human physiology 100 - digestive system

What are the functions of the digestive system?

1. Ingestion
2. Mechanical digestion and propulsion
3. Chemical digestion
4. Secretion
5. Absorption
6. Excretion / defecation

What are the digestive organs and their functions?

1. mouth
- ingestion
- mechanical digestion with accessory organs
- moistening, mixing with salvory glands

2. pharynx
- muscular propulsion of materials into the esophagus

3. esophagus
- transport of materials to the stomach

4. stomach
- chemical digestion (acid and enzymes)
- mechanical digestion (muscular contraction)

5. small intestine
- enzymatic digestion and absorption of water, organic substrates, vitimins and ions

6. large intestine
- dehydration and compaction of indigestible materials

What are the accessory organs and their functions?

1. Teeth
- mechanical digestion (mastication)

2. Tongue
- assists mechanical digestion with teeth and sensory analysis (taste)

3. Salivary glands
- secretion of lubricating fluid containing enzymes that break down carbohydrates

4. Liver
- secretion of bile
- storage of nutrients

5. Gallbladder
- storage and concentration of bile

6. Pancreas
- exocrine cells secrete buffers and digestive enzymes
- endocrine cells secrete hormones

What are mesenteries?

Mesenteries is a serous membrane with two sheets. Areolar tissue in between the sheets that contain blood supply nerves, and lymphatics. It stabilises and attaches the digestive organs to peritoneal cavity. Stops digestive organs with entangling

What is the mucosa?

- inner lining
- mucous membrane

- simple columnar or stratified squamous
- enteroendocrine cells secrete hormones
- moistened by glandular secretions
- longitudinal folds
- circular folds
- villi to increase surface area

What is the lamina propria of the mucosa?

- areolar tissue containing:
• blood and lymphatic vessels
• sensory nerve endings
• lymphiod tissue

What are the three components of the muscles membrane of the mucosa?

- digestive epithelium
- lamina proper
- smooth muscle cells (muscularis mucosae)

What is the digestive epithelium?

- stratified squamous
• oral cavity, pharynx, oesophagus
• mechanical stresses

- simple columnar
• stomach, small intestine, large intestine
• absorption
• presence of goblet (mucous) cells

Protection against:
- digestive acids and enzymes
- mechanical stresses, such as abrasion
- bacteria
• ingested with food
• reside in digestive tract

What are the specialised epithelial cells ?

- stem cells
• constant cell renewal

- enteroendocrine cells
• secrete hormones

- goblet cells
• secrete mucous

- paneth's cells
• secrete antimicrobial peptides

What is the submucosa?

binds mucosa to muscular layer Has large blood. and lympathic vessels and may contain exocrine glands. Submucosal Plexus innervates the mucosa and submucosa.

What is the muscular layer / muscularis externa?

Muscularis Externa is smooth muscle arranged in inner circular layer and outer longitudinal layer. Involved in mechanical processing and movement of materals along digestive tract.

Primarily innervated by parasympathetic division of the ANS

What is the serosa?

•Serous membrane covering muscular layer (muscularis externa)

•Attachment to mesentery
- except in oral cavity, pharynx, esophagus and rectum

What is the serosa in oral cavity, pharynx, esophagus and rectum?

Adventitia
- firmly attaches the digestive tract to adjacent structures
- dense sheath of collagen fibres

How does digestive movement work?

- involves contraction and relaxation of smooth muscle

Pacesetter cells:
- spontaneous depolarisation
- trigger waves of contraction that spread throughout the entire muscular sheet
- located in the muscularis muscosae and muscularis externa

- coordinated contraction is vital for movement of materials along the digestive tract

What is peristalsis?

The process of moving food/material along the digestive tract

- waves of muscular contraction
- circular muscles - push forward
- longitudinal muscles shorten
- propels bolus forward
- bolus moves along small intestine in approx 90-120 min

What is segmentation?

- is the mixing / churning of food
- cycles of contraction
• churn and fragment bolus
• mix with intestinal secretions

- not directional
- purpose is to mix food

What is the makeup of saliva?

99.4% water

Remaining 0.6% is:
- igA (prevents entry of unwanted things)
- lysozyme (destroys pathogens)
- enzymes (salivary amylase)
- buffers, mucins, electrolytes and waste products

Where does chemical digestion start?

In the mouth

- salivary amylase breaks down starch
- lingual lipase breaks down lipids

What are the four stages of deglutition?

1. Buccal stage
- voluntary
- bolus —> oropharynx

2. Pharyngeal stage
- involuntary
- bolus —> stomach

3. Esophageal stage

4. Bolus enters stomach

What digestion occurs in the stomach?

