Lecture 18 The Digestive System. Food for Energy and Growth Food provides animals with energy and essential nutrients for growth A healthy diet contains.

Lecture 18The Digestive System

Food for Energy and Growth

Food provides animals with energy and essential nutrients for growth

A healthy diet contains more carbohydrates than fatsIt also contains a lot of proteins

The pyramid of nutrition

Who Is Overweight?

The body mass index is used to determine who’s overweight

BMI =body weight in kg

(height in m)2 (height in in)2

=(body weight in lbs) X 703

Obesity

Many vertebrates are unable to manufacture one or more of the 20 amino acids needed to make proteins Humans are unable to synthesize 8 amino acids These essential amino acids must be obtained through food

In addition, all vertebrates cannot synthesize certain polyunsaturated fats

Trace elements Minerals required in small amounts Iodine, cobalt, zinc, molybdenum, manganese

Vitamins Essential organic substances required in small amounts Humans require at least 13 different vitamins

Vitamin C (ascorbic acid) if not in diet, the disease scurvy will develop

Essential Substances for Growth

Evolution of the Digestive System

Digestive System Overview

Consists of tubular gastrointestinal tract and accessory digestive organs Mouth and pharynx Esophagus – Delivers food to

the stomach Stomach – Some preliminary

digestion Small intestine – Digestion and

absorption Large intestine – Water and

mineral absorption Rectum – Waste excretion

General Structure of Digestive Organs

The gastrointestinal tract has a characteristic layered structure

MucosaEpithelium

SubmucosaConnective tissue

MuscularisSmooth muscles

SerosaConnective tissue

Breaking Down Food in the Mouth

Many vertebrates have teeth that are used for chewing (mastication)

Carnivores have pointed teeth adapted for cutting and shearing

Herbivores have large flat teeth well suited for grinding plant cellulose

Omnivores have carnivorous teeth in front and herbivorous teeth in the back

The Structure of a Tooth

Tooth and Gum Disease: Periodontitis

Dental caries – gradual demineralization of enamel and dentin by bacterial action Dental plaque, a film of sugar, bacteria, and mouth debris, adheres to teeth Acid produced by the bacteria in the plaque dissolves calcium salts Without these salts, organic matter is digested by proteolytic enzymes Daily flossing and brushing help prevent caries by removing forming plaque

Gingivitis – as plaque accumulates, it calcifies and forms calculus, or tartar

Accumulation of calculus: Disrupts the seal between the gingivae and the teeth Puts the gums at risk for infection

Periodontitis – serious gum disease resulting from an immune response Immune system attacks intruders as well as body tissues, carving pockets

around the teeth and dissolving bone

Tongue

Superior surface bears three types of papillae Filiform – give the tongue roughness and provide friction Fungiform – scattered widely over the tongue and give it a reddish hue Circumvallate – V-shaped row in back of tongue

Sulcus terminalis – groove that separates the tongue into two areas: Anterior 2/3 residing in the oral cavity Posterior third residing in the oropharynx

In The Mouth

The tongue mixes food with a solution called saliva Saliva moistens and

lubricates food and contains amylase which initiates breakdown of starch into maltose

The secretions of the salivary glands are controlled by the nervous system A continuous secretion of

about 0.5 milliliters per minute keeps the mouth moist

The presence of food in the mouth triggers an increased rate of secretion

Prior to swallowing, the tongue moves food to the back of the mouth

The soft palate elevates, pushing against back wall of pharynx This stimulates neurons to send impulses to the swallowing center in

the brain Muscles contract and raise the larynx The glottis is pushed against the epiglottis which keeps food out of the

respiratory tract, and into the esophagus

Swallowing

The esophagus is a muscular tube that connects the pharynx to the stomach

The swallowing center stimulates successive waves of contraction

Peristalsis moves food along the esophagus to the stomach

The Esophagus and Stomach

Movement of food from esophagus into stomach is controlled by a ring of smooth muscle, the sphincter Humans lack a true

sphincter and thus, can regurgitate

The stomach is a saclike portion of the tract with a convoluted surface enabling expansion It contains an extra layer

of smooth muscles for mixing food with gastric juices

Two kinds of secretory cells

Parietal cells – Secrete hydrochloric acid

Chief cells – Secrete pepsinogen

The human stomach produces about 2 liters of HCl and other gastric secretions every day This gastric juice has a pH of ~ 2

It kills most bacteria ingested with food and also denatures food proteins

The mixture of partially digested food and gastric juice is termed chyme

Chyme leaves the stomach to the small intestine through the pyloric sphincter

The stomach and gastric glands

The hormone gastrin regulates the synthesis of HCl

Overproduction of gastric acid can occasionally eat a hole through the stomach wall These gastric ulcers are rare

Susceptibility increases when mucosal barriers are weakened by Helicobacter pylori infection

Over 90% of gastrointestinal ulcers are duodenal ulcers Caused by too much chyme in the small intestine

Ulcers

The small intestine is the body’s true digestive vat It breaks down large

molecules into smaller ones which are then absorbed into the bloodstream

The small intestine is ~ 6 m long The first 25 cm (~ 4%)

constitute the duodenum

The duodenum is the actual site of digestion The pancreas secretes

digestive enzymes into it The liver secretes bile salts

into it, to make fats easier to digest

The Small Intestine

The ileum is the rest of the small intestine (~ 96%) It is devoted to absorption

The lining is covered with finger-like projections called villi

Each cell covering a villus has cytoplasmic projections called microvilli which increase the absorptive surface

Anatomy of the Small Intestine

The Large Intestine

The large intestine, or colon is only 1 meter long But has a larger diameter than the small intestine

No digestion and little absorption take place in the large intestine Its primary function is to act as a refuse dump by collecting and compacting

solid wastes Feces pass through the rectum as a result of peristalsis and leave the body

through the anus

Accessory Digestive Organs

Pancreas Functions as both an exocrine & endocrine

gland Exocrine: Cell clusters called acini secrete

Trypsin and chymotrypsin which digest proteins

Amylase which digests starch Lipase which digests fats Bicarbonate which neutralizes HCl in

chyme Endocrine: Cell clusters called Islets of

Langerhans secrete Insulin and glucagon which regulate sugar

levels in blood

Liver Largest internal organ of the body Its main exocrine secretion is bile

Aids in the digestion of fats in the duodenum

Chemically modifies substances absorbed in the gastrointestinal tract

Converts poisons into less toxic forms Produces most of the proteins found in plasma

Gall bladder Stores and concentrates bile Delivers it to the duodenum

via the bile duct

Microscopic Anatomy of the Liver

Liver sinusoids – enlarged, leaky capillaries located between hepatic plates

Kupffer cells – hepatic macrophages found in liver sinusoids

Hepatocytes’ functions include: Production of bile Processing

bloodborne nutrients

Storage of fat-soluble vitamins

Detoxification

Secreted bile flows between hepatocytes toward the bile ducts in the portal triads

Homeostasis

Homeostasis is the dynamic constancy of the internal environment Conditions fluctuate continuously within narrow limits

Homeostasis is essential for life

Play Hormones & Gastric Secretion

Blood glucose levels are monitored by the Islets of Langerhans in the pancreas

Regulating Blood Glucose

When levels are high, insulin is released

When levels are low, glucagon is released