21.4–21.11: The human circulatory system consists of a heart, blood vessels, and blood.

Blood flows from the heart to the tissues of the body and back to the heart again through a series of endothelium-lined blood vessels.

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Question 21.23

Why do most fishes swim with their mouths open?

  • a) They do this to take in any food particles suspended in the water.
  • b) This is a cooling mechanism to help regulate body temperature.
  • c) This allows a fish to swim faster.
  • d) This helps prevent infection by mouth parasites.
  • e) They do this to ventilate their gills.

Question 21.24

Carbon dioxide transport differs from oxygen transport in the circulatory systems of mammals, because:

  • a) CO2 is transported mostly in the plasma, while O2 is transported mostly in the red blood cells.
  • b) CO2 is transported mostly in the platelets, while O2 is transported mostly in the red blood cells.
  • c) CO2 is transported mostly via the lymphatic system, while O2 is transported mostly in the red blood cells.
  • d) CO2 is bound primarily to receptors within the red blood cell membrane, while O2 is bound primarily to hemoglobin inside the red blood cells.
  • e) CO2 is transported mostly in the plasma, while O2 is transported mostly in hemoglobin within the plasma.

Question 21.25

Breathing in birds is more like “breathing” in fishes than in mammals, because:

  • a) water and high-altitude air have similar viscosity.
  • b) oxygen never runs into a dead end.
  • c) both birds and fishes have gill arches.
  • d) the blood vessels in bird lungs, like those in fish gills, are insulated with a fatty coating, reducing heat loss.
  • e) in birds and fishes, but not mammals, the heart pumps in a two-circuit system of blood flow.

Question 21.26

Which of the following are the main muscles used in breathing?

  • a) back muscles and thoracic muscles
  • b) intercostal muscles and pectoral muscles
  • c) diaphragm and thoracic muscles
  • d) diaphragm and back muscles
  • e) intercostal muscles and diaphragm

Question 21.27

Under resting metabolic conditions, what percentage of oxygen is released from the blood to the tissues?

  • a) 1%
  • b) 75%
  • c) 25%
  • d) 100%
  • e) 50%