1. The Digestive System and Body Metabolism
Digestion
Breakdown of ingested food
Absorption of nutrients into the blood
Metabolism
Production of cellular energy (ATP)
Constructive and degradative cellular activities

2. Processes of the Digestive System
Ingestion – getting food into the mouth
Propulsion – moving foods from one region to another
Mechanical digestion
Chemical digestion
Absorption
Defecation
One way disassembly line. Several steps done by dift organs. Some overlap in function.

3. Stomach Functions Acts as a storage tank for food
Mechanical, chemical breakdown of protein begins
Delivers chyme to the small intestine
Chyme = processed food
Convolutions of interior surface allow stomach to expand.

4. Food Breakdown in the Stomach
Gastric juice is regulated by neural and hormonal factors
Presence of food or falling pH causes the release of gastrin
Gastrin causes stomach glands to produce protein-digesting enzymes
Hydrochloric acid makes the stomach contents very acidic
Protein digestion enzymes
Pepsin – an active protein digesting enzyme
Rennin – works on digesting milk protein
The only absorption that occurs in the stomach is of alcohol and aspirin
Necessity of an Extremely Acid Environment in the Stomach
Activates pepsinogen to pepsin for protein digestion
Provides a hostile environment for microorganisms

5. Chemical Digestion in the Small Intestine
Pancreatic enzymes digest starch, proteins, nucleic acids, fats; reset pH.
Gall bladder -
Enzymes from the brush border
Break double sugars into simple sugars
Complete some protein digestion
Pancreatic enzymes play the major digestive function
Help complete digestion of starch (pancreatic amylase)
Carry out about half of all protein digestion (trypsin, etc.)
Responsible for fat digestion (lipase)
Digest nucleic acids (nucleases)
Alkaline content neutralizes acidic chyme
Endocrine products of pancreas
Insulin
Glucagons
Figure 14.6

6. Propulsion in the Small Intestine
Peristalsis is the major means of moving food
Segmental movements
Mix chyme with digestive juices
Aid in propelling food
Esophagus moves food bolus by peristalsis
Small intestine uses segmental movements to mix food, push it up agst lining for better absorption.

7. Absorption in the Small Intestine
Site of nutrient absorption into the blood
Suspended from the posterior abdominal wall by the mesentery
Water is absorbed along the length of the small intestine
End products of digestion
Most absorbed by active transport
Lipids are absorbed by diffusion
Most absorbed by active transport through cell membranes
Substances are transported to the liver by the hepatic portal vein or lymph

8. Absorption in the Small Intestine
Site of nutrient absorption into the blood
Villi = Fingerlike structures formed by the mucosa
Microvilli on each cell
Increases surface area
Each villus is made of many cells. Each cell has tiny microvilli (1700) on its free surface. = brush border.
Note vascular connections and lymph collecting vessels (green)
Most absorbed by active transport through cell membranes
Substances are transported to the liver by the hepatic portal vein or lymph
Figure 14.7a

9. Absorption of Proteins and Carbohydrates
Amino acids, sugars move by diffusion into blood vessels
Next stop: liver, where glucose glycogen
amino acids protein
Excess molecules
are converted to fats
Figure 14.13

10. Absorption of Fats
Lipids are absorbed into lymph system, which drains into heart
Emulsified with bile salts
Digested into fatty acids, glycerol
With bile salts, diffuse into cells
Reassembled into fats, exocytosis
Figure 14.14

11. Food Breakdown and Absorption in the Large Intestine
No digestive enzymes are produced
Resident bacteria digest remaining nutrients
Produce some vitamin K and B
Release gases
Water and vitamins K and B are absorbed
Undigested fiber keeps materials moving and is eliminated via feces

12. Control of Digestive Activity
Regulation dependent on volume and content of food
Nervous system: stretch receptors in stomach
Hormones:
Gastrin: stimulates release of gastric juice
Secretin: stimulates pancreas to secrete water and bicarbonate
Cholecystokinin (CCK): signals pancreas to secrete digestive enzymes
Chemical and mechanical receptors in organ walls trigger reflexes
Table 14.1, p475. Stomach is stimulated by food entering it. Produces 3 hormones:
Gastrin - stimulates release of gastric juice, stimulates small intestine
Histamine - stimulates parietal cells to release HCl. pH drops
Somatostatin - inhibits secretion of gastric juice and pancreatic juice. Inhibits emptying of stomach.
2-3 L of gastric juice per day, normally.
Stimuli include:
Stretch of the organ
pH of the contents
Presence of breakdown products
Reflexes include:
Activation or inhibition of glandular secretions
Smooth muscle activity
NOTE: this is not appetite control mechanism. Separate system.
Appetite stimulated by: CCK (from pancreas) , low glucose levels in blood.
Suppressed by leptin (from adipose cells). Phen-fen drug.

13. Nutrition Carbohydrates: major energy source, simple or complex
Lipids: cell components and energy sources, saturated or unsaturated
Proteins: 20 amino acids
Vitamins: fat soluble and water soluble
Minerals: recommended daily allowance
Fiber
Major biological polymers, made of repeated monomers. Same categories as listed on food nutrition labels. These molecules make up the vast bulk of our food, organic or otherwise.
Carbs - energy, fiber
Lipids, energy and membs
Proteins - made up of amino acids.20 essential our bodies must ingest 9 of these, can make the rest.
Vitamins - coenzymes. Essential to function of other enzymes, but not consumed in reaction.
Minerals - specific functions. Fe in hemoglobin.
Water - vital. Part of everything

14. Food Guide Pyramid
Carbohydrates are The body’s preferred source to produce cellular energy (ATP)
Glucose (blood sugar) is the major digestive product and serves as fuel to make ATP
Brain uses 65% of blood glucose. Neurons take lots of energy, prefer to use glucose.
Other cells will use fats, proteins if glucose is unavailable.
Caffeine stimulates enzymes which break down fats AND those which make storage fats. No net gain.
Figure 14.16

15. Body Energy Balance
Energy intake = total energy output (heat + work + energy storage)
Energy intake from food oxidation
Energy output
Heat is usually about 60%
Storage energy is in the form of fat or glycogen
Energy intake - combat obesity by blocking conversion of food.
Olestra - fat which cannot be broken down (5 Carbon sugar with 5 fatty acids)
gastric bypass surgery
artificial sweeteners - same sweetness, fewer molecules = fewer Calories
Enegy output -
maintain body temp. Heat is usly lost to environment. So if two people weigh the same, which uses more energy for body temp stability: short, wide vs tall, thin? Why?
Storage energy - carbo loading before a competition is putting glycogen into muscle cells and liver.
Longer term storage uses fats in adipose tissue, usly underneath skin.

16. Regulation of Food Intake
Mechanisms that may regulate food intake
Levels of nutrients in the blood
Hormones
Body temperature
Psychological factors
Anorexia
bullemia

17. Basal Metabolic Rate
BMR– amount of heat produced by the body per unit of time at rest
Factors that influence BMR
Surface area
Gender
Age – children and adolescents have a higher BMR
thyroxine from thyroid gland
Give reasons why each one of these factors should influence BMR.
Factors that influence BMR
Surface area - loss of body heat. Surface area/volume
Gender - males highe b/c of lower % BODY FAT. Adipose tissue w/ low metabolic rate.
Age – children and adolescents have a higher BMR b/c of growth, not just repair of cells.
thyroxine from thyroid gland.

18. Total Metabolic Rate (TMR)
Total amount of kilocalories the body must consume to fuel ongoing activities
TMR increases with an increase in body activity