Nutrition Ch.3 Digestion and absorption

Liver/Gallbladder secretions

*Bile* also flows into the duodenum. The liver continuously produces bile, and then stores it in the gallbladder. The gallbladder secretes bile into the duodenum of the small intestine *when fat arrives*. Bile is not an enzyme, it is an *emulsifier* that causes fat to mix with water so that enzymes can break it down

Muscularis

3rd layer made of 2 layers of smooth muscle that move food along the GI tract

Submucosa

A layer of the human digestive system that contains nerves, blood vessels, and lymph nodes

The lymphatic system

A loosely organized system that carries products of fat digestion lymph circulates between the cells (*lymph=blood-like fluid that has no RBCs or platelets) Circulates nutrients (fats and fat-soluble nutrients) from one portion of the body to another Eventually enters the thoracic duct behind the heart, where it then enters the vascular system (will bypass the liver before it circulates to other cells).

Large intestine

After traveling through the small intestine, the remaining contents passes through another sphincter muscle: the *ileocecal valve*, which marks the entrance to the large intestine (colon) The contents passes another opening, but does not go through it (opening to the appendix). As the contents travels to the rectum, the colon withdrawals water, leaving semisolid waste. The rectum and anal muscles hold this waste until it is time to poop. Then, the rectum muscles relax and the two sphincters of the anus allow the waste to pass.

Vascular system and the digestive system

Blood is carried to the digestive system through arteries which branch into capillaries to reach the cells. The blood leaves the capillaries through the *hepatic portal vein*, which actually goes to the *liver* rather than back to the heart. The *hepatic vein* branches into capillaries so that every cell of the liver has access to the blood. Blood leaving the liver goes through the *hepatic vein* back to the right side of the heart.

Pancreatic secretions

By the time enzymes leave the stomach, digestion of proteins, lipids, and carbohydrates has already begun. Digestion pick up in the small intestine, though. The pancreas secretes *pancreatic juice* into the duodenum, which acts on all 3 macronutrients Pancreatic juice contains enzymes, and *bicarbonate*. Bicarbonate is basic (in contrast to the stomach's acidity). This neutralizes the acidic chyme, allowing the enzymes from the intestine and pancreas to function the best.

Three phases of digestion

Cephalic phase: "wake up" -when we smell/food, central nervous system is triggered and we start salivating/stomach will start producing juices -Chemoreceptors and mechanical receptors Gastric phase: "engines are starting" -Food is in stomach -Stomach senses that there's pressure -signal sent to brain -brain says "more gastrin" Intestinal phase: begins when chyme enters small intestine. Lower pH, lower gastric juices, tell pancreas to release secretin (and maybe CCK)

Regulation of the digestive system

Combined work of two systems: *Hormonal (endocrine) system* and the *nervous system* To maintain homeostasis, their is a feedback loop between the two systems. When something is off balance, a response is triggered.

Esophagus

Contains an *esophageal sphincter* muscle at both ends. When food is swallowed, the upper sphincter opens and allows the food to pass through a hole in the diaphragm that leads to the stomach. The lower sphincter at the opening of the stomach than closes so that the bolus will not re-enter into the esophagus.

hydrochloric acid

Found in gastric juices, uncoils (digests) proteins in the stomach

Secretions that break down proteins

Gastric juices from the gastric glands in the stomach Pancreatic juices in the small intestine Intestinal juices from the small intestine

GI Hormones

Gastrin, Secretin, Cholecystokinin (CCK)

The muscular action of digestion

In mouth, we consciously chew and swallow. But the other muscles of the digestive system perform involuntarily. The keep the process slow enough for the job to get done, and fast enough to make progress.

Gastric glands and their secretions

In the stomach, *gastric glands* secrete *gastric juices*. Gastric juices contain water, enzymes, and *hydrochloric acid* which acts primarily in *protein digestion* Gastric juices make the stomach highly acidic. In order to protect stomach cells from being destroyed from the acidity, the stomach cells secrete *mucus*. The mucus coats cells, protecting them from the acidity, enzymes, and harmful bacteria. Salivary enzymes do not function in the acidic environment of the stomach, therefore the salivary digestion of carbs ceases int he stomach, and the salivary enzymes become proteins that the stomach digests.

