1. Chapter 13: the digestive system
Ms. Luaces – honors anatomy & physiology

2. Read & answer question at the end
Csi case #13 – pg. 488
Read & answer question at the end

3. Overview
The digestive system allows the body to break down complex molecules into simple ones needed for body energy, tissue and cell development
Made up of 2 components:
The digestive tract (alimentary canal): Mouth, esophagus, stomach, small intestine, large intestine and rectum
Accessory digestive organs (produce secretions to help with digestion and food absorption): salivary glands, pancreas, liver, and gallbladder

4. overview
The digestive system is one of the first organs to develop from the endoderm during week 4 of development – gastrulation
Accessory organs don’t begin to develop until week 5 (liver and pancreas) up until week 40 (liver). The liver only produces blood cells for the fetus at this time
At birth, the digestive system is fully formed and releases meconium (waste product – green/black, sticky substances made of bile, mucus, and epithelial cells)

5. Mouth & pharynx
The mouth is involved in both chemical (saliva) and mechanical (breakdown) digestion
The anterior portion is made up of the lips (labia) which hold sensory receptors that detect temperature and texture of food
Inside you find the oval (buccal) cavity, housing the teeth and where food is moistened

6. Mouth & pharynx The palate forms the roof of the buccal cavity
Hard palate (roof – maxillary and palatine bones)
Soft palate (posterior back portion – contains uvula)
At the base of the buccal cavity, we have the Oropharynx
Contains the epiglottis – separates the respiratory system from the pharynx
Tongue (lingual membrane) – contains taste buds, 4 intrinsic muscles, and 3 extrinsic muscles
Intrinsic muscles allow the tongue to take a variety of shapes, assist with speech and swallowing while extrinsic muscles form most of the muscle mass and allows the tongue to retract, raise, and depress the tongue

7. Mouth & pharynx
Lingual tonsils are found at the base of the tongue and are lymphatic tissues to fight throat infections
Salivary glands release saliva and moisten food to start chemical digestion
3 major salivary glands
Parotid: in cheeks; largest; watery section
Sublingual: underneath the tongue; mucous section
Submandibular: under the mandible; mixture of water and mucous

8. Mouth & pharynx
Mechanical breakdown of food involves the 2 jaw bones, hard palate, and the teeth
Permanent teeth consist of 16 teeth in each jaw – 32 total
4 central teeth in each jaw bone – incisors (cut up food)
4 canine (cuspid) teeth – hold and tear food
8 bicuspids (premolars) – break down food into fine particles
6 molars – grind food
Extra 2 molars (wisdom teeth) – farthest back ; may/may not develop between years old depending on space left

9. Esophagus & Stomach The esophagus: Located behind the epiglottis
Muscular tube that carries food & liquids to the stomach
Has an upper sphincter (closes the larynx during swallowing) and a lower sphincter – cardiac sphincter (closes entrance to stomach)
Has 4 distinct tissue layers: mucosa, submucosa, muscularis layer, and serosa/adventitia

10. Esophagus & Stomach Mucosa: layer of epithelium that secretes mucus
Innermost layer
Connected to lamina propria and has a boundary before submucosa made of smooth muscle and connective tissue called muscularis mucosae layer – thicker in the esophages to assist with passage of food
Submucosa: layer of connective tissue that supports the shape of the esophagus
Muscularis Layer: layer of smooth muscles to contract and move food / liquid through the digestive tract
Serosa / Adventitia: thick covering of connective tissue filled with nerves to send signals to the muscles

11. Esophagus & Stomach
Sphincters prevent backflow of material into previous organs
Cardiac sphincter (at base of esophagus) prevents acid reflux (heartburn)
Pyloric sphincter closes off the bottom of the stomach

12. Esophagus & Stomach The stomach:
Large, sac-like organ with 4 layers that can store 3 pints of food for digestion
Stomach’s mucosa secretes acid and protein digestion enzymes and is designed to protect the stomach from digesting itself
Divided into upper (cardiac), middle (fundic) and lower (pyloric) regions

13. Esophagus & Stomach Cardiac region: Fundic region: Pyloric region:
Secretes mucus
Fundic region:
Contains parietal cells (produce hydrochloric acid), chief cells (produce digestive enzymes), mucous neck cells (produces mucus when stimulated) and gastric stem cells (replaces gland cells)
Pyloric region:
Also secretes mucus

14. Esophagus & Stomach
The stomach is composed of 3 thick layers of smooth muscle:
Oblique: directly underneath the submucosa
Stomach squeezing
Circular: thick bands of muscle that mix food
Longitudinal: mix and move digested food out of the stomach

15. Small Intestine Long narrow tube that tightly loops back and forth
Composed of the:
Duodenum: receives partially digested food; where most digestion takes place
Jejunum: where nutrients are absorbed
Ileum: remaining nutrients are absorbed
All areas contain fingerlike projections called villi and microvilli (increase surface area for absorption) and lacteals (collections of lymphatic vessels)

16. Small Intestine
Most common cells found in the small intestine are the enterocytes (absorptive cells) – found mostly in the jejunum
Enteroendocrine cells produce hormones that regulate digestion
Paneth cells found in mucosa areas called crypts produce antibacterial enzymes that kill harmful bacteria but promote beneficial microorganism growth
Peyer’s patches (collections of lymphatic tissue) in the duodenum help fight infection and are involved in immune response to food allergies

17. Small Intestine
Composed of an inner circular layer (mixing food) and outer longitudinal layer (transport of food) of muscle
Contains many extensive networks of nerves to send signals
Attached to mesenteries to hold it in place and are also places for blood vessels to innervate
Separated from the large intestine by the ileoceccal valve

18. Large intestine
Also known as the colon – much larger in diameter and shorter than small intestine
Contains:
Cecum: small, swollen region that contains the appendix (common site of infection - appendicitis)
Ascending colon: travels vertically up on right side to hepatic flexure (below liver)
Transverse colon: runs parallel and connects at the splenic flexure (below spleen)
Descending colon: travels vertically down on left side
Sigmoid colon: s-shape end that spills into the anal canal

19. Large intestine
Main functions: absorb electrolytes, vitamins and water ; remove undigested materials
Contains no villi but many absorptive cells that use active transport to move electrolytes into its many capillaries
Contains many bacteria and a type of yeast to help break down waste – essential!

