Unit 4: The circulatory and excretory systems                       1   2   3   4

1. The internal medium

The internal medium is the set of liquids which forms the surrounding environment of cells in multicellular organisms.

In humans these liquids are mainly blood, lymph and interstitial plasma (extracellular liquids)

- Interstitial plasma. It is a liquid which are among cells. It comes 

  from blood plasma that exits from capillaries. 

- Blood. It is the transport medium of substances which flows within

  blood vessels.

- Lymph. It is a liquid formed from the interstitial plasma that flows

  within lymphatic vessels. The lymphatic system ends into circulatory system.

The optimal operation of cells depends on the maintenance of constant conditions of interstitial plasma that surround them. These conditions include temperature, chemical composition, pH, etc. The optimal situation is called homeostasis. To keep homeostasis, blood and lymph renew constantly the interstitial plasma.

1.1. Listen and identify what component of the internal medium is described: