Two of the most important facets of homeostasis are the exchange of gasses between cells and the environment and the maintenance of the body's water balance. In our discussion of metabolism, we learned that aerobic respiration pathways convert sugars and oxygen to carbon dioxide and water. Since these pathways function in all cells, there must be some way to deliver food and oxygen, and to take away carbon dioxide and other waste. This transport is the function of the circulatory system. As shown in figure 29.3, the circulatory system interfaces with the outside world through the respiratory, digestive, and urinary systems. In the next two lectures, we will look at some of the details of the connection between internal and external environments via the circulatory system.

Remember that the generation of ATP to do cellular work requires a constant supply of glucose and oxygen. In the lecture on hormonal control we looked at glucose homeostasis. Now we can consider oxygen / carbon dioxide homeostasis. Gas concentrations are measured in partial pressures. The atmospheric concentration of oxygen is about 160 mmHg, while the partial pressure of carbon dioxide is about 0.3 mmHg (see figure 29.4). Oxygen in the atmosphere can be made available to cells in one of two basic ways. If the animal is small enough, then the volume will be small in relation to the surface area. Since surface area is the limiting factor in gas exchange, small animals will have enough surface area to supply their limited volume of cells. Recall from section 3.2 that as the size of a structure increases, the surface area grows much faster than the volume. This means that as the animal grows beyond a certain size, some sort of oxygen transport must exist. Insects (figure 29.6) solve this problem with a network of branching trachea that deliver oxygen to (and remove carbon dioxide from) tissues.

Most vertebrates use gas exchange organs (lungs or gills) along with active circulation to transport oxygen and carbon dioxide. Note that the gas exchange in fish uses a countercurrent exchange mechanism (figure 29.7) To maximize the transfer of oxygen from the water to the blood. In humans, air is moved into the lungs by the concerted action of the rib cage and diaphragm to change the shape of the chest cavity. When the chest cavity goes from small to large, air is drawn into the lungs, when it resurns to a smaller size, the air is expelled. Most of the lung volume is taken up by alveoli (small inflatable air sacs surrounded by capillaries; figure 29.10). It is here that gas exchange occurs. Figure 29.14 shows the partial pressures (concentrations of oxygen and carbon dioxide in various places in the body. Note that oxygen and carbon dioxide always move in the direction of diffusion - down the concentration gradient. The function of the circulatory system is to transport oxygen from where it is abundant (in the atmosphere) to where it is needed ( in the tissues). Carbon dioxide is transported from the tissues where it is generated to the atmosphere.

Another important interface of the internal and external environments is mediated by the urinary system. The amounts of water and salts that enter the body through metabolism and the digestive system and the amounts that leave through the feces and the evaporation of sweat and other fluids are inherently unpredictable. It is the remaining avenue of water and solute (salt) loss, the urinary system, that provides most of the control of water and salt concentration in the body. Some dissolved solutes, like urea, the product of amino acid degradation, are purely waste molecules, and are excreted. Other solutes, like sodium, potassium, and calcium, have optimal concentrations in the blood that must be maintained by homeostasis. The kidneys are the main organs of the urinary system. In the kidney, blood enters Bowman's capsule (figures 31.4 and 31.5) and there looses most of its water and solutes to the collecting tubules of the kidney. Then, as the filtrate moves down the collecting tubule, desired salts are pumped back into the blood by active transport, and water follows by osmosis. It is the reabsorption process that is regulated. An average human generates about 180 liters of filtrate per day. Obviously, no one has this kind of urine volume. The amount of water that is reabsorbed is regulated by the action of the pituitary hormone ADH on the cells involved in reabsorption. Other hormones regulate the bulk flow of blood to the kidneys in order to influence the rate of urine generation. The brain can also regulate (to some degree) the amount of water entering the body by prompting thirst behavior.