The document provides information about the digestive system. It discusses: 1. The movement of food through the alimentary tract and the secretion of digestive juices and absorption of nutrients. 2. It describes the structure and function of different components of the digestive system like the mouth, stomach, small intestine, large intestine. 3. It discusses the role of organs like the liver, pancreas and gallbladder in digestion and the absorption of nutrients like carbohydrates, fats and proteins in the small intestine.

1. Digestive System

2. Digestive System
What you will learn:
Digestion, absorption, homeostatic mechanisms of energy balance, BMR
Structure and function of different components of digestive system in different
animals, Absorption of CHO, fats, proteins, bile salts in digestion and absorption, HCl
in stomach, enzymes and hormones in digestive process, evolutionary adaptations of
vertebrate digestive system
1. Movement of food through the alimentary tract
2. Secretion of digestive juices for digestion of food
3. Absorption of water, electrolytes and digestive products
4. Circulation of blood through GI tract to carry absorbed substances
5. Control of all functions by local, nervous and hormonal systems

3. Animals
Herbivores Carnivores Omnivores
Consume plants consume animals consume both animals and plants, algae
All consume prokaryotes
Nutritional needs
Fuel Organic material Essential nutrients
Chemical biosynthesis to make vitamins, Ess. Fatty acid
Energy for carbon skeleton
Cellular work

4. HOMEOSTATIC MECHANISMS OF ENERGY BALANCE
ENERGY BUDGET (ATP)
Metabolism (resting)
Various activities
Thermoregulation (endotherms)
Oxidation of energy rich molecules
(CHO, prts, fats)
ATP
Cellular respiration

5. RECAPPING THE LAST CLASS
1.What makes BAT brown in color?
2.What is the full form of UCP1?
3.a) What is heterothermy? b) How is it beneficial?
4.Which of the organs is not part of GI tract but part
of Digestive system?
1.Speen 2. Liver 3. Pancreas 4. Gall Bladder
5.What components contribute to the energy budget?
6.Digestive system contributes toward BMR. How?
7.What are the parts of GI tract?

6. Regulation of glucose in the body is an example of
homeostasis
Calories (glucose) ----> ATP ------> cell respiration
excess
biosynthesis
fat
Liver: glycogen
Muscle: glycogen
Glucose is regulated by hormones
if glycogen reserves are in excess and still glucose intake continues
excess is stored as fat and also due o lack of exercise and vice versa.
First liver glycogen is used and 2nd muscle glycogen and fat reserves are used

7. INSULIN
Decr glucose level
GLUCAGON
Incr glucose level
PANCREAS

8. Caloric Imbalance
1. If stores of glycogen and fat are used up body breaks proteins and muscle
decreases in size and brain is protein deficient
Energy intake < expenditure
Death
2. Anorexia nervosa: compulsive starvation
3. Obesity/ over nourished

9. Food components other than CHO, Fats and Proteins
1. Essential Amino Acids: some amino acids which humans cannot synthesize out of
20aa are EAA. 8aa + His for infants
Protein deficiency: Kwashirkor, deficiency of blood proteins causing edema of belly
Sources of EAA: Meat, eggs, cheese, animal products. Plant sources are generally
deficient in one or more eg corn is deficient in Lysine and hence a combination of plant
proteins must be eaten.
2. Essential Fatty Acids: animals can synthesize FA but not EFA eg. Unsaturate FA
Like linoleic acid: for phospholipids in membranes
3. Vitamins: requirement is less (0.01-100mg/day) but essential for survival
They are organic molecules required in minute quantities.
13 essential vitamins
Water soluble Fat soluble
B (coenzymes),C (conn. Tiss) A(eye),E (anto Oxi.),D (Ca absorp),
K (blood clotting)
Excess not harmful Excess is harmful

10. 4. Minerals: Less than 1mg to 2500mg per day
Humans
Vetebrates
Ca, P (bone, nerves, muscles, ATP, nucleic acid)
Fe (Hb, cytochromes)
Mg, Zn, Cu, Mn, Se Cofactor of enzymes
I Thyroid Hormones
Na, K, Cl nerve function, osmotic balance of cells
and interstitial fluid
(excess is harmful)

