Firstly, the digestive system works by breaking down complex molecules (such as carboydrates and proteins) into smaller molecules which can be absorbed into the bloodstream by diffusion in the villi and microvilli of the small intestine.
The main energy source for cellular respiration is glucose, which comes from simple carbohydrates (mono-saccharides) as well as more complex polymers such as starch - they are broken down by carbohydrases (enzymes whose specific substrates are carbohydrates) in the digestive system to form glucose molecules, which can be carried in the blood plasma to reach cells through the circulatory system.
The carbohydrases, like any enzyme, have optimum temperatures which allow them to work at their most efficient rate. This is due to the 'lock-and-key' hypothesis, which describes how the substrate (the substance being broken-down) binds with the active-site of the enzyme in question - enzymes are highly-specific, meaning that their active site is the correct shape to bind to a specific substrate only.
Therefore as the temperature increases, both the enzymes and substrates have more kinetic energy, moving faster as a consequence in their aqueous state within the digestive system. Consequently the percentage of 'successful collisions' (collisions which result in the breakdown of the substrate in question) is greater because more exceed the minimum activation energy of the reaction - more collisions occur per second also, raising the rate of reaction too. However, over a certain temperature, the bonds between atoms within the enzymes begin to break, changing the shape of the active site so that it is no longer useful - this process is called denaturing, and is the same reason why eggs cook. It therefore follows that there is an optimum temperature for the enzyme, at which it works best, and the nervous system helps to regulate this by inducing vasodilation/vasoconstriction, amongst other processes, on a negative feedback loop, to keep the core body temperature at 37 degrees celcius (the optimum for most human enzymes). Keeping the perfect balance of internal body conditions with the use of negative feedback loops is called homeostasis.
Cellular respiration, or at least the main aerobic type, requires oxygen too to meet the requirements of the equation:
C6H12O6 + 6O2 --> 6CO2 + 6H2O
Glucose + Oxygen --> Carbon Dioxide + Water
(sorry about not using subscript, but I can't work it out in this textbox)
Oxygen is gained through the respiratory system, where the difference in air-pressure inside and outside the lungs is created by the movement of the diaphragm, causing air to move into/out of the body cyclically. The alveoli, or 'air-pockets' of the lungs, have a huge surface area and have a good blood-supply through many capillaries close to the surface, maximising diffusion of oxygen gas into the bloodstream, and carbon dioxide (the waste-product of cellular respiration) out of it.
When the oxygen enters the bloodstream, it binds with haemoglobin (the main iron-based pigment on red blood cells which makes blood red) to form oxyhaemoglobin; this is a reversible reaction, allowing oxygen to leave the blood when it reaches the cells requiring it for respiration:
Oxygen + Haemoglobin <--> Oxyhaemoglobin
Red blood cells are adapted to maximise the amount of oxygen they can carry: they have a bi-concave shape (doughnut-shaped with a thin membrane in the middle instead of a hole) to maximise the surface area; they have no nucleus, saving the space instead for oxygen; they are flexible, which means they can easily squeeze their way down the microscopic capillaries to reach every cell in the body. The carrying capacity of red blood cells is reduced by smoking and regular exposure to incomplete combustion fires which produce carbon monoxide (CO) - CO binds to red blood cells much more strongly that O2 gas, taking up space, and also takes a long while to clear out of the system. Consequently, smokers are often less physically fit in terms of endurance, because their CO-laden cardiovascular systems cannot compete with non-smoking individuals.
Even the skeletal system is involved in allowing cellular respiration. This is because the red bone marrow, within the 'long bones' (e.g. the femur and humerus), creates red blood cells, which are the carriers of oxygen to the cells of the body.
In conclusion, the systems that work together are:
- cardiovascular (circulatory)
- respiratory (bringing oxygen in and excreting carbon dioxide, the waste product)
- nervous (temperature-control for enzymes)
- digestive (supplying glucose from food)
- skeletal (the bone marrow produces red blood cells to carry oxygen)
