9.1 Muscles

Muscles are organs made of cells that are specialized to generate forces. There are three types of muscle tissue: skeletal, smooth, and cardiac

9.2 Structure of a Skeletal Muscle

Skeletal muscles are composed of nervous, vascular, and various other connective tissues, as well as skeletal muscle tissue.

1. Connective tissue coverings

   1. Fascia covers each skeletal muscle

   2. Other connective tissues surround cells and groups of cells within the muscle’s structure (epimysium, perimysium, endomysium).

   3. Fascia is part of a complex network of connective tissue that extends throughout the body.

2. Skeletal muscle fibers

   1. Each skeletal muscle fiber is a single muscle cell, the unit of contraction

   2. Muscle fibers are cylindrical cells with many nuclei

   3. The cytoplasm contains mitochondria, sarcoplasmic reticulum, and myofibrils of actin and myosin.

   4. The arrangement of the actin and myosin filaments causes striations forming repeating patterns of sarcomeres (I bands, Z lines, A bands, Hzone, and M line)

   5. Troponin and tropomyosin molecules associate with actin filaments

   6. Transverse tubules extend from the cell membrane into the cytoplasm and are associated with the cisternae of the sarcoplasmic reticulum

9.3 Skeletal Muscle Contraction

Muscle fiber contraction results from a sliding movement of actin and myosin filaments overlapping that shortens the muscle fiber.

1. Neuromuscular junction

   1. The functional connection between a neuron and another cell is a synapse. The neuromuscular junction is a synapse.

   2. Motor neurons stimulate muscle fibers to contract

   3. The motor end plate of a muscle fiber lies on one side of a neuromuscular junction

   4. In response to an action potential, the end of a motor neuron axon releases a neurotransmitter, which diffuses across the junction and stimulates the muscle fiber

2. Stimulus for contraction

   1. Acetylcholine released from the end of a motor neuron axon stimulates a muscle fiber

   2. Stimulation causes a muscle fiber to conduct an impulse that travels over the surface of the sarcolemma and reaches the deep parts of the fiber by means of the transverse tubules

3. Excitation-contraction coupling

   1. In response to stimulation the sarcoplasmic reticulum releases calcium ions

   2. Calcium ions combine with troponin, causing the tropomyosin to shift and expose active sites on the actin for myosin binding

   3. Cross-bridges form between myosin and actin, and the actin filaments move inward, shortening the sarcomere

4. The sliding filament model of muscle contraction

   1. The sarcomere is a functional unit of the skeletal muscle

   2. When the overlapping thick and thin myofilaments slide past one another, the sarcomeres shorten. The muscles contract.

5. Cross-bridge cycling

   1. A myosin head can attach to an actin binding site to form a cross-bridge which pulls on the actin filament, the myosin head can then release the actin and attach to another active binding site farther down the actin filament and pull again

   2. The breakdown of ATP releases energy that provides the repetition of the cross-bridge cycle

6. Relaxation

   1. Acetylcholinesterase rapidly decomposes acetylcholine remaining in the synaptic cleft, preventing continuous stimulation of a muscle fiber

   2. The muscle fiber relaxes when calcium ions are transported back into the sarcoplasmic reticulum

   3. Cross-bridge linkages break and do not reform– the muscle fiber relaxes

7. Energy sources for contraction

   1. ATP supplies the energy for muscle fiber contraction

   2. Creatine phosphate stores energy that can  be used to synthesize ATP as it is decomposed

   3. Active muscles require cellular respiration for energy

8. Oxygen supply and cellular respiration

   1. Anaerobic reactions of cellular respiration yield few ATP molecules whereas aerobic reactions of cellular respiration provide many ATP molecules

   2. Hemoglobin in red blood cells carries oxygen from the lungs to body cells

   3. Myoglobin in muscle cells temporarily stores some oxygen

9. Oxygen debt

   1. During rest or moderate exercise, oxygen is sufficient to support the aerobic reactions of cellular respiration

   2. During strenuous exercise, oxygen deficiency may develop, and lactic acid may accumulate as a result of the anaerobic reactions of cellular respiration

   3. The oxygen debt includes the amount of oxygen required to react accumulated lactic acid to form glucose and to restore supplies of ATP and creatine phosphate

10. Muscle fatigue

    1. A fatigued muscle loses its ability to contract

    2. The causes of muscle fatigue are not fully understood

    3. Athletes usually have an increased ability to supply oxygen and nutrients to muscles

11. Heat production

    1. Muscular contraction generates body heat

    2. Most of the energy released by cellular respiration is lost as heat.

9.4 Muscular Responses 

1. Threshold stimulus is the minimal stimulus needed to elicit a muscular contraction

2. Recording of a muscle contraction

   1. A twitch is a single, short contraction of a muscle fiber

   2. A myogram is a recording of the contraction of an electrically stimulated isolated muscle or muscle fiber.

   3. The latent period is the time between stimulus and responding contraction

   4. The length to which a muscle is stretched before stimulation affects the force it will develop

      1. Normal activities occur at optimal length

      2. Too long or too short decreases force

   5. Sustained contractions are more important than twitch contractions in everyday activities

3. Summation

   1. A rapid series of stimuli may produce summation of twitches and sustained contraction

   2. At higher frequencies of stimulation, contraction with little time for relaxation is called partial tetany

   3. Forceful, sustained contraction without any relaxation is called complete (fused) tetany

4. Recruitment of motor units

   1. One motor neuron and the muscle fibers associated with it constitute a motor unit

   2. Muscles whose motor units have few muscle fibers produce finer movements

   3. Motor units respond in an all-or-none manner

   4. At low intensity of stimulation, other motor units are recruited until the muscle contracts with maximal tension

5. Sustained contractions

   1. Tetanic contractions are common in everyday activities

   2. Even when a whole muscle appears at rest, some of its fibers undergo sustained contraction. This is called muscle tone.

