1. Module 8 Intro

1.29. Page 5

Lesson 4

Module 8—Circulation, Immunity, and Excretion

Lesson Summary

 

In this lesson you explored the following essential question:

  • At the capillary level, how does the circulatory system aid the digestive, excretory, respiratory, and motor systems’ exchange of matter with the environment?

Matter exchange between the circulatory system and all other body systems occurs because of fluid pressure in the interstitial fluid and the capillary. The pressures determine relative concentration gradients of other molecules that must diffuse in and out of the capillaries. Substances will move from high concentrations to low concentrations. These concentrations are mediated by the difference between osmotic pressure and hydrostatic pressure.

 

As blood moves from the arteriole to the venous end of a capillary, pressures will change. The pressure is maintained because fluid leaving the capillary equals the fluid entering the capillary. Because of concentration gradients, substances such as oxygen will diffuse out of the capillary while carbon dioxide will diffuse into the capillary.

 

Lesson Glossary

 

hydrostatic pressure: the pressure exerted by a fluid on any contacting surface, such as the force exerted by blood on a capillary wall

 

Hydrostatic pressure is greater on the arteriole end of a capillary.

 

osmotic pressure: hydrostatic pressure produced by a solution that is separated by a semi-permeable membrane

 

The pressure is created due to different concentrations of solutes. It is required to maintain equilibrium with no net movement of solvent. In other words, osmotic pressure stays the same at all times unless there is a problem. You can think of osmotic pressure as the pressure required to prevent water from leaving.