Animal Physiology Nutrition, Feeding, & Digestion

Is the lumen considered inside or out of the body in terms of physiology?

outside the body since it is a closed space/tube inside the gut

Order of chemical compounds that compose the body

proteins>lipids>minerals>nucleic acids>carbohydrates

these make up 40% of the body

Water components of the body

makes up 60% of the body

intracellular fluid>interstitial fluid>blood plasma

Why do carbohydrates make up such a small portion of the body?

they are already used up for energy purposes

Non-essential vs. Essential Nutrients

non-essential are the things you can synthesize from other things while essential vitamins are the ones you must take up from your diet

Vitamins and minerals are considered…

cofactors (any non-protein substance required for a protein to be catalytically active)

Serengeti Migration

-wildebeest travel from the north to the south in order to have offspring in the south where the volcanic soil is rich in calcium compared to the soil in the north.

-calcium increases milk and bone production which is important for offspring

-they return back to the north during dry season

this is an example of how nutritional needs drive behavior

Spider example of nutritional needs and behavior

if the spider is being fed a lipid-rich diet, it will choose to eat protein-rich flies and vice versa

Filter feeders

have filter combs that they drive through the water to catch tiny things

Fluid feeders

hang out in the gut and absorb nutrients from the fluid in the gut

-ex. tapeworms

Detritivores

go around and eat sand, pull off organic materials from the sand in their gut

this cleans the environment for us

ex. sea cucumbers

Carnivores vs. Herbivores vs. Omnivores

carnivores: meat eaters

herbivores: plant eaters (can be non-specific or specific; pandas are specific)

omnivores: eat both

Symbiotic Autotrophs

microorganisms that do photosynthesis to feed the organisms they live on

-ex. zooxanthellae on corals

Photoautotroph vs. Chemoautotroph

photoautotroph: uses sunlight to make their food (zooxanthellae)

chemoautotroph: uses inorganic chemicals to create energy and organic biomass (animals that live where there is no light use these, riftia worms in hydrothermal vents)

Foregut Fermenter advantages and disadvantages

Advantage: can manage much lower qualities of food (can eat lots of cellulose)

Disadvantage: bacteria get first shot at any good nutrients

Examples of Foregut Fermenters

cows, deer, llamas, sheep, giraffes, etc.

Cows are ruminants, what does that entail?

-rumen and reticulum: non-acidic, where cows are always chewing and swishing things around which allows for fermentation by symbiotic microbes to take place

• rumen is also where urea diffuses into and bacteria can convert it back to ammonia to synthesize amino acids that then go down and feed the cow (example of recycling)

-omasum: filter that makes sure small, good things get passed on

-abomasum: acidic part that everything passes through before going to the small intestine

Hindgut Fermenter advantages and disadvantages

advantage: have first dibs on all nutrients

disadvantage: food must be of much better quality than the food of foregut fermenters

Coprophagy

eating feces

rabbits do this regularly where they crap big and soft cecotropes that are then eaten to gain extra nutrients that are left behind by the hindgut microbes after first digestion

How do horses establish hindgut microbes?

eating mother’s poop

What is the significance of the appendix in terms of hindgut fermenters?

it stores bacteria to sort of protect them; when antibiotics wipe out gut fermenting bacteria, they can then be repopulated with the bacteria that is stored in the appendix

Midgut Fermenters

bacteria live in the small intestines

examples include herbivorous fish like tilapia and catfish

What is the main reason that animals need to ferment in their digestive systems?

there are things that we cannot break down (i.e. cellulose) and microbes help to synthesize things like vitamins and amino acids

Peristalsis

directional movement where food enters the tube and the circular muscles continue to squeeze it down and push it through the length of the tube (esophagus)

What is the major function of the midgut?

breaking stuff down (digestion) and absorption (get it into body)

Segmentation

muscle contraction in the intestines that moves food back and forth

chance of something being absorbed relies on this because mixing up food molecules increases the chance that they hit a particular receptor/transporter on the wall of the gut for absorption

What is the advantage of having a tube as opposed to having a sac/dead end gut?

With a tube, fresh stuff is not being mixed with recently digested stuff, and multiple functions can be carried out at the same time so it is more efficient

with a sac, waste and new food are mixed together, creating inefficient direction flow

Four basic processes of the digestive tract

-Motility: peristalsis and segmentation

-Secretion: secreting things from inside the body and putting them “outside the body” into the lumen (i.e. stomach acid, proteases, bile acids, water, etc.)

-Digestion: mechanical (breaking down large things into smaller things) and chemical (taking full protein and chopping it up)

-Absorption: point at which something happens inside the body (things are absorbed into it); water is also absorbed in addition to nutrients

What is the function of the headgut? What structures are included in it?

-to capture and engulf food while preparing it for digestion

-includes the lips, buccal cavity, tongue, pharynx, etc.