6. Types of contractions

   1. One type of contraction called isotonic occurs when a muscle contracts and it ends are pulled closer together. Because the muscle shortens, it is called a concentric contraction. 

   2. In another type of isotonic contraction the force a muscle generates is less than that required to move or lift an object. This lengthening contraction is an eccentric contraction

   3. When a muscle contracts but its attachments do not move, the contraction is isometric.

   4. Most body movements involve both isometric and isotonic contractions

7. Fast- and slow-twitch muscle fibers

   1. The speed of contraction is related to a muscle’s specific function

   2. Slow-contraction, or red muscles can generate ATP fast enough to keep up with ATP breakdown and can contract for long periods

   3. Fast-contracting, or white muscles have reduced ability to carry on the aerobic reactions of cellular respiration and tend to fatigue rapidly.

9.5 Smooth Muscle

The contractile mechanisms of smooth and cardiac muscle are similar to those of skeletal muscle

1. Smooth muscle cells

   1. Smooth muscle cells contain filaments of myosin and actin, but these filaments are not organized into myofibrils

   2. They lack transverse tubules, and the sarcoplasmic reticula are not well developed

   3. Types include multiunit smooth muscle and visceral smooth muscle

   4. Visceral smooth muscle displays rhythmicity

   5. Peristalsis aids movement of material through hollow organs

2. Smooth muscle contractions

   1. In smooth muscle, calmodulin binds to calcium ions and activates the contraction mechanism

   2. Both acetylcholine and norepinephrine are neurotransmitters for smooth muscles

   3. Hormones and stretching affect smooth muscle contractions

   4. With a given amount of energy, smooth muscle can maintain a contraction longer than skeletal muscle

   5. Smooth muscle can change length without changing tautness

9.6 Cardiac Muscle

1. Cardiac muscle contracts for a longer time than skeletal muscle because transverse tubules supply extra calcium ions

2. Intercalated discs connect the ends of cardiac muscle cells and hold the cells together

3. A network of cells contracts as a unit and responds to stimulation in an all-or-none manner

4. Cardiac muscle is self-exciting, rhythmic, and remains refractory to further stimulation until a contraction is completed

9.7 Skeletal Muscle Actions

1. Body movement

   1. Bones and muscles function together as levers

   2. A lever consists of a rod, a fulcrum (pivot), a resistance, and a force that supplies energy

   3. Parts of a first-class lever are arranged resistance-fulcrum-force; of a second-class lever, fulcrum-resistance-force; and of a third-class lever, resistance-force-fulcrum

2. Origin and insertion

   1. The less moveable end of an attachment of a skeletal muscle to a bone is its origin, and the more movable end is its insertion

   2. The origin may also be described as more superior or medial, with the insertion more distal or inferior

   3. Some muscles have more than one origin or insertion

3. Interaction of skeletal muscles

   1. Skeletal muscles function in groups 

   2. A muscle that causes a specific movement is an agonist for that movement; antagonists can resist a movement; synergists work together to perform a specific movement

   3. Smooth movements depend upon antagonists giving way to the actions of agonists

9.8 Major Skeletal Muscles

Muscle names often describe sizes, shapes, locations, actions, number of attachments, or direction of fibers

1. Muscles of facial expression

   1. These muscles lie beneath the skin of the face and scalp and are used to communicate feelings through facial expression

   2. They include the epicranius, orbicularis oculi, orbicularis oris, buccinator, zygomaticus major, zygomaticus minor, and platysma

2. Muscles of mastication

   1. These muscles are attached to the mandible and are used in chewing

   2. They include the masseter, temporalis, medial pterygoid, and lateral pterygoid

3. Muscles that move the head and vertebral column

   1. Muscles in the neck and back move the head

   2. They include the sternocleidomastoid, splenius capitis, semispinalis capitis, scalenes, quadratus lumborum, and erector spinae.

4. Muscles that move the pectoral girdle

   1. Most of these muscles connect the scapula to nearby bones and are closely associated with muscles that move the arm

   2. They include the trapezius, rhomboid major, rhomboid minor, levator scapulae, serratus anterior, and pectoralis minor.

5. Muscles that move the arm

   1. These muscles connect the humerus to various regions of the pectoral girdle, ribs, and vertebral column

   2. They include the coracobrachialis, pectoralis major, teres major latissimus dorsi, supraspinatus, deltoid, subscapularis, infraspinatus, and teres minor.

6. Muscles that move the forearm

   1. These muscles connect the radius and ulna to the humerus and pectoral girdle

   2. They include the biceps brachii, brachialis, brachioradialis, triceps brachii, supinator, pronator teres, and pronator quadrus.

7. Muscles that move the hand

   1. These muscles arise from the distal end of the humerus and form the radius and ulna.

   2. They include the flexor carpi radialis, flexor carpi ulnaris, palmaris longus, flexor digitorum profundus, flexor digitorum superficialis, extensor carpi radialis longus, extensor carpi radialis brevis, extensor carpi ulnaris, and extensor digitorum

   3. An extensor retinaculum forms sheaths for tendons of the extensor muscles

8. Muscles of the abdominal wall

   1. These muscles connect the rib cage and vertebral column to the pelvic girdle

   2. