The digestive system consists of a long, muscular tube, also called the gut or alimentary canal (Figure 6.2). Associated with it are a number of glands that secrete enzymes and other digestive juices. The gut extends from the mouth to the anus and is specialised for the movement, digestion and absorption of food.
mouth tongue salivary gland
oesophagus
stomach
pancreas
colon pancreatic duct
rectum anus appendix
small intestine bile duct gall bladder liver
large intestine
Figure 6.2 The human digestive system.
Mouth, oesophagus and stomach
In the mouth, food is broken into small pieces by the jaws and teeth, and mixed with saliva containing the enzyme salivary amylase, which begins the digestion of any starch the food contains.
The food is then passed down the oesophagus to the stomach by a sequence of muscle contractions known as peristalsis. The stomach is a muscular sac that holds the food for up to four hours while digestion proceeds inside it. As muscles of the stomach contract, food and enzymes are mixed – this gives maximum contact between food and enzyme molecules, and speeds up the digestive process.
Digestion of protein begins here, catalysed by the enzyme pepsin, which is secreted in gastric juice produced by millions of gastric glands in the stomach wall. Gastric juice contains pepsin in an inactive form.
6 HUMAN HEALTH AND PHYSIOLOGY 1 133 Hydrochloric acid activates the pepsin and maintains a pH of 1.5–2.0
in the stomach. This pH is the optimum for protein digestion and also kills many of the bacteria present in the food we eat. Goblet cells in the stomach lining secrete mucus to protect the interior of the stomach from the acid and enzymes, which might otherwise digest it.
Food is transformed in the stomach to a semi-liquid called chyme and is then ready to move on to the next stage of digestion in the small intestine.
Roles of the small intestine
Little by little, chyme leaves the stomach via a valve at the lower end and moves into the fi ve-metre long small intestine. Digestion is completed in the fi rst section of the small intestine, (Figure 6.2). Digestive juices are secreted from the liver, gall bladder, pancreas and the intestine walls. Bile is added from the liver and gall bladder, and the pancreas secretes pancreatic juice containing trypsin (a protease), lipase, amylase and bicarbonate ions.
The acidity of the chyme is reduced by these ions, allowing the enzymes to work at their optimum pH.
The inner surface of the small intestine is greatly folded to form
thousands of tiny villi (Figure 6.3, overleaf ). Each villus contains a network of capillaries and a lacteal. (A lacteal is a small vessel of the lymphatic system.) Villi greatly increase the surface area of the small intestine and improve its effi ciency as an absorbing surface. As small molecules such as glucose, amino acids, fatty acids and glycerol, come into contact with a villus, they are absorbed, either passively or by active transport, into the single layer of epithelial cells that cover it. Amino acids and glucose then enter the capillaries and are carried away in the bloodstream. Fatty acids and glycerol are taken into the lacteal and travel in the lymphatic system.
After digested food has been absorbed, it is assimilated into the body and enters cells to become part of the body’s tissues or reserves. Glucose is transported to the liver, which maintains a constant level of blood sugar.
Amino acids form part of the reserve of amino acids used to build new proteins in cells all over the body, and fatty acids and glycerol enter the bloodstream from lymph vessels near the heart to be used as an energy source or to build larger molecules.
Role of the large intestine
By the time food reaches the end of the small intestine, most useful substances have been removed from it. Any remaining undigested material passes into the large intestine, which also contains mucus, dead cells from the intestine lining and large numbers of naturally occurring bacteria. Bacteria living here are mutualistic organisms, gaining nutrients and a suitable habitat, while synthesising vitamin K for the benefi t of their human host.
The main role of the large intestine is reabsorbing water and mineral ions such as sodium (Na+) and chloride (Cl−). Water in the gut contents comes not only from our diet, but also from the many additional litres that are added to the intestine in digestive juices. What remains of the original food is now referred to as faeces and is egested, or eliminated from the body, via the anus.
Absorption the process by which small molecules are taken through the cells of the intestine and pass into the bloodstream
Assimilation the process by which products of digestion are used or stored by body cells
Egestion the process by which undigested material leaves the body at the end of the gut
1 List the reasons why digestion is essential.
2 Distinguish between ‘absorption’ and ‘assimilation’.
3 State why enzymes are needed in digestion.
4 List the ways in which a villus is adapted to increase the effi ciency of absorption of nutrients.
6.2 The transport system
Figure 6.3 The inner surface of the small intestine is highly folded, with millions of fi nger-like villi. Each epithelial cell is covered in minute microvilli, so the total surface area for absorption is vast.
Villus (enlarged view)
columnar epithelium cells
capillary network
lacteal
smooth muscle
venule arteriole
Small intestine with villi
Microvilli on surface of epithelial cells
Assessment statements
•
Draw and label a diagram of the heart showing the four chambers, associated blood vessels, valves and the route of blood through the heart.•
State that coronary arteries supply the heart muscle with oxygen and nutrients.•
Explain the action of the heart in terms of collecting blood, pumping blood and opening and closing of valves.•
Outline the control of the heart beat in terms of myogenic muscle contraction, the role of the pacemaker, nerves, the medulla of the brain and adrenalin (epinephrine).•
Explain the relationship between the structure and function of arteries, capillaries and veins.•
State that blood is composed of plasma, erythrocytes, leucocytes (phagocytes and lymphocytes) and platelets.•
State that the following are transported by the blood: nutrients, oxygen, carbon dioxide, hormones, antibodies, urea and heat.6 HUMAN HEALTH AND PHYSIOLOGY 1 135 Our circulatory system provides a delivery and collection service
for the whole body. The heart, blood and blood vessels make up a most effi cient transport system that reaches all cells, bringing the substances they need and taking away their waste. Humans and other mammals have what is known as a closed circulatory system with blood contained inside a network of arteries, veins and capillaries.