alimentary canal

• the tube via which food passes through the body - from mouth to anus, its lining is the surface in which nutrients are absorbed
• order : mouth cavity, pharynx, oesophagus, liver, stomach, small intestine, large intestine
• Ingestion occurs at mouth 

  food is chewed in a process called mastication  

  Chemical and mechanical digestion commence before food is swallowed
• Pharynx is the back of mouth cavity. the tongue moves backwards and upwards to swallow, pushing food into pharynx
• Wall of alimentary canal has double layer of muscle :

  • circular - smooth muscle with fibres arranged in a circle around the organ
  • longitudinal - smooth muscle with fibres arranged lengthwise along an organ
• Peristalsis is waves of muscular contraction that push food along the alimentary canal
• oesophagus is the tube that carries food from the pharynx to the stomach (23 - 25 cm long). as food enters, the circular muscle behind it contracts to narrow the tube, peristalsis moves the food and is assisted by the secretion of mucus that lubricates inner lining
• the liver produces bile
• Mucosa is the mucous membrane that lines the alimentary canal
• Mucosa in the stomach is specialised to secrete gastric juice by gastric glands located in gastric pits which are narrow, tube-like structures
• Gastric juice is the digestive juice secreted by the glands of the stomach. it contains hydrochloric acid, mucus and digestive enzymes; each being secreted by different types of cells in the gastric pit
• pH in the stomach is approximately 2-3 due to the hydrochloric acid. it allows enzyme pepsinogen to convert into pepsin, its active form. cells lining stomach are protected by layer of mucus.
• Nutrients are not absorbed into the bloodstream through the stomach because of the thick layer of mucus. however alcohol and other drugs are.
• Thickening of circular muscle at lower end of stomach creates constriction called pyloric sphincter, a ring of smooth muscle between the stomach and the duodenum which prevents stomach contents moving through unless pushed along by peristalsis
• the small intestine is the longest part (6 - 7 metres), but has a small diameter. it receives materials pushed through the pyloric sphincter
• the small intestine has three regions :

  • Duodenum - first part: it is the shortest section at about 25cm, extending from the bottom end of the stomach in a curve around the pancreas. This is where most chemical digestion occurs
  • Jejunum - middle section: its lining allows for effective absorption of carbohydrates and proteins
  • Ileum - final part: where vitamin B12, bile salts and remaining products of digestion are absorbed
• the large intestine is 1.5 meters long with a large diameter
• large intestine parts :

  • Caecum - 6cm long. it is the pouch where the small intestine joins to large intestine
  • Appendix - a small tube attached to caecum
  • Colon - the longest part with an inverted U shape
  • Rectum - the last part. semi-solid material is pushed through by peristalsis. As walls stretch, they trigger defecation, relaxing muscles around anus to let faeces out 
  • Anus - the external opening at end of rectum. Anal sphincter is the circular muscle around anus
• the large intestine contains no villi and does not secrete digestive juices, but its lining does secrete a large amount of mucus
• in the large intestine, most of the water is absorbed, making contents solid. Some bacteria also produce vitamins which is absorbed through walls. Mineral nutrients are also absorbed
• faeces is made of water, undigested food material (particularly cellulose), bacteria, bile pigments (gives colour), cells that have broken away from internal lining. it passes through the rectum and anus to the exterior