recap

49.1 recap

Circulatory systems consist of a pump and an open or closed set of vessels through which a fluid transports oxygen, nutrients, wastes, and a variety of other substances. Flow to and from tissues in closed systems of vessels can be more rapid and can be directed to specific tissues according to their needs.

learning outcomes

You should be able to:

  • Describe the diverse functions of circulatory systems.

  • Explain why some animals can function without a circulatory system.

  • Describe the differences among hemolymph, blood plasma, and interstitial fluid.

  • Identify features that make a closed circulatory system more efficient than an open circulatory system.

Question 1

Describe three ways that circulatory systems support exercising muscles.

Circulatory systems supply exercising muscle cells with oxygen and nutrients. They also take CO2 and heat out of muscles.

Question 2

A sponge is neither small nor thin. How can it survive without a circulatory system?

A sponge has water channels throughout its tissues, which means that the external medium can circulate close to all of the sponge’s cells, where exchanges of nutrients, oxygen, and wastes take place.

Question 3

Explain the importance of extracellular fluid in large mobile animals.

Hemolymph is the extracellular fluid in animals with an open circulatory system. In animals with a closed circulatory system, blood plasma is the extracellular fluid contained in the heart and blood vessels, and interstitial fluid is the extracellular fluid outside the circulatory system. Blood contains cellular elements in addition to plasma.

Question 4

How does a closed circulatory system facilitate the fight-or-flight response?

The fight-or-flight response involves increasing blood flow to tissues that are necessary for action, such as the skeletal muscles, and decreasing blood flow to tissues that are not necessary for action, such as the gut. A closed circulatory system can increase blood flow by increasing pressure, and it can also direct that blood flow to critical tissues by changing the resistance in the vessels leading to those tissues.

Our overview of the open and closed systems found among invertebrates introduced some basic concepts about circulatory systems. Next we will turn to describing the closed circulatory systems of vertebrates.