The digestive and circulatory systems

Digestion breaks down large food molecules into smaller ones for easier absorption. Digested nutrients are delivered to cells in the bloodstream — part of the circulatory system. .

Principles of organisation

The cell is the ‘basic building block of life’ and is the smallest functioning part of an organism. A group of cells working together is called a tissue and a collection of tissues all performing a specific function is called an organ. Multiple organs which are connected together are referred to as an organ system.

The heart, the lungs, the liver, kidneys, the intestines and the stomach are all organs.

Examples of organ systems include the respiratory system, circulatory system, reproductive system and digestive system.

The human digestive system

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

Large food molecules need to be broken down into smaller ones to be absorbed by the villi in the small intestine. Enzymes catalyse the breakdown of food molecules and increase the rate of reaction at body temperature. A region of an enzyme called the active site binds to the substrate (e.g. starch) and is then converted into product (e.g. glucose). The shape of the active site varies depending on the enzyme. According to the lock and key model, only substrate molecules with a matching (complementary) shape to the enzyme’s active site will be able to be broken down by that enzyme.

Digestive enzymes

Digestion in humans involves the following enzymes: 

• Amylase is responsible for the breakdown of starch into a two-sugar molecule called maltose. Another enzyme called maltase breaks this down further into glucose, which is a single sugar molecule. Starch breakdown occurs as soon as we place food in our mouth, as amylase is present in our saliva. Amylase is also produced by the pancreas and small intestine.

• Proteases are the name given to a group of enzymes which can breakdown protein into amino acids. A specific protease is called pepsin which is found in the harsh, acidic environment of our stomach, where most of the protein we consume is digested.

• Lipases are the enzymes which break down lipids into fatty acids and glycerol. Lipases are produced by the pancreas and small intestine.

The products of digestion are used to build new carbohydrates, proteins and lipids. Some glucose will be used in respiration. Whatever isn’t used will be converted into glycogen and stored in cells of the liver.

The alimentary canal

In the mouth, carbohydrate is broken down by amylase enzymes in saliva. Chewing breaks up food, increasing the surface area for the digestive enzymes to act on.

A structure at the back of our throat called the epiglottis prevents food from being inhaled through our windpipe during eating. Once we swallow, a muscular tube called the oesophagus moves the food into the stomach. 

The stomach contains pepsin, an enzyme which breaks down proteins into amino acids. The stomach is filled with gastric juices containing hydrochloric acid, which provide optimal pH conditions for pepsin to work effectively.

The partially digested food and gastric juices — known as chyme — is quite acidic. Bile, which is produced by the liver and stored in the gall bladder, neutralises the chyme on its way to the small intestine, and emulsifies fats to facilitate their breakdown.

In the first part of the small intestine — called the duodenum — food continues to be broken down. Digestive enzymes are produced by the small intestine and pour into the small intestine from the pancreas. When digestion is complete, sugars, amino acids, glycerol and fatty acids are absorbed into the bloodstream through finger-like structures called villi which line the walls of the second part of the small intestine, the ileum.

The remaining undigested material travels into the large intestine, which is also composed of two parts - the colon and the rectum. In the colon, water is absorbed into the bloodstream leaving behind the waste material faeces. This is stored in the rectum before removal from the body through the anus.

The circulatory system

Humans have a double circulation: blood is transported in two loops and passes through the heart twice.

In the first loop, blood is pumped from the right ventricle of the heart to the lungs, where is gets rid of carbon dioxide and collects oxygen before returning back to the heart. In the second loop, blood is pumped from the left ventricle of the heart to the rest of the body tissues. There it drops of oxygen that cells need for respiration and picks up the carbon dioxide that has been generated as a waste product. The walls of the left side of the heart are thicker than the right side since it needs to transport blood a greater distance to reach all of the body tissues.

Arteries carry blood away from the heart. The aorta is the largest artery in our body and delivers blood from the heart to all of our tissues. The pulmonary artery takes deoxygenated blood from the heart to pick up more oxygen in the lungs. Arteries pump blood under high pressure so they have thick muscular walls to withstand the high pressure without becoming damaged.

Veins carry blood towards the heart. The vena cava brings blood from the body to the heart and the pulmonary vein carries oxygenated blood between the lungs and the heart. Veins carry blood much more slowly at lower pressure, so they have thinner walls and a large lumen, with valves to prevent the backflow of blood. The low pressure of blood, large lumen and the fact that we can find veins closer to the surface of our skin makes them much easier to obtain a blood sample from compared to arteries.

Capillaries are tiny blood vessels which connect veins and arteries. They are extremely small (just one cell thick) and contain small holes called pores which allows substances to pass through them easily.

Lungs

When we breathe, air enters our bodies through our nose and mouth and makes its way past the larynx and down the trachea. The trachea branches into two smaller tubes, called bronchi, which send air to each lung. The bronchi divide into even smaller tubes called bronchioles which finally send the air into air-sacs called alveoli. The alveoli look like a bunch of grapes and it’s this structure where gas exchange takes place.

Oxygen diffuses from a region of high concentration in the alveoli to a region of low concentration in the bloodstream, where it travels to different tissues of the body and is used for respiration. Carbon dioxide travels in the other direction, from a region of high concentration in the bloodstream to a region of low concentration in the alveoli, where it travels up the trachea and is breathed out.

The lungs themselves are surrounded by the pleural membrane, a moist membrane which forms an airtight seal around the lungs. The rib cage protects the organs of the respiratory system and are surrounded by intercostal muscles, which move the rib cage during breathing to help bring air into or out of the lungs.

