Download Lecture 2: Physiological principles in the alimentary tract

Lecture 2: Physiological principles in the alimentary tract
1) Label a diagram of the alimentary system identifying the following:- mouth, oesophagus, salivary glands, stomach, pancreas,
liver, gall bladder, duodenum, jejunum, ileum, colon, rectum, and anus. Mark on the diagram the position of the sphincters –
gastro-oesophageal, pyloric, ileo-caecal, anal and sphincter of Oddi

GI tract is hollow tube from mouth to anus made of smooth muscle and lined by epithelium + associated structures (salivary glands,
liver and pancreas). Mouth – oesophagus – stomach – small intestine (duodenum, ileum and jejunum) – colon (ascending, transverse
and descending) – rectum - anus

Flow through tube regulated by activity smooth muscle and 4 sphincters (specialised regions of smooth muscle)

The positions of the sphincters are numbered on the diagram to the left as:
(1) Gastro-oesophageal
(2) Pyloric
(3) Ileo - caecal
(4) Anal
A fifth sphincter, Sphincter of Oddi guards common duct entrance to duodenum (Liver (bile) and Pancreas secrete into duodenum via
Common bile duct throught this sphincter.
2) Label a simple diagram of a typical gut wall section identifying: epithelial lining of gut lumen, mucosa, circular and
longitudinal muscle, myenteric and submucosal plexuses and extrinsic nerves
Epithelial lining

Mucosal function shows specialisation for secretion or absorption. May be invaginated to form glands (gastric glands, Brunner’s
glands)

Gut neural plexi have multiple interconnections (“gut brain”) both within/between plexi as well as connections from external nerves of
ANS. Both local and “long” (involving brain) reflexes are present
3) Briefly explain the terms “intrinsic” and “extrinsic” nerve supply to the gut

Intrinsic comprises of interconnections between ganglia within a plexus and between the submucosal (secretion and blood flow) and
the myenteric (GI Motility) plexus (gut brain)

Extrinsic comprises of extrinsic nerve supply to the enteric nervous system (with autonomic innervation coming from vagus and pelvic
nerves and also the sympathetic ganglia (parasympathetic – mainly excitatory; sympathetic – mainly inhibitory)
4) State that these nerves may modify secretion, absorption, motility and blood flow of the gut

intrinsic and extrinsic nerves may modify secretion, absorption, motility and blood flow of the gut
5) State that the extrinsic nerve supply has both a parasympathetic (mainly excitatory) and a sympathetic (mainly inhibitory)
component

the extrinsic nerve supply has both a parasympathetic (mainly excitatory) and a sympathetic (mainly inhibitory) component
6) State that mechano- and chemoreceptors in the gut wall may invoke both local reflexes involving the intrinsic nerve system
and “long” reflexes involving the brain via afferent nerve fibres in the vagus and splanchnic nerves

mechano- and chemoreceptors in the gut wall may invoke both local reflexes involving the intrinsic nerve system and “long” reflexes
involving the brain via afferent nerve fibres in the vagus and splanchnic nerves
7) State that blood flow to the gut is about 30% of resting cardiac output and that venous drainage is via the hepatic portal vein
which passes through the liver before returning to the heart

Gut receives blood flow of about 30% resting cardiac output but this may fall during heavy cvs demand (exercise, haemorrhage);
venous drainage is via the hepatic portal vein which passes through the liver before returning to the heart
8) State that activity of the gut muscles mixes contents with the digestive secretions and also aids efficient absorption of
nutrients

activity of the gut muscles mixes contents with the digestive secretions and also aids efficient absorption of nutrients
9) Distinguish between muscle activity involved in “mixing” movements and that involved in “propulsive” (peristalsis) activity

Contractions smooth muscle both mix contents of lumen and also propel (peristalsis) contents towards anus

Gut has 2 layers smooth muscle - Circular and Longitudinal

Stomach has 3rd smooth muscle layer - Oblique -powerful contractions to grind food into small particles before entry into duodenum
(larger particles retained)

Mixing (segmentation activity) is governed by the myenteric plexus, occurring in areas of the intestine with large volumes of chyme; a
segmentation is initiated with the contraction of circular muscle which segments the intestine; then the muscle fibers within each
segment contract; then the original contraction relaxes, and new contractions form; this creates a “sloshing” effect, occurring most
rapidly in the duodenum (12/min); it mixes the food with digestive juices and also aids absorption by presenting “fresh” chyme to the
mucosa

Propulsive (peristalsis) – migrating motility complex) is also governed by the myenteric plexus, and is characterised by a series of
progressive contractions beginning in the lower part of the stomach and down to the terminal ileum within 2hrs; it is an organised
movement and so requires an intact nervous system. In peristalsis relaxation precedes region contraction
10) Describe how the structure of the mucosal lining of the gut leads to a large surface area for absorption

There are several features which lead to a large surface area for absorption these include:
(1) mucosal folds (increases area by 3 fold)
(2) villus structure (increases area by 20 fold)
(3) Brush border of luminal membrane of epithelial cells (increases area by 10 fold)

Overall there is a 600 fold increase of area for absorption compared with a smooth surface
11) State that the maximum rates of absorption of fat, protein and carbohydrate are about 10x greater than the normal daily rates
The gut has a large capacity to absorb nutrients.
Max rate of absorption can be 10x normal daily requirements – this leads to the problem of obesity
12) State that some 7 litres of fluid are secreted into the gut lumen during a day
13) Explain the importance of reabsorbtion of this secreted fluid

About 7.5l/day of water + ions are secreted into the gut – this is equivalent to 50% of the extracellular volume- together with some 1.5l
in the diet.

Therefore there is a need to reabsorb most of this – to leave a faecal volume of normally 100-150ml/day

It is important to reabsorb secreted fluid in order to recycle secretions
14) State that nearly all of the nutrients and most of the fluid secreted by the gut is reabsorbed in the small intestine

Total daily input (diet + secretion) is about 9.5 l; Fluid absorbed by osmosis following absorption dietary components & salts

Note that there is no absorption in stomach

Most of the absorption occurs in the small intestine - about 8 l (Large intestine - about 1.4 l)

Faecal output - about 100 ml
15) Describe how gut function is regulated by a combination of neural (both central and local), humoral and local paracrine
activity. Illustrate this with a suitable example

Control of GI functions involves both local and central effects.
Local – neural plexus reflexes and “hormones” (paracrine) such as histamine and 5HT.
Central - long reflexes involving CNS plus circulating “true” hormones produced both by the gut itself (secretin, gastrin, CCK)
and by other endocrine glands (aldosterone)

These mechanisms regulate secretion, absorption, motility and blood flow.