A-level Biology/Human Health and Disease/smoking and disease

Smoke

Cigarette smoke is composed of over 4000 different chemicals, many of which are toxic. Both smoke that the smoker inhales (through the filter) and the smoke from the burning end is incredibly toxic. There are three main components that are hazardous to health.

• Tar - settles in the lungs and stimulates a series of changes that lead to obstructive lung disease and lung cancer
• Nicotine - is the addictive element of cigarettes, stimulates the nervous system to reduce arteriole diameter and release adrenaline - increasing heart rate and blood pressure. Causes increased stickiness of blood platelets, which increases the risk of blood clotting.
• Carbon monoxide - combines irreversibly with haemoglobin meaning that oxygen cannot bind effectively. This causes a strain on the heart muscle because it must pump more to provide the same amount of oxygen

Lung Disease

Chronic obstructive lung diseases (asthma, chronic bronchitis and emphysema) are now prevalent because of smoking and vehicle/industrial emissions. Lung diseases can be caused by a variety of things such as potentially harmful gases and particles, being deposited in the alveoli, and this can enhance the risk of viruses such as influenza.

Allergens that cause large coughing fits, such as those found in asthmatic attacks can cause long term damage to the alveoli.

Lung Cancer

Tobacco smoke contains tar, and tar contains several carcinogens. These can make DNA in epithelial cells lining the lungs mutate, which is the first step towards a malignant tumour. This tumour then develops into the lymphatic system after spreading through bronchial epithelium and it is here, in the lymph system where cancerous cells can break away and spread around the organ, causing secondary tumours.

Emphysema

Emphysema is the condition in which bronchioles collapse, leaving large spaces where surface area for gaseous exchange used to be. This is because the lungs are constantly infected, causing phagocytes to line the airways, and to reach them they release a protein digesting enzyme known as elastase - this destroys the elastin in the walls of the alveoli allowing the phagocytes to enter and remove bacteria. However, this elastin is vital, without it, the alveoli do not stretch and recoil when breathing in and out, causing bronchioles to collapse. The initial inflammation is caused by infections which in turn is caused by the accumulated mucus as a result of the tar in the lungs. This also leads to the destruction of the alveoli walls, which reduces the surface area of gas exchange surface, meaning less gas exchange can occur.

People with emphysema cannot oxygenate their blood very well, and thus breathe rapidly - blood pressure also increases as blood vessels in the lungs become more resistant to the flow of blood. Lung function will deteriorate and people with severe emphysema may need a supply of oxygen through a face mask to stay alive.

Chronic bronchitis

Tar destroys and weakens cilia, inhibiting the cleaning of the airways. It also stimulates goblet cells to secrete more mucus, making mucus accumulate in the bronchioles, and thus mucus starts to fill up the holes and cannot be moved by the cilia. Thus bacteria and viruses collect and block the bronchioles, causing the smoker to cough in an attempt to move the mucus up the airways.

The damaged ciliated epithelial cells are replaced by scar tissue, thickening the airways and making it more difficult to move air in and out of the lungs. Infections also easily develop in the accumulated mucus, inflaming the airways. The damage and blocking of the airways is chronic bronchitis.

Chronic obstructive pulmonary disease

Emphysema and chronic bronchitis together are known as chronic obstructive pulmonary disease, a progressively degenerating disease. Reversal is very rare, the damage to the lungs can sometimes be reversed if smoking is given up when young. COPD is responsible for >300,000 deaths a year in the UK.

Evidence

Epidemiological

Cigarette smoking began en masse in the beginning of the twentieth century, and doctors started noticing a huge increase in cases of lung cancer from 1930 onwards, and by 1950s it was declared an epidemic. For comparison, in 1912 there were 374 lung cancer cases, and now there are over 35,000 deaths a year, an increase of nearly 100 times.

The correlation between lung cancer and cigarette smoking is plain in the chart to the right - it shows the 20 year 'lag' between the rise of cigarettes and the rise of lung cancer. Epidemiological data links smoking and cancer, and up to 50% of smokers may die of smoking-related diseases.

One third of cancer deaths are as a result of cigarette smoking, and a quarter of smokers die of lung cancer. Chronic obstructive pulmonary disease is very rare in non smokers, less than 10% of victims are non-smokers, and less than 2% of people with emphysema are non-smokers. One fifth of smokers suffer from emphysema, and as a result deaths from pneumonia and influenza are twice as high amongst smokers.

Epidemiological studies have ruled out other factors, that is to say that they cannot find any other factor with a close correlation with smoking. Smoking has been found to have a direct link with lung cancer, as smoking is the common fact in almost all cases. Smoking also contributes to many other cancers of the mouth, larynx, bladder, kidney and pancreas.

Experimental

Epidemiological evidence is sometimes questioned, as it does not 'prove' sufficiently for some that there is a definite link between lung cancer and smoking. However, there is also experimental evidence to prove a direct causative link between smoking and lung cancer.

In the 1960s, an ethically questionable experiment was set up to investigate the effect of cigarette smoke on lungs. One group of 24 dogs were forced to smoke filter-tipped cigarettes and did not develop cancer, whilst 24 other dogs smoked plain cigarettes and developed tumours similar to those found in human lung cancer patients. COPD was also recognised in the plain-cigarette dogs. The healthy group of 24 dogs did not develop tumours, but this is thought to be due to the relatively (to the time a smoker may smoke in their lifetime) short time of the experiment.

