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Human Anatomy & Physiology: Digestive System; Ziser Lecture Notes, 2014.4 1
The Digestive System
We need food for cellular utilization: !nutrients as building blocks for synthesis !sugars, etc to break down for energy most food that we eat cannot be directly used by the body !too large and complex to be absorbed !chemical composition must be modified to be useable by cells digestive system functions to altered the chemical and physical composition of food so that it can be absorbed and used by the body; ie Functions of Digestive System:
**1. physical and chemical digestion
- absorption
- collect & eliminate nonuseable** components of food Human Anatomy & Physiology: Digestive System; Ziser Lecture Notes, 2014.4 2
Anatomy of the
Digestive System
organs of digestive system form essentially a long continuous tube open at both ends ! alimentary canal (gastrointestinal tract) mouth! pharynx! esophagus! stomach! small intestine! large intestine attached to this tube are assorted accessory organs and structures that aid in the digestive processes salivary glands teeth liver gall bladder pancreas mesenteries The GI tract (digestive system) is located mainly in abdominopelvic cavity surrounded by serous membrane = visceral peritoneum this serous membrane is continuous with parietal peritoneum and extends between digestive organs as mesenteries ! hold organs in place, prevent tangling The wall of the alimentary canal consists of 4 layers: outer serosa : visceral peritoneum, mainly fibrous and areolar CT with some pockets of adipose CT muscularis several layers of smooth muscle submucosa blood vessels, lymphatic vessels, nerves, connective tissue inner mucosa : small band of muscle tissue, muscularis mucosa mucus membrane lining contains goblet cells that secrete mucous for protection these layers are modified within various organs ! some have muscle layers well developed ! some with mucous lining modified for secretion of digestive juices ! some with mucous lining modified for absorption
1. Mouth (Buccal Cavity, Oral Cavity) bordered above by hard and soft palate forms partition between mouth and nasal passages uvula is suspended from rear of soft palate blocks nasal passages when swallowing tongue lines ventral border of mouth cavity is skeletal muscle covered with mucous membrane contains taste buds frenulum is thin fold of mucous membrane on ventral surface of tongue that anchors the tongue to the floor of the mouth short frenulum! “tongue tied” Teeth two sets deciduous (=baby teeth) (20) begin at 6 months; shed 6-13 yrs permanent teeth (32) each tooth has a crown (above gum) neck is where crown, gum and root meet root (below gum) imbedded in socket gingivitis = inflammation of gum surrounding teeth; can lead to
Human Anatomy & Physiology: Digestive System; Ziser Lecture Notes, 2014.4 5 periodontal disease kinds of teeth modified for specific functions incisors – 4+4; cut, knip canines – 2+2; holding onto prey premolars – 4+4; cutting, crushing molars – 6+6; chewing, grinding, crushing each tooth is composed of several layers: enamel very hard outer surface on upper exposed crown only resists bacterial attack cannot regenerate if damaged dentin below enamel less hard, similar to bone matrix decays quickly of enamel is penetrated pulp living portion of tooth consists of blood vessels, nerves cementum on root of tooth only outer surface holds root into socket in jaws Human Anatomy & Physiology: Digestive System; Ziser Lecture Notes, 2014.4 6 Salivary Glands 3 Pairs of salivary glands : sublingual submandibular parotid largest, below ears mumps = acute infection of parotid gland secrete saliva (enzymes and mucous for digestion)
2. Pharynx (throat) already discussed 3. Esophagus collapsible tube ~ 10” long extends from pharynx to stomach !gets food through thorax to abdominal cavity posterior to trachea and heart pierces diaphragm uses peristalsis to move food to stomach ! can swallow upsidedown drains into stomach through the cardiac orifice surrounded by the **lower esophageal sphincter
- Stomach** muscular sac just below diaphragm and liver alimentary canal expands to form stomach 50 mL when empty; up to 1.5 L after meal Major functions of stomach:
- physical digestion – churning action
- chemical digestion – esp proteins
- limited absorption (some water, alcohol, certain drugs) divided into 4 regions: cardiac fundus body pyloris cardioesophageal sphincter lesser curvature cardiac fundus pyloric sphincter body greater curvature pyloris Muscle layers are very well developed in stomach circular longitudinal oblique Help to break up food by churning action results in milky white liquid = chyme sphincter muscles close both stomach openings: cardioesphageal sphincter (=lower esophageal sphincter) heartburn !doesn’t close properly pyloric sphincter cholic in babies! doesn’t open properly given smooth muscle relaxers mucosal lining of stomach is folded into rugae to allow for expansion with a meal within the mucous lining of stomach are glandular tubes called gastric pits !within gastric pits are numerous microscopic gastric glands : ! secrete mucous for protection ! secretes various digestive enzymes ! secretes HCl
Human Anatomy & Physiology: Digestive System; Ziser Lecture Notes, 2014.4 13 !help break down hard to digest fibers and starches !make essential vitamins and additional nutrients !protect us from pathogens, toxins and some carcinogens !activate our immune systems to better resist infections gut bacteria change and adapt as your foods change ! those better able to metabolize dominant food tend to increase gut bacteria affect our mood and behavior: correlations have been found between gut flora and some psychiatric disorders such as depression, autism and schizophrenia obesity, diabetes, Crohn’s disease, colitis, celiac disease, irritable bowel syndrome all may be the result of an imbalanced microbial ecosystem in our guts some forms of severe malnutrition have been linked to a particulary group of intestinal bacteria promising research has found that fecal transplants have cured symptoms of Parkinsons, diabetes and obesity eg. 100% cure rate for C. difficile infections, a deadly disease common in patients on antibiotic therapy Human Anatomy & Physiology: Digestive System; Ziser Lecture Notes, 2014.4 14 use of antibiotics can cause dramatic and long term changes in our gut flora and increase risk of some chronic diseases in the future: eg. might be able to test for changes in kinds and numbers of species as an early indication of certain diseases eg. doctors may prescribe bacterial supplements to improve physical health eg. fecal transplants: restores bowel flora to a healthy state
7. Serous Membranes body wall and organs of abdomen are lined with peritoneum !parietal peritoneum !visceral peritoneum most, but not all, of the visceral organs are completely lined with visceral peritoneum these layers are continuous with thin flaps of serous tissues = mesenteries mesenteries allow free movement while holding organs in place and prevent them from tangling greater omentum fold of mesentery extending from stomach and duodenum loosely covers the small intestine like an apron contains fat deposits lesser omentum smaller fold of mesentery between liver and stomach Accessory Organs of Digestive Tract A. Liver is the largest gland in body lies immediately under the diaphragm consist of 2 lobes separated by falciform ligament receives blood from the Hepatic Artery and the Hepatic Portal Vein Hepatic Artery Hepatic Vein Hepatic Portal Hepatic Bile Duct Vein
Liver
blood leaving the liver enters the Hepatic Vein to the Vena Cava bile leaves the liver through the Hepatic Bile Duct B. Gall Bladder lies on undersurface of liver 3 - 4” long and 1.5” wide liver produces 0.6 – 1.2L of bile/day bile travels up Cystic Duct to gall bladder for storage can hold 30-50 ml of bile gall bladder stores and concentrates bile When needed bile travels down Cystic Duct to Common bile Duct to the duodenum C. Pancreas most digestion is carried out by pancreatic enzymes in curve of duodenum and dorsal to greater curvature of the stomach (retroperitoneal) 6 - 9 “ long
Human Anatomy & Physiology: Digestive System; Ziser Lecture Notes, 2014.4 17 composed of 2 kinds of glandular tissue: endocrine! secretes hormones islets = 2% of total mass of pancreas their secretions pass into circulatory system secrete insulin and glucagon exocrine! digestive function pancreatic digestive secretions average ~2L/day ! mainly on demand, in short timespans pancreatic secretions are collected in pancreatic duct and usually a smaller accessory pancreatic duct that both drain into the duodenum Human Anatomy & Physiology: Digestive System; Ziser Lecture Notes, 2014.4 18