11. Hormones Regulating Appetite
Adipose tissue LEPTIN L Suppresses appetite
Body fat L incr appetite
Stomach walls GHRELIN G triggers hunger
Small intestine PYY appetite suppressant after meals
counters G
Pancreas INSULIN suppresses appetite by acting on brain

12. FOOD PROCESSING
1. Ingestion: mastication, swallowing
2. Digestion: breakdown into small molecules for absorption
Polysacc. & Disacc. ----> simple sugars; fats ----> glycerol & FA;
proteins ---> aa, Nucleic acid -----> nt
Enzymatic hydrolysis: by use of hydrolytic enzymes which starts in mouth (salivary amylase)
3. Absorption of small molecules from digestive compartments
4. Elimination of wastes and undigested material
Intracellular Digestion Extracellular Digestion
•Inside cells - outside cells
•Phogocytosis/pinocytosis - lumen of alimentary canal or
gastrovascular cavity
•Food vacuoles - no FV
•FV fuses with lysosomes - enzymes are secreted by cells
in lumen
•Digestion in alkaline medium - digestion in acidic (pepsin) and
i.e. Tryptic digestion alkaline (trypsin) medium
peptic and tryptic digestion
•Distribution to other parts by - Distribution by blood
•Endoplasmic streamlining, streaming movt.

13. Gastrovascular Cavity Alimentary Canal
Organisms belonging to two major phyla,
the Cnidaria and the Platyhelminthes,
possess gastro vascular cavities.
Extracellular digestion takes place within
the central cavity of the sac-like body.
This cavity has only one opening to the
outside and, in most cnidarians, that is
surrounded by tentacles which serve to
capture prey.
Example: Digestion in hydra occurs in
gastrovascular cavity.
Presence of a digestive tube extending
between mouth and anus
Food moves in a single direction and
specialized organs and regions carry
digestion and absorption
Ability to ingest more food before earlier
food is completely digested

14. Human Alimentary Canal

15. RECAPPING THE LAST CLASS
1.What makes BAT brown in color?
2.What do you mean by essential AAs and FAs?
3.Where is glycogen stored in mammalian body?
4.a) What is gastrovascular cavity? b) Do we have it?
5.Which one is the major hunger hormone?
1.Insulin 2. Ghrelin 3. PYY 4. Leptin
6.What difference between extracellular and intracellular
digestion?
7.What are the sphincters located in stomach?

16. PERISTALSIS:rhythmic waves of contraction by smooth muscles in walls of canal
which push food along the tract
SPHINCTER:regulate passage of materials b/w chambers Pyloric sphincter
Cardiac sphincter
Anal sphincter
Upper Esophageal sphincter
Ileocaecal sphincter
Accessory Glands
Salivary Glands: Parotid
sub mandibular
sub lingual
Pancreas
Liver
Gall Bladder
Lower esophageal sphincter or
gastroesophageal sphincter

17. Oral cavity, pharynx, oesophagus:
Salivary amylase hydrolyses starch (plant polymer) and glycogen (animal polymer)
Medula and lower pons regulate swallowing in swallowing centre which are transmitted to
pharynx and esopagus to 5th, 9th, 10th and 12th cranial nerves
Reflex act
upper

18. Stomach:
Fundus
Pylorus
Rugae
Cardiac
Rugae: series of ridges produced by folding of the wall of an organ. Most commonly the term
is applied to the internal surface of the stomach (gastric rugae)
Antrum
Body

19. Stomach:
Contains gastric juices, cause churning action of smooth muscles in stomach wall
Gastric juice has pH ~2, KCl and NaCl enough to dissolve iron nails
Ability to disrupt extra cellular matrix of food and kills bacteria
Why stomach cells are not destroyed by the acid?
+ve feedback causing more pepsinogen to be produced
+
Pepsinogen ---------> Pepsin
Inactive active
HCl
Chief cells
Parietal cells
Secretion of mucus by epithelial lining which protects cells
New cells are made by mitosis, ulcers are generally caused by Helicobacter pylori infection

20. Gastric Gland
Mucus Cells
Or goblet cells
Mucus
Chief Cells or
peptic cells
Pepsinogen
Parietal Cells or oxyntic
HCl

21. Acid Chyme: gastric juices + food + churning-------> nutrient rich chyme (murky, semi
fluid)
In 2-6h stomach is empty into small intestine via the pyloric sphincter
SMALL INTESTINE
Ist 25cm is called DUODENUM
Here the acid chyme mixes with digestive juices from
1. Pancreas
2. Liver
3. Gall Bladder
4. Gland cells of interstitial walls

