Digestion Part 1

What is the order of biological organization from smallest to largest?

atoms → molecules → cells → tissues → organs → organ systems → organism

Atom

the smallest unit of an element that retains the properties of that element

Molecule

a group of 2 or more atoms (same or different elements) bonded together

How many types of tissue does the human body contain, and what are they?

4 types - muscle, nerve, epithelial, and connective

Can an organ belong to more than one organ system?

yes - the pancreas is both an endocrine and exocrine organ

How many organ systems does the human body have?

11

Which organ systems help eliminate waste from the body?

urinary, respiratory, and integumentary systems

What are the 3 macronutrients that must be broken down before absorption, and what are they broken into?

proteins → amino acids; carbohydrates → sugar molecules; fats → fatty acids

Which nutrients do NOT need to be broken down before absorption?

water, vitamins, and minerals

Why can’t fiber be absorbed into the body?

it cannot be digested - it adds bulk to fecal contents and helps clear out the gut instead

Feces

body waste containing unabsorbed food residue, bacteria, mucus, and dead cells, eliminated through the anus

How long is the GI tract, and what other names does it go by?

about 30 feet long; also called the gut, alimentary canal, or digestive tract

Lumen

the hollow central cavity inside the GI tract; food here is technically not yet “inside the body” until absorbed

How long do mucosal cells lining the GI tract live, and why does this matter?

only about 2-5 days - they’re constantly replaced, so the mucosa has high nutrient requirements and is one of the first areas affected by nutrient deficiencies

What is transit time, and what’s the normal range in a healthy adult?

the time food takes to travel from mouth to anus; normally 24-72 hours, varying with diet, activity, emotional state, health, and medications

Name the 3 main accessory organs of digestion

liver, gallbladder, and pancreas

What are the two main digestive roles of the liver?

main site of nutrient metabolism, and production of bile acids for fat digestion

What does the gallbladder do?

store bile acids made by the liver and releases bile when fat enters the intestines

What are the pancreases two digestive jobs mentioned?

releases bicarbonate to neutralize stomach acid, and releases digestive enzymes/chemicals via a duct into the intestine

How many teeth do adult humans have?

32

What enzyme is in saliva, and what does it digest?

salivary amylase - digests carbohydrates only

Name the 3 pairs of salivary glands and their locations

sublingual, submandibular, and parotid

Where are most taste buds located, and what is this structure called?

on the tongue, in the circumvallate papilla

Why does chewing matter for nutrient absorption?

it breaks up fiber that traps nutrients - poorly chewed foods isn’t as well digested/absorbed, and reduced saliva, worsens tooth decay and swallowing difficulty

What’s the difference between soluble and insoluble fiber in terms of digestion?

soluble fiber can be partially digested; insoluble fiber cannot be digested at all

Epiglottis

a valve-like flap of tissue that blocks the airway during swallowing so food goes to the esophagus, not the lungs

What happens if the epiglottis fails to close properly?

food can enter the airway, usually dislodged by coughing, but can cause choking requiring the Heimlich maneuver

Peristalis

rhythmic, wave-like, involuntary muscle contractions that push food through the GI tract

Sphinchter

a ring-like muscle that acts as a valve in the digestive tract - closed when contracted, open when relaxed

What causes GERD (heartburn)?

the lower esophageal sphincter allows acidic stomach contents to leak backward into the esophagus

What are the 3 muscle layers of the stomach wall, from outer to inner?

longitudinal, circular, diagonal

Chyme

the semi-liquid food mass formed when food mixes with gastric secretions in the stomach

What does gastric juice contain?

water, mucus, hydrochloric acid, and inactive pepsinogen

Why is pepsinogen secreted in an inactive form?

so it doesn’t damage the gastric glands that produce it

What 3 things does hydrochloric acid in the stomach do?

kills most food bacteria, initiates protein digestion, and stops the activity of salivary amylase (carb digestion)

Besides water, alcohol, and a couple of drugs, does much absorption happen in the stomach?

no - very little absorption occurs in the stomach

How long does it typically take the stomach to empty, and what affects the rate?

2-6 hours; affected by meal size and composition - liquid empty fastest, then high-starch/sugar meals, then high-fiber/protein meals, with high-fat meals taking the longest

Does carbohydrates digestion occur int he stomach?

no - only protein digestion and limited fat digestion occur there

How long is the small intestine, and what are its 3 parts?

about 20 feet long duodenum, jejunum, and ileum

What is segmentation?

rhythmic contractions that slosh chyme back and forth to mix it

Villi

finger-like projections covering the entire inner surface of the small intestine that increase surface area for absorption

Microvilli (brush border)

tiny projections on the mucosal cell membrane covering each villus. further increasing surface area

What two structures does each villus contain, and what does each absorb?

a capillary and a lacteal

About how large is the total absorptive surface area of the small intestine?

roughly the size of a tennis court

Why does pancreatic bicarbonate matter in the small intestine?

it neutralizes acidic chyme from the stomach, making the small intestine neutral/slightly basic so its enzyme can function

What does pancreatic amylase do?

continues breaking down starches into sugars

Name two pancreatic proteases mentioned and their role

trypsinogen and chymotrypsinogen - break proteins into shorter amino acid chains

What do lipases need to work with to digest fat efficiently?

bile slats

What do intestinal digestive enzymes do?

break disaccharides into single sugars, and peptides into amino acids

Disaccharide

a sugar made of 2 sugar units, requiring further digestion by lactase, sucrase, or maltase

Where do absorbed sugars and amino acids go first after entering the intestinal cell?

into the portal circulation, which carries them to the liver before general circulation

How does bile help with fat digestion?

it mixes with fat and breaks large lipid globules into smaller droplets/globules, increasing surface area so lipases can digest fat more efficiently

Micelle

a spherical structure formed from fat, bile, and phospholipids, with water-soluble components on the surface and fat-soluble components in the core - allows fat to be absorbed into intestinal cells

Chylomicron

a transport particle formed inside intestinal cells from absorbed fat components, which enters the lymph vessels rather than going directly into the blood

Why must fat-transport particles enter the lymph instead of blood directly?

they must mature before entering the bloodstream, so they’re absorbed into the lacteals/lymph first