The Heart

• between the 2 lungs
• enclosed by the pericardium
• peridcardiul fluid is secreted between them to aid movement and protects the heart from over expansion
• the walls of the heart are made of cardiac muscle (myocardium)
• never gets tired
• needs oxygen all the time

Arteries-

• carry blood away from the heart (always oxygenated apart from the pulmonary artery which goes to the lungs)
• have thick muscular walls
• have small passageways for blood (internal lumen)
• contain blood under high pressure

Veins-

• carry blood to the heart (always de-oxygenated apart from the pulmonary vein which goes from the lungs to the heart)
• have thin walls
• have larger internal lumen
• contain blood under low pressure
• have valves to prevent blood flowing backwards

Capillaries-

• found in the muscles and lungs
• microscopic- one cell thick
• very low blood pressure
• where gas exchange takes place. Oxygen passes through the capillary wall and into the tissues, carbon dioxide passes from the tissue into the blood

These substances are all exchanged between the blood and the cells in capillary beds. Substances do not acctually move directly between the blood and the cell, first they diffuse into the tissue fluid that surrounds all cells, and then diffuse from there into the cells.

Plasma-

• fluid part of blood
• carries carbon dioxide, hormones and waste
• red blood cells
• contains haemoglobin which carries oxygen
• made in the bone marrow, the more you train the more red blood cells are made

White blood cells-

• an important part of the immune system
• produce antibodies
• made in the bone marrow
• destroy harmful micro-organisms

Platelets-

• clump together to form clots
• protect the body by stopping bleeding

Heart consists of myogenic muscle (it does not need electrical impulse from a nerve to make it contract). Nerves do supply the heart but these are to increase or decrease the number of heart beats. The normal heart beat is 60-80 beats per minute.

• blood comes into the heart from the body
• if then has to pass to the lungs to collect oxygen
• this is called a double circulatory system
• after it returns

body -> vena cava -> right atrium -> tricuspid valve -> right ventricle -> semi-lunar valve -> pulmonary artery -> lungs -> pulmonary vein -> left atrium -> mitral valve -> left ventricle -> aorta -> body

• heat beat is initiated in the sinoatrial (SAN) otherwise known as the pacemaker which is positioned in the right atrium
• waves of impulses from the SAN spread along the muscle cells across the walls of both atria
• causes muscles to contract (atrial systole) forcing blood into the ventricles

• impulses do not travel directly to ventricles because of the ring of non-conducting tissue between both atria and the ventricles. There is a short period of delay as it would be dangerous if the ventricles contracted at the same time as the atrium.
• impulses reach atrioventricular node (AVN)
• impulses conducted down specialised muscle cells in walls of septum called the bundle of his. The purkinji fibres make sure the contraction starts from the base up.

• one nerve, the atrioventricular nerve rums from the cardio acceleratory centre in the medulla of the brain to the SAN.
• another, the vagus nerve, runs from the cardio inhibitory centre in the medulla of the brain to the SAN.
• these nerves are stimulated in various situations, e.g. during exercise the accelerator nerve is stimulated. It releases noradrenaline at the SAN resulting in the heart rate increasing due to a decreased delay at the AVN and increasing the force of the contractions
• if the vagus nerve is stimulated, acetylcholine is released at the SAN. The delay at the AVN increases and the cardiac output falls

ECG can be a very powerful diagnostic tool, however they can occasionally yield misleading or false information.

• sinus tachycardia- sinus rhythm is faster than 100 beats per minute, rhythm is similar to normal but RR interval is shorter. Different causes and symptoms, a normal response to exercise, stress or anger. Can also indicate blood flow is restricted or isn't circulating properly (sometimes after a heart attack).
• bradycardia- heart rate is slower than 60 beats per minute. Rhythm is similar to normal but RR interval is longer. Extreme tiredness, dizziness, fainting and shortness of breath, difficulty or painful breathing.
• sinus arrhythmia- disturbances in the hearts regular rhythm. RR and PP intervals are not regularly spaced. Can be life threatening causing cardiac arrest and sudden death. Other causes are milder such as being aware the heart is beating differently or palpitating.
• ventricular fibrillation- parts of the ventricles depolarize repeatedly in an erratic uncoordinated way. Little or no blood being pumped, so the patient will loose consciousness, go into cardiac arrest and die without CPR.

The cardiac cycle is the cycle of events that occur as the heart contracts. There are 2 phases of the cardiac cycle; systole and diastole. At the diastole phase, the heart ventricles are relaxed and the heart fills with blood. In the systole phase, the ventricles contract and pump blood into the arteries.