Chapter 3 - The Human Body: Are We Really What We Eat?

What is the organization of the human body from smallest to largest?

Atoms → molecules → cells → tissues → organs → organ systems

What is an atom?

The smallest unit of matter; atoms bond together in specific configurations to form molecules

What is a molecule?

A group of atoms bonded together in a specific configuration; examples include H2O and CO2

Why does the body digest carbohydrates proteins fats and vitamins?

Digestion breaks large molecules into smaller molecules that can be absorbed and used to provide energy and build maintain and repair body structures

What is a cell?

The smallest unit of life, cells can grow, reproduce, take in nutrients, produce chemicals, and excrete waste

Why must many body cells be continually replaced?

Many cells have relatively short life spans and must be replaced to maintain body tissues and functions

How are nutrients from food used by cells?

Molecules produced by digestion are absorbed and used to provide energy and to build maintain and repair cells and other body structures

What is a tissue?

A group of cells that work together to perform a common function

How does muscle tissue contribute to digestion?

Muscle tissue mixes food with digestive juices and moves food through the GI tract

How does nervous tissue contribute to digestion?

Nervous tissue helps control and coordinate body functions including digestive activities

What is an organ?

A sophisticated organization of different tissues that performs a specific function

What are examples of organs involved in digestion and body regulation?

The stomach mixes digests and stores food; the heart pumps blood; the brain coordinates functions including hunger and digestion

What is an organ system?

A group of organs that work together to perform a particular function

What are the major functions of the gastrointestinal system?

It moves food through the body digests food absorbs nutrients and eliminates waste

What are enterocytes?

Specialized cells lining much of the small intestine that absorb nutrients from digested food

What is the cell membrane?

The outer layer surrounding a cell that controls what enters and leaves the cell

What is the structure of the cell membrane?

It consists of two layers of phospholipids with hydrophobic lipid tails facing inward and hydrophilic phosphate heads facing outward

What do cholesterol and proteins do in the cell membrane?

Cholesterol contributes to membrane flexibility, while proteins help transport substances across the membrane

What does selectively permeable mean?

A membrane allows some substances to pass through while restricting others

What does hydrophobic mean?

Repelled by or does not mix with water

What does hydrophilic mean?

Attracted to or mixes with water

What is cytoplasm?

The semiliquid material inside a cell that contains organelles and is the site of many cellular processes

What are organelles?

Specialized structures within cells that perform particular cellular functions

What is the function of the nucleus?

It contains DNA which holds the instructions used to make proteins

What is the function of ribosomes?

They use genetic instructions to assemble proteins

What is the function of the endoplasmic reticulum?

It helps further process and package proteins for transport and use within or outside the cell

What is the function of mitochondria?

They produce ATP from food components; ATP is the usable energy source for many cellular activities

What is hunger?

A physiological drive to eat that occurs when the body senses that it needs food

What is appetite?

A desire to eat stimulated by factors such as the sight smell or thought of food; it can occur without an immediate physiological need for food

What is the cephalic phase of digestion?

The earliest phase of digestion in which the nervous system stimulates digestive secretions in preparation for food before it reaches the stomach

What is anorexia?

The absence of appetite

What role does the hypothalamus play in hunger and satiety?

It helps regulate hunger, satiety, and other body functions by receiving and responding to signals from the digestive system and hormones

What is the feeding center?

A group of nerve cells in the hypothalamus that stimulates hunger and drives eating

What is the satiety center?

A group of nerve cells in the hypothalamus that promotes feelings of fullness and helps signal when to stop eating

How does the digestive tract communicate with the hypothalamus about hunger and fullness?

Nerve cells in the stomach and small intestine detect conditions such as whether the organs are empty or distended while hormones also send chemical signals to the brain

How does an empty stomach influence hunger?

Signals indicating that the stomach is empty suppress the satiety center allowing the feeding center to dominate and promote eating

What are hormones?

