2026-04-14 20:39

Status: #new

Tags: #digestivesystem #animals #biology

The Digestive System

Digestion

4 Steps of digestion

Ingestion

• For simple organisms, they have gastrovascular cavities, where nutrients and waste end up using the same opening
• In more complex organisms, they have a alimentary cavity, where nutrients enters one end and, and waste exits the other.

Digestion

Pre-Intestine

• In humans, we have a mouth, consisting of a physical part (teeth/tongue) and a chemical part (saliva) to break down food.
• The tongue and teeth break food down into smaller pieces
• Saliva aids in digestion and helps moisturize (acts a lubricant) food so it goes down faster
  • It also contains amylose, to break down food starches.
  • As it dissolves food particles, it also stimulates taste buds.
    • 5 tastes that are evenly distributed in the tongue
      • Sweet, salty, bitter, sour, umami (savour)
• The tongue is covered in papillae, which are covered in taste buds.
• Uvula - When swallowing, it moves to block the nasal cavity.
• Epiglottis - A flap of tissue that covers the pathway to the lungs
• Esophagus -Muscles that push food down in a wave pattern known as peristalsis.
  • Food gets pushed down into the stomach
  • 3 Layers of muscle squeeze the contents of the stomach
• The stomach secretes fluid to help in digestion
  • HCL, KCL, NaCL and other digestive enzymes (pepsin) are dissolved in water
• The stomach creates a thick mucus lining that creates a protective layer against stomach acid
• Acid Reflux
  • Capsaicin (from chilli peppers) dissolves in oil rather than in water, so both sit on top of the water (made worse by fatty or spicy foods)
• Heartburn
  • Caused by acid moving up the body, which can be neutralized with a base.
• When an organism lacks a mouth it may have something called a gizzard (in addition to the stomach)
  • ie. Birds consume tiny rocks that are stored in this gizzard to help crush food.
• Chyme - A highly mixed slushy substance of grinded down food
• Cud - Vomit that certain organisms such as birds chew to break down more nutrients

The small intestine

• Chyme enters the small intestine, where digestion can be completed

Duodenum

• The pancreas and gall bladder secrete additional enzymes into the duodenum to complete digestion.
  • Carbohydrates (amylase), proteases, lipases (mainly from gall bladder)
  • Without a pancreas and gallbladder
    • There wouldn't be energy needed for cellular respiration
• Pancreas - Responsible for making insulin (regulates certain nutrients)

Jejunum

• Mesentery - Thin tissues holding the small intestines in place
  • Ensures that the small intestines don't tangle
  • Holds all parts of the small intestine

Absorption

• Occurs in the small intestine
  • The inside of every small intestine is folded, for more surface area
• Each fold is covered by finger-like projections (Villi)
• Villi:
  • Blood Vessels (Absorb water-soluble nutrients to transport to the rest of the body)
  • Lacteal (A network of blood vessels that absorb fat-soluble molecules)
  • Pink Area (Chyme travels through this)
  • Microvilli - Finger-like projection of cells on the surface of the villii
    • Is bumpy, with each bump having its own fingers
    • Each cell in the microvilli has their own hair
• Ileum
  • Where digestion stops, and where absorption takes place
  • This is where digestion stops, as no more enzymes are created

Elimination

• Any nutrient that does not get sucked into the blood steam has to be eliminated.
• This starts in the large intestine (also known as the colon)

Large Intestine

• Caecum, Ascending/Transverse/Descending Colon, Rectum, Anus
• Components not absorbed pass through the ileocecal valve.
• Appendix: A small tail as the end of the Secum, projection at the bottom of the Caecum.
• Caecum - Pouch that marks the beginning of the colon
• 90% of water passing through the colon is reabsorbed.
• Indigestible plant matter is broken down by bacteria in the colon.
• Gases released by bacteria contributes to the odour of feces.
• Antibiotics can kill bacteria regardless of its benefit, so eating healthy is needed in order to replenish the gut microbiome in the colon.
• Feces is stored in the rectum until elimination through anal sphincter (anus)
• Coprophagy (Rabbits eat their own feces)

Digestive System Disorders

• Colonoscopy - Used to diagnose IBD such as Crohn's or Ulcerative Colitis
• Endoscopy - Examine upper gastrointestinal tract
• Peptic Ulcers - Observed in the stomach
• Canker Sores - Painful sores in the mouth that heal quickly
• Capsule Endoscope - Ingested Camera that goes into the digestive system
• Ulcers - Heliobacter pylori are responsible for peptic disorders
• Jaundice - Yellow Pigmentation on the skin
  • Hepatitis (liver disease induced jaundice)
  • Babies can get jaundice as their blood type attacks other blood during labour
  • It is fixed by sticking babies in a box, shining on them, and letting babies excrete it out of their system.
• Dark Urine - Urine without a colour range
  • Normal Urine is clear and transparent
• Edema - Water collecting in the limbs, appears swollen

Liver - Healthy to eat (lots of iron and nutrition)

• Responsible for detoxification
• Enzymes break down chemicals typically not found in the system
• A increase in enzyme concentrations makes it work harder
• Blood Test - Gives a clear indication that a liver is not functioning, which is useful given early liver damage exhibits very few symptoms

References

3.0 Animal Molecules
3.1 Animal feeding