Maintaining your blood pressure is animportant part of keeping your body inbalance. Blood is put under pressure when it

passes through your heart. The main function of theleft ventricle when it contracts is to pressurise bloodso that it is forced through the blood vessels whichserve the organs of your body (Figure 1). The tinycapillaries that distribute blood through your tissuesand organs have a very small diameter and so offer ahigh resistance to the flow of blood through them.Your heart needs to work hard to provide enoughpressure to push blood through the miles and miles ofcapillary beds.

Inside a capillary bed in an organ, blood pressure ispart of the mechanism that causes water to leave the blood and enter the tissues carrying nutrients,

Blood pressure Jane Taylor

Have you ever fainted? Fainting can be theresult of a sudden drop in your bloodpressure, which is often part of a shockreaction. Read on to find out more about howand why your blood pressure may vary.

17September 2006

GCSE key wordsHeart

CirculationBlood pressure

Valvesforcedopen

Aorta

Blood under highpressure (120 mmHg)

Contracting muscle pushes blood throughvalves into aorta

Closed valveprevents backflow

Atrium

Ventricle

Tendons stopvalves turninginside out

Figure 1 One half of aheart during ventricularcontraction

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Above: A capillary inthe liver. Its smalldiameter (note thesize of the red bloodcells) creates a highresistance to bloodflow

hormones and other useful substances (Figure 2). Bythe time blood has travelled to the end of a capillarybed it has lost much of its volume and pressure. Thepressure of fluid in the tissues is now higher thanblood pressure, so water can return to the blood fromthe tissues by osmosis.

Measuring blood pressureYour blood pressure is given as two f igures, forexample 120 over 80 mm of mercury (mmHg).Although we should use pascals to measure pressure(the reading above would be 16 over 10 kPa), thegeneral public is more familiar with the older units.

A doctor or nurse measures your blood pressureusing a sphygmomanometer. Usually a cuff iswrapped round your upper arm to measure thepressure in the main artery supplying your arm. Thisartery is one of the first branches from the aorta sothe pressure is high.

As the cuff is inflated the pressure of the air inside itpushes on the artery and stops blood flow through it.The air pressure is then reduced slowly. When the airpressure is low enough for blood pressure to forceblood through again the nurse can hear the bloodflow. The pressure can be read off the scale. This givesthe higher f igure (120 in the example above). Itrepresents the pressure of the blood as the leftventricle contracts to push blood out of the heart.This is the systolic pressure.

The lower figure (80 in the example above) is whenthe nurse can no longer hear the flow. It is the bloodpressure in the arteries while the heart is relaxed andfilling with blood. This is the diastolic pressure. Manyhospitals now use digital blood pressure meters whichare thought to be more accurate than a nurse’sjudgement of the flow.

Normal blood pressureHealthy people have blood pressures around thefigures given above (120/80 mmHg). If you need tohave your blood pressure taken you may be asked tosit quietly for a while beforehand because when youare active your blood pressure rises slightly. Duringactivity your body produces a hormone calledadrenaline that causes you to tense your muscles andincreases your heart rate. When you are relaxed your blood pressure drops slightly. Figures of140/90 mmHg or more indicate you could have aproblem with your blood pressure.

Blood pressure and the kidneysYour blood pressure is not just due to heart action.Several different mechanisms are involved, includingyour kidneys. The water content of the blood isregulated in your kidneys. If you expel a lot of water

18 Catalyst

Water and dissolvedsubstances leavethe blood through thethin capillary walls

If the blood pressure is too high,water cannot re-enter capillaries

Tissue fluid pressurerises relative to blood pressure, and with osmosiswater re-enters blood

Capillary walls are oneflattened cell thick

Low bloodpressure

High bloodpressure

Bloodflow

Tissue

Blood pressurefalls

Osmosis is animportant part of theexchange mechanism incapillary beds.

The amount of fluidthat can flow through apipe is proportional tothe radius4. A pipe cancarry only 1/16th of thevolume of a pipe twiceas wide.

Figure 2 What happens in a capillary bed

Above: A doctormeasuring a patient’sblood pressure

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from blood into urine then the blood volume, andhence pressure, will be lower. When you are short ofwater a chemical is produced in the body calledangiotensin II which causes your arteries to constrict.The pressure of blood inside these narrower arteriesrises. Angiotensin also makes you feel thirsty.

HypertensionSome people have high blood pressure. This is knownas hypertension. Their blood pressure is greater than 140/90 mmHg. There are several causes, but acommon one is fatty deposits in the arteries aroundthe heart. These reduce the internal diameter of theblood vessels so much that less f luid can passthrough. This raises the pressure and puts a strain onthe heart.

If someone’s blood pressure is too high, thepressure will not have dropped enough by the end ofthe capillary bed to allow fluid to return to the blood.Instead it stays in the tissues and makes them swollenand puffy. People with high blood pressure maydevelop swollen ankles and feet.

High blood pressure also damages some of the verytiny capillaries serving important organs such as thekidneys and brain. If blood vessels are damaged, ortheir lining is roughened by fatty deposits, there is agreater risk of a blood clot forming in the blood vessel— a condition know as thrombosis. Blood clots can cause local problems where they form, or theymay be dislodged, to block blood flow elsewhere inthe circulatory system.

Living with high blood pressurePeople with high blood pressure are encouraged tomake some changes to their lifestyle. Taking moreexercise and eating a balanced diet help to reduce the‘furring up’ of their arteries by reducing the amount offats in the blood. Reducing salt intake helps thekidneys regulate the water concentration in blood.Stress and anxiety release adrenaline and nor-adrenalin that raise blood pressure, so managingstress also becomes important.

Some people need treatment to reduce their bloodpressure. Drugs called beta-blockers reduce the rateat which the heart beats and the force of theventricular contractions. As a consequence, bloodpressure is reduced. ACE-inhibitors reduce the effectsof angiotensin and dilate the arteries and veins sothat blood flows more easily.

Jane Taylor teaches biology and is an editor of CATALYST.

19September 2006

Answer to Alchemical wordsearch, page 13The word not in the grid is spirit.

The right ventriclepumping blood to thelungs generates only 25 mmHg (3.3 kpa).

Over 10 million peoplein the UK havehypertension.

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Above: The tiny capillaries that make up the glomerulusin a kidney tubule are a filter for the blood. High bloodpressure or very low blood pressure can lead to kidneyfailure

Artwork of a coronaryartery that has beenpartially blocked by thebuild-up of fatty deposits