The cardiac center in the medulla of the brain increases the rate of heart beat (and blood pressure) through the accelerator nerve, and slows the heart rate (and lowers blood pressure) through the vagus nerve.   The respiratory center in the medulla increases our breathing rate by stimulating the diaphragm via the phrenic nerve when blood CO2 levels rise (as happens during exercise).    A chemoreceptor in our aorta monitors the amount of CO2 in the blood and stimulates the cardiac center when levels of CO2 begin to raise (as happens during strenuous exercise).  This same chemoreceptor also stimulates the respiratory center when CO2 levels become high.   A baroreceptor in our aorta monitors our blood pressure and stimulates the cardiac center when the pressures become high (as happens during strenuous exercise).   Our hypothalamus monitors the osmotic pressure of our blood and the posterior lobe of the pituitary gland releases antidiuretic hormone (ADH) when the blood becomes deficient of water (as happens when one perspires; ADH shuts off when there is excess water in the blood).  ADH travels to the kidney and stimulates it to concentrate our urine by reabsorbing more water back into the blood. 

OK, those are the systems involved in this assignment. Now, suppose you are in a boat race on a hot summer day. You begin the race in great shape, but after the race begins, you begin breathing faster, you begin sweating profusely. After the race, in the bathroom, you notice that you do not release a lot of urine, but what you have is dark. You drink a lot of water and rest. After a while your breathing slows, heart slows, and you have got to go pee again. This time, though, you release more urine and it is very clear. 

Your assignment: Walk me through how these systems interact and work to satisfy the body’s needs and eventually reestablish homeostasis (i.e., from the beginning of the race until after you have rested and released a lot of urine). How did they interact to change your breathing rate, heart rate, and maintain a relatively constant amount of water in your blood? (Hint: As you answer this question, make a diagram that links these systems with arrows. Go one step at a time, from the beginning of the race until after you have sat and relaxed. Homeostasis is a dynamic process-follow how the systems reacted during this time.)