Circulatory system structure

• Multicellular organisms require a transport system as they have a low SA:V and cannot rely on diffusion to supply all their cells and tissues with the substances they need, or to remove waste substances because the diffusion distance is too long
• Circulatory systems can either be open, eg in insects or closed, like in fish and mammals where the blood is confined to blood vessels only
• Closed circulatory system - Blood is enclosed within vessels

  • Contains a pump (heart), vessels (arteries, capillaries and veins) and a medium (blood) to transport substances around the body
• Closed circulatory systems come in two forms, either single or double.

  • Single form consists of a heart with two chambers meaning the blood passes through the heart once for every circuit of the body
• Double circulatory system - The heart has four chambers and blood passes through the heart twice for every circuit of the body
• Double (closed) circulatory system is more efficient than single as it increases pressure and therefore speed of delivery of oxygenated blood to tissues
• The circulatory system is made up of the heart which pumps blood through blood vessels to reach different parts of the body
• Blood transports: (around the body)

  • Gases
  • Products of digestion
  • Metabolic wastes
  • Hormones
• The heart has its own blood supply - the left and right coronary arteries
• Blood vessels:

  1. Arteries carry blood away from heart and into arterioles
  2. Arterioles are smaller than arteries and connect to the capillaries
  3. Capillaries connect the arterioles to the veins
  4. Veins carry blood back to the heart
• Blood always flows from high to low pressure

  • The vena cava is the final blood vessel to return to the heart, so it has the lowest pressure
• Blood pressure is the highest in the aorta immediately after a ventricular contraction
• Blood loses pressure as it moves along the vessels from the aorta due to it:

  • Branching into many vessels
  • The increasing of lumen size of the vessels
  • Friction
• There are fluctuations in the blood pressure of arteries and arterioles as:

  • Elastic tissue stretches when blood moves through at high pressure
  • (when the ventricle contracts/systole)
• There are fluctuations in the blood pressure of arteries and arterioles as:

  • Elastic tissue recoils as blood pressure drops
  • (when the ventricle relaxes/diastole)
• There are fluctuations in the blood pressure of arteries and arterioles due to elastic tissue stretching and recoiling

  • This helps to even out the pressure of blood flowing through the artery
• Advantage of the double circulation found in mammals:

  • Increases blood pressure, therefore increases the rate of blood flow to the tissues
• Renal vein - Carries deoxygenated blood away from the kidney
• Renal artery - Carries oxygenated blood to the kidney
• Need for a specialised transport system with a pump:

  • Low surface area to volume ratio
  • High metabolic rate