22. DUODENUM
Pancreas: produces hydrolytic enzymes and an alkaline solution rich in bicarbonate
HCO3
2- is buffer which neutralizes acid chyme and contains proteases (protein
digestion) in inactive form
Liver: production of Bile. Has no digestive enzymes but has bile salts which act as
emulsifiers to aid in digestion and absorption of fats and pigments which are by
products of RBC destruction in liver which are eliminated with faeces.
Epithelial Lining of Duodenum: it is called Brush border and has digestive enzymes
Inactive Active
Trypsinogen -------> Trypsin
+
Inactive Proteases ---------> Active proteases
+
Membrane bound
enteropeptidases
+
Small intestine rest of the portion is responsible for absorption of nutrients and water

23. Hormonal regulation of movement of food
between Stomach & Duodenum

24. Absorption of Nutrients
SMALL INTESTINE
Rest of the portion of SI (jejunum and ileum) is responsible for absorption of nutrients
and water.
Huge surface area-300m2 (size of a tennis court)
Finger like projections called villi and epithelial cell of a villi have microvili exposed
to intestinal lumen
Core of each villus has a network if blood vessels and small vessels of lymphatic
system called LACTEAL which absorbs nutrients.
Absorption is passive (diffusion) for some molecules from the lumen into intestinal
cells then to capillaries, for others its active (aa, small peptides, vitamins, glucose)
and finally its passed onto blood. AT concentrates more than PT (why??)
Glycerol and fatty acids are absorbed by epithelial cells --->form fats
Fats mix with cholesterol and proteins to form ----> chylomicrons, transported by
exocytosis into lacteals
From Lacteals lymph + chylomicrons combine and drain into the big lymphatic
system ---> large veins -----> blood----> heart

25. Absorption of Nutrients
SMALL INTESTINE
Those cappillaries and veins which drain nutrients away from the villi converge into
HEPATIC PORTAL VEIN (blood vessel leading to liver)
Hence liver gets the first supply of aa, sugars absorbed after a meal
Vein that leaves has different mix of nutrients -----> travels to heart

26. SMALL INTESTINE

27. LARGE INTESTINE
colon
Ileo-caecal sphincter
Ileum connects to the cecum which has finger like projections like appendix (has
lymphoidal tissue)
Absorption of water which is not absorbed by small intestine: 90% of water is
reabsorbed by SI and LI
Wastes become more solid as water is absorbed and as they move by peristalsis takes
12-24h to travel
[if bacterial or viral infection is the lining of colon is irritated and less water is
absorbed resulting in diahorrea
If peristalsis moves the faeces too slowly and excess water is absorbed making feaces
more compact resulting in constipation]
Gut microflora
Rich in micro-organisms which are harmless bacteria eg. E.coli.
They live on undigested organic material and as by products produce gases like
methane, H2S. Some produce important vitamins (B and K) which are absorbed by
humans as essential nutrients.

28. Gut microbiome in human at different ages

29. LARGE INTESTINE
rectum
Faeces are stored for a long duration till eliminated
Involuntary and voluntary anal sphincters
Reflex is defecation reflex.

30. RECAPPING THE LAST CLASS
1.What is the role of omega-3-FA in mammalian body?
2.Which part of SI has extensive villi and microvili?
Why?
3.What is the role of gut microbiota in health?
4.What is the role of “LACTEAL” in absorption process?
5.What is the role of enterogastrone?
6.What step “Esophageal sphincter” plays a role in
digestive system?
7.Which cells in stomach wall secreate HCl?
1.Parietal 2. Chief 3. Goblet

31. Absorption of Nutrients
Stimulation of Alimentary canal
1. Contact with food: secretes enzymes and stimulation of nervous system occurs by
a. tactile stimulation
b. chemical irritation
c. distension of gut--stimulates mucus cells in gut and glands to secrete
secretions
2. Autonomic stimulation: Parasympathetic (glossopharryngeal and vagus
parasympathetic nerves stimulate salivary glands, esophageal glands, gastric
glands, pancreas, brunners gland in duodenum)
3. Autonomic stimulation: Sympathetic: stimulation of nerves which increase
secretion of local glands and constriction of blood vessels
4. Hormones regulating secretions of gastrointestinal hormones stimulated by
presence of food in lumen of the gut.
Hormones---> Blood ---> glands -----> secretion eg. Gastric and pancreatic juices are
secreted in this manner
Seceretions for

