Do you think that you are the same person you were a month ago? You probably act the same, but the cells of your body are constantly renewing themselves, and, apart from sparking a somewhat unwinnable philosophical debate, this brings us to the topic of assimilation. Everything (almost) that you ingest is incorporated into your body via assimilation.
Assimilation is the process by which the products of digestion absorbed are transported around and utilized by the body. The products of digestion are incorporated into the structure of your body.
As previously discussed, absorption begins in the small intestine. Here glucose, fructose, fatty acids, and proteins are absorbed (through the villi) into the blood stream. From there, they reach tissues throughout the body and are assimilated. What isn't absorbed in the small intestine is then passed into the large intestine where water and minerals are absorbed.
The enzymes of your digestive system break down the complex molecules you ingest into their simplest forms. Carbohydrates are absorbed as monosaccharides, proteins are absorbed as amino acids and fats/lipids are absorbed as fatty acids and glycerol.
Your body uses the products of digestion in several ways:
Monosaccharides
Monosaccharides are carried via the hepatic portal vein in the blood, and the liver converts any non-glucose monosaccharides to glucose. The glucose is carried into general circulation, where:
The cells of the body use it in respiration to produce energy.
Excess glucose entering the body is condensed into the large polymer, glycogen, by the hepatocytes (cells of the liver). This process is known as glycogenesis.
Excess glucose may also be converted to fatty acids and triglycerides by the liver and adipose tissue underneath the skin and around organs. The fat made in the adipose tissue is stored, and the fat converted in the liver is transported in the blood to adipose tissue and stored.
Amino Acids
Like monosaccharides, amino acids are carried via the hepatic portal vein. Amino acids
then enter general circulation, where:
The body may use them to synthesize proteins for cell growth and repair.
They can be used to make enzymes.
Endocrine glands use them to synthesize hormones.
They are used in the creation of antibodies.
Hepatocytes use them in the synthesis of most of the plasma proteins. Albumin, the major plasma protein, is synthesized almost exclusively by the liver. Also, the liver synthesizes many of the clotting factors necessary for blood coagulation.
Excess amino acids are deaminated by the liver (because they cannot be stored). The nitrogen containing amine group (NH2) is removed from the molecule and converted to urea (CO(NH2)2). The urea is carried by the blood to the kidneys, which filter it out and excrete it. The remaining non-nitrogenous part of the amino acid molecules are converted to glycogen or fat and stored.
Fatty Acids and Glycerol
Fatty acids and glycerol are carried by the lymph into general circulation, where:
They are used in creating the cell membranes of new cells.
The cells of the body use them for respiration in some circumstances.
Excess are converted to fat and stored in adipose tissue.
CSEC also requires that you understand the regulation of blood sugar levels in the body. As you know, your body needs to maintain a somewhat stable equilibrium of internal chemical and physical conditions known as homeostasis.
Your blood sugar levels are controlled by the action of both the pancreas and the liver. The pancreas secretes two hormones (insulin and glucagon) directly into the blood which are responsible for keeping blood glucose levels constant. Blood sugar regulation is one aspect of homeostasis.
When the blood sugar level rises, such as after eating a meal high in sugars, the pancreas will secrete insulin into the blood. This triggers the cells of the body to absorb glucose for respiration and causes the liver cells to convert excess glucose to glycogen (via glycogenesis) for storage.
When blood sugar level falls, such as between meals or during intense exercise, the pancreas will secrete glucagon which stimulates the liver's cells to convert glycogen back to glucose (via glycogenolysis), which is released into the blood.
This process of regulation can be summarized like this:
(You can read some more on liver metabolism here)