Mucosa

Inner most layer of the digestive tract; composed of…

-mucosa membrane: epithelial layer on basal lamina, tight junctions between epithelial cells to prevent things from the lumen from escaping easily

-muscularis mucosa: thin layer of muscle that is not strong enough for peristalsis or segmentation but does shake the epithelial layer enough for mixing, prevents things from getting stuck in the finger-like folds of the gut

Submucosa

layer of connective tissue in the digestive tract separating the mucosa and muscularis externa

Muscularis externa

achieves peristalsis and segmentation, 2 parts:

-inner circular muscle: sphincter-like muscle that closes the lumen

-outer longitudinal muscle: runs along the length of the tube

Serosa

outer-most layer of the digestive tract

the mucus membrane that holds everything together

Intrinsic Nerve Plexuses of the Digestive Tract

-submucosal plexus: controls muscularis mucosa; located in the submucosa

-myenteric plexus: controls muscular layers of the gut; located between the longitudinal and circular muscles of the muscularis external

Epithelia of the gut

-epithelial cells with a basal surface (sits toward body) that sits on the basement membrane and an apical surface (faces the lumen)

-microvilli on apical side; increases surface area which makes room for more transporters for more absorption

-all epithelial cells are joined together by tight junctions

Physiological mechanisms working together in the gut examples

-Primary Active Transport: Na+/K+ ATPase maintains the sodium gradient (keeps sodium inside the cell low by pumping it into space on opposite side of lumen

-Secondary Active Transport: SGLT; sodium is high in the lumen and low in the cell (thanks to sodium potassium pump) so sodium moves along the gradient releasing energy that can then be used by the SGLT transporter to transport glucose into the cell against its gradient

-Passive/Facilitated Diffusion: GLUT; high amounts of glucose in the cell diffuses to outside the cell where it is low (opposite side of lumen and into bloodstream)

Membrane vs. Bilayer

Membrane refers to the bilayer plus the proteins embedded in it

Bilayer is the phospholipid bilayer in itself

Which parts of the esophagus are voluntary/involuntary?

the top third of the esophagus is voluntary while the rest in involuntary so it can carry out peristalsis

Gastroesophageal Sphincter

protects the esophagus from acidic stomach acid (chyme)

heartburn occurs when this sphincter fails

Stomach/Foregut

considered a storage organ

very muscular, allowing for churning of contents and chyme; only a little bit of this mixture comes out at a time into the duodenum to protect it against the acid by giving it time to neutralize the small amount it was previously given

remotely zero absorption (things like alcohol and NSAIDs do though)

Pepsin

-protease: breaks down proteins into smaller peptide fragments

-secreted by chief cells as inactive pepsinogen (exocrine secretion)

-pepsinogen (zymogen) becomes activated to pepsin following exposure to HCl in the stomach

HCl

-exocrine secretion from parietal cells

-protons are byproduct from water that are then transported outside the cell and into the gut along with chloride ions to form HCl

-contributes to acidic environment in the stomach needed for digestion

Mucus

-Mucus is an exocrine secretion from mucus cells

-it is thick and basic (alkaline) which acts as a protection mechanism against self-digestion

What measures are in place to prevent against self-digestion in the stomach?

-pepsin is released in its zymogen form

-mucus lining is alkaline and thick

-gastric pits are lined with epithelial cells (they are highly regenerative and have tight junctions between them)

Bicarbonate

-made from CO2 and water via carbonic anhydrase enzyme and is secreted by duct cells in the pancreas

-present to neutralize acidic chyme from the stomach

-released into the duodenum when needed through the Spincter of Oddi

Food is primarily…

-carbohydrates: broken down to monosaccharides and absorbed into the epithelial cells of the gut and then into the blood

-proteins: broken down into amino acids and brought into cells and then the bloodstream

-lipids: fats, broken up into fatty acids and then brought into cells and taken up through lymphatic vessel

Chitinase

-breaks down chitin but only some animals have it

-chitin is found in invertebrate shells (shrimps, lobsters, crawfish, etc.)

-insectivores produce chitinase and are able to eat shells whole

Amylase

-comes primarily from the pancreas (some from salivary glands but this is negligible)

-breaks down starches to disaccharides which can then be digested by other enzymes (maltase, sucrase, lactase) into monosaccharides

-final breakdown of disaccharide happens anchored at the membrane of epithelial cells; important to absorb plenty of sugars so transporter can be placed right beside where this is bound so that the monosaccharide can be taken up and absorbed immediately

Enterokinase

bound to duodenum epithelial cells

activates trypsin when inactive trypsinogen comes to the wall of the duodenum (trypsin then activates the others)

**kinase name but does not add phosphates

Proteases in the midgut

-Trypsin and Chymotrypsin: cleave peptide somewhere in the middle

-Carboxypeptidase: cleaves peptide from C-terminal end

-All are secreted by the pancreas in their zymogen forms to prevent self-digestion (makes sure it does not digest your proteins instead of the proteins in your food)