32. Salivary Glands
Parotid -----------serous
Submandibular---serous and mucus
Sublingual--------serous and mucus
1. Ptyalin (a amylase) (serous)
2. Mucin (mucus)
pH 6-7
Saliva has:
1. Thiocyanate ions
2. Lysozyme
3. Digestion
4. Protein antibodies
Destroy oral bacteria

33. IONS IN SALIVA
K+ and HCO3-
NaCl are less in saliva than in Plasma
Na+ reabsorbed ----> Na+
K+ secreted in saliva K+ <-------
Hence Na+ is less causes Cl- to be passively reabsorbed leading to low Cl-
HCO3- secreted passive exchange for Cl-
NET RESULT:
NaCl: 15mEq/L (1/10 less than plasma)
K: 30mEq/L (7x greater than plasma)
HCO3-: 50-70mEq/L (3X more than plasma)
Na+ Active absorption
Cl- Passive
K+ Active
HCO3- secretion
Saliva
Oesophagus also has mucus secretions

34. Glandular secretion
Secretion is dependent on hormonal regulation or signal, till then they are stored.
Mechanism
Control signal-----> incr of cell permeability to Ca++ -----> Ca enters
Membrane vesicles
fuse with apical cell
membrane
<---
Exocytosis to exterior
Secretion takes place
capillary
Nerve fibre
RER
nucleus
golgi
Zymogen grannules
secretion
Basement
membrane

35. Oxyntic glands
HCl Secretion
Stimulation
Parietal cells
HCl (160mEq/L (isotonic with body fluids)
pH 0.8)
H+ is 3 million times than of arterial blood
H+ is required to be concentrated
Requires 1500 Cal energy/L of gastric juice
canaliculi
secretion
Oxyntic
(Parietal)
cells
Mucus
cells
Mucus
cells

36. Gastric Secretions: tubular glands
Oxyntic glands Pyloric glands
(Parietal and Chief cells)
HCl
Pepsinogen
Intrinsic factor
Mucus
Mucus
Gastrin: stomach walls
80% stomach 20% stomach
Mucus cells -----> mucus
Peptic/chief cells ------> Pepsinogen
Parietal cells------> HCl + intrinsic factor

38. Activation and secretion of Pepsinogen
Different types of pepsinogen are secreted by peptic or chief cells and mucous
cells
Initially pepsinogen (MW 42,000) is inactive and gets activated by HCl and forms
Pepsin (MW 35,000)
Functions as a proteolytic enzyme in highly acidic medium (pH 1.8-3.5) but above
pH 5 it has no proteolytic activity and is inactivated
Secretion of Intrinsic Factor
It is essential for absorption of VitB12 in ileum. It is secreted by parietal cells along with
HCl.
DISORDER: When parietal cells get destroyed, leads to lack of HCl secretion
(achlorhydria), this leads to perinicious anemia due to faliure of RBCs to mature (in bone
marrow) due to absence of Vit B12

39. Pyloric glands Mucus and Gastrin
Contains peptic cells but no parietal cells; but has lot of mucus cells for lubrication to
protect stomach walls (mucus is alkaline)
Produce pepsinogen and gastrin (controls gastric secretion)
Stimulation of Gastric acid Secretion
Secretion of HCl by parietal cells is under control of nervous and endocrine signals
Enterochromafin-like-cells (ECL) function is to secrete HISTAMINE and are in close contact
with Parietal cells
Amount of Histamine ∝ amount of HCl secreted by Parietal cells
Stimulated by:
1. By hormone gastrin produced by antral region in response to protein in food
2. By acetylcholine produced by stomach vaga nerve endings
3. Enteric nervous system of stomach

40. Stimulation of Gastric acid Secretion by gastrin
G cells ---> gastrin cells ----> gastrin
Pyloric glands
G-34 (34aa)
G-17 (17aa)(abundant)
Meat or prt containing foods stimulate pyloric
glands (antrum)
G-cells
GASTRIN
Transported to ECL cells in stomach
Histamine release in deep oxyntic cells
HCl secretion

