Small Intestine - Anatomy & Physiology

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Small Intestine

Introduction

The small intestine extends from the pylorus of the stomach to the caecum. The small intestine recieves chyme from stomach. Most chemical degradation of the chyme and absorption takes place in the small intestine. Fats are exclusively broken down in this part of the alimentary tract. Carbohydrates and proteins that are not degraded in the small intestine are available for microbial fermentation in the large intestine. The small intestine produces enzymes for digestion of protein, carbohydrate and fat and absorbs the products of their digestion. Enzymes are produced by glands in the intestinal wall and the pancreas. The gall bladder produces bile which emulsifies fats for digestion. Absorption is facilitated by ridges in the small intestine and by the presence of villi and microvilli.

The small intestine consists of three parts:

• Duodenum - Anatomy & Physiology
• Jejunum - Anatomy & Physiology
• Ileum - Anatomy & Physiology

Development

The small intestine develops from the digestive tube, which exists ventrally in the developing embryo. The digestive tube is endoderm, and therefore the small intestine and it's associated glands develop primarily from endoderm. Splanchnic mesoderm surrounds the digestive tube and gives rise to muscles of the tunica muscularis (for peristalsis), blood vessels in the submucosa and connective tissue of the serosa.

The intestines begin as a straight tube, suspended by the dorsal mesentry. They then loop ventrally. This slips out of the abdomen into the umbilical cord where growth continues. The cranial loop becomes the small intestine. It elongates and becomes extensively coiled. It then undergoes a 270° rotation about the cranial mesenteric artery so that the caecum ends up on the right.

PICTURE

Structure

• Attached along it's whole length to the dorsal abdominal wall by mesentry.
• The mesentry is relatively long for its most part, giving the small intestine a great deal of mobility.
• The basic structure of the intestinal wall is conserved throughout the whole length of the alimentary tract, but there is greatest diversity in the epithelial layer.
• Within the tunica muscularis are muscles present for peristalis and mixing of food.
  • There are two muscle layers; an inner circular and outer longitudinal layer.
  • Between the two muscle layers is the myenteric plexus.
  • Between the inner circular layer of muscle in the tunica muscularis and the submucosa is the submucosal plexus. (see regulation and control).

Function

• To mix content and transport chyme.
• To secrete emzymes for the digestion of chyme and absorb the products of digestion.

• Contraction of the two muscle layers facilitates mixing and transportation.
• There are two types of muscle contraction:
  • Segmental
    • Rhythmic contraction of circular muscle.
    • Creates ring like contractions
    • Divides content into many segments
    • Moves segments backwards and forth causing mixing with digestive juices
    • This type of contraction predominates during digestion
  • Peristaltic
    • Antagonistic contraction of longitudinal and circular muscle
    • Transports chyme through the small intestine but contractions can be weak to allow time for absorption
• When a peristaltic ‘’wave’’ of contraction reaches the end of the ileum, another starts in the duodenum. This is known as a migrating myoelectric complex.

