Open Circulatory System – Class 11 | Chapter – 18 | Biology Short Notes Series PDF

Open circulatory system are systems where blood, rather than being sealed tight in arteries and veins, suffuses the body and may be directly open to the environment at places such as the digestive tract. Open circulatory systems use hemolymph instead of blood. This “hemolymph” performs the functions of blood, lymph, and intestinal fluid – which are three different, highly specialized fluids in animals with closed circulatory systems.

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Open circulatory system

Instead of a complex and closed system of veins and arteries, organisms with open circulatory systems have a “hemocoel.” This is a central body cavity found inside most invertebrate animals where both digestive and circulatory functions are performed. This hemocoel may have “arteries” through which the blood can reach tissues – but these arteries are not closed and do not circulate blood as quickly as closed, muscle-assisted arteries.

Within the hemocoel, hemolymph directly absorbs nutrients from food and oxygen from the lungs or breathing pores. It also contains immune cells – but hemolymph does not have red blood cells like our own. Instead of using hemoglobin to carry oxygen, organisms with open circulatory systems use blue or yellow-green pigments to carry oxygen throughout the body.

Many animals that use open circulatory systems do have a heart – but the heart only pumps hemolymph to different cavities in the hemocoel. From these branches of the central body cavity, blood and the oxygen and nutrients it contains must penetrate the tissues and then return to the heart without the help of highly specialized pathways or muscle-assisted arteries like vertebrates possess.

Open circulatory systems are used by arthropods and most mollusks. This is one or several reasons why there are no giant insects; open circulatory systems are less efficient than closed circulatory systems, and cannot move oxygen efficiently enough to power large bodies.

Interestingly, while octopi and squid are considered mollusks, they have evolved closed circulatory systems. This is why they are able to reach huge sizes – colossal squid are thought to reach almost 50 feet in length, and weigh up to 1,650 pounds!

Function of Open Circulatory System

In all animals, circulatory systems perform several vital functions. The circulatory system can be thought of as a river connecting the specialized cells of the body, which allows them to perform the trade and communication upon which their survival depends.

There are a few vital functions that all circulatory systems must serve. These include:

  • Transporting the oxygen that is necessary for cellular respiration
  • Transporting nutrients from food, which are necessary for cellular respiration and other functions
  • Transporting waste products of cellular respiration and other functions, which could otherwise build up to toxic levels within the body
  • Transporting any necessary messages between cells, such as hormones signaling hunger, thirst, oxygen deprivation, or other bodily needs.
  • Transporting immune cells which can fight infection to any area of the body where they might be needed.

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By Team Learning Mantras