Lecture 18

Gut adaptations

Some gut adaptations: valves, crops, villi and gizzards

• The gut is an assembly line where food is processed. Specialized regions are given different names and separated by valves that regulate the progress of the food, e.g., pyloric valve between the stomach and small intestine.

• Most animals don’t eat continuously and must store food: both stomachs and crops accumulate food. Stomachs also serve the function of mixing. Crops are blindly ending sacs (diverticulae) early in the gut tube that serve expressly to store food.

Leech sacculate crop: discontinuous feeder

The leech (Annelida) has a disproportionately large crop in which it stores blood; this crop is comprised of many blindly ending lateral sacs that increase its capacity. Leeches feed on a food that is ready to digest so no teeth are required to ‘chew’ it. But the leech does have teeth to make the entry wound in its host.

Diagram from sharon-taxonomy on the web

Many animals have internal teeth

Sometimes a notable feature of an animal is what it doesn’t have. Birds don’t have teeth in their bills, yet many birds need to grind their food, e.g., seed-eaters. They have a region of the gut following behind the stomach where small ingested stones (‘gravel’) are used to triturate seeds advanced from storage within the crop. The gizzard is very muscular and lined with a material that resists mechanical damage.

Behind the crop in a locust lies a gizzard with chitinous teeth that grinds food material; gastric caecae ring the gut at the entry to the midgut just past the

gizzard.

Secretion and absorption: mammalian intestine

• Like gas exchange systems and particulate feeder filtering systems, digestive regions are marked by devices that increase surface area. And also in animals with closed circulatory systems [as too in the case of gas exchange systems where the blood carries oxygen and carbon dioxide] the region is heavily vascularized. That is, many fine branching vessels form a plexus (network) for picking up the products of digestion and conveying them away to the various body tissues.

• There are morphological devices to slow the travel of the food along the tube: folds, spirals (shark), simple lengthening of the tube, i.e., making the ‘chyme’ take longer to travel. And the tube is throughout invested with muscle, everywhere along its length that churns and kneads, pushes (peristalsis) and mixes.

• Mucous membrane: this is the innermost layer of epithelium: it functions to secrete enzymes and absorb digestive products and it is thrown up into villi (sing. villus). These are tiny finger-like processes like the pile of rug. Villi line the intestine and greatly increase the surface area available for secretion and absorption.

• Density of villi in humans 10-40 per square mm. In some animals in some parts of the intestine (duodenum) the villi may be more leaf-shaped than finger-like. The villi have intrinsic muscles and are independently mobile.

• Valvulae conniventes: the gut wall in the small intestine is thrown up into gross folds upon which the villi sit. These folds help to roll the chyme.