Small Intestine Physiology

Question 1

what are the 4 major structural characteristics of the small intestine?

Answer 1

1. mucosa
2. cells
3. microvilli
4. blood and lymphatic vessels

Question 2

what all does the small intestine contain to increase surface area for absorption? (3)

Answer 2

1. microvilli (also in large intestine)
2. vili
3. plicae circulares (muscular ridges)

Question 3

what are the 4 major functions of the mucosa of the small intestine?

Answer 3

1. barrier
2. digestion
3. absorption
4. communication with microbiota

Question 4

list and give 2 examples of cell types found in the small intestine

Answer 4

1. enterocytes (epithelial cells) of mucosa: form tight junctions on lateral membrane
2. intestinal stem cells: active cell division to differentiate into other cell types; migrate from crypt to tip and are exfoliated into lumen every 2-3 days

many more cell types tho (paneth, M cells, etc.)

Question 5

describe the microvilli of the small intestine (4)

Answer 5

1. located on apical membrane of enterocytes
2. contain brush border enzymes like enterokinase, disaccharides, aminopeptidases for digestion
3. contain transporters and ion channels for absorption of digested nutrients, electrolytes, and vitamins
4. combination of muscular ridges, villi, and microvilli increases surface area by 600x

Question 6

what are the 4 primary functions of the small intestine?

Answer 6

1. break down food
2. absorb nutrients
3. extract water
4. move food to large intestine/cecum

Question 7

describe the blood and lymphatic vessels of the small intestine (3)

Answer 7

1. capillaries: countercurrent flow of arteriolar and venular blood for direct diffusion of dissolved gases (O2 and CO2) along their concentration gradient
2. enterocytes at the tip of villi are exposed to relative hypoxia, meaning that rapid turnover of these cells is possible and also that these cells are very prone to hypoxic injury
3. central lacteals/lymphatic vessels: one for each villus, for transport of chylomicrons

Question 8

what are gastroenteropancreatic endocrine cells (GEP)?

Answer 8

the largest group of endocrine cells in the body

Question 9

list and describe an example of hormones secreted by GEP (4)

Answer 9

1. glucose-dependent insulinotropic polypeptide (GIP): formerly known as gastric inhibitory peptide
2. secreted by enteroendocrine K cells, which are located in high density in the duodenum and upper jejunum, but also all along the SI
3. secreted in response to glucose, which is a potent stimulator of insulin secretion, which is why oral ingestion of glucose results in more insulin than IV glucose
4. also decreased G cell secretion of gastrin and decreases parietal cell secretion of HCl

Question 10

explain major functions of each part of SI

Answer 10

duodenum: digestion (I cells secrete CCK which leads to secretion of enzymes for digestion)

jejunum: mechanical digestion and absorption; is hella long and has segmental motility (which is much slower than peristalsis), length and motility type mean food is in there a long time for digestion and absorption

ileum: fat absorption, then bile resorption; has peristaltic contractions to get food quickly to cecum for bacteria to do fermentation

Question 11

compare digestion of lipids, carbohydrates, and protein in SI

Question 12

describe digestion and absorption of carbohydrates in the SI (5)

Answer 12

1. dietary carbohydrates begin as polysaccharides (starch/amylose and/or fiber/cellulose); are then broken down to oligosaccharides, then disaccharides (lactose, maltose, or trehalose), then to monosaccharides (glucose, fructose, galactose)
2. alpha amylase in saliva of herbivores and omnivores
3. pancreatic amylase hydrolyzes starches
4. brush border enzymes break oligosaccharides to monosaccharides
5. monosaccharides are absorbed on the apical membrane mostly in the jejunum by carrier mediated transporters (SGLT1 and GLUT5) or facilitated diffusion (Glu transporters)

Question 13

describe liver digestion and metabolism

Answer 13

liver is in charge of glucose, amino acids, and fatty acids that are used for either

catabolism: results in ATP production + metabolism wastes (CO2, urea, etc.) or
anabolism: synthesis of glycogen, protein, and triacylglycerol (but also need ATP)

liver decides what do based on if

absorptive state: excess of energy substrates in blood, so liver does anabolic metabolism (glycogenesis, lipogenesis, protein synthesis) for storage when the insulin to glucagon ratio is high or

post-absorptive state: low insulin to glucagon ratio, leads to catabolic processes (glycogenolysis, lipolysis, protein degradation, gluconeogenesis and ketogenesis)

Question 14

define gluconeogenesis, glycogenolysis, glycogenesis, and glycolysis

Answer 14

gluconeogenesis: produce glucose from non-carbohydrate sources

glycogenolysis: breakdown of glycogen to make glucose

glycogenesis: production of glycogen

glycolysis: breakdown of glucose to lactate

Question 15

describe major SI motility patterns in broad terms including relating function to each pattern (3)

Answer 15

1. digestive phase:
   1a. mixing contractions (segmentation): distention of gut elicits localized concentric contractions at regular intervals of 8-13 times per minute
   1b. propulsive movement: peristaltic wave (peristalsis) moves food aborad
2. inter-digestive phase:
   2a. migrating myoelectric (motor) complex (MMC): cyclic motor movements cause peristaltic waves that move from the stomach to the ileum at regular intervals every 1-2 hours, mediated mainly by motilin, interrupted by eating; function is to sweep digestive contents, cellular debris, and bacteria into the colon to limit bacterial overgrowth
3. powerful rapid peristalsis: caused by physical or chemical irritation to the ANS or ENS; movements are caused by muscularis mucosa (lamina muscularis of t. mucosa) and begin in duodenum before moving through the entire length to the ileocecal valve, sweeping the contents into the colon and possibly resulting in diarrhea