Chemical messengers usually made of proteins or lipids that travel through the bloodstream to target organs and regulate body functions

What is ghrelin and how does it affect hunger?

A hunger-promoting hormone released by the stomach; its levels decrease after eating, and it helps signal the brain that it is time to eat

What is cholecystokinin or CCK?

A hormone produced by the small intestine that promotes satiety or feelings of fullness

What is leptin?

A hormone produced by adipose cells and enterocytes that helps regulate hunger and satiety by signaling the brain that energy stores are available

What did the 2016 The Biggest Loser study suggest about drastic weight loss?

Contestants lost more than 100 pounds on average but experienced sharply decreased leptin levels and slower metabolism; six years later most regained weight suggesting that hormonal and metabolic adaptations can promote weight regain

What are the functions of insulin and glucagon?

They help maintain blood glucose; insulin lowers blood glucose by promoting glucose uptake and storage while glucagon raises blood glucose by signaling the release of stored glucose

Which macronutrient generally has the highest satiety value?

Protein generally has the highest satiety value

How do carbohydrates compare with fats in satiety value?

Carbohydrates generally have a lower satiety value than fats

Why do foods high in fiber and water promote satiety?

They add bulk to the stomach and digestive tract without necessarily adding many calories which can increase feelings of fullness

How do solid foods compare with liquids for satiety?

Solid foods are generally more filling than semisolid foods or liquids; for example whole grapes are more filling than grape juice with the same calories

What is the gastrointestinal or GI tract?

A series of organs arranged as a long tube through which food passes as it is moved digested and absorbed before waste is eliminated

What is the general path of food through the GI tract?

Mouth → esophagus → stomach → small intestine → large intestine → elimination

What are sphincters?

Muscular structures that open and close to control the passage of material from one organ of the GI tract to another

What are accessory digestive organs?

Organs that assist digestion but are not part of the continuous GI tube; they include the salivary glands, liver, gallbladder, and pancreas

What happens during mechanical digestion in the mouth?

Chewing breaks food into smaller pieces increasing its surface area so digestive enzymes can act more effectively

What is salivary amylase?

An enzyme produced by the salivary glands that begins the chemical digestion of carbohydrates in the mouth

What is the function of the epiglottis?

It covers the opening to the trachea during swallowing helping prevent food from entering the respiratory tract

What is peristalsis?

A series of muscular contractions that move food forward through the GI tract

What is the function of the gastroesophageal sphincter?

It separates the esophagus from the stomach regulates movement into the stomach and helps prevent stomach contents from flowing backward into the esophagus

What are the main functions of the stomach?

The stomach stores, mixes, and digests food and gradually releases chyme into the small intestine

What is gastrin?

A hormone produced by G-cells of the stomach that stimulates secretion of gastric juice and promotes stomach activity involved in digestion

What is gastric juice?

An acidic digestive fluid containing hydrochloric acid, intrinsic factor, pepsin, and gastric lipase

What are the functions of hydrochloric acid in the stomach?

HCl creates an acidic environment, denatures proteins, and activates pepsin for protein digestion

What does denaturation mean in digestion?

Denaturation is a change in the structure of a protein that makes it easier for digestive enzymes to break it down

What is intrinsic factor and why is it important?

Intrinsic factor is a protein produced by parietal cells that is necessary for the absorption of vitamin B12

What is pepsin?

A stomach enzyme that begins the chemical digestion of proteins

What is gastric lipase?

A stomach enzyme involved in the digestion of fats and associated with chief cells

What is chyme?

The semisolid mixture produced by mechanical and chemical digestion in the stomach that is gradually released into the small intestine

How is the stomach protected from its acidic digestive contents?

Gastric glands produce mucus that protects the lining while other cells secrete bicarbonate which neutralizes acid near the stomach surface

What can happen if the stomach's protective mucus and bicarbonate defenses are disrupted?

