Note: I will keep things generalizable to both hand rolled and manufactured cigarettes
Incomplete Combustion
Let's start with lighting the cigarette. Cigarettes are designed to burn but to burn poorly. They slowly smoulder rather than bursting into flames; this is so that the nicotine vapourizes so it can be inhaled rather than just burning into carbon-dioxide and water. Burning, aka combustion, is the rapid addition of oxygen to a substance. Organic molecules, such as those found in plants, are mainly composed of carbon and hydrogen; complete combustion breaks these atoms apart and sticks them onto oxygen forming carbon dioxide and water but incomplete combustion means there isn't enough oxygen to stick onto them causing them instead to rearrange themselves producing a huge variety of hydrocarbons as well as pure carbon (aka charcoal). This resulting mess is called tar but contains a different complement of hydrocarbons than the tar used on roads - for one road-tar is mainly long chain like hydrocarbons whereas tobacco-tar is more aromatic (benzene-ring containing) hydrocarbons - because plants are mainly based on carbohydrates which are rings of carbon atoms.
 |
| Images of smoke particles from a wild-fire which is another source of incompletely combusted organic matter. Smooth particles are coated in various hydrocarbon products, spiky ones are uncoated carbon soot. Source: http://phys.org/news/2013-08-insights-wildfire-climate.html Credit: Swarup China imag |
Inhalation
This tobacco tar is inhaled with the vapourized nicotine where it comes in contact with your mouth, throat, windpipe and lungs. In general, the pure carbon in the tar is not in nice smooth round balls it will be spiky shards which can scratch and physically damage everything it comes in contact with. This damage can cut open cells which release chemicals triggering inflammation (and you get bronchitis/asthma). The larger particles of carbon you will breath out but smaller particles will get stuck in the vortexes of air swirling about in your lungs as well as get stuck in the thin layer of water & surfactant (biological soap) over the cells in your lungs. Your white blood cells can find and remove these particles but since they are only a long-term threat, your white blood cells are rather inefficient at it so even just the exposure to vehicular exhaust from living in a city is enough to gradually build up carbon particles in your lungs - slowly turning them black. The accumulated damage from these carbon spikes is what causes emphysema. The fluid release from pricked cells and the over production of mucus by cells trying to protect themselves from these prickles makes a nice home for bacteria (causing pneumonia).
Meanwhile the aromatic hydrocarbons along with the nicotine can be absorbed into the lung cells and the bloodstream. The aromatic hydrocarbons are usually really good at getting into cells so a lot of them just hang out in the lung cells where some of them will cause mutations in DNA and/or promote the formation of cancers (hence the fact that 90% of lung cancers can be attributed to smoking).
In the blood stream
Ok so now the nicotine and some of the nasty aromatic hydrocarbons have dissolved into the blood. The blood with transport this stuff everywhere. The nicotine will float around not doing much until it is sent up to the brain where in can dissolve across into the neural tissue it also enters the adrenal glands where it causes the release of adrenaline and glucose. The aromatic hydrocarbons aren't so good at crossing the blood-brain barrier so will travel around stopping at the other organs with large blood supplies (ie. liver, pancreas) where they contribute to causing cancers then end up being filtered out through the kidneys, bladder and exit in your urine.
Nicotine in the brain
Within 10 second of smoking the 1-3% of the tobacco that is actually nicotine has reached the brain where it will stay for 1-2 hours before being flush out the kidneys as well. In the brain, nicotine interacts with neural cells to release many neurotransmitters and hormones. Dopamine is release enhancing the reward/pleasure system which contributes to nicotine's addictive properties. Nicotine has an unusual combination of mood alterations both stimulating (through the release of adrenaline) and relaxing (through the release of serotonin and beta-endorphin). Other effects include increased concentration (acetylcholine) and pain-killing (acetylcholine, serotonin). Nicotine can also enhance the opiate system (the same system activated by heroin, cocaine) increasing its addictiveness. Some neurotransmitters and all hormones re-enter the bloodstream to affect other parts of the body.
Effects outside the brain
Epineferine (adrenaline) and norepineferine released by nicotine stimulation of the adrenal gland & brain cause blood vessels to constrict reducing blood-flow to the extremities and increasing blood pressure (contributes to most cardiovascular diseases). In addition, arginine vasopressin (aka anti-diuretic hormone) released from the brain travels to the kidney and increases water retention from urine which increases blood pressure further. The constriction of blood vessels affects pregnancy contributing to stillborns, miscarriages, premature birth, and low birth-weight babies. In men the decreased bloodflow to the penis causes impotence.
Closing
Various filters for cigarettes reduce the quantities of carbon particles and might catch some of the aromatic hydrocarbons but the amount of smoke one exhales even using a filter gives an indication of just how ineffective they are. Pipe-smoking due to a lack of filter is even worse than cigarettes.
Interestingly, nicotine and tobacco plants have be used for other things as well. Concentrated nicotine is a natural insecticide which has been used in agriculture. Nicotine may also be useful for treating the symptoms of Alzheimer's & Parkinson's disease due to its effects on the dopamine system (prior to onset it actually increases risk of disease). While tobacco plants are now being developed using genetic engineering to produce useful pharmaceutical products such as antibodies to treat cancers, influenza vaccines, and aprotinin (reduces bleeding during surgery).
