Final answer:
In humans, the alveoli are the structures where oxygen diffuses into the bloodstream, and this gas exchange process is facilitated by the close relationship between the respiratory and circulatory systems.
Step-by-step explanation:
The structure in humans that allows oxygen to pass into the blood is the alveoli. These are tiny air sacs in the lungs where gas exchange occurs. Oxygen enters the alveoli during inhalation, and because of the concentration gradient, it diffuses through the alveolar walls and into the capillaries surrounding the alveoli. Once in the bloodstream, the oxygen-rich blood returns to the heart, where it is then pumped throughout the body to deliver oxygen to the tissues and cells.
Gas Exchange and the Respiratory System
Gas exchange is the main function of the respiratory system, which requires the cardiovascular system to transport oxygen to the cells and remove carbon dioxide, a waste product of cellular respiration. The respiratory system and circulatory system work closely together to maintain the exchange of oxygen and carbon dioxide, not only at the level of the lungs but also at the cellular level where capillaries exchange gases with body cells.
Path of Air and Blood Flow
Air enters the body through the nasal cavity and passes through the pharynx, larynx, trachea, bronchi, bronchioles, and finally the alveoli. In reverse order, the path is followed from the alveoli back out to the external environment. The blood flow from the heart out to the body and back follows this order: arteries, arterioles, capillaries, venules, and veins, with the aorta being the main artery leaving the heart and the vena cava being the main vein entering the heart.