Excretion

Metabolic activity produces metabolic waste Systems that deal with waste: -Respiratory system, Skin (releases water) -Urinary system: (released nitrogen) Food goes through digestive system, nutrients is absorbed, anything not absorbed goes out the anus Cells exchange nutrients with blood, and also drop off waste. That waste is sent to the urinary system, kidneys *filtrate* waste from blood, important substances are returned to circulatory system to be *reabsorbed*, then you pee out the bad stuff CO2 released by lungs

Anatomy of the Absorptive System

Most absorption occurs in the small intestine. The small intestine is wrinkled into hundreds of folds with finger like projections called *villi* A single villi is composed of hundreds of cells, and each of these cells are covered with their own microscopic hairs called *microvilli* Between each villi are *crypts*, which are tubular glands that secrete intestinal juices. Nearby *goblet cells* secrete mucus. Villi are lined with sheets of muscle, and are in constant motion. Any nearby nutrients that are small enough to be absorbed are trapped by the microvilli and then drawn into the intestinal cells. Microvilli, which protrude from an intestinal cell's membrane, contain different kinds of pumps and enzymes that act on different kinds of nutrients. The cells in different portions of the digestive tract are specialized to absorb specific nutrients

Segments of the GI tract (top to bottom)

Mouth Pharynx Esophagus Stomach Small Intestine Large Intestine (Colon)

Neural and Hormonal Mechanisms

Neural: -Central nervous system -Enteric Nervous system (sensory receptors) Hormonal: -GI hormones

The circulatory system

Once a nutrient enters the bloodstream, it is delivered wherever it is needed by the circulatory system.

Secretin

Responds to: Acidic chyme in small intestine Secreted from: Duodenal wall Stimulates: the pancreas Response: pancreas secretes bicarbonate juices to neutralize acidity

Cholecystokinin

Responds to: fat or protein in the small intestine Secreted from: Intestinal wall Stimulates: Gallbladder and pancreas Response: Secretion of Bile to emulsify fats and AND Secretion of bicarbonate and enzymes from the pancreas to maintain slightly basic pH, digest proteins and fats, and slow GI tract motility

Gastrin

Responds to: food in the stomach Secreted from: Stomach wall Stimulates: Stomach glands Response: Hydrochloric acid secreted into the stomach to maintain an acidic pH

Secretions that break down carbohydrates

Saliva from the salivary glands Pancreatic juices in the small intestine Intestinal juices from the small intestine

5 organs that contribute to secretion

Salivary glands Gastric glands Pancreas Liver/gallbladder Intestinal glands

Bicarbonate

Secreted by the Pancreatic glands, neutralizes chyme so it can be broken down further in small intestine

Absorption of nutrients (diffusion and active transport)

Simple diffusion: some nutrients (water and small lipids) cross into intestinal cells freely (low to high concentration gradient). No energy required Facilitated Diffusion: Some nutrients (water soluble vitamins) need a specific carrier to transport them from one side of the cell wall to the other. No energy required Active transport: requires energy to move against concentration gradient (glucose, amino acids). Utilizes a carrier also.

Intestinal glands and their secretions

Small intestine secretes intestinal juice that contains enzymes. These enzymes break down carbs, lipids, and proteins. Also, the small intestine secretes mucus to protect the intestinal lining

The muscular action of digestion: Sphincter Contractions

Sphincters, specifically the esophageal ones, prevent *reflux* of the stomach content. All sphincters are muscles that are designed to keep the contents flowing in the right direction at the right pace.

Digestion

The body breaks down food into nutrients and prepares it for absorption

Small intestine

The chyme passes through the opening of the common bile duct, which drips fluids from the pancreas and the gallbladder. The chyme travels down the three sections of the small intestine: -*duodenum* -*jejunum* -*ileum*

The muscular action of digestion: Segmentation

The circular muscles of the intestine rhythmically contract and squeeze their contents. These contractions, called *segmentation*, mix the chyme and promote close contact with digestive juices and the absorbing cells of the intestinal walls before letting the contents move along.