Digestive Physiology
Muscular Movements (=motility) in GI Tract as materials are being processed they are moved through alimentary canal by by several muscular processes: chewing voluntary movements of skeletal muscles swallowing coordinated activity of skeletal and smooth muscles reflex controlled by medulla pharynx to esophagus peristalsis propulsive movements sequential smooth muscle contractions in adjacent segments !pushes food forward esophagus, stomach, small intestine, large intestine segmentation mixing movements alternating contractions and relaxations of adjoining portions of intestine food is moved backward and foreward !helps to physically break up and mix contents for better digestion & absorption mass movements occur 1-3 times/day when all circular muscle constricts in a long stretch of intestine to push food toward anus ! main propulsive force in large intestine sphincters tonic contractions of smooth and skeletal muscles that control the emptying and filling of various portions of the GI tract Digestion digestion = all food changes that occur in the alimentary canal need to convert food into a form that can be absorbed and used by body cells two types of digestion: physical digestion breaking large pieces down into smaller pieces chemical digestion breaking large molecules (proteins, fats, starches, etc) into small molecules (amino acids, fatty acids, sugars, etc)
1. Mouth food entering mouth is physically broken down teeth mixed with saliva lubricant enzyme = amylase ! begins carbohydrate digestion at end of digestion in mouth, food = bolus 2. Pharynx bolus is swallowed uvula closes off nares epiglottis closes off glottis of larynx 3. Esophagus wave of reflex contractions = peristalsis 4. Stomach muscular contractions separate and mix food particles and move them toward the pylorus in stomach bolus is mixed with gastric juices gastric juices low pH ~ ! ideal for breaking proteins into smaller fragments gastric ulcers : Helicobacter pylori part of normal flora of stomach can neutralize stomach acids excessive growth can irritate stomach lining to produce ulcers physical digestion is completed in stomach once digestion in stomach is competed have a white milky liquid = chyme stomach takes about 2-6 hours to empty after a meal
Human Anatomy & Physiology: Digestive System; Ziser Lecture Notes, 2014.4 25 Ileum reclaims some additional bile salts Small intestine is greatly modified for absorption
- epithelial cells are joined by tight junctions better control of what is absorbed substances cant move between cells materials must pass through cells to get to interstitial spaces (=transepithelial transport)
- surface area is greatly increased for more efficient absorption of nutrients: 1” diameter x 10’ long ! if smooth tube = 0.33 m^2 (3 sq ft) but: interior is folded ! increases area ~3 x’s also: fingerlike projections = villi ~1mm tall contain capillary beds contain lacteals ! increases area another 10x’s also: each epithelial cell of villus has microvilli up to 1700/cell =brush border ! increases area another 20x’s Total Area = 200m^2 (1800 sq ft) Human Anatomy & Physiology: Digestive System; Ziser Lecture Notes, 2014.4 26 Large Intestine additional water if body needs it some Vit K and B’s made by bacteria there Mechanisms of Absorption absorption can be an active or passive process: 1. most nutrients are absorbed by active transport eg. glucose amino acids some minerals 2. water is absorbed by osmosis 3. large molecules are absorbed by pinocytosis eg. a few large fats and proteins; fats passed to lacteals with other fats 4. some lipids are absorbed by diffusion to lacteals Feces = “residue of digestion” cellulose connective tissues, fibers, toxins from meats undigested fats and mucous bacteria (~50%) feces may also contain recognizable remnants of poorly digested foods: corn, peanuts, peas, carrots, cereals, beans Liver Processing the liver is main organ for metabolic regulation in the body ! over 200 specific functions
- stores iron, vitamin A, B 12 & D
