Yawning is a reflexive action that is often associated with sleepiness. when a person yawns, the mouth opens, and a longer than normal breath of air is inhaled, followed by a longer than normal exhalation. hypothesizing that yawning has a regulatory mechanism similar to breathing, scientists conducted an experiment on college students to test the effect of ambient carbon dioxide levels on yawning. the scientists investigated carbon dioxide levels in their experiment because high carbon dioxide levels in the signal control centers in the brain to increase rate and depth of inhalations and exhalations.

The correct answer in the blank is arterial blood.

The elevated levels of CO2 in the arterial blood signals the brain to increase the rate and depth of inhalations and exhalations.

Blood in the veins normally has the high concentration of CO2.

Arterial blood contains very low level of CO2 and High level of CO2 in this blood signals brain to increase breathing rate.

1. a host loses nutrients to a parasite

2. a parasite only benefits when the host is alive (cause if the host dies the parasite dies with it )

3. producers to consumers

4. the tapeworm is a parasite

5. they convert energy from the sun (photosynthesis)

6. sunflower-robin (i think im 80% sure)

7. producer

8. consumer

9. prey

10. fresh water snails ( cause the green algea is the producer and the snails are the primary consumers and the small fish consumer so is the big fish)

the level of progesterone is low at the pre-ovulatory phase of the cycle but it gradually increases and the levels are high at the luteal phase. if pregnancy occurs, the level of progesterone will be high throughout the pregnancy.

if egg is fertilized and the mother gets pregnant, progesterone starts preparing the uterus for the implantation. it starts building up the lining of uterus and make it hostile of the period of pregnancy.