Question 16

what 3 reflexes are associated with SI motility? describe

Answer 16

1. gastroileal reflex: increased gastric activity leads to increased ileum activity and increases movement of food through ileocecal valve to cecum
2. ileogastric reflex: distension of ileum decreases gastric motility (SI pumps brakes bc full)
3. intestino-intestinal reflex: over-distension of one portion of the intestine leads to relaxation of other portions; involves short reflexes (local) and long reflexes (CNS)

Question 17

describe digestion and absorption of protein in the small intestine (4)

Answer 17

1. proteolytic enzymes break peptide bonds through hydrolysis, generating free amino acids, dipeptides, and tripeptides
   1a. the stomach actually begins breakdown via chief cells that secrete pepsinogen that is activated to pepsin
   1b. the pancreas contributes trypsinogen, which is activated to trypsin and then activates many other pancreatic digestive enzymes (also activated by pH and enterokinase)
   1c. in the duodenum and jejunum, proteolytic enzymes (endopeptidases like trypsin, chymotrypsin, etc.) cleave peptide bonds in the interior of the polypeptide and exopeptidases cleave bonds from the ends of the polypeptide
2. the amino acids generated move from the lumen, past the brush border, and enter the cells of the small intestine
3. the dipeptides and tripeptides are transported and absorbed via pept-1, which requires a hydrogen ion gradient; the free amino acids are transported via multiple carrier-mediated active and facilitated transport proteins for absorption
4. this absorbed protein is then sent to the liver, which decided what to do with it

Question 18

describe the one exception to regular protein digestion and absorption

Answer 18

pinocytosis/endocytosis of protein in neonates:

neonates can absorb the whole protein (no breakdown) from colostrum during the 1st 48 hours of life; this confers passive immunity to the bby as it allows maternal IgA to be absorbed as an intact molecule; is hella important for horses, ruminants, pigs, and dogs

Question 19

describe dietary fats

Answer 19

mostly triglycerides that get broken down into-monoglycerides and 3 fatty acids

Question 20

give 3 sources of lipases

Answer 20

1. saliva
2. gastric
3. pancreas: colipase (stabilizes lipase), lipase, phospholipase A2, and cholesterol ester hydrolase

Question 21

what inactivates lipases?

Answer 21

bile salts

Question 22

what are bile salts and what stimulates their release?

Answer 22

duodenum senses fatty acids, so releases secretin to tell the liver to make more andCCK tells the gallbladder to contract to release more

Question 23

describe digestion and absorption of fats (5)

Answer 23

1. emulsification by bile salts breaks fat globules into smaller droplets called micelles that increase surface area for lipases to break down
2. micelles are moved to the brush border of the small intestine for absorption
3. absorption occurs both actively and passively
4. absorbed fats enter the endoplasmic reticulum of enterocytes and are resynthesized into triglycerides and phospholipids
5. the resynthesized triglycerides and phospholipids are combined with lipoprotein to form chylomicrons that are secreted into central lacteals, meaning they get to bypass the liver, enter the thoracic duct instead, mix with venous blood at vena cava, and enter circulation

Question 24

how are lipids carried through blood? generally and then give 3 types

Answer 24

as lipoproteins (lipid + protein)
1. chylomicrons: when they travel from intestine to lymphatics to blood
2. VLDL, LDL, IDL: when traveling from liver
3. HDL: from extra-hepatic cells

Question 25

what occurs in the blood vessels of adipose tissue regarding fat as an energy source?

Answer 25

lipoprotein lipase (insulin sensitive) cleaves triglycerides into monoglycerides and fatty acids that are incorporated into fat or sent to other cells, where they are reformed into triglycerides for storage

Question 26

what are the 2 routes of electrolyte absorption?

Answer 26

transcellular route: movements across apical membranes

paracellular route: movement through tight junctions

Question 27

how are monovalent ions like Na+, K+, Cl-, I-, and F- abrsorbed?

Answer 27

via multiple mechanisms, see nearly complete absorption independent of need as other organs will absorb, use, or excrete the ions as needed

Question 28

how are divalent ions like Ca2+ and Fe2+ absorbed?

Answer 28

dependent on animal need, monitored by hormones

Question 29

summarize absorption of nutrients in the small intestine (4)

Answer 29

1. stomach: doesn’t absorb much, mainly alcohol and some drugs like aspirin
2. duodenum and jejunum are major absorbers!!: break carbs to monosaccharides, proteins to amino acids and di-/tripeptides, and absorb electrolytes and water
3. distal jejunum and proximal ileum: absorb water soluble vitamins mainly by passive diffusion; utilize carrier mediated transport for thiamin, folic acid, and vitamin C
4. distal ileum: absorption of vitamin B12, bile salts, and conjugated bilirubin (enterohepatic circulation), and fat-soluble vitamins (using chylomicrons and lymphatic vessels)