Chemical analysis of cigarette smoke shows that tar contains a wide variety of carcinogens, and when this tar is painted on the skins of mice, cancerous growths are shown to develop.

Cardiovascular diseases

All cardiovascular disease are degenerative diseases of the heart and/or circulatory system, and are a major cause of death in developed countries. They are multifactorial, meaning that many factors contribute to the development of these disease, and smoking is a big one.

Atherosclerosis

Atherosclerosis is a chronic inflammatory response in the walls of arteries, in large part due to the accumulation of macrophage white blood cells. The accumulation of fatty material in artery walls damages the arteries, which produces the invasion of macrophages whose secretions stimulate the growth of muscle cells to aid repair and the accumulation of cholesterol.

Cholesterol is an essential biochemical, but it causes problems if it is to accumulate in tissues. This is due to its insolubility - if it is transported in the blood it is done so in tiny balls of lipid and protein known as a lipoprotein. Lipoproteins are in two groups, LDLs and HDLS (low and high density respectively). These LDLs transport cholesterol to the artery walls and unfortunately deposit their cholesterol at damaged sites, whereas HDLs transport it to the liver to be excreted.

These cholesterol deposits form plaques in the arteries, causing them to be less elastic and providing greater resistance to the flow of blood. These plaques cause the blood to flow roughly, making it likely to clot, forming a thrombus, disrupting blood flow even more. See the picture on the right for the stages of atherosclerosis and more detail (click to enlarge).

Strokes

Stroke is the rapidly developing loss of brain functions due to either a blood vessel in the brain bursting or a blocking in a brain artery due to atherosclerosis or a thrombus. They can be instantly fatal or severely disabling to the victim - it depends on how much of the brain is affected.

Coronary Heart Disease

Coronary heart disease (CHD) is also a cardiovascular disease and is disease of the two coronary arteries (the ones that supply the muscles of the heart). It is as a result of atherosclerosis narrowing the coronary arteries, restricting the blood flow, forcing the heart to work harder to force the blood through, raising the blood pressure. It can cause anything from chest pain to a fatal heart attack.

Angina

Angina is the main symptom of CHD, and is characterised by severe chest pain during exercise, caused by a shortage of blood to the heart muscle.

Heart failure

A heart failure is the result of CHD that does not end in a heart attack, and the heart simply gives out due to the gradual damage of the heart muscle - eventually it fails to pump efficiently and the victim dies.

Heart attack

Also known as a myocardial infarction - this is when a large enough branch of the coronary artery is obstructed by a blood clot, and thus part of the heart is starved of oxygen and dies. Depending on how much of the heart dies, this can be fatal, and brings severe and sudden chest pain.

Epidemiological evidence

Coronary heart disease remains a major killer across the world in 2008, and is most prevalent in Russia, and least prevalent in Japan (counting only the developed world). Coronary heart disease death rates differ also within countries, and between socio-economic stratas - in the UK, poor people, certain ethnic groups and men are more at risk. This data tells us that people are not equally at risk of developing CHD.

Risk Factors

There are many known CHD risk factors. You can control some risk factors, but not others. Risk factors you can control include:

1. High blood cholesterol and triglyceride (tri-GLIS-er-ide) levels (a type of fat found in the blood) 2. High blood pressure 3. Diabetes and prediabetesexternal link icon 4. Overweight and obesity 5. Smoking 6. Lack of physical activity 7. Unhealthy diet 8. Stress

Table 1: Risk

Risk factor	Effect
Alcohol	Causes high blood pressure, this and high intake to increase risk of atherosclerosis. Risk may decrease with moderate intake.
Age	Risk increases as age increases
Blood cholesterol	Direct correlation between high cholesterol concentration and CHD risk
Diabetes	Anyone with diabetes is at increased risk
Diet	Risk increases with high intake of saturated fat and/or salt, since these factors increase the risk of atherosclerosis. Decreases risk for antioxidants, soluble fibres and moderate amounts of unsaturated fats.
Heredity	Genetic predisposition does exist, and genes can cause people to develop high blood cholesterol concentrations.
Gender	Men are at greater risk than women
Level of exercise	Risk decreases if more exercise is done, aerobic activity helps to control blood cholesterol and obesity. Lowers blood pressure.
Obesity	Being obese greatly increases risk, puts a strain on the heart and rises blood pressure
Social Class	Poverty increases the risk

Prevention & cure

Coronary heart disease is a major killer in the UK and is also one of the biggest examples of avoidable ill health. For this reason governments are encouraging people to take more exercise, give up smoking and reduce the fat in their diet in an effort to reduce coronary heart disease death rates.

Treatment includes drugs to reduce blood clotting, lower blood pressure and decrease the concentration of cholesterol in the blood. If these are not successful, a coronary by-pass may be carried out - this operation takes a blood vessel from elsewhere on the body, and attaching it to the coronary artery, 'by-passing' the clot in the artery blockage. If this does not or cannot work, a complete heart transplant may be given - but this is very difficult to since a donor must be found that matches the recipients tissues etc. Heart transplants are usually refused to people who have not completely given up drink and smoking, in an effort to improve the health of the waiting list and to ensure that the few hearts they do get are not then ruined.