The topic your are touching on here is called pharmacokinetics, which is a branch of pharmacology dedicated to determining the fate of substances administered to a living organism. The substances of interest include any chemical xenobiotic such as: pharmaceutical drugs (as nicotine), pesticides, food additives, cosmetics, etc. It attempts to analyze chemical metabolism and to discover the fate of a chemical from the moment that it is administered up to the point at which it is completely eliminated from the body.
So the digestive system is a huge process and consists of many various areas of the body. Let's dig deeper on this; the first step of digestion starts in your mouth, this is where anything you ingest orally starts it's journey, getting covered in saliva. Saliva contains water, electrolytes, mucus and enzymes. Important enzymes are amylase which catalyses the hydrolysis of starch into sugars, and lysozyme (or muramidase) which functions by attacking, hydrolyzing, and breaking glycosidic bonds in peptidoglycans.
Whatever you have ingested then travels down your throat and into your stomach, where it is mixed with hydrochloric acid, enzymes and water. Hydrochloric acid kills bacterias and helps with decomposition. Your stomach is basically preparing the ingested substance, to then release portions into your intestine. There are little to no absorption of nutrients in the stomach.
Chemical degradation and absorption of food in the body finds almost exclusively in the small intestine. Anatomically, the small intestine is divided into three sections: duodenum, jejunum and ileum.
The enzymes in the pancreas attack all main groups of nutrients. Among the most important enzymes are:
trypsin and chymotrypsin, which continues the pepsin's degradation of proteins
pancreatic amylase, which, like amylase in saliva, cleaves starch and glycogen
pancreatic lipase that cleaves fatty acids (triglycerides) into glycerol and fatty acids.
Nutrients broken down into molecules are absorbed by the villus cells, partly by diffusion (it is here nicotine gets absorbed), and by active transport through the cell membrane. Carbohydrates are preferably taken as monosaccharides (e.g. glucose), and proteins are taken up in the form of amino acids.
Along with salt and water soluble vitamins, monosaccharides and amino acids are then secreted into small blood vessels in the intestines. From here they are led to the liver with the blood. The degradation products from the fat in the food (fatty acids, mono- and diglycerides and glycerol) are reintroduced into the intestinal cells of triglycerides and are provided with a capsule consisting of protein, cholesterol and phospholipids, before they pass into the lymph, known as kylomicrons.
The small intestine extends into the large intestine. The large intestine has no villi's. Once the intestinal content has come so far, the decomposition and absorption of nutrients are virtually completed. In the colon, almost exclusively water is absorbed, so that the thin-flowing waste from the digestive process eventually becomes concentrated. The colon also has a strong peristaltic that transports the waste to the last section of the digestive tract, rectum.
Let's talk about our lungs a little bit.
Our lungs contain something called bronchi. Bronchial arteries are the respiratory tract pathways (bifurcatio tracheae). The bronchioles divide into ever smaller branches. The smallest are called respiratory bronchioles because they no longer have a continuous smooth wall, but are partly composed of pulmonary ulcer or alveoli (alveoli pulmonis). Through the alveolar veins they end up blindly in collections of airy lung ulcers - the lungs functional units.
The thin blister walls form a grid that together constitutes a surface area of approximately 140 m². They consist of a double layer of plate epithelial cells (pneumocytes type I), one layer from each alveolar, with a capillary between. The capillaries are formed by branches of the lung artery that lead to oxygen-low blood.
The gas exchange thus takes place by diffusion through two very thin walls (below 0.001 mm): an alveolar wall and a capillary wall. It is here that nicotine is absorbed from aerosol (vapor), bypassing all those digestive steps we talked about earlier.