41. Pyloric glands
Pepsinogen regulation
2 signals:
1. Stimulation of peptic cells by acetylcholine released from vagus nerve or gastric
enteric nerve plexus
2. Stimulation of peptic cell secretion in response to acid secretion
Vagus nerve ----> Ach ------> gastric enteric nerve plexus
Peptic cells
Acid
Pepsinogen (inactive)

42. RECAPPING THE LAST CLASS
1.Which digestive juice contain Ptyalin?
2.Which are the major ions of saliva?
1.Ca2+ 2. Na+ 3. K+ 4. HCO3-
3.What are the 4 layers of the wall of digestive tract?
4.Where from intrinsic factor is secreted?
5.What is the role of intrinsic factor in digestion
process?
6.What are ECL cells? What is their function?
7.Which ones regulate HCl secretion in the stomach?
1.Mucin 2. Histamine 3. Acetylcholine 4. Gastrin

43. PHASES OF GASTRIN SECRETION
Cephalic phase
Before food enters i.e. during
eating (sight, taste, smell,
appetite (intensity)
Neurogenic signals: cerebral
cortex and appetite centre in
amygdala and hypothalamus
Transmitted thro ’ vagus to
stomach
20% secretion
Gastric phase
Food enters stomach
Parasympathetic excite pepsin and
acid production
1. Vagal reflex from vagus to
stomach and back
2. Local enteric relfexes
3. Gastrin-histamine stimulation
mechanims
Intestinal phase
Entry of food in
upper portion
of SI
(duodenum)
1. Nervous mech.
2. Hormonal mech.
Secretin
CCK
Enterogastrone
1. Food in SI stimulates reverse enterogastrone reflex (sympathetic nerves) inhibits stomach
secretions.
2. Prts, fats, acid in SI, irritant factor stimulates production of intestinal hormones: secretin
(inhibits stomach secretion; control of pancreatic secretion)
3. Gastric inhibitory peptide, vasoactive intestinal polypeptide and somatostatin also inhibit.
INHIBITING GASTRIC SECRETIONS

44. Pancreatic Secretion Endocrine (Islet of Langerhans; secrete in blood) and
Exocrine (pancreatic acini secrete enzymes through ducts)
Acini secrete NaHCO3
-, enzymes which joins hepatic duct before it empties into duodenum
ENZYMES
Trypsinogen ---> Trypsin Proteins (not indv. aa)
Chymotrypsinogen -----> Chymotrypsin
Procarboxypolypeptidase -------> Carboxypolypeptidase peptides to indv. aa)
Pancreatic amylase: hydrolyzes starch, glycogen, other CHO (except cellulose)
Pancreatic lipase: hydrolyzes neutral fat to FA and monoglycerides
Chlolesterol esterase: hydrolysis of cholesterol esters
Phospholipase: hydrolysis of FA from phospholipids
PROTEINS
CHO
FATS
Inactive but get activated when
secreted into intestine
Enterokinase (intestinal mucosa
when chyme in contact)
Trypsinogen ---> Trypsin
Chymotrypsinogen -----> Chymotrypsin
Procarboxypolypeptidase -------> Carboxypolypeptidase

45. Pancreatic Secretion Bicarbonate secretion 145 mEq/L
5x that of plasma
Blood Ductule cells Lumen
CO2 CO2
H2O H2O
H2O
+
H2CO3
H+ HCO3- HCO3-
Na+ Na+
H+
Na+
AT
AT
To maintain electrical neutrality
Osmotic pressure gradient
Isosmotic bicarbonate solution

47. Regulation of Pancreatic Secretion
1. Acetylcholine: From parasympathetic vagus nerve and cholinergic nerves into entric NS
2. Cholesystekinin: secreted by duodenal and upper jejunal mucosa when food enters SI
3. Secretin: secreted by duodenal and upper jejunal mucosa when highly acidic food enters
SI
1 and 2 stimulate acinar cells to secrete digestive enzymes but less water and HCO3-
3 stimulates secretion of Large quantities of water solution of HCO3- to neutralize acid
HCl Soap (fat) Peptone
Rate of
pancreatic
secretion
Water, HCO3
Enzymes
CCK
Secretin