- helps stabilize blood glucose levels by storing excess glucose or synthesizing glucose if needed
- carries out most of body’s fat synthesis including cholesterol and phospholipids
- synthesizes plasma proteins & degrades excess amino acids
- phagocytes remove old/damaged blood cells and pathogens
- detoxify blood from digestive system removes drugs, alcohol, antibiotics, etc
- is largest blood reservoir in body receives 25% of cardiac output
- collects and removes metabolic wastes such as cholesterol, products of RBC destruction, etc
- secrete bile to aid in digestion (~1pt /day) Liver Lobule lobule is functional unit of liver !each liver lobe is divided into 1000’s of lobules tiny hexagonal cylinders (~2mm x 1mm) ~ 1 million lobules in human liver small branches of hepatic vein extend through middle of each lobule as central vein sinusoid spaces lined with hepatic cells extend outward from central vein around periphery of each lobule are branches of hepatic portal vein hepatic artery hepatic bile ducts ! arterial blood brings oxygen to liver cells ! venous blood from hepatic portal vein delivers blood through lobule for “inspection”: a. phagocytic cells remove toxic compounds and convert them to nontoxic compounds b. some vitamins and nutrients are removed and stored c. synthesis of starches, lipids and proteins for storage ! cholesterol, bile pigments and bile salts are secreted
Human Anatomy & Physiology: Digestive System; Ziser Lecture Notes, 2014.4 29 into bile ducts for later use in digestion of fats hepatic bile ducts join cystic duct! store bile in gall bladder Hepatic Artery Hepatic Vein Hepatic Portal Hepatic Bile Ducts Vein Hepatic Duct Cystic Duct Common Bile Duct sinusoids oxygen removes toxins stores vitamins stores nutrients Human Anatomy & Physiology: Digestive System; Ziser Lecture Notes, 2014.4 30
The Aging Digestive System
shows significant senescence in old age: less saliva ! food less flavorful, harder swallowing ~half of those over 65 yrs wear dentures gastric mucosa secretes less acid ! reduces absorption of Calcium iron, zinc and folic acid gastric mucosa secretes less intrinsic factor ! reduces absorption of vitamin B ! leads to pernicious anemia Heartburn becomes more common most common digestive complaint of older people is constipation !due to: less muscle tone weaker colon peristalsis reduced sensitivity to neurotransmitters less fiber & water in diet less exercise activity of liver, gall bladder and pancreas are reduced only slightly in old age
Digestive Problems
1. Choking food in air passages usually meats, hot dogs, grapes, carrots, hard candy, popcorn, peanut butter may not be able to make a sound DON’T hit on back 2. Vomiting symptom of many diseases waves of reverse peristalsis if severe may empty duodenum as well rest and drink small amounts of fluids guard against massive fluid loss 3. Bulemia self induced vomiting may cause damage and infection of esophagus, pharynx, or salivary glands erosion of teeth, more dental caries esophagus may rupture or tear 4. Diarrhea frequent loose watery stool intestinal contents moving too fast for fluid absorption to occur main danger is fluid loss also upsets acid/base balance 5. Constipation caused by: lifestyle! inadequate water input lack of physical activity side effect of medication controlled by increase in fiber, prunes, laxatives ! attracts water! softens stool Colonic Irrigation alternative medical practice potentially harmful unneccessary can rupture the intestine frequent use of laxatives and enemas: can lead to dependency upset body’s fluid balance mineral oil can interfere with absorption of fat soluble vitamins 6. Belching results from swallowed air carbonated drinks and chewing gums can contribute occasionally can be a sign of a more serious disorder: gall bladder pain, colonic distress eat slowly, chew thoroughly relax while eating 7. Hiccups repeated spasms of diaphragm may be triggered by eating or drinking too fast 8. Gas large intestine generates 7-10 L of gas/day and normally we expel ~500ml of gas/day the rest is reabsorbed most is odorless 1% are “volatile” gasses high carb foods known to produce excess gas 9. Heartburn (& gastroesophageal reflux disease) cardiac sphincter doesn’t close properly affects 50% of US, esp white males
