Exan 3 Bio 211

Absorption

Is the process whereby nutrients enter the bloodstream

Ingestion

Take in food

Physical digestion

Breaking large pieces of food into smaller pieces of food

Chemical digestion

Breaking large organic molecules (carbohydrates, proteins, lipids) into individual monomers; requires specific enzymes

Lumen

Space inside a hollow organ

Accessory digestive organs: aid GI tract organs

▪ Teeth

▪ Tongue

▪ Gallbladder

▪ Digestive glands: produce secretions that help break down foodstuffs

- Salivary glands

- Liver

- Pancreas

Physical (mechanical) digestion

Process whereby larger food pieces are broken down into smaller food pieces

Carbohydrates

Chemically digested into monosaccharides

Triglycerides

Chemically digested (Lypolysis) into glycerol and fatty acids

Proteins

Chemically digested into amino acids

Propulsion

Process whereby food moves through the alimentary canal. Some propulsions are caused by skeletal muscle contractions and some by the smooth muscle

Small intestine

Most absorption of nutrients takes place at the...

Water

Is only major nutrient absorbed at stomach, small intestine and large intestine

Peritoneum

Serous membrane of abdominal cavity that consists of the visceral and parietal peritoneum

Peritoneal cavity

Fluid-filled space between two peritoneums

Mucosa, Submucosa, Muscularis externa, Serosa (most superficial)

All digestive organs have the same four basic layers, or tunics

GI tract organs

Mechanical and chemical receptors controlling digestive activity exist in walls of..

Splanchnic circulation

- Arteries that branch off aorta to serve digestive organs

▪ Hepatic, splenic, and left gastric arteries

▪ Inferior and superior mesenteric arteries

Hepatic portal circulation

▪ Drains nutrient-rich blood from digestive organs

▪ Delivers blood to liver for processing

Enteric nervous system

GI tract has its own nervous system, referred to as....

- Also called the gut brain

Enteric neurons

Gut brain is made up of ____ that communicate extensively with each other

- Tongue

- Salivary glands

- Teeth

Associated organs of the mouth include...

- Parotid

- Submandibular

- Sublingual

Major salivary glands include...

- Cleanses mouth

- Dissolves food chemicals for taste

- Moistens food; compacts into bolus

- Begins breakdown of starch with enzyme salivary amylase

Functions of saliva

Parotid

Anterior to ear and external to masseter muscle

Submandibular

Medial to body of mandible

Sublingual

Anterior to submandibular gland under tongue

- Mostly water

- Slightly acidic (pH 6.75 to 7.00)

- Electrolytes: Na+, K+, Cl−, PO42−, HCO3−

- Proteins: mucin, lysozyme, and antibodies

Composition of saliva

Salivary amylase

Begins chemical digestion of starch

Allows passage of food, fluids, and air

Pharynx

The oropharynx and then into laryngopharynx

Food and beverages pass from mouth into...

Upper esophageal sphincter

Located between pharynx and esophagus

Esophagus

Flat muscular tube that runs from laryngopharynx to stomach

- Is collapsed when not involved in food propulsion

- No physical digestion

- Chemical digestion of starch due to salivary amylase

Gastroesophageal (cardiac) sphincter

Also called lower esophageal sphincter

Alimentary canal organs & Accessory digestive organs

Organs of the digestive system fall into two groups

Mouth, pharynx, esophagus, stomach, small intestine, large intestine, anus

Organs of the alimentary canal

Peristalsis

Adjacent segments of the alimentary canal organs alternately contract and relax

Stomach

Is temporary storage tank that starts chemical breakdown of protein digestion

- Converts bolus of food to paste-like chyme

- When empty it has ~50 ml volume but can expand to 4 L

- When empty, stomach mucosa forms many folds called rugae

- Cardiac region (cardia): at cardiac sphincter (lower esophageal sphincter)

- Fundus: dome-shaped region beneath diaphragm

- Body: midportion

- Pyloric region

▪ Pyloric region connects to duodenum (first region of small intestine) at pyloric sphincter

Major regions of the stomach

Circular (most superficial) and longitudinal smooth muscle layers (middle), as well as extra third layer, the oblique (diagonal) layer (deepest)

- Together, smooth muscles allow stomach not only to churn, mix, and move chyme, but also to pummel it, which increases physical digestion and ram it into small intestine

Muscularis externa has...

Chyme

Is the semisolid "mix" of food, digestive enzymes, acid, mucus that is propelled into the small intestine

Mucosa of Stomach Wall

- Consists of simple columnar epithelium entirely composed of mucous cells

▪ Secrete two-layer coat of alkaline mucus; protects stomach

- Dotted with gastric pits, which lead into gastric glands

▪ Gastric glands produce gastric juice

Mucous neck cells

Secrete thin, acidic mucus of unknown function

Parietal cells

▪ Secretions include:

- Hydrochloric acid (HCl)

- Intrinsic factor

Hydrochloric acid (HCl)

• pH 1.5-3.5; denatures protein

• Breaks down plant cell walls; kills many bacteria

• Activates pepsinogen (inactive protein enzyme) to pepsin (activeprotein enzyme) when mixed with food in stomach

Intrinsic factor

• Required for absorption of vitamin B12 in small intestine

• Only function of the stomach that is vital for life!