48. Secretion of Liver (Bile)
Bile: 600-1000ml/day
I. Fat digestion and absorption (bile acids)
1. Emulsify large fat particles of food into minute particles attacked by lipase
enzymes in Pancreatic Juice (PJ)
2. Absorption of digested fat end products through intestinal mucosal membrane
II. Bile serves as means of excretion of important waste products from blood.
Bilirubin, end product of Hb destruction and excess of cholesterol

50. Bile salts: Fat Digestion and Absorption
Cholesterol --------------> Bile Salts (soluble)
precursor
Cholic acids or chenodeoxycholic acids
Glyco-and tauro conjugated bile salts
Glycine or taurine
Na+ salts
Bile
Functions:
1. Detergent action on fat particles:Decrease
in surface tension and allows agitation in
intestinal tract to break fat
(emulsification)
2. Absorption of FA, monoglycerides,
cholesterol and other lipids of intestinal
tract by forming micelles (semi soluble in
chyme) without bile salts 40% of fat will be
lost to faeces (metabolic deficit and
nutritional loss)…lacteal

51. Secretions of Small Intestine
Brunners Gland (duodenum)
Secerete mucus in response to;
protects duodenal wall from
digestion by acid. Large amounts
of bicarbonate are secreted by
pancreas which adds to bile to
neutralize acid
Tactile
Vagus stimulation (stomach enzymes)
secretin
Crypts of Lieberkuhn (small intestine)
Crypts secrete various enzymes, including sucrase
maltase, isomaltase and lactase (disacc to monosacc),
along with endopeptidases and exopeptidases and
intestinal lipase.

52. Secretions of Large Intestine
Mucus secretions with HCO3-
Crypts of Lieberkuhn but no villi
No enzymes
Parasympathetic control by pelvic nerves from spinal cord also cause incr in mucus
During extreme PS stimulation and emotional disturbances large amts of mucus is
secreted leading to ropy mucus
Mucus is for adherence of fecal particles and holding it together
Bacterial activity in LI: mucus protects intestinal wall from bacterial activity, because
of bicarbonate and mucus (pH8) provides a barrier for acids formed in the faeces
from attacking intestinal walls.
In case of bacterial infection mucosa is irritated and mucus is produced in large
quantities also large quantities of water and electrolytes (to dilute irritating
factors) are secreted resulting in watery faeces or diarrhea.
Result is loss of water and electrolytes which must be replenished but loss of
irritant factors also occurs which promotes early recovery also.

54. Evolutionary Adaptations of Vertebrate Digestive System
DENTAL ADAPTATIONS
2123
2123
Man 212
212
5134
4134
Opposum
(non placental
mammal)
placental
mammal
cat
3131
3121
rabbit
2033
1023

55. STOMACH AND INTESTINAL ADAPTATIONS

56. RECAPPING THE LAST CLASS
1.Define phases of gastrin secretion.
2.What are the two major functions of bile salts?
3.Which are the major ions of pancreatic secretion?
1.Ca2+ 2. Na+ 3. K+ 4. HCO3-
4.Distinguish between hepaticportal vein and hepatic vein.
5.What biomolecule is used as precursor for bile salts?
6.Canine teeth is prominent in which type of placental
mammals?
7.Which type of mammals do not have incisor teeth?
8.What type of mammals large caecum? Why?
9.Why rabbits are coprophagus?

57. SYMBIOTIC ADAPTATIONS Symbiotic bacteria and protists in gut digest cellulose
Rumen—rechewed–
Reticulum– Omasum--
Abomasum
Which chamber is
equivalent to human
stomach?

61. What is the major cause of diarrhea?
1. Reduced water absorption in large intestine
2. Absence of water absorption in stomach
3. Reduction in secretion of digestive juice in the stomach
4. Absence of bile due to defect in gall bladder
Where is chymotrypsinogen secreted?
1. Stomach
2. Small intestine
3. Large intestine
4. Mouth
What is the role of the hormone “Ghrelin”, also known as lenomorelin?
1. Signal brain to stimulate hunger
2. Reduce hunger
3. Works opposite to leptin in the brain
4. Activate heart rate
Which are true for Cholecystokinin (CCK)?
1. Secreted from duodenum
2. Stimulate gall bladder to secrete bile
3. Stimulate pancreas to secrete digestive enzymes
4. Its secretion is stimulated by entry of HCL, proteins, fatty acids into duodenum
What is leptin?
What are the functions of liver?
How are nucleic acids digested in mammals?