Animal tissues, organs and organ systems

The digestive system is an organ system in which several organs work together to break down and absorb food.

• •Food passes through the mouth, oesophagus, stomach, small intestine and large intestine, with digestive juices added along the way.
• •The stomach produces protease (pepsin) and hydrochloric acid, which kills bacteria and gives the optimum pH for protease.
• •The small intestine absorbs soluble food molecules into the blood; its lining is folded into villi to increase surface area.
• •The large intestine absorbs water from the remaining undigested material.

• •Enzymes are large proteins that speed up (catalyse) reactions; each has an active site with a specific shape that only fits its own substrate — the 'lock and key' model.
• •Amylase (a carbohydrase) breaks down starch into sugars; made in the salivary glands, pancreas and small intestine.
• •Protease breaks down proteins into amino acids; made in the stomach (as pepsin), pancreas and small intestine.
• •Lipase breaks down lipids into fatty acids and glycerol; made in the pancreas and small intestine.
• •Bile is made in the liver, stored in the gall bladder, and released into the small intestine — it neutralises stomach acid and emulsifies fat into droplets, increasing its surface area for lipase.

• •Each enzyme works fastest at its optimum temperature (around 37°C, body temperature) and optimum pH — most enzymes work best around pH 7, but pepsin's optimum is low (acidic) to match the stomach.
• •Above the optimum temperature, or too far from the optimum pH, the enzyme's active site changes shape so the substrate can no longer bind — the enzyme is denatured and stops working.

1. 1Place drops of iodine solution into each well of a spotting tile.
2. 2Add amylase solution and a buffer of a chosen pH to a test tube, then add starch solution and start a timer.
3. 3At regular intervals, transfer a sample of the mixture into a fresh well of iodine using a dropper.
4. 4Record the time taken for the iodine to stop turning blue-black — this shows all the starch has been broken down.
5. 5Repeat with buffers of different pH, keeping temperature, volume and concentration the same.
6. 6Plot time taken against pH to find the optimum pH for amylase activity.

Blood passes through the heart twice on each full circuit of the body.

• •The right side of the heart pumps deoxygenated blood to the lungs; the left side pumps oxygenated blood to the rest of the body.
• •Deoxygenated blood enters the right atrium via the vena cava and leaves the right ventricle via the pulmonary artery to the lungs; oxygenated blood enters the left atrium via the pulmonary vein and leaves the left ventricle via the aorta to the body.
• •The left ventricle has a thicker muscular wall than the right, since it pumps blood at higher pressure all the way around the body.
• •A group of cells in the right atrium act as a natural pacemaker, controlling resting heart rate.

The lungs are the site of gas exchange, supplied by a system of airways: the trachea branches into two bronchi, which branch further into the alveoli — tiny air sacs surrounded by a dense network of capillaries.

• •Alveoli are adapted for efficient gas exchange: a huge surface area (millions of alveoli), thin (one cell thick) walls for a short diffusion pathway, and a good blood supply to maintain a steep concentration gradient.
• •Oxygen diffuses from the air in the alveoli into the blood in the surrounding capillaries; carbon dioxide diffuses the opposite way, from the blood into the alveoli, to be breathed out.

Each component of blood has a distinct role in transport, defence or clotting.

The coronary arteries supply the heart muscle itself with oxygenated blood.

• •Narrowing of the coronary arteries reduces blood flow to the heart muscle, increasing the risk of a heart attack.

Different types of disease, and physical and mental ill health, are often linked rather than separate.

• •A defective immune system makes a person more susceptible to communicable (infectious) diseases.
• •Some viruses living in cells can trigger cancer — e.g. HPV infection is linked to cervical cancer.
• •An immune response to a pathogen can wrongly trigger allergic reactions, such as skin rashes or asthma.
• •Severe physical ill health, or a serious diagnosis, can lead to depression and other mental health issues.

Certain lifestyle choices are strongly linked with a higher risk of non-communicable diseases.

• •Poor diet, low exercise and obesity increase the risk of cardiovascular disease and type 2 diabetes.
• •Alcohol damages liver cells (leading to cirrhosis) and can impair brain function and development.
• •Smoking damages the lungs and increases the risk of lung disease, lung cancer and cardiovascular disease, due to substances including tar, nicotine and carbon monoxide.
• •Smoking or drinking alcohol during pregnancy increases the risk of low birth weight and developmental problems in the fetus.
• •Exposure to carcinogens, including ionising radiation, increases the mutation rate and so the risk of cancer.

Uncontrolled cell division forms a mass of cells called a tumour, which may be benign or malignant.

• •Lifestyle risk factors for cancer include smoking, obesity, and exposure to UV radiation or other carcinogens.
• •Genetic risk factors include inheriting mutated genes that increase susceptibility to certain cancers.