3.2.2 Circulatory Systems

The need for a circulatory system

• The cells of all living organisms need a constant supply of reactants for metabolism, e.g. oxygen and glucose
  • These materials are gained from the environment via exchange surfaces
• Single celled organisms can gain oxygen and glucose directly across their surface membranes and the molecules can diffuse to all parts of the cell quickly due to short diffusion distances
• Larger organisms gain these reactants via specialised exchange surfaces, but because they are made up of many layers of cells, the time taken for substances such as glucose and oxygen to diffuse to every cell in the body would be far too long
  
  • The diffusion distances involved are too great
• To solve this problem their exchange surfaces are connected to a mass transport system, for example
  • The digestive system is connected to the circulatory system
  • The lungs are connected to the circulatory system
• Circulatory systems are systems that transport fluids containing oxygen, nutrients and waste   

Single and double circulatory systems

• There are two different models of circulatory systems, single circulatory systems and double circulatory systems
  • In a single circulatory system, the blood passes through the heart once during one complete circuit of the body
  • In a double circulatory system, the blood passes through the heart twice during one complete circuit of the body

• Fish have a single circulatory system while mammals have a double circulatory system

Single circulatory system in fish

• Deoxygenated blood is pumped to the gills from the heart
• The gills are the exchange site where oxygen and carbon dioxide are exchanged with the atmosphere and the blood
• The oxygenated blood flows from the gills to the rest of the body
  • It travels through the capillaries in organs, delivering oxygen and nutrients

• The blood returns to the heart
• The heart only has one atrium and one ventricle

The circulatory system of a fish. The blood passes through the heart only once.

Double circulatory system in mammals

• In mammals the blood passes throught the heart twice during a single circuit of the body
• As a result the mammalian heart has a left side and right side with a wall (septum) dividing the two
  • The left side contains oxygenated blood and the right side contains deoxygenated blood

• Blood in the right side of the heart leaves and travels to the lungs
• The blood returns to the left side of the heart before being pumped around the rest of the body
• Once the blood has passed through all the other organs and tissues it returns to the right side of the heart
  • In general, any blood that has just passed through an organ goes straight back to the heart, not to another organ
  • The hepatic portal vein is the exception to this rule, it allows blood from the gut to flow to the liver

The double circulatory system in mammals

Main circulatory system structures table

Advantages of double circulation

• It is believed that a double circulatory system has evolved from the single circulatory system as there are several benefits to a double circulatory system
• When blood enters a capillary network the pressure and speed drops significantly
• In a single circulatory system, the blood has to pass through two capillary networks before returning to the heart
• In a double circulatory system, the blood only passes through one capillary network before returning to the heart
• As a result, the double circulation maintains higher blood pressure and average speed of flow
  • This increased pressure and speed helps to maintain a steeper concentration gradient which allows for the efficient exchange of nutrients and waste with the surrounding tissues

Open and closed circulatory systems

• Circulatory systems are either open or closed
• In a closed circulatory system, blood is pumped around the body and is always contained within a network of blood vessels
  • All vertebrates and many invertebrates have closed circulatory systems

• In an open circulatory system, blood is not contained within blood vessels but is pumped directly into body cavities
  • Organisms such as arthropods and molluscs have open circulatory systems.

• Humans have a closed double circulatory system: in one complete circuit of the body blood passes through the heart (the pump) twice
• The right side of the heart pumps blood deoxygenated blood to the lungs for gas exchange; this is the pulmonary circulatory system
• Blood then returns to the left side of the heart, so that oxygenated blood can be pumped efficiently (at high pressure) around the body; this is the systemic circulatory system

Circulatory system in insects

• Insects have one main blood vessel - the dorsal vessel
• The tubular heart in the abdomen pumps haemolymph (this is what blood in insects is called) into the dorsal vessel
• The dorsal vessel delivers the haemolymph into the haemocoel (body cavity)
• Haemolymph surrounds the organs and eventually reenters the heart via one-way valves called ostia
• Unlike the blood in a mammals circulatory system, the haemolymph is not specifically directed towards any organs in an insect
• Insects are able to survive with this less efficient circulatory system because oxygen is delivered directly to their tissues via tracheae (a system of tubes) that connect directly to the outside

The circulatory system of an insect