Brent Cornell

    
The human intestines function to absorb the products of digestion and have specialised structures to fulfil this function

• The small intestine absorbs usable food substances (i.e. nutrients – monosaccharides, amino acids, fatty acids, vitamins, etc.)
• The large intestine absorbs water and dissolved minerals (i.e. ions) from the indigestible food residues

Structure of the Small Intestine

The small intestine is composed of four main tissue layers, which are (from outside to centre):

• Serosa – a protective outer covering composed of a layer of cells reinforced by fibrous connective tissue
• Muscle layer – outer layer of longitudinal muscle (peristalsis) and inner layer of circular muscle (segmentation)
• Submucosa – composed of connective tissue separating the muscle layer from the innermost mucosa
• Mucosa – a highly folded inner layer which absorbs material through its surface epithelium from the intestinal lumen

Below is a cross-section of the ileum – the final section of the small intestine (click on image to view coloured layers):

    
The inner epithelial lining of the intestine is highly folded into finger-like projections called villi (singular: villus)

• Many villi will protrude into the intestinal lumen, greatly increasing the available surface area for material absorption

Features of Villi

Intestinal villi contain several key features which facilitate the absorption of digestive products (monomers, ions and vitamins):

• Microvilli – Ruffling of epithelial membrane further increases surface area
• Rich blood supply – Dense capillary network rapidly transports absorbed products
• Single layer epithelium – Minimises diffusion distance between lumen and blood
• Lacteals – Absorbs lipids from the intestine into the lymphatic system
• Intestinal glands – Exocrine pits (crypts of Lieberkuhn) release digestive juices
• Membrane proteins – Facilitates transport of digested materials into epithelial cells

Mnemonic:  MR SLIM

Features of Intestinal Villi

Structure of Villus Epithelium

The epithelial lining of villi contains several structural features which optimise its capacity to absorb digested materials:

Tight Junctions

• Occluding associations between the plasma membrane of two adjacent cells, creating an impermeable barrier
• They keep digestive fluids separated from tissues and maintain a concentration gradient by ensuring one-way movement 

Microvilli

• Microvilli borders significantly increase surface area of the plasma membrane (>100×), allowing for more absorption to occur
• The membrane will be embedded with immobilised digestive enzymes and channel proteins to assist in material uptake 

Mitochondria

• Epithelial cells of intestinal villi will possess large numbers of mitochondria to provide ATP for active transport mechanisms
• ATP may be required for primary active transport (against gradient), secondary active transport (co-transport) or pinocytosis

Pinocytotic Vesicles

• Pinocytosis (‘cell-drinking’) is the non-specific uptake of fluids and dissolved solutes (a quick way to translocate in bulk)
• These materials will be ingested via the breaking and reforming of the membrane and hence contained within a vesicle

Cross-Section of Villus Epithelium

                  Structure:          Pinocytotic Vesicles                 Tight Junction                  Microvilli                  Mitochondria                  All