The muscular action of digestion: Peristalsis

The entire GI tract is rigged with circular muscles that are surrounded by longitudinal muscles. When the rings tighten and the long muscles relax, the tube is constricted. When the rings relax and the long muscles contract, the tube bulges. This action-- *peristalsis* --occurs continuously and pushes the intestinal contents along. The waves of contraction ripple along the GI tract at varying rates and intensities depending on the part of the GI tract and on whether food is present.

Mucosa

The innermost layer of the human digestive tract; in some parts of the digestive system, it contains mucus-secreting cells and glands that secrete digestive enzymes

The Liver's role in nutrient transportation

The liver wants to be the first to receive the nutrients absorbed from the GI tract, because it prepares nutrients to be used by other parts of the body. *The liver is the most metabolically active organ of the body*

Lumen

The space within the tube of the GI tract

The muscular action of digestion: Stomach Action

The stomach has the thickest walls and strongest muscles of the GI tract. Along with the circular and longitudinal muscles, it has diagonal muscles that alternatively contract and relax. Together, these three sets of muscles force the chyme downward. The pyloric sphincter remains tightly closed to keep chyme from spilling into the small intestine. The stomach walls secrete fluids as the chyme is churned to liquefy it, and then the sphincter will periodically open to allow small portions into the small intestine.

Stomach

The stomach retains the bolus for awhile in the upper portion. Little by little, the stomach send the food to the lower portion where it is grinded and has fluid added to it to make a paste called *chyme*. Bit by bit, the stomach transfers the chyme through the *pyloric sphincter* into the small intestine, and the sphincter closes behind the chyme. *pyloric sphincter*: a circular muscle in the stomach that releases chyme to the small intestine

After absorption

When nutrients have been absorbed, they either enter the *bloodstream* or the *lymphatic system*. Both transport systems supply vessels to the villi Water soluble nutrients and small lipids go directly into the bloodstream where they are transported to the liver via the hepatic vein. Larger fats cannot be sent into bloodstream because they do not like water. They are assembled into larger molecules and then carried by special proteins and released into the lymphatic system. They move through lymph until they can enter the bloodstream at a point near the heart

Mouth

Where digestion begins Chewing crushes food, and fluids from salivary glands and food blend/soften these pieces to ease swallowing. Fluids also dissolve the food so that the tongue can taste it. Food is swallowed and passed to the pharynx (shared by respiratory system and digestive system). To bypass the entrance to the lungs, the epiglottis closes off the airway to prevent choking. After a mouthful of food has been chewed and swallowed, it is called a bolus.

Enzyme

a protein that facilitates chemical reactions by: -synthesizing/breaking down molecules -changing the arrangement of molecules -exchanging parts of molecules The suffix "ase" indicates that the substance is an enzyme. The word will often include the organ in which it comes from, and be compound it works on.

Bile

emulsifier, made by the liver, stored in the gallbladder which secretes it into the small intestine to mix fat with water so that it can be broken down.

Catalysts enzymes

enzyme remains unchanged

Gastrointestinal tract

flexible muscular tube that extends from the mouth to the anus

layers of digestive system

mucosa, submucosa, muscularis, serosa

Serosa

outermost layer

Secretions that break down lipids

pancreatic juice in the small intestine Intestinal juice in the small intestine

Digestive enzymes

proteins found in digestive juices that act on food substances, causing them to break down into simpler compounds

Accessory organs of the GI tract

salivary glands, liver, gallbladder, pancreas

Salivary glands and their secretions

secrete saliva to moisten food and make swallowing easier. Saliva contains enzymes that *initiate the digestion of carbohydrates* Saliva also protects the teeth, esophagus, mouth, and stomach from substances that may cause damage

Vascular system (blood circulatory system)

system of vessels through which blood flows continuously and the hearts serves as the pump. As the blood circulates, and distributes/picks up materials as needed. All body tissues derive nutrients and oxygen from the blood, and also release waste into the blood. The nutrients are supplied by the digestive system. Blood leaving the right side of the heart circulates through the lungs and then back to the left side of the heart. The left side of the heart pumps blood out of the aorta into arteries to all systems of the body. The blood circulates in the capillaries (small vessels that branch from arteries) where it exchanges materials with cells, and then goes to veins which bring it back to the left side of the heart.