.
Sixty per cent of your body weight consists of water. Two thirds of this water is found inside your cells (intracellular) while the remaining third is found outside your cells (extracellular) in the so-called internal sea. This extracellular fluid is divided into the fluid that bathes your cells (interstitial fluid) and the fluid that circulates in your blood stream (plasma) and which contributes to your blood pressure. Water molecules constantly pass from one fluid compartment to another as nutrients and wastes are passed to and from your cells, so your fluid balance is in a constant state of flux.
During an average day in a temperate climate, you lose around 2.4 litres of fluid through your kidneys as urine, through your lungs as water vapour and via your skin as sweat. A small amount is also lost in your bowel motions. If you take vigorous exercise or visit a hot climate, it is easy to lose twice this amount. However, as long as you replace your daily water losses through adequate drinking and eating, the amount of fluid in your cells, interstitial fluid and blood stream is kept within narrow limits.
Excess fluid passes into your circulation and is quickly filtered out by your kidneys. If you drink more fluid than you need, you will void increased amounts of urine. If you are slightly dehydrated, you will pass less urine than normal as your body will carefully conserve its water stores and also trigger sensations of thirst.
If the fluid balance of the body becomes disturbed, excess water may seep into the interstitial fluid bathing your cells rather than passing into your blood stream, leading to swollen ankles (oedema). If excess fluid finds its way into your blood stream, your blood pressure is not affected initially as your veins can distend to accommodate the extra fluid, but as they become more and more distended, you will reach a point where your blood pressure starts to rise.
Similarly, if you lose excessive amounts of water for any reason, the veins can collapse down to a certain extent to protect the pressure in the system. If fluid loss from the circulation is excessive (eg haemorrhage) your blood pressure will fall rapidly, leading to clinical shock.
How blood pressure is normally controlled
Your body normally keeps your blood pressure within safe limits by adjusting several different factors, including the:
- pumping action of your heart (cardiac output)
- diameter of your resistance vessels (mainly the arterioles)
- amount of blood pooled in your capacitance vessels (veins)
- balance of salt and fluids in your circulation.
These factors are controlled by nerve signals from the brain, and by hormones or related substances released from various parts of the body including the kidneys (renin hormone), the adrenal glands (eg aldosterone, adrenaline), the pituitary gland (antidiuretic hormone) and even the heart (atrial natriuretic peptide).
Your body monitors the pressure within your circulation via special sensors, or baroreceptors, found in the walls of the heart and in strategic blood vessels throughout your circulation.
These baroreceptors respond to the degree of stretch your artery or vein walls are experiencing. At normal blood pressures, the baroreceptors fire at a slow rate. If your blood pressure falls, they discharge at an even lower rate, and if your blood pressure rises, they fire at an increased rate.
Baroreceptors send nerve signals to the brain that that trigger nerve reflexes which:
- slow or speed up your pulse rate and cardiac output
- dilate or constrict blood vessels
These changes decrease or increase your blood pressure until it returns to its previous normal level.
The baroreceptors in the heart are also part of a reflex that regulates the volume of water in your blood stream and in the extracellular fluid bathing your tissues. This is achieved by altering the amount of certain hormones secreted (anti-diuretic hormone, renin and aldosterone) which regulate the amount of fluid and salts retained by your kidneys.
When you stand suddenly from a lying position, the pressure in arteries taking blood to the brain falls, and blood also pools in your lower leg veins. The rapid drop in blood pressure is detected by baroreceptors in your heart and in the carotid artery in your neck, triggering a reflex increase in cardiac output, constriction of arterioles, dilation of blood vessels within the brain, and a prompt increase in circulating levels of renin and aldosterone.
These quick responses help to maintain blood pressure so that blood flow to the brain only decreases by around 20% on standing, rather than falling by 60% or more, which would quickly make you black out. If this mechanism fails, then you may develop postural hypotension in which you feel dizzy on standing.
The renin-angiotensin system
The most important hormones involved in blood pressure control is known as the renin-angiotensin system.
Renin is produced in the kidneys and passes into the blood stream to trigger a series of reactions that cause your blood pressure to rise.
This series of reactions starts when renin interacts with a circulating sugar-protein known as angiotensinogen. Angiotensinogen is made in the liver and circulates within the blood stream until it meets up with renin. Renin then splits angiotensinogen into two to release a small chain of ten amino-acids (protein building blocks) known as angiotensin I.
Angiotensin I is inactive, but as it travels around in the circulation, it interacts with an enzyme found in blood vessel linings. This enzyme, angiotensin converting enzyme (ACE) cuts a further two amino acids away from angiotensin I to form a chain of 8 amino acids known as angiotensin II. This in turn is acted on by other enzymes (together known as angiotensinase) to produce angiotensin III.
Angiotensin II makes arterioles constrict and is one of the most powerful vasoconstrictors known. It rapidly produces a rise in both systolic and diastolic blood pressures. It also acts on the brain to trigger a nerve-mediated rise in blood pressure, causes feelings of thirst (so you drink more fluid) and increases the secretion of anti-diuretic hormone and aldosterone hormone, both of which cause you to retain fluid. All these actions result in a rise in blood pressure.
Angiotensin III has similar actions to angiotensin II but is only 40% as powerful in boosting blood pressure.
Some antihypertensive drugs target fluid retention in the kidneys (diuretics), or by blocking the action of angiotensin converting enzyme (ACE) or angiotensin II. Some (beta blockers) work partly by decreasing renin secretion.
Some forms of high blood pressure may be linked to over-secretion, or over-sensitivity to the effects of renin or the angiotensins.
Anti-diuretic hormone (ADH or vasopressin) is secreted by the pituitary gland at the base of your brain. As its name suggests, it acts to lower urine production in the kidney. This maintains the level of fluid in your circulation and helps to maintain blood pressure if, for example, you are dehydrated or had a haemorrhage.
Anti-diuretic hormone works by increasing the permeability of urine collecting ducts in your kidneys so that water that’s already filtered into the urine can be reabsorbed into the body even as it drains away as waste. This hormone makes your urine increasingly concentrated (darker in colour and lower in volume) when you haven’t drunk enough fluids. Some forms of high blood pressure may be linked to over-secretion, or over-sensitivity to, anti-diuretic hormone.
Aldosterone is a steroid hormone produced in your adrenal glands. It acts to increase the reabsorption of sodium (in exchange for potassium) from the urine, sweat, saliva and intestinal juices to conserve your sodium stores. This in turn draws water back into your body (through the process of osmosis) and plays an important role in maintaining blood pressure when salt intake is low. As sodium ions are swapped for potassium ones, the overall effect is a rise in sodium levels and blood pressure and a fall in potassium levels.
One form of secondary hypertension is due to over-secretion of aldosterone (primary aldosteronism) due to an adrenal gland tumour.
Atrial natriuretic peptide
Atrial natriuretic peptide is released from the upper chambers of the heart (atria) in response to mechanical stretching. This sends a message that the pressure within the heart is too high, and the hormone acts to quickly lower blood volume in at least three ways, by increasing the loss of salt and water through the kidneys, causing blood vessels to dilate, and causing blood vessels to become more leaky so fluid can escape into the tissues (causing swelling).
Your blood pressure control depends on constant tweaking of many different factors, so it’s not surprising that things can go wrong. Age-related hardening and furring up of the arteries, and high dietary intakes of salt can cause your blood pressure to rise. Drugs that target these different control mechanisms, and which cause blood vessels to dilate are the mainstay of medical treatment.
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