Essay 3

Do not memorise these answers! These are guidelines. Be creative, devise your own expression and organisation! You can elaborate on the points in your own language.
Remember that there is no 'perfect' answer - a little shortfall in content is easily compensated by presentation.
You should aim at writing an essay in this examination in approximately 20 minutes.

Correlation between development and blood supply of the digestive system.

Key features of development : The digestive system begins development as an endodermal tube. Endoderm gives rise to the lining epithelium and parenchyma of the glands. The connective tissue and muscle layers develop from the mesoderm.

The digestive tube is divided into three parts - foregut, midgut and hindgut. The foregut gives rise to the oesophagus, stomach and half of the duodenum. The remaining small intestine, ascending and most of the transverse colon develop from the midgut. The descending colon, sigmoid colon, rectum and most of the anal canal develop from the hindgut. The liver, gall bladder and the pancreas develop from the junction of the foregut and midgut.

The oesophagus is largely supplied by arteries in the neck and the thorax. The abdominal part of the gut is supplied by three unpaired arteries - coeliac, superior mesenteric and inferior mesenteric for the foregut, midgut and hindgut respectively. The abdominal part of the gut is drained by veins that join the portal vein leading to the liver.

The blood vessels form a plexus in the submucosa of the gut wall. Thus, at the lower end of the oesophagus and in the anal canal there are anastomoses between the veins of the portal and systemic circulation.

Discussion

It is important to realise the level of the answer expected. Details of the branches of the three major arteries are not expected! If you wish, you may comment on the fact that the main arteries branch and anastomose repeatedly along the borders of the gut tube.

The pancreas is supplied by branches from both coeliac and superior mesenteric arteries. The liver is also a "junctional" organ. Its blood supply is described as coming from the coeliac arteries, though as very common variations, branches from the superior mesenteric do supply the liver and gall bladder.