Cardiovascular System

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The heart

Your heart is roughly the size of your fist.

The heart is divided into 2 halves – left and right.

The 2 halves of the heart are separated by a muscular wall called the septum

The heart is divided into 4 chambers – 2 on the left and 2 on the right

The top chambers are called the left and right atria – atria is plural for atrium

The bottom chambers are called the left and right ventricles

There are 4 valves in the heart which stop blood flowing back when it has been squeezed into the next chamber or blood vessel.

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Blood Flow

Blood enters the right side of the heart through the superior (SVC) and inferior vena cava (IVC)

The SVC brings de-oxygenated blood from the upper part of the body

The IVC brings de-oxygenated blood from the lower part of the body

The SVC & IVC deliver blood into the 1st of the 4 chambers of the heart, the right atrium.

When the right atrium is full it contracts and blood is pushed through the tricuspid valve into the 2nd chamber of the heart – the right ventricle. When the right ventricle is full, blood is pumped through the pulmonary valve into the pulmonary artery which carries blood to the lungs where it becomes oxygenated.  The pulmonary artery  is the only artery to carry de-oxygenated blood. Once the blood has been oxygenated in the lungs it comes back to the heart via the pulmonary vein (this is the only vein to carry oxygenated blood). The pulmonary vein brings the oxygenated blood to the 3rd  chamber of the heart – the left atrium, through the mitral valve. When the right atrium is full it contracts and pumps the blood through the aortic valve out of the heart via the aorta. The aorta splits off into many different blood vessels which carry oxygenated blood all over the body. The left ventricle has the thickest muscle of all of the heart chambers because it has to pump blood to the rest of the body.

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Summary of blood flow

  • Superior and inferior vena cava
  • Into right atrium
  • Through the tricuspid valve
  • To the right ventricle
  • Through the pulmonic valve
  • To the pulmonary arteries to the lungs 
  • The blood picks up oxygen in the lungs
  • Then flows from the lungs
  • To the pulmonary veins
  • To the left atrium
  • Through the mitral valve
  • To the left ventricle
  • Through the aortic valve
  • To the aorta
  • To the body
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Important info about blood flow

You need to know the journey blood takes through the heart but you MUST remember that blood enters the right and left atria from the vena cavae & pulmonary vein at the same time.

This blood is then squeezed into the left and right ventricles at the same time.

And then squeezed into the aorta & pulmonary artery at the same time.

Our heartbeat is the contraction & relaxation of the atria and then the contraction & relaxation of the ventricles – which means that the L&R atria contract at the same time& the L&R ventricles contract at the same time.

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An ECG records the rhythms and electrical activity of your heart. 

A number of electrodes (small metallic discs) are placed on your arms, legs and chest. The electrodes are connected to a machine that records the electrical signals of each heartbeat.

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The Cardiac Cycle

The cardiac cycle is what happens when the atria and ventricles of the heart contract and relax during a heartbeat.

Electrical impulses start at the SA (Sino Atrial) node in the right atrium and are then transmitted across the heart setting off the chain of events that form the cardiac cycle.  The SA node is the heart’s natural pacemaker.

Each part of the cardiac cycle can be seen in a ECG.

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PQRST wave

P wave is when the L&R atria contract and blood is pushed into the L&R ventricles

The L&R ventricles are filling with blood between the P and Q waves

Q wave is when the electrical signal tells your L&R ventricles to contract

R wave is when the right ventricle contracts

S wave is when the left ventricle contracts

T wave is when the walls of the L&R ventricles are relaxing

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Blood Pressure

Blood pressure is the pressure of the blood in your arteries. You need a certain amount of pressure in your arteries to keep the blood flowing around your body.

The pressure of blood flowing through the arteries varies at different times in the heartbeat cycle.

Systolic blood pressure is the highest level your blood pressure reaches. This is when your heart contracts and blood is forced through the arteries.