Regulation & Control

• The functions of the small (and large) intestine are regulated by three mechanisms:
  • Endocrine hormones
  • Paracrine hormones
  • Neural transmitters
• Unlike the stomach, control is mainly local, with superimposed co-ordination through the extrinsic ANS.
• All signals affect the small intestine via sensory neurones.
  • Neurones interact with the two plexuses in the intestinal wall.
  • Neurones in the myenteric plexus produce changes in muscle contractility
  • Neurones in the submucosal plexus produce changes in secretion and blood flow to the small intestine
  • Plexuses are connected via interneurones.
  • Excitatory neurotransmitters of the parasympathetic nervous system include:
    • substance P
    • acetylcholine (via muscarinic M1 and M2 receptors)
    • serotonin (5-HT)
  • Inhibitory (sympathomimetic) neurotransmitters include:
    • vasoactive intestinal polypeptide (VIP)
    • nitric oxide (NO)
    • ATP
    • enkephalins
• The autonomic nervous system (ANS) control is superimposed over local control.
  • The ANS acts via the same two plexuses.
  • Parasympathetic innervation stimulates motility and secretion
  • Sympathetic innervation antagonises the parasympathetic nervous system and inhibits motility and secretion.
• Generation of action potentials is intrinsic to the smooth muscle in the intestine.
  • Interstitial cells of Cajal act as muscle pacemakers and generate a basic electrical rhythm (BER).
  • The BER is able to pass between cells due to the presence of gap junctions. The muscle therefore acts as a syncytium.
  • Some BER are sufficient to generate an action potential.
• Food in the intestine stimulates peristalsis to occur, and increases the likelihood of an action potential occuring.
• Enteroendcrine cells in the crypts of the small intestine produce substances that profoundly influence GIT motility.
  • Hormones produced by enteroendocrine cells in the small intestine include; secretin; gastric inhibitory peptide; and gastrin.

Lymphatics

• The products of fat digestion are transported in the lymph, and when fat has been absorbed, it turns the lymph a milky colour.
• Lymph travels to the mesenteric lymph nodes.
• Lymph drains into the chyle cistern.
  • This is a dilation of the thoracic duct at its origin.
  • It lies dorsally to the aorta and extends from the diaphragmatic crura to the renal arteries.
  • Flow of lymph is facilitated by movement of the diaphragm in ventilation of the lungs and by pulsation of the aorta.

Species Differences

• In the dog, mesenteric lymph nodes are found mainly around the root of the mesentry.
• In the pig, they form a long raised band in the middle of the mesentry.
  • They are routinely examined after slaughter as enlargement indicates enteric disease or tuberculosis.

Histology

Electron Micrograph Duodenum - Copywright RVC 2008

Caudal Duodenum (cat) - Copywright RVC 2008

• The mucosa is arranged into villi that provide a large surface area for absorption.
• Epithelium is simple columnar - ideal for absorption.
  • Epithelial cells are known as enterocytes.
  • A single layer of enterocytes overlies the lamina propria.
  • Enterocytes originate from progenitor cells that migrate from mucosal crypts. They differentiate as they migrate up the villus.
  • Enterocytes are absorptive and posses microvilli.
• Membrane bound enzymes and transport proteins are also within the epithelium.
• Each villus houses a generous blood supply of capillaries that transport amino acids, monosaccharides and other digestive products and lacteals that transport triacylglycerides. Lacteals drain into the lymphatic system.
• Crypts are present at the base of each villus in the mucosa. Cell types in mucosal crypts (from luminal to basal):
  • goblet at the tip of the crypt. Produce mucous by exocytosis.
  • entero-endocrine in the middle of the crypt. Produce many important substances for the regulation of GIT motility. See Regulation & Control.
  • paneth at the base of the crypt. Function unknown. Contain eosinophilic granules.
• Lamina muscularis is smooth muscle.
  • Contraction of smooth muscle shortens the villus. This helps to pump out absorbed products of digestion. Relaxation of smooth muscle lengthens the villus which increases surface area, facilitating absorption.
• Submucosa is loose connective tissue and it contains blood vessels and nerve fibres.
• Brunner's glands are present in the submucosa of the duodenum.
  • Secrete a protective mucous.
  • Produce an alkaline secretion which neutralises stomach acid.
  • Open into the crypts in the mucosa above.
• Tunica muscularis is smooth muscle.
  • It consists of two muscle layers (see regulation & control).
    • An inner circular layer
    • An outer longitudinal layer
• Serosa is loose connective tissue.

Species Differences

Length of Intestine

• Carnivore - relatively short intestine; in dogs it is approximately three times the body length.
• Horse - 25m in the carcass (in life the figure is much less).
• Sheep - up to 25 times body length.
• Ox - up to 50m.

Links

Pathology of the Small and Large Intestines