How Animals Absorb Nutrients and Dispose of the Rest
Nutrients that the body can use are absorbed into the cells lining the small intestine. The rest of the material that cannot be further digested or used passes onto the large intestine.
Worthy nutrients remain in your system
Sugars such as glucose that were gleaned from carbohydrates, as well as amino acids that previously made up proteins in your food, pass directly into the cells of the small intestine by active transport. This means that energy — in the form of adenosine triphosphate (ATP) molecules — is used to move the sugar and amino acids into the intestinal cells. Capillaries, which are the smallest type of blood vessels, surround the outside of the small intestine.
By way of capillary exchange, the sugars and amino acids get into the bloodstream. Capillary exchange serves as a trading system. The small intestine sends the beneficial nutrients into the capillaries of the circulatory system, and the capillaries dump off the cellular garbage collected from around the entire body (for example, dead blood cells) into the small intestine so that they can continue on through the excretory system.
The sugars and amino acids that are now inside the capillaries get shuttled through the bloodstream to the liver. However, the products of fat digestion get coated with proteins and are then called chylomicrons. Instead of being carried through the bloodstream, the chylomicrons get transported through the lymph system, which deposits lymph fluid into veins near the heart.
Callin’ on the colon to dump the junk
Once nutrients have passed out of the small intestine, whatever material remains continues on to the large intestine (the colon). Here, most of the water contained in the leftover material gets reabsorbed back into the body. An error in this absorption results either in constipation (too much water is absorbed) or diarrhea (not enough water is absorbed). After the water is reabsorbed, the waste materials compact into a solid (feces).
The large intestine absorbs ions (such as sodium) into its cells from the material passing through it. Sodium ions are necessary for many cellular processes, such as the active transport of materials across cell membranes. The large intestine also collects (from the bloodstream) ions to be excreted, helping to regulate the amount of ions in the body.
If the amount of ions in your body (also called electrolytes) is not in the normal range, serious effects occur. For example, if your level of sodium and potassium electrolytes is abnormal, the ability for muscles to contract properly or for nerves to send impulses correctly is affected, and that can affect your heartbeat, possibly causing a heart attack.
Several types of bacteria call the large intestine home. During digestion, some of the bacteria produce vitamin K, which humans need but cannot produce. This necessary product from these friendly little beneficial bacteria is absorbed through the lining of the large intestine.
When digestion and absorption are finished, the body has what it needs (or what it has to work with at the time), and the feces pass out of the colon into the rectum. The rectum is like a holding tank. When the rectum is full, you feel the need to defecate (meaning remove fecal material).
Back to the liver
Sugars and amino acids get transported to the liver for a very specific reason. The liver is like the quality control department in this factory called your body. Blood flows through the liver, which can detect any abnormalities in blood levels of various substances and start to correct them.
For example, the liver can detect the level of glucose in the blood. If the level is too high (hyperglycemia), the liver removes some of the glucose from the blood and turns it into glycogen to store it. If excess glucose is still in the blood after the liver has made enough glycogen, the liver switches its metabolic process to storing the extra glucose as fat. The fat molecules are then carried by the bloodstream and deposited around the body.
If the level of glucose in the blood is too low, the liver breaks down some of its stored glycogen back into glucose and puts the glucose into the blood. If all of the glycogen stores are used up, the liver starts to break down some stored fats to get some glucose.
If fat stores are used up (such as during starvation), the body starts to break down amino acids to get the carbon-oxygen-hydrogen molecules the body so desperately needs. To get the amino acids, however, proteins (such as muscles) in the body are broken down. Remember that the heart is a muscle, so eventually starvation leads to death.
Although sugar seems to be an evil (because an excess of it is stored as fat), just the right amount of glucose needs to be in the bloodstream. Why? Because glucose is the main fuel for your brain.