Homeostasis- The Circulatory System

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Homeostasis Lesson Notes

Why are transport systems necessary?

Exchange of materials and energy occur at cellular level. Nutrients and oxygen move across the plasma membrane to the cytoplasm. Diffusion alone is insufficient over distances of more than a few millimetres. Therefore, the circulatory system solves this problem by ensuring that no substance must diffuse very far to enter or leave a cell. Large organisms require an effective transport system as the larger the organism, the smaller the SA: V ratio. This affects the rate of diffusion.

The circulatory system

The human circulatory system works with the respiratory (gaseous exchange), digestive (nutrients and wastes) and excretory (waste to kidney for removal) systems. The human circulatory system is made up of the cardiovascular system and the lymphatic system. The cardiovascular system is further divided into the pulmonary (lungs), coronary (heart) and systemic systems. The cardiovascular system is made up of blood, arteries and veins, capillaries and the heart.

The cardiovascular system

The heart

  • Left/ right ventricle
  • Left/ right atrium
  • Tricuspid, mitral, pulmonary, aortic valve
  • Chordae tendineae, papillary muscles


Consists of 2 ventricles, 2 atria and 4 valves. The ventricle pumps blood at high pressure out to the arteries. The pressure generated by the left ventricle is greater than that generated by the right ventricle as the systemic circuit is more extensive than the pulmonary circuit. The left ventricle is thus more muscular than the right ventricle.

The atria receive blood at low pressure from the veins. The pressure generated by the atria is less than that generated by the ventricles since the distance from atria to ventricles is less than that from ventricles to circulatory system. Thus, the muscle around the atria is thinner than the muscle around the ventricles.

Valves are important as they ensure that blood flows in the right direction. The tricuspid valve separates the right atrium from the right ventricle. It opens to allow deoxygenated blood to flow into the right ventricle. It closes as the right ventricle contracts, preventing blood from returning to the right atrium and forcing blood to exit thorough the pulmonary valve into the pulmonary artery. The bicuspid valve separates the left atrium from the left ventricle. It opens to allow oxygenated blood collected in the left atrium to flow into the left ventricle. It closes as the left ventricle contracts, forcing blood to exit through the aortic valve into the aorta. The pulmonary valve separates the right ventricle from the pulmonary artery. As the ventricles contract, it opens to allow the deoxygenated blood collected in the right ventricle to flow to the lungs. The aortic valve separates the left ventricle from the aorta. As the ventricles contract, it opens to allow oxygenated blood from the left ventricle to flow to the rest of the body. It closes as the ventricles relax to prevent the backflow of blood.

Chordae tendineae are tendons that connect papillary muscles to the tricuspid and mitral valves. They prevent flaps of the valves from being inverted into the


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