Describe glucose absorption mechanism in the midgut

-secondary active transporter on lumen side using energy produced from sodium going down its gradient to transport glucose against its gradient and into the epithelial cell (SGLT1)

-Primary Active Transporter Na+/K+ ATPase pump on other side facing the bloodstream maintaining sodium gradient in the cell (keeping sodium low)

-Facilitated diffusion transporter on basal membrane facing bloodstream pushing glucose down its gradient and into the bloodstream (GLUT2)

Lipid Digestion

-lipase is excreted through the Sphincter of Oddi

-bile salts (amphipathic and made from cholesterol) keep fats in small enough balls and prevents them from re-coagulating so that lipase can more effectively break them down (maintains high surface area)

-bile salts are reabsorbed at the end of the small intestine, absorbed into the blood where it then travels back to the liver via the hepatic portal vein (allows for them to be recycled and reused in diet absorption again)

Fatty Acid Micelles

-when fats are broken down into fatty acids, they form micelles in the lumen that eventually dissociate

-once they dissociate, they can diffuse through the membrane where they are then packaged into chylomicrons

-these chylomicrons are then excreted via exocytosis

-fats are then absorbed into lymphatic vessels that are white in color

-lymphatic system dumps into the subclavian vein near the vena cava, so then these fats would be dumped into the bloodstream

Circulatory system of the gut

-blood comes from the heart to midgut capillary

-vein brings it to the liver where there is another capillary bed it goes through before going back to the heart (two capillary beds=portal system)

Hepatic Portal Vein

picks up all the things you’re absorbing (except fats) and sends them to the liver first

-liver reabsorbs the bile salts and detoxifies any bad stuff that may be in what you absorbed (i.e. alcohol)

Bilirubin excretion

hemoglobin from fragile blood cells is converted to bilirubin which is then put into bile; produced from breakdown of RBC

-bilirubin is yellow so when the liver isn’t functioning properly that is when jaundice occurs since bilirubin is hanging out throughout the body

Gluconeogenesis

Metabolic process that occurs in the liver to produce glucose from its own breakdown products or from the breakdown products of lipids (fats) or proteins; creates glucose from non-carbohydrate sources

Hindgut fermentation happens regardless of…

whether you are a foregut or hindgut fermenter

Gastrin

-endocrine secretion that comes from G cells

-G cells secrete gastrin when you eat which then activates pepsinogen secreting cells, acid-secreting cells, and muscle motility for digestion

Secretin

secretion of secretin is activated by chyme from the stomach being in the duodenum

activates bicarbonate secretion from the pancreas to neutralize

**negative feedback (set point is neutral environment)

CCK

-cholecystokinin

-stimulates the release of proteases (from pancreas) and contraction of the gallbladder (full of bile); induces relaxation of the Sphincter of Oddi so everything can get out into the duodenum

-gallbladder squeezes bile that it had been storing out into the duodenum

-can inhibit gastric emptying/food intake

Gastric inhibitory peptide

comes from the duodenum and inhibits the mechanisms sending stuff down from the stomach (inhibits gastric emptying)

if duodenum is full, then it prevents more stuff from coming down on top of the stuff it’s already working on

endocrine

Effectors in the digestive system…

are smooth muscles and glands that are controlled by local nerve plexuses that respond to intrinsic signals or gastrointestinal hormones

Vomiting

is NOT reverse peristalsis

relaxing of sphincters and use stomach muscles (external skeletal muscles) to get bad stuff up and out

Gastroileal Reflex

when you have food in your stomach and it starts activity in the midgut

Intestino-intestinal Reflex

when you have distention in the back end of the midgut, it slows activity in the front end of the midgut so you have more time to process

Defecation Reflex

two sphincters at the end of the tube (one involuntary and one voluntary); squeeze with abdominal muscles to get crap out

Ghrelin

endocrine control

hunger hormone: when stomach is empty it releases ghrelin and it goes into the blood and then to the brain (hypothalamus), which then increases appetite

increases body weight

NPY/AgRP neuron in brain

Leptin

endocrine control

when you are full, fat cells release leptin into the blood and then to the brain to decrease appetite and indicate fullness/satiety

decreases body weight

NPY/AgRP neuron in brain

What do taste and smell protect you from?

from eating things that will make you sick

-people who cannot do this have higher levels of food poisoning

Immunosurveillance

Peyer’s patches in ileum that detect when something is wrong

-Celiac’s disease occurs when this doesn’t work because it interprets things that are part of you as foreign

Communication as a Non-nutritional function of digestive system

Atlantic puffin has back barbs in its beak that allow him to line up fish caught in his mouth so he can parade them around for mating purposes

Crop Milk

-doves and pigeons do not have mammary glands

-they eat seeds and grains that they store in a crop (below their beak)

-the epithelium that lines the crop divides and adds fat to what is churning around in their crop

-this is then regurgitated and fed to offspring (white in color due to fat content)

-done in response to prolactin which is the same hormone that controls breastfeeding in mammals