Control of heart rate

The natural resting heart rate is controlled by a group of cells located in the wall of the right atrium that act as a pacemaker. This group of cells is able to increase or decrease our heart rate to match our levels of activity, when stimulated by signals from our brain. Artificial pacemakers are electrical devices that are used to correct arrhythmias (irregularities in the heart rate).

Blood

Our blood contains four main components: red blood cells, white blood cells, platelets and plasma. 

• Red blood cells: the haemoglobin inside red blood cells binds to and transports oxygen. Red blood cells are unusual in that they do not have a nucleus – this provides more space to store oxygen. Their biconcave shape gives them a large surface area for the diffusion of oxygen.

• White blood cells: detect any pathogens which are circulating in the blood and initiate an immune response. There are different types — some engulf and destroy pathogens (these are known as phagocytes), while others manufacture antibodies or antitoxins.

• Platelets: small fragments of cells which clot at the site of a wound to prevent excessive blood loss and entry of microorganisms through the skin.

• Plasma: a liquid containing carbon dioxide, digested food molecules, urea, hormones and heat energy

Coronary Heart Disease: a non-communicable disease

Coronary heart disease occurs when the arteries which supply the heart with blood (the coronary arteries) become blocked.

It is an example of a non-communicable disease because it is not caused by pathogens and is not infectious. Certain lifestyle factors, such as diet and smoking, can increase the risk of coronary heart disease.

Coronary arteries become blocked due to an accumulation of fatty material containing cholesterol. If the fatty deposit becomes so big that it completely blocks the coronary artery, blood cannot flow to the heart muscle. Without a blood supply, the heart muscle cells cannot receive oxygen and glucose for aerobic respiration and the heart muscle dies. Coronary heart disease can be treated with:

• Stents – arteries which have become narrower due to a build-up of fatty plaques in the artery walls can be stretched open using a device called a stent. The stent pushes against the walls of the artery, holding it open and maintaining a flow of blood through the artery. Stents can be used in cases where drugs are less effective and are a long-term solution.

• Statins – these are drugs which reduce cholesterol levels and are prescribed to people who are at high risk of coronary heart disease. Statins work by reducing how much cholesterol the liver synthesises. There are side effects associated with the use of statins, such as headache and memory loss. They cannot be given to pregnant women or people with liver disease.

Coronary heart disease can lead to heart failure, which is when the blood supply to the heart is completely blocked off and the heart starts to die. This is treated with a heart transplant. Artificial hearts are sometimes used to keep patients alive whilst waiting for a heart transplant, or to allow the heart to rest as an aid to recovery.

Heart transplants are not a perfect solution though, as the patient’s immune system may recognise the heart as foreign and initiate an immune response. This means that the patient has to take immunosuppressant drugs for the rest of their life, leaving them at greater risk of becoming ill from other infections due to their weakened immune systems. There is a long waiting list for heart transplants as there is a shortage of donor hearts.

Faulty valves

In some people, it may not be the whole heart that is damaged but just the valves that are located between the atria and ventricles. Faulty heart valves may not open properly, which means that less blood enters the heart chambers. Less blood is delivered to the body, causing pressure to build up and the lungs to swell with fluid. Faulty valves might not close properly which means that blood will leak back from the ventricle into the atrium. Faulty valves can be replaced with:

• Biological valves – from an animal (e.g. a pig or cow) or from a human donor

• Mechanical valves – made of strong materials such as titanium or carbon

Health issues

Health is the state of being physically and mentally well. Disease is a major cause of a lack of health and can be classed as communicable (can be transmitted from person to person) or non-communicable (cannot be passed between people). Other causes of a lack of health arise from diet, stress and life situations.

Different types of diseases may interact, for example:

• Defects in the immune system makes an individual more susceptible to infectious disease. For example, the HIV virus kills white blood cells, which makes people with AIDS vulnerable to a range of infectious that people with a healthy immune system would be able to cope with.

• Viruses living in cells can trigger cancers – e.g. the HPV virus can cause cervical cancer.

• Immune reactions initially caused by a pathogen can trigger allergies such as asthma and skin rashes.

• Living with a physical disease can be stressful and impact a person’s quality of life. This can lead to depression and other forms of mental illness.

The effect of lifestyle on some non-communicable diseases

Risk factors are things which increase an individual’s likelihood of having a particular disease. Risk factors can be:

• An aspect of a person’s lifestyle – e.g. diet, exercise, smoking

• Substances in the person’s body or environment – e.g. asbestos, pollution, radiation

Scientists have proved the association between some risk factors and certain diseases, for example:

• The effect of smoking on lung disease and lung cancer

• The effect of smoking and alcohol on unborn babies

• The effects of smoking, diet and exercise on cardiovascular disease

• The effects of obesity on type 2 diabetes

• The effects of alcohol on liver damage

• The effects of carcinogens such as ionising radiation on cancer

For other diseases, the association with particular risk factors has not been conclusively proved. It is often difficult to show that a risk factor causes a disease, because there may be a number of interacting risk factors which individually exert a small effect.

Cancer

Cancer occurs when the instructions inside cells changes – in other words, the DNA mutates. The mutation will be in a gene that is responsible for controlling cell division and growth. When the gene is mutated, cells can divide uncontrollably to form a mass of abnormal cells, called a tumour.

• Benign tumours are a growth of abnormal cells which are contained in one area, usually within a membrane, and do not invade other parts of the body. Benign tumours are not cancerous and so long as they are not pressing on any blood vessels, are harmless.

• Malignant tumour cells are cancerous tumours. These invade neighbouring tissues and travel in the bloodstream to different parts of the body where they form secondary tumours.

Lifestyle factors such as smoking, stress and obesity have been associated with an increased risk of various types of cancer. Inheriting a faulty gene can also increase the risk of certain cancers e.g. mutations in a gene called BRCA1 lead to an increased risk of breast cancer.