Chief cells

▪ Secretions include:

- Pepsinogen: inactive enzyme (cannot chemically digest protein) that is activated to pepsin by HCl

Pepsin

Is active protein enzyme (can chemically digest protein)

- Protein chemical digestion begins in the stomach

Enteroendocrine cells (G cells)

▪ Secrete the hormone gastrin

▪ Gastrin acts as local chemical messenger and circulating hormone

- Serves as holding area for food; temporarily stores food

- Secretes intrinsic factor

- Physical digestion of food by rhythmic contractions

▪ Rhythmic contractions also mix food with gastric juice

- Denatures proteins by HCl

- Pepsin begins chemical digestion of proteins

- Water absorption (only major nutrient absorbed at stomach)

▪ Lipid - soluble alcohol, aspirin absorbed here

- Delivers chyme to small intestine

Processes carried out by stomach

Gastrin

Stimulates HCl secretion by stomach in the presence of proteins and peptide fragments

Gastric secretion

Secretin hormone inhibits...

Vagus nerve

(parasympathetic) stimulation increases secretion

Neural mechanisms & Hormonal mechanisms

Gastric mucosa secretes > 3 L of gastric juice/day and are regulated by...

Enterogastrones

Duodenal enteroendocrine cells release two important circulating hormones that inhibit gastric secretion; Secretin & Cholecystokinin (CCK)

Secretin

Secretion stimulated by acidic chyme

Cholecystokinin (CCK)

Stimulus stimulated by fatty chyme

Gastric emptying

Contents of stomach (chyme) exits through pyloric sphincter into duodenum

Duodenum

(10.0 in) long; curves around head of pancreas

Jejunum

(8 ft) long; attached posteriorly by mesentery

Ileum

(12 ft) long; joins large intestine at ileocecal valve

Liver

Digestive function is production of bile; bile secreted into duodenum

- Digestive component of bile: bile salts that emulsify fats

Emulsification

Breakdown of fat droplets; type of physical digestion

Gallbladder

Chief function is storage of bile, but also secretes bile into duodenum

Pancreas

Supplies most of enzymes (pancreatic juice) needed to chemically digest chyme, as well as bicarbonate to neutralize stomach acid

Liver, gallbladder, and pancreas

Are accessory organs associated with small intestine (primarily, duodenum)

Gallbladder functions

Functions to store and concentrate bile by absorbing water and ions

• Muscular contractions release bile via cystic duct, which flows into bile duct

▪ Watery, alkaline solution (pH 8) to neutralize acidic chyme coming from stomach

▪ Digestive enzymes

- Proteases (for proteins)

- Amylase (for carbohydrates)

- Lipases (for lipids and triglycerides)

Composition of pancreatic juice

Neural and hormonal controls

Bile and pancreatic juice secretions are both stimulated by...

Small intestine

Is the major organ of digestion and absorption

- Most chemical digestion takes place here

- Most absorption of nutrients/electrolytes take place here

huge surface area for nutrient absorption

Small intestine's length and other structural modifications provide...

▪ Circular folds

▪ Villi

▪ Microvilli

Modifications to increase surface area include...

Large intestine

Contains thicker mucosa made up of simple columnar epithelium and abundant deep crypts with many mucus-producing goblet cells

Digestive Processes in the Large Intestine

• Residue remains in large intestine 12-24 hours

• No food breakdown occurs except what enteric bacteria digest

• Bacteria ferment indigestible carbohydrates

• Water absorbed

• Propulsion of feces to anus and defecation

Haustral contractions

Most contractions of colon, where haustra sequentially contract in response to distension

Gastrocolic reflex

Initiated by presence of food in stomach

▪ Sensory receptors in stomach

▪ Results in mass movements

Mass movements

Slow, powerful peristaltic waves that are activated three to four times per day

Nutrient

Substance in food needed for growth, maintenance, repair

Small intestines

Nutrients, from the foods and beverages ingested, are absorbed into the bloodstream at the...

Vitamins and minerals

Two other nutrients that are required, but only in small amounts

Water

Is required, so technically it is a nutrient, most important inorganic molecule in the human body

Essential nutrients

Nutrients that must be eaten because body cannot synthesize these from other nutrients

Nonessential nutrients

Are also vital to life, but if not enough is available, liver canusually convert another nutrient into one needed; don't have to be present in the diet

Monosaccharides

Digestive tract breaks down foods/beverages that contain carbohydrates into

Insoluble fiber

Cellulose in vegetables provides roughage; not digested

Soluble fiber

Pectin in apples and citrus fruits reduces blood cholesterol levels

Dietary sources

Include sugars (monosaccharides and disaccharides) and starches (polysaccharides)

Glycogen

Stored glucose; liver and skeletal muscle

Glucose

Most important monosaccharide

Glucose

Fuel most used by cells to make ATP: primary energy source

- Neurons and RBCs rely entirely on this

• Neurons die quickly without this

Dietary requirements of carbohydrates

Recommended daily intake: 45-65% of total calories

▪ Should consist mostly of complex carbohydrates (whole grains and vegetables)

Simple carbohydrates

Should be limited (sugars, processed sugars)

Unsaturated fats

Found in plant products: seeds, nuts, olive oil, and most vegetable oils; liquid at room temperature

Cholesterol

Found in egg yolk, meats, organ meats, shellfish, and milk products

▪ Liver makes ~85% of it

Triglycerides

(neutral fats): most abundant form

▪ Found in saturated fats in animal products: meat, dairy foods, tropical oils, or hydrogenated oils (trans fats); solid at room temperature

Digestive tract

Breaks down triglycerides into fatty acids and glycerol

- Fatty acids and glycerol used to make ATP after glucose: secondary energy source

Adipose tissue stores triglycerides

Offers protection, insulation, fuel storage

▪ Fats provide more energy/gram than any other nutrient

Phospholipids

Essential in myelin sheaths and all cell membranes

Prostaglandins

Smooth muscle contraction, BP control, inflammation

Dietary requirements of lipids

- Fats: should represent 20-35% of total caloric intake

- Saturated fats: limited to 10% or less of total fat intake

- Cholesterol:

▪ not required in diet

▪ Intake should be kept as low as possible