1. Respiratory System If cells are not directly exposed to the outside environment, then some mechanism must provide gas exchange to internal cells, delivering O 2 and removing waste CO 2

2. Gas exchange mechanisms Direct with environment –Phylum Porifera, Phatyhelminthes –Typically have large surface areas, and every cell either is exposed to the outside environment or is close enough that gases are avialable by diffusion through adjacent cells In larger animals, such as Annelida, gas exchange through the skin is augmented by the circulatory system just inside the skin.

3. Gas exchange mechanisms Gills –Countercurrent exchange Tracheae –Insects have chitin-lined tubes, or tracheae, that permeate their bodies –Oxygen enters (or CO 2 exits) the tracheae through openings called spiracles –Diffusion occurs across moistened tracheal endings Lungs

4. Gas exchange in Humans 1. Mouth 2. Pharynx 3. Larynx (contains the vocal cords) 4. Trachea 5. Bronchi, bronchioles 6. Alveolus (alveoli) 7. Diffusion between alveolar chambers and blood 8. Bulk flow of oxygen

5. Gas exchange in Humans 7. Diffusion between blood and cells 8. Bulk flow of CO 2 –Most CO 2 is transported as dissolved bicarbonate ions (HCO 3 -) in plasma –CO 2 + H 2 O  H 2 CO 3  H+ + HCO 3- The formation of HCO 3 - occurs in RBC where the fromation of carbonic acid is catalyzed by the enzyme carbonic anhydrase 9. Bulk flow of air into and out of the lungs –Diaphragm and intercostal muscles

6. Gas exchange in Humans 10. Control of respiration –Chemoreceptors in the carotid arteries monitor the pH of the blood –Blood pH drops as CO 2 enters the blood –In response, the chemoreceptors send nerve impulses to the diaphragm and intercostal mucles to increase respiratory rate. –This results in a faster turnover in gas exchange, which returns blood pH to normal –Homeostasis in maintained by negative feedback