20. Large intestine
Ends in the rectum, a muscular storage area for undigested wastes
Anal canal is the transitional region to the anus, which is rich in apocrine and eccrine sweat glands
The anal sphincter has internal and external bands of muscle

21. Csi – case break
Knowing the major parts of the alimentary canal (digestive tract) now, think about what area of this is malfunctioning in our patient. Turn to Pg. 503 and read the CSI Case Break, and respond in your notes

22. Glandular structures: pancreas
Lies posterior to the stomach and divided into glandular lobules surrounded by connective tissue that is each connected to blood vessels, nerves and ducts
Has both endocrine and exocrine functions
Endocrine: islets of Langerhans, which secrete glucagon – raises blood glucose (alpha cells) and insulin – lowers blood glucose (beta cells)
Exocrine: saclike cluster of serous glands called acini – have granules with active and inactive digestive enzymes

23. Glandular structures: pancreas
Exocrine (cont):
Inactive enzymes: zymogens, converted into active enzymes once they enter the digestive tract
Ductal system of the pancreas originates in the acini that feeds into large duct called pancreatic duct  common bile duct  duodenum
The yellow-green bile contains acids, cholesterol, glyceride fats, and salts to help in fat digestion
Cholesterol is secreted from the liver through bile (also contains bilirubin, pigments, and broken down RBC’s)

24. Glandular structures: liver
Carries out the most complex functions using 4 lobes: left and right (make up most of the mass) and quadrate and caudate lobe.
Left & right lobe separated by the falicform ligament

25. Glandular structures: liver
Two blood vessels enter the liver at the hilum
Hepatic artery: provides blood for the liver cells
Hepatic vein: carries wastes from the liver lobes to the inferior vena cava
Haptic portal vein transfer food from the small intestine to the liver

26. Glandular structures: liver
Composed of hepatocytes: unusual cells with 2 or more nuclei that live about 5 months and are slow at carrying out mitosis
Capable of regeneration (small pieces of liver have regrown lobes)
Arteries going to the hepatocytes empty into sinusoids (exchange atmospheric gases, nutrients and wastes) and contain Kupffer cells that remove microorganisms
Please refer to Pg. 507 for some of the many jobs the liver does

27. Glandular structures: gallbladder
Located underneath the right lobe of the liver
Receives bile through the bile ductwork and releases it through the cystic duct, where it joins the common hepatic duct to form the common bile duct into the small intestine
Main job: store and concentrate bile
If bile has too much bilirubin, cholesterol, or bile salts, bile flow is blocked by the formation of gallstones (very painful!)

28. The Digestive process
Pre-gastric factors (hunger and thirst) are released from the hypothalamus to signal the beginning of digestion
Attitudes about eating and one’s upbringing help determine a person’s food intake
Satiety center (also in the hypothalamus) will later signal to stop eating, depending on blood sugar levels, fat levels, appearance and taste of food
Abnormalities in this hypothalamus area can trigger certain eating disorders

29. The Digestive process
Digestion begins with ingestion (taking in food through the mouth)
If taken by injection into muscles of veins, known as parenteral nutrition (IV fluids or food)
Mastication (chewing) is the first step in mechanical digestion, while chemical digestion also begins as saliva and salivary amylase enzyme begin to breakdown starch into glucose
Foods and liquids travel down the esophagus by peristalsis (smooth- muscle contractions) and enter the stomach when the cardiac sphincter relaxes
Reverse peristalsis (vomiting) can occur if bitter receptors triggered

30. The Digestive process
Pregastric factors and parasympathetic nervous system release the following stomach secretions before and when food enters the stomach:
Protease zymogens (become proteases)
Hydrochloric acid
Proteases (digest protein – require the acidic environment of pH 1-3 in order the work)
Bicarbonate and mucus (to protect stomach from digesting itself)

31. The Digestive process
Hormones released by the stomach and small intestine further stimulate the digestive system:
Cholecystokinin (CCK): stimulates pancreas to produce pancreatic juice and gallbladder to empty
Gastrin: produces hydrochloric acid in the stomach (assist with protein digestion)
Secretin: stimulates pancreas to secrete digestive juices including bicarbonate, stomach to produce protease, and liver to produce bile

32. The Digestive process
Chyme enters the duodenum after the pyloric sphincter relaxes, and the duodenum mixes the food with bile (breaks down fat) and pancreatic secretions (breaks down carbs, lipids, and proteins).
Enterokinases activate zymogens, which aid the absorption of nutrients into the blood and lacteals via the villi in the jejunum
Proteins and carbs go to the liver through the hepatic portal system for further digestion into ammonia

33. The Digestive process
Any substance not absorbed passes the ileocecal valve enters the large intestine, where bacteria digest and absorb much of the material (this produces flatulence)
Peristalsis continues to transport undigested material, while fluids and electrolytes are reabsorbed in the ascending and transverse colon
The final waste is compacted into feces (made of bacteria, carbohydrate polymers, fat, etc) in the descending and sigmoid sections of the colon, where mucus is secreted to aid in the expulsion by muscular contractions through the rectal sphincter