The stomach lining becomes vulnerable to hydrochloric acid and digestive enzymes which can contribute to gastritis or ulcers

What is the small intestine?

The longest portion of the GI tract and the primary location where most digestion and nutrient absorption occur

What are the three sections of the small intestine?

Duodenum → jejunum → ileum

What is the function of the duodenum?

The first section of the small intestine; it receives chyme from the stomach plus pancreatic secretions and bile and is a major site of chemical digestion

What is the function of the jejunum?

The middle section of the small intestine where digestion continues and substantial nutrient absorption occurs

What is the function of the ileum?

The final section of the small intestine where absorption continues and which connects to the large intestine

What is the pyloric sphincter?

A muscular valve between the stomach and small intestine that controls the gradual release of chyme into the small intestine

Why does most digestion and absorption occur in the small intestine?

The small intestine receives digestive enzymes and bile and has specialized structures that provide a very large surface area for nutrient absorption

What are the functions of the salivary glands?

They produce saliva which moistens food and contains salivary amylase to begin carbohydrate digestion

What are the major functions of the liver in digestion?

The liver produces bile, processes and stores products of digestion, releases processed nutrients into the blood, and detoxifies blood by processing substances such as medications and alcohol

What is bile and what does it do?

Bile is produced by the liver and emulsifies fats by breaking large fat droplets into smaller droplets which increases surface area for digestive enzymes

What is emulsification?

The physical breakdown of large fat droplets into smaller droplets that increases the surface area available to fat-digesting enzymes

What are the digestive functions of the pancreas?

The pancreas produces digestive enzymes including pancreatic amylase lipase and proteases and produces bicarbonate to neutralize acidic chyme

What does pancreatic amylase digest?

Carbohydrates

What does pancreatic lipase digest?

Fats

What do pancreatic proteases digest?

Proteins

Why does the pancreas produce bicarbonate?

Bicarbonate neutralizes acidic chyme entering the small intestine from the stomach

What is the function of the gallbladder?

It stores bile produced by the liver and releases it into the small intestine when needed for fat digestion

What is the ileocecal valve?

A sphincter-like valve that controls movement of material from the ileum into the large intestine and helps prevent backward movement

What are the main functions of the large intestine?

It stores digestive materials absorbs water and some nutrients and forms feces; very little digestion occurs there

How long can digestive materials be stored in the large intestine before elimination?

Approximately 12 to 24 hours

What are feces?

The semisolid waste material formed in the large intestine from undigested food components and other waste

What is elimination?

The removal of undigested food materials and other waste from the body

What is absorption?

The process of moving molecules across a cell membrane and into cells of the body

What substances can be absorbed in small amounts from the stomach?

Water, fluoride, lipids, and lipid-soluble substances such as some drugs, including aspirin and alcohol

Where does most nutrient absorption occur?

In the small intestine across its highly specialized lining

How do folds, villi, and microvilli improve nutrient absorption?

They progressively increase the surface area of the small intestine allowing more nutrients to contact the intestinal lining and be absorbed

What are villi?

Fingerlike folds of the small intestinal lining that increase absorptive surface area and contain blood vessels and a lacteal

What are microvilli?

Tiny projections on the surface of intestinal cells that form the brush border and greatly increase the surface area available for absorption

What is the brush border?

The surface of intestinal cells formed by microvilli; it greatly increases the surface area available for nutrient absorption

What is the function of capillaries in intestinal villi?

They absorb water-soluble nutrients and transport them into the bloodstream

What is a lacteal?

A small lymphatic vessel within a villus that absorbs fat-soluble nutrients

What is the correct progression of structures that increases small intestinal surface area?

Folds → villi → microvilli

Which nutrients generally enter the portal vein after intestinal absorption?

Water-soluble nutrients such as products of carbohydrate and protein digestion, minerals, and some vitamins

What is the function of the portal vein in digestion?

It carries absorbed water-soluble nutrients from the small intestine to the liver for processing storage or release into the bloodstream