13-14 - Digestive System

7/27/2019 13-14 - Digestive System

1/28

Lecture 13, 14Coelomic Cavities, Mouth and Digestive System

Kardong Chapter 13, Hildebrand Chapter 12

KK 13.1


2/28

Origin of the Coelom and Gut

Organs of the gut

develop asevaginations of the gut

into the mesenteries.

The coelomic space is

a split in the

hypomere.

KK 13.2, H&G 12.1


3/28

The coelom or peritoneal cavityis undivided in hagfish, some

sharks.

In most fishes and Amphibia

there is a separate pericardial

cavity just behind the gills. It is

separated from the peritoneal

cavity by the transverse septum.

In reptiles, which have a neck,the heart moves back under the

lungs.

KK 5.34, H&G 11.2


4/28

In mammals, the lungs are ina separate pleural cavity.

The new membrane

separating the pleural and

peritoneal cavities houses the

diaphram.

The lungs surround the heart

in the adult and the

membrane separating the

pleural and pericardial

cavities is the mediastinum.

KK 5.34, H&G 11.3


5/28

The mouth develops de novo in

deuterostomes. The invagination that

meets the gut and creates the mouth is

called the stomodeum.

The nasal epithelium and the

hypophyseal (Rathkes) pouch develop

on the head. But, are drawn in by the

deepening stomodeum (a).

In cyclostomes (b) both remain on top of

the head adjacent to the shallowstomodeum.

In most fishes (c) only the hypophyseal

pouch is drawn into the stomodeum.

In vertebrates with choanae or internalnares (a,d) the mouth is deeper, and both

the hypophyseal pouch and the nasal

epithelium are drawn into the

stomodeum. The nasal placode deepens

to emerge on the top of the head creating

the external nares.

Origin of the Mouth

KK 13.4


6/28

Mouths in Sagittal SectionKK 13.3, 13.37, H&G 12.2&12.3

Tetrapods changes

include salivary glands,

and, especially inmammals, a much larger

and more mobile tongue.

Fish have no neck, their

heart is in their pharyngeal

region, and the opening to

lungs (if present) is dorsal.


7/28

Specialization of Homodont Dentition

Teeth vary greatly among fish

and ectothermic tetrapods,

but generally all are the same

within a species. Many have

sharply pointed teeth to graspprey. Chewing is minimal.

KK 13.13, H&G 7.5


8/28

Heterodont

Dentition of

Mammals

Most mammals and synapsids

had differentiated teeth: incisors,

canines, premolars and molarsThese teeth are usually replaced

once. Premolars and molars are

very similar, the difference being

that molars are not replaced.

They can be collectively referredto as cheek teeth.

Carnassial teeth = molars

specialized for shearing.

KK 13.7, H&G 7.7, 30.17


9/28

Adaptations for Herbivory

Herbivores may have

fewer or no incisors and

canines, but high and

deeply folded cheek

teeth. Why?

KK 13.14, H&G 30.22


10/28

Adaptations for tooth wear

Tooth wear limits

the lifespan ofmany herbivores.

Rodents (e.g.,

beaver) have

opening-rooted

incisors that keepgrowing.

Elephants use their

6 immense cheek

teeth one at a time.

KK 13.9, H&G 30.20, 30.21


11/28

Gut Functions

Transportation (via peristalsis) and storage

Physical treatment: mixing and churning

Chemical treatment: breakdown ofmacromolecules into subunits by acids and enzymesprior to assimilation. May also be biologicaltreatment by microbes.

Absorption Production of feces, reclamation of H2O


12/28

Structure of the GutKK 13.22, H&G Fig. 12.4

The muscularis

externa and the

serosa are from the

splanchnic hypomere.

The mucosa andsubmucosa are

derived from

endoderm.

Note that the ventral

mesentery mostlydisappears.


13/28

Structure of the

small intestine

In the absorbtive area of

the gut, i.e., the small

intestine, the internalsurface of the gut is

intensely folded to increase

the surface area. There is a

rich blood supply. Note

also lymph vessels

(lacteals).

KK 13.25, H&G 12.4


14/28

Fish Guts KK 13.29, H&G 12.6

Fish (ectothermic predators) generally have

short and simple guts. The esophagus isshort and starts right behind the mouth.

The large intestine is not distinct.

The shark and some Osteichthyes have

increased the area for absorbtion via aspiral valve in the intestine.

Teleosts like the perch and trout have

pyloric ceca to increase absorbtive area

instead of a spiral valve. The smallintestine may loop back on itself.

Note that the liver and pancreas (where

present) are not shown.


