The Circulatory System

The Heart

• The heart organ is a double pump
  • Oxygenated blood from the lungs enters the left side of the heart and is pumped to the rest of the body (the systemic circuit)
    • The left ventricle has a thicker muscle wall than the right ventricle as it has to pump blood at high pressure around the entire body,
  • Deoxygenated blood from the body enters the right side of the heart and is pumped to the lungs (the pulmonary circuit)
    • The right ventricle is pumping blood at lower pressure to the lungs
  • A muscle wall called the septum separates the two sides of the heart
• Blood is pumped towards the heart in veins and away from the heart in arteries
• The coronary arteries supply the cardiac muscle tissue of the heart with oxygenated blood
  • As the heart is a muscle it needs a constant supply of oxygen (and glucose) for aerobic respiration to release energy to allow continued muscle contraction
• Valves are present to prevent blood flowing backwards

Structure of the Heart

The pathway of blood through the heart

• Deoxygenated blood coming from the body flows through the vena cava and into the right atrium
• The atrium contracts and the blood is forced through the tricuspid (atrioventricular) valve into the right ventricle
• The ventricle contracts and the blood is pushed through the semilunar valve into the pulmonary artery
• The blood travels to the lungs and moves through the capillaries past the alveoli where gas exchange takes place
  • Low pressure blood flow on this side of the heart prevents damage to the capillaries in the lungs
• Oxygenated blood returns via the pulmonary vein to the left atrium
• The atrium contracts and forces the blood through the bicuspid (atrioventricular) valve into the left ventricle
• The ventricle contracts and the blood is forced through the semilunar valve and out through the aorta
  • Thicker muscle walls of the left ventricle produce a high enough pressure for the blood to travel around the whole body

Exam Tip

Remember : Arteries carry blood Away from the heart

When explaining the route through the heart we usually describe it as one continuous pathway with only one atrium or ventricle being discussed at a time, but remember that in reality, both atria contract at the same time and both ventricles contract at the same time

Also, the heart is labelled as if it was in the chest so the left side of a diagram is actually the right hand side and vice versa

The Circulatory System

• The circulatory system consists of a closed network of blood vessels connected to the heart
  • Oxygenated blood is carried away from the heart and towards organs in arteries
  • These narrow to arterioles and then capillaries as they pass through the organ
  • In the organs, respiring cells use up the oxygen from the blood
  • The capillaries widen to venules and finally veins as they move away from the organs
  • Veins carry deoxygenated blood back towards the heart
• A different network of lymphatic vessels collect all the excess tissue fluid that leaks out of the capillaries and delivers it back to the circulatory system

The circulatory system

Main Blood Vessels of the Circulatory System Table

Cardiac Output

• Cardiac output (CO) is the term used to describe the volume of blood that is pumped by the heart (the left and right ventricle) per unit of time
  • An average adult has a cardiac output of roughly 4.7 litres of blood per minute when at rest
• Individuals who are fitter often have higher cardiac outputs due to having thicker and stronger ventricular muscles in their hearts
• Cardiac output increases when an individual is exercising
  • In order to supply oxygen and glucose for respiration
• The CO of an individual can be calculated using their heart rate and stroke volume
  • Heart rate is the number of times a heart beats per minute
  • Stroke volume is the volume of blood pumped out of the left ventricle during one cardiac cycle

Calculating cardiac output

• Cardiac output is found by multiplying the heart rate by the stroke volume:

Cardiac output (cm³min^-1) = heart rate (bpm) x stroke volume (cm³)

• The equation can be rearranged to find the heart rate and stroke volume if require