Heart and Blood

• The heart is divided into four chambers, two atria (right and left) and two ventricles (right and left).
• Oxygenated blood then returns to the left side of the heart through pulmonary veins.
• Blood flows from the right side to the lungs where it picks up oxygen.
• Blood has four main components: red blood cells, white blood cells, platelets all of which are carried in a fluid called plasma. Within the plasma dissolved substances like glucose (plus other products of digestion), urea and carbon dioxide will also be carried.
• Red blood cells carry the oxygen from the air in our lungs to our respiring cells. Red blood cells have certain adaptations that make them efficient at their job. They are biconcave disks which increases their surface area to volume ratio which increase diffusion. They are packed with haemoglobin which binds to oxygen. They also have no nucleus which makes space for more haemoglobin.
• White blood cells fight infection by engulfing bacteria or viruses using phagocytosis. There are different types of white blood cell including neutrophils, eosinophils, basophils, monocytes and lymphocytes.
• Platelets are small fragments of cytoplasm found in the blood. Their function is to clot the blood when we cut ourselves so that we don't bleed out. When platelets come across damaged tissue they stick together forming a plug which stops further bleeding.
• Arteries carry blood away from the heart to the organs or body. The blood is usually oxygenated with the exception of the pulmonary artery. Blood in the arteries is under high pressure. Arteries have a thick layer of muscle and elastic fibres along with thick walls to allow them to withstand the high pressure and to stretch. Arteries have a small lumen. 
• Veins carry blood towards the heart. The blood is low in oxygen, except for the pulmonary vein. Veins have a larger lumen and relatively thin muscular and elastic walls. This is because the blood is under less pressure. Veins have valves to prevent blood flowing backwards. 
• Capillaries connect arteries to veins. Capillaries are very narrow with thin walls. This ensures there is a short diffusion distance the inside of the capillary and surrounding cells. This enables substances such as glucose and oxygen to easily diffuse out of your blood into cells. Conversely carbon dioxide can easily do the opposite. Capillaries have very narrow lumens which only allow 1 cell to pass through at a time and their walls are only 1 cell thick.
• The circulatory system consists of the heart, blood vessels (arteries, veins and capillaries) and blood. It transports nutrients around the body, removes waste products and helps regulate temperature.
• Blood carries oxygen, nutrients, hormones and other important molecules around the body. It also takes away waste products like carbon dioxide.
• The heart is made up of four chambers; two atria and two ventricles. Each chamber has its own one-way valve to prevent backflow of blood. The right side of the heart pumps deoxygenated blood into the lungs where it picks up oxygen. The left side of the heart then pumps this oxygenated blood around the rest of the body.
• Blood is carried round our body in three main types of vessel: arteries, veins and capillaries.
• Humans have a double circulation system. One transport system carries blood from your heart to the lungs and back again and the other system carries blood from the heart to all other organs of your body and back again. This double circulation system is advantageous to us as we need lots of oxygen and glucose transported round our body. The double circulation system allows lots of oxygenated blood to be transported quickly where it needs to go. 
• Blood enters the right atrium through the vena cava, a vein that brings deoxygenated blood back to the heart.
• Blood travels from the right atrium to the right ventricle.
• When the right ventricle contracts, deoxygenated blood is forced into the pulmonary artery, which travels to the lungs to pick up oxygen.
• The newly oxygenated blood is returned to the heart by the pulmonary vein into the left atrium.
• Blood flows into the left ventricle.
• The left ventricle pumps oxygenated blood around the body via the aorta.
• The muscle wall of the left ventricle is thicker than elsewhere, allowing the blood leaving the left ventricle to be under the high pressure needed to pump it around the body.
• Coronary heart disease is caused by the narrowing of the coronary arteries that supply the heart muscle with blood. This is caused by a build-up of fatty material on the lining of the vessels which reduces the supply of oxygen to the heart.  If the heart muscle cells do not receive enough oxygen then they cannot respire and therefore die.  When this occurs it may lead to a heart attack.   
• Coronary heart disease can be treated with a stent. A stent is a metal mesh placed in the artery.  A tiny balloon is then inflated to open the blood vessel and the stent. The balloon is then removed but the stent ensures the blood vessel remains widened.  Stents don’t require general anaesthetic and can be placed anywhere in the body.
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  Another option to treat coronary heart disease is to prescribe statins. Statins reduce blood cholesterol levels (LDLs) and slows down the rate at which fatty material is deposited. This treatment is used to prevent the development of coronary heart disease and can’t be used to unblock an already blocked coronary artery.
• Heart valves must withstand a lot of pressure. As such they may start to leak. Doctors can operate and replace faulty valves with mechanical valves made of titanium. These mechanical valves are long lasting but require medication to prevent blood clotting around it. Biological valves are based on valves from pigs or even human donors. These do not require any medication but only last 12-15 years.
• The heart normally beats at 70bpm. The beating of the heart is controlled by a group of cells found in the wall of the right atrium. This cluster of cells is known as the pacemaker.  It can stop working and needs replacing with an artificial pacemaker. 
• If a person’s heart beats too slowly, they won’t get enough oxygen. The most common disease treated by pacemakers is arrhythmia (abnormal heart rhythm).  An artificial pacemaker is an electronic device that weighs between 20-50g and is connected to your heart by two wires. They control when the heart beats by sending strong, regular electronic signals to the heart. 
• Artificial hearts are used when a person’s heart stops working completely. It can take a long time for a donor heart to become available, so artificial hearts are used to keep the patient alive in the meantime. There is a risk of blood clotting with artificial hearts. 
• At the top of the right atrium is a bundle of cells known as the pacemaker.  When at rest the pacemaker keeps the heart beating in a regular rhythm.  The cardiac cycle is made up of three parts:​

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  Atrial systole – both atria contract and move blood into the ventricles​

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  Ventricular systole – both the ventricles contract and move blood out of the heart​

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  Diastole – the whole heart relaxes.​

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  This cycle typically takes about 0.8s.  
• A complete heart cycle includes atrial and ventricular systole and then diastole.  The total time for this is in seconds.  ​

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  Scientists record how often the heart beats in minutes.​

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  There are 60 seconds in one minute. Make sure you convert all your numbers into seconds​

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  If one cycle take 1s, calculate beats per minute by:​

  Time given (s) / Time for one cycle (s)
• Blood pressure is measured using a sphygmomanometer.​

  Normal blood pressure is below 120/80 mm Hg and above 90/60 mm Hg in an adult.  The first number is the pressure exerted on the blood when the left ventricle contracts and the second number is the pressure in the blood when the heart is relaxed.
• The amount of blood pumped out of the heart can be calculated using the equation:​

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  CO = SV x HR​

  Where,​

  CO = cardiac output, the amount of blood leaving the heart every minute​

  SV = stroke volume, the amount of blood leaving the ventricle on each contraction of the heart​

  HR = heart rate, the number of ventricle contractions each minute.
• Plasma is the liquid part of blood.
• If someone loses a lot of blood following an accident, artificial blood is a substitute that can keep people alive even if they lose 2/3 of their blood. It is a salt solution (saline) which is safe and will give patients enough time to make new blood cells or have a blood transfusion.
• Valves prevent the backflow of blood.