15/28

Tetrapod Guts 1Tetrapods have a more distinct large intestine, and most have a urinary

bladder. They never have pyloric ceca or a spiral valve. Instead, the small

intestine is long and coiled rather than a single loop.


16/28

Tetrapod Guts 2

Birds and many reptiles lack a urinary bladder. They both have a gizzard.

Birds also have a crop.

KK 13.32,

H&G 12.7


17/28

Tetrapod Guts 3

Amphibia have a

short esophagus,

like fish.

Birds have a cropand a gizzard.

They also have

colic ceca that are

enlarged in

herbivorousspecies.

KK 12.27

Tetrapods increase absorbtive area by lengthening the small intestine.


18/28

Functional variation in mammal

gutsKK 13.28, H&G 12.8

Simple guts of

carnivores and

invertivores.

Complex

guts of

herbivores.


19/28

Foregut vs. Hindgut Fermentation

Foregut fermentation (artiodactyls, kangaroos, sloth,

hippopotamus, ostrich) is ahead of the small intestineso absorbtion is favoured, and allows for re-chewingorchewing the cud in some species. But it is slow.

Hindgut fermentation (horses, rodents, rabbits,rhinoceros) is below the small intestine involving thelarge intestine or the cecum, so absorbtion of

products is limited but it allows faster throughput.

Hindgut fermenters cannot regurgitate and re-chew,but many are coprophagous.


20/28

Vertebrate stomachs

are rather similar. In

birds (and some

diapsids) the pylorus

is particularly

muscular, and called

the gizzard. It often

contains stones.

In ruminants, e.g., the

cow, the foregut is

expanded as an

microbial fermentation

chamber, allowingthem to extract

nutrition from coarse

plant material.

The Stomach

KK13.31, H&G 12.5

G

G


21/28

The RumenKK 12.34, H&G 12.9

The rumen is actually a part of theesophagus upstream from the true

stomach.

The rumen allows ruminant

animals to extract nutrition from

poor quality food via microbialfermentation, and to minimize

their exposure to predators. They

can consume large amounts of

food quickly, move to cover, then

regurgitate and re-chew it at their

leisure.

Methane gas production from

ruminant animals is a significant

contribution to atmospheric

methane, a potent greenhouse gas.


22/28

Liver FunctionsThe liver is the largest organ in the body, and universal (and very

similar) in all vertebrates.

Digestive: - produces bile acids, which emulsify fats in the

digestive tract. Bile acids are stored in the gall bladder.

Metabolic: - Carbohydrate metabolism regulates blood glucose

by synthesizing and de-synthesizing glycogen.- Stores fat to various degrees (e.g., shark, cod).

- Protein metabolism De-aminates amino acids and

synthesizes urea with the waste N.

Also: - stores iron and some vitamins

- detoxifies toxins

- phagocytosis of old blood cells, foreign cells

- blood cell formation in fish and embryos


23/28

The Liver in Relation to the

Circulatory System

Interlobular veins

Capillaries

Central Veins

Livers Metabolic

Functions

Hepatic Vein

Heart

Absorption in Gut Capillaries Hepatic Portal

Vein

Mesenteric arteries

Hepatic Artery


24/28

Liver Lobule in Transverse Section

KK 13.39, H&G 12.11

Liver functions

occur as the blood

from the hepatic

portal vein passesthrough the liver

lobules to collect in

central veins and

ultimately the

hepatic veins.


25/28

Pancreas Functions

As an exocrine digestive gland (secretions to the outside, ie,the gut lumen) it produces proteolytic enzymes as zymogens or

pro-enzymes. These are in an inactive state until worked on byproteolytic enzymes already in the gut.

As an endocrine gland (secretions into the body, i.e., theblood) it secretes insulin (stimulates deposition of glycogen)and glucagon (stimulates release of glycogen)

While the liver is universal in vertebrates, the pancreas is not adiscrete organ in most fish. Instead, tissue with pancreaticfunction is scattered in the mesentery adjacent to the liver.


26/28

Development of the Liver

and Pancreas

The pancreas develops from

separate structures in the

dorsal and ventral

mesenteries that fuse duringdevelopment in many

groups.

KK 13.38, H&G 12.1


27/28

The Pancreas

In many vertebrates, thedorsal and ventral

pancreas are more or

less separate and two

ducts may persist, as in

the panda. The ductscarry digestive enzymes

to the gut.

KK 13.40


28/28

The PancreasIn other vertebrates, including humans, the

dorsal and ventral pancreas are intimatelyfused, and endocrine functions of the dorsal

pancreas are carried out in the pancreatic

islets.

KK 15.11, H&G 12.12

13-14 - Digestive System

Documents

Transcript of 13-14 - Digestive System