The Heart and circulatory system (also known as the cardiovascular system) is a Blood distribution network responsible for transporting nutrients and removing waste products. It is made up of the heart (cardio) and blood vessels (vascular), which includes arteries, veins and capillaries. This system also plays an important role in maintaining all of the body’s functions.

The heart is located slightly to the left of the sternum, in the chest cavity. A fibrous double walled sack called the pericardium surrounds the heart. The heart muscle is called the myocardium and is largely composed of cardiac muscle tissue. The heart is a four-chambered muscular pump, roughly the size of a fist. It contracts rhythmically to pump blood around the body, beating continuously without conscious involvement and therefore the muscles of the myocardium are classed as involuntary muscle.

Coronary [cardiac] circulation

Blood is supplied to the myocardium (heart muscle) by the coronary arteries, which branch off from the aorta. The flow of blood through the numerous vessels that pierce the myocardium is called the coronary circulation. Its function is to continually supply the myocardium with oxygenated blood so that the chambers of the heart can function i.e. contract and relax. The myocardium extracts about 70 – 75% of the available oxygen at rest, this is high in comparison to skeletal muscle, which extracts only about 25%.

The myocardium has virtually no anaerobic capacity, therefore it is crucially dependant on blood flow through the coronary arteries. The majority of blood flow through the coronary arteries occurs during the diastolic [relaxation] phase of the cardiac cycle.

The left and right coronary arteries originate at the bottom of the aorta from openings called the coronary ostia located behind the aortic valve.

• Left circumflex: Supplies oxygenated blood to the left atrium and left ventricle
• Left anterior descending: Supplies oxygenated blood to the left atrium and left ventricle
• Right coronary artery: Supplies oxygenated blood to the right atrium and right ventricle

The coronary arteries and their branches lie on the surface of the heart, and therefore are sometimes referred to as the epicardial coronary vessels.

The heart is divided into two halves, a left side and a right side, each side has two chambers. The two upper chambers where blood is collected on entry to the heart are called atria. The two larger, lower chambers, where blood is pumped are called ventricles.

Valves in the heart

The flow of blood through the heart is a one-way system through the atria, into the ventricles and out through the aorta and pulmonary artery. The four heart valves maintain the direction of blood flow. The valves open and close passively in response to changes in Blood Pressure as the atria and ventricles contract and relax, keeping the blood flow in the right direction. As the atria contract, blood is pushed into the ventricles, and as the ventricles contract, blood is pushed out of the heart and into the aorta and pulmonary artery.

Atria-ventricular valves

The bicuspid valve [mitral] lies between the left atrium and left ventricle and the tricuspid valve lies between the right atrium and right ventricle.

Semi-lunar valves

The entrances to the aorta and pulmonary arteries also contain valves. These are called the aortic and pulmonary semi lunar valves. The aortic valve is located between the left ventricle and the aorta; it prevents the back flow of blood into the left ventricle from the aorta. The pulmonary valve is located between the right ventricle and the pulmonary artery, it prevents back flow into the right ventricle from the pulmonary artery.

Valves are made of dense fibrous connective tissue, each valve consisting of cup-shaped fibrous flaps. When the ventricles contract (systolic pressure}, the increased pressure in the ventricles forces the semi­ lunar valves open and closes the atria-ventricular valves. The thickened rims of each flap prevent them from inverting. When the ventricles relax (diastolic pressure), the atria contract and force open the atrio­ ventricular valves to allow blood to pass through into the ventricles.

Atria

These are collecting chambers. Their walls are thin and weak and do not have much contractile power. The left atrium receives oxygenated blood from the lungs through four veins, two from each lung called the pulmonary veins. The right atrium receives de-oxygenated blood from the body’s muscles, tissues and organs through the inferior and superior vena cava.

Ventricles

These are the pumping chambers that pump the blood to all of the body’s tissues. The ventricles are much thicker than the atria. The left ventricle has a much more demanding role than the right as it has to pump the blood out of the heart and into the whole of the body, including the muscles, brain and organs. Therefore it is thicker than the right ventricle that only has to pump the blood as far as the lungs. The left ventricle pumps oxygenated blood to the body’s tissues through the aorta. The right ventricle pumps de­ oxygenated blood to the lungs through the pulmonary artery.

Blood Pressure

Blood pressure (BP) is the term used to describe the force created by the blood as it passes through the arteries with each heartbeat. Each heartbeat consists of a contraction or systolic phase, and a relaxation or diastolic phase, this is known as the cardiac cycle. Therefore when measuring blood pressure two readings are taken and it is measured in milligrams of mercury (mmHg):

Optimal BP –       120 mmHg  (systolic)

80 mmHg  (diastolic)

Systolic blood pressure is greatly affected by the speed at which the heart beats. Any factors that cause the heart rate to increase will also increase systolic blood pressure.

Diastolic blood pressure remains relatively unchanged by heart rate and other cardiovascular demands although long term adaptations to exercise and lifestyle can bring about change in both systolic and diastolic blood pressure.

Blood pressure is very important:

• It helps push blood through the Without this pressure, blood would not flow through these very narrow vessels.
• It helps push blood uphill against the force of gravity. Without this pressure the blood would pool in the lower extremities of the body.

Hypotension (low blood pressure) is generally not considered a health risk, but may be an indication of related issues. Hypotension is hard to define, it can occur naturally in people who exercise regularly, keep a normal weight, and eat a healthy diet. In some cases, it accompanies certain medical conditions.

and may be a sign of life-threatening illness. Hypotension may be a sign of multiple conditions, including dehydration, severe infection with septic shock, endocrine (hormonal) conditions, some heart diseases, and taking certain medications. It is also common for women to experience hypotension during pregnancy.

Hypertension (high blood pressure) is a health risk in terms of cardiovascular diseases, stroke, kidney dysfunction and certain types of dementia. Regular exercise helps keep arteries elastic and the ventricles strong, which in turn ensures smooth blood flow and normal blood pressure. Increasing activity levels can lead to a reduction/ normalisation in blood pressure.

Note: ‘threshold’ refers to the first number of that classification e.g. the threshold for Stage 2 would be 160/100.

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