Diastolic blood pressure is the lowest level your blood pressure reaches. This is when your heart relaxes between each beat.

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Measuring Blood Pressure

Your blood pressure is expressed as two numbers – for example, 120/80mmHg. (‘mmHg’ is the unit used for measuring blood pressure. It stands for millimetres of mercury.) 

The first number is the systolic pressure (when the heart is contracting) and the second is the diastolic pressure (when the heart is relaxing). According to the BHF  the target for the general population is to have a blood pressure below140/90 mmHg.

Blood pressure is measured using a sphygmomanometer.

When you have your BP taken - the cuff is inflated until it stops blood passing through the brachial artery in your arm.  At this point there will  be no pulse.

The cuff is then slowly deflated until the pulse is felt – this is the systolic pressure. The cuff continues to deflate until the sound of the pulse fades – this is the diastolic pressure.

In general high blood pressure (or hypertension) is more of a health problem than low blood pressure (hypotension).  High blood pressure is caused by hardening of the arteries caused by a loss of elasticity in the arteries.  Many things contribute to this loss of elasticity.

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Your pulse is the wave of your heart contracting as it pumps blood out.

Heart rate = the no. of times your heart beats in a minute

Most adults have a resting heart rate of 72-80 bpm.

To work out your heart rate find your pulse, count the number of beats for 10 seconds then x that no. by 6.

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  • Have narrow lumen

  • Have thick, highly muscular walls (except for the arteries of the cranium and vertebral column)

  • Carry oxygenated blood (exception= the pulmonary artery which carries de-oxygenated blood from the heart to the lungs)

  • Carry blood away from the heart

  • Do not have valves

  • Blood in them moves under higher pressure and has a pulse

  • Arteries branch into smaller arterioles
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  • Have wide/large lumen
  • Have thinner walls
  • Carry de-oxygenated blood (exception=the  pulmonary vein which carries oxygenated blood from the lungs to the heart)
  • Carry blood to the heart
  • Have valves to prevent the back-flow of blood
  • Blood in them moves under very low pressure and does not have a pulse
  • Veins branch into smaller venules
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These are the smallest blood vessels and the walls are only 1 cell thick.

This means that nutrients and oxygen can diffuse OUT through the walls of the capillaries to surrounding cells and carbon dioxide and other waste products can diffuse IN from the surrounding cells into the capillaries and then taken away to be excreted

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Red Blood Cells

  • These blood cells are formed in the red bone marrow and contain haemoglobin.  They live for approx. 120 days

  • Haemoglobin is an iron rich protein which picks up oxygen as the cells pass through the lungs

  • These blood cells transport oxygen around the body and release it to organs and tissues throughout the body 

  • They have a bi-concave shape which gives them a large surface area which means they can pick up more oxygen

  • They are broken down in the spleen & then the liver where any spare iron is retrieved & recycled
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  • These blood cells help 
  • prevent abnormal or excessive bleeding by forming clots .  They are formed in the bone marrow from parts that break off large cells

  • They activate the clotting process in the presence of air and 
  • foreign material 

  • They are  tiny, very fragile & irregularly shaped.  They need to be fragile as they need to break to release thromboplastin which starts the clotting reaction
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White Blood Cells

  • These blood cells play a major role in defending the body against disease  producing bacteria, viruses and fungi 

  • They are irregularly shaped and produced in the bone marrow.  Their life span ranges from hours to years depending on the type of cell

  • There are three main types of leukocytes, & each type performs a specific infection-fighting function 

  • Monocytes – defend against bacterial infection
  • Granulocytes – there are  several different types of these.  They defend the body by rapidly increasing in number and engulfing  & destroying foreign substances
  • Lymphocytes - consist of two types of cells which work together to create a complex interaction to regulate the immune response.  
  • T cells attack virus-infected and malignant cells.
  • B cells produce and release antibodies, or protein 
  • substances, which bind to infectious agents and help prevent them from doing damage to the body
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