Revise The circulatory system for Cambridge 9700 (8.1)

The mammalian heart is a muscular, four-chambered organ acting as a double pump, ensuring efficient double circulation. This means blood passes through the heart twice for every complete circuit of the body: once for the pulmonary circulation (to and from the lungs) and once for the systemic circulation (to and from the rest of the body).

The heart is divided into two sides by the septum. The right side receives deoxygenated blood from the body and pumps it to the lungs, while the left side receives oxygenated blood from the lungs and pumps it to the rest of the body.

Key structures include:

• Atria (sing. atrium): Two upper, thin-walled chambers that receive blood. The right atrium receives deoxygenated blood from the body via the vena cavae. The left atrium receives oxygenated blood from the lungs via the pulmonary veins.
• Ventricles: Two lower, thick-walled muscular chambers that pump blood out of the heart. The right ventricle pumps deoxygenated blood to the lungs via the pulmonary artery. The left ventricle, with its much thicker muscular wall, pumps oxygenated blood to the rest of the body via the aorta at high pressure.
• Valves: Crucial for preventing the backflow of blood.
  • Atrioventricular (AV) valves: Found between the atria and ventricles. The tricuspid valve is on the right side, and the bicuspid (mitral) valve is on the left. They close when ventricles contract.
  • Semilunar valves: Located at the base of the aorta (aortic valve) and the pulmonary artery (pulmonary valve). They open during ventricular contraction and close when ventricles relax, preventing blood from flowing back into the ventricles.

Blood vessels are a network of tubes uniquely adapted to transport blood throughout the body. Their structure dictates their function:

• Arteries: Carry blood away from the heart, usually oxygenated (except the pulmonary artery).
  • Structure: Thick, strong, and elastic walls containing smooth muscle and elastic fibres. Narrow lumen.
  • Function: Withstand and maintain high pressure from heart contractions, smoothing out blood flow. Elastic recoil helps propel blood.
• Arterioles: Smaller branches of arteries that lead into capillaries.
  • Structure: Walls contain a relatively high proportion of smooth muscle.
  • Function: Control blood flow into capillaries through vasoconstriction (narrowing lumen to reduce flow) and vasodilation (widening lumen to increase flow), regulating blood pressure and distribution.
• Capillaries: Microscopic vessels forming vast networks within tissues.
  • Structure: Walls are a single cell thick (endothelium), with a very small lumen, just wide enough for red blood cells to pass in single file.
  • Function: Site of exchange of substances (oxygen, nutrients, waste products) between blood and tissue fluid due to their thin walls and large total surface area. Low blood pressure and slow flow maximise exchange time.
• Venules: Small vessels that collect blood from capillaries.
  • Structure: Thin walls, larger lumen than capillaries.
  • Function: Merge to form veins, beginning the return journey of deoxygenated blood.
• Veins: Carry blood towards the heart, usually deoxygenated (except the pulmonary vein).
  • Structure: Thinner walls and larger lumen than arteries. Contain valves at intervals.
  • Function: Return blood to the heart at low pressure. Valves prevent backflow, and skeletal muscle contractions aid blood movement.

The cardiac cycle describes the sequence of events that occurs during one complete heartbeat. It consists of two main phases: diastole (relaxation) and systole (contraction).

1. Atrial and Ventricular Diastole (Relaxation and Filling):
   • All chambers relax.
   • Blood from the vena cavae (deoxygenated) fills the right atrium and blood from the pulmonary veins (oxygenated) fills the left atrium.
   • Atrioventricular (AV) valves are open, allowing blood to flow passively into the ventricles (about 70% of ventricular filling).
   • Semilunar valves are closed, preventing blood from re-entering the ventricles from the aorta and pulmonary artery.
   • Pressure in atria and ventricles is low and rising slowly as they fill.
2. Atrial Systole (Atrial Contraction):
   • The atria contract, pushing the remaining blood (about 30%) into the ventricles.
   • AV valves remain open.
   • Ventricular pressure rises slightly as they fill completely.
3. Ventricular Systole (Ventricular Contraction):
   • The ventricles begin to contract.
   • The rise in ventricular pressure forces the AV valves to snap shut, producing the 'lub' sound of the heartbeat.
   • As ventricular pressure continues to rise and exceeds the pressure in the aorta and pulmonary artery, the semilunar valves are forced open.
   • Blood is ejected from the left ventricle into the aorta and from the right ventricle into the pulmonary artery.
   • Aortic and pulmonary artery pressure rises sharply.
   • As ventricles relax and pressure falls, blood tries to flow back, forcing the semilunar valves shut, producing the 'dub' sound. This marks the beginning of the next diastole.