1. Mechanical digestion
- mixing waves every 15-25 seconds
- forms chyme (partially digested semi fluid material)
- forces chyme into duodenum

2. Chemical digestion
- HCl (denatures proteins)
- pepsinogen
• converted into pepsin when reacts with HCl
• hydrolyses peptide bonds
- gastric lipase
• hydrolyses triglycerides

What is the stomach mucosa?

Secretory cells from gastric glands and gastric pits

- mucous cells
- parietal cells
• HCl (H+ and Cl- released separately)
• intrinsic factors (required for absorption of B12)
- chief cells
• pepsinogen
• gastric lipase
- G cells
• gastrin (hormone)
~ stimulates parietal and chief cells
~ increases secretions and motility

What is the regulation of gastric activity?

1. Cephalic phase
- begins on sight, smell, taste or thought of food
- prepares stomach for arrival of food
• impulses to submucosal plexus (vagus nerve)
• increases gastric secretion from gastric glands
• increases motility

2. Gastric phase
- arrival of food into stomach
- presence of undigested food/ materials in stomach triggers stretch receptors and chemoreceptors (pH)
• enhances secretion of gastrin, pepsinogen and HCl
• stimulate gastric motility

3. Intestinal phase
- begins when chyme enters duodenum
- intestinal secretion increases
• secretin
• CCK
• GIP
- controls rate of chyme exiting from stomach

What hormones control the rate of food intake?

- ghrelin = appetite stimulant
- leptin = appetite suppressant
- Insulin and CCK = depress hunger

What is the function of the lamina proper in the intestinal villi?

Contains nerve endings

Contains network of capillaries:
- carry absorbed nutrients to liver then around body

What is the function of lacteals of the intestinal villi?

- transport material that cannot enter capillaries
- fatty acids (chylomicrons)

What are the parts of the small intestine?

1. Duodenum (25 cm)
- receives chyme from stomach
- mixes with digestive enzymes from pancreas
- neutralises acid
• avoid damage to absorptive surfaces
• avoid inactivating digestive enzymes

2. Jejunum (2.5m)
- chemical digestion and nutrient absorption

3. Ileum (3.5 m)
- nutrient absorption
• vitamin B12, bile salts, leftover undigested products
- controls flow of contents from ileum to the large intestine

What is the function of the pancreas?

- pancreas has distinct lobules - contain acini and islets
- exocrine
• acinar glands secrete pancreatic juice
• digestive enzymes and buffers
- endocrine
• pancreatic islet cells
• secrete insulin and glucagon into bloodstream
- secretes 1L of pancreatic juices per day

What do pancreatic secretions contain?

- water, salts, bicarbonate and phosphate buffers
- digestive enzymes:
• pancreatic alpha amylase (carbohydrates)
• proteolytic enzymes / proteases (proteins)
• pancreatic lipase (triglycerides)
• nucleases (RNA and DNA)

What occurs in the lobules of the liver?

- hepatocytes arranged around sinusoids and central vein
• adjust levels of circulating nutrients
- blood passes through sinusoids and drains into central vein
- satellite macrophages
• phagocytic - engulf pathogens, cell debris and damaged RBCs

What does the hepatic artery of the liver do?

Delivers oxygenated blood

What are the functions of the liver?

metabolic regulation
- carbohydrate, lipid and protein metabolism
- waste removal
- vitamins and mineral storage
- processing of drugs

haematological regulation
- removal of bacteria
- removal of old RBC and WBC
- make plasma proteins
- removal of hormones and antibodies
- activation of vitamin D
- removal or storage of toxins

Bile production
- synthesis and secretion of bile into duodenum

What is catabolism?

The breakdown of structure of food, disassembling of molecules

How are monosaccharides absorbed?

2nd degree active transport (cotransport) with sodium

Facilitated diffusion for fructose

How are amino acids, dipeptides and tripeptides absorbed?

Amino acids via 1st degree and 2nd degree active transport

Di and tripeptides via 2nd degree transport

How are lipid absorbed?

Simple diffusion
- SCFA move into capillaries in villus
- others move into lacteals

Bile combines with LCFA and monoglycerides to form micelles

What is haustra?

Is located in the large intestine

Are present instead of villi or circular folds in mucosa to increase surface area

Are expandable pouches that aid in mechanical digestion
- mixing chambers
- segmentation

What are the functions of the large intestine?

- reabsorption of water
- compaction of intestinal contents into feces
- absorption of vitamins (K, B12, B5, biotin)
- breakdown of remaining products
- storage of feces before removal

What digestion occurs in the large intestine?

mechanical digestion
- slowed peristalsis
- haustral churning
- mass movement

Chemical digestion
- complex polysaccharides are able to be digested due to bacteria present