Avian
Birds have particular difficulties getting the nutrition they need from food. Since they lack
teeth, Figure 143's depiction of their digestive system implies that it is capable of processing
unmasticated food. Birds' diverse beak types have evolved over time to reflect the wide range of foods
they consume, from seeds and insects to fruits and nuts. Most birds can fly, so they have high
metabolic rates to process food quickly and maintain low body weights. The stomach of birds has
two chambers: the
proventriculus
, where gastric juices are produced to digest the food before it
enters the stomach, and the
gizzard
, where the food is stored, soaked, and mechanically ground. Food
pellets made of the undigested material are occasionally regurgitated. The intestine is where most
chemical digestion and absorption takes place, and the cloaca is where waste is expelled.
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Figure 143.
The crop, a pouch found in the avian esophagus, is used to store food. Food travels from the
crop to the proventriculus, the first of the two stomachs, where it is broken down by digestive juices. The
food enters the gizzard, also known as the second stomach, from the proventriculus, where it is ground.
Some birds ingest grit or stones that are kept in their gizzards to help with the grinding process. In birds, the
openings used to expel feces and urine are not separate. Instead, waste products from digestion are mixed
with uric acid secreted from the kidneys into the large intestine. Through a hole called the cloaca, this waste
is expelled.
Ruminants
Ruminants
are mainly herbivores like cows, sheep, and goats, whose entire diet consists of
eating large amounts of
roughage
or fiber. Due to their highly developed digestive systems, they can
easily digest large amounts of cellulose. The absence of upper incisor teeth in ruminants' mouths is
an intriguing trait. To tear and chew their food, they use their lower teeth, tongue, and lips. The food
moves from the mouth through the esophagus and into the stomach.
Ruminants have multi-chambered organs in their stomachs to aid in the digestion of the
substantial amount of plant material, as shown in Figure 144. The rumen, reticulum, omasum, and
abomasum are the names of the stomach's four distinct compartments. Numerous microbes that
ferment ingested food and break down cellulose can be found in these chambers. The equivalent of
the monogastric stomach chamber, the abomasum is the "true" stomach and is where gastric juices
are secreted. The four-compartment gastric chamber gives ruminants the extra room and microbial
support they need to digest plant matter. Large amounts of gas are produced during the fermentation
process and must be expelled from the stomach chamber. Similar to other animals, the small intestine
is crucial for nutrient absorption, and the large intestine aids in waste removal.
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