Cellular respiration occurs in the mitochondria when one uses up oxygen and glucose to create adenine triphosphate [energy], water, and carbon dioxide. Without it, ones’ cells would die.
Your cells acquire oxygen through inhaling air, which is about 80 percent nitrogen[N], 20 percent oxygen[O], and 1 percent other things like carbon dioxide, and this combination is sent to your lungs through your trachea, it travels through your bronchiole tubes [very small bronchial tubes], and into your alveoli [tiny air flaps in the lungs]. Capillaries are laced around all your alveoli, and they allow oxygen to diffuse in, and carbon dioxide to diffuse out. (When you exhale, the air contains about 16% air, and 4% carbon dioxide).
Also, your capillaries deliver nutrients, which they pick up from your villi [which are small finger like extensions that are located within your small intestine]. When you consume foodstuffs, it contains glucose, on the premises that you are not allergic, and the food travels down your esophagus, down the esophagial sphincter [which is like a door closing and opening holes in your body. You have multiple, such as your anal sphincter, which keeps your feces in], and into your stomach. Acidic and enzymeal [their names generally end in -ine] reactions break down the food particles only part way. It then goes down the duodenum, and bile and pancreatic juices drip on them, especially if it’s a fatty meal. Your food then continues to break down and down and down into its building blocks? Since your small intestines are so elongated with folds and bends which create surface area.
It actually takes approximately two days for your food to pass through. The nutrients left behind then diffuse into your villi through capillaries, and if it is not needed then, it is sent cell to cell to cell, and continues until it is needed. The waste goes through your large intestine, and traverses through your rectum.
Ergo, it all ties together so that your cells can complete cellular respiration. Blood matter acquires oxygen within the alveoli [again, tiny air flaps in the lungs], nutrients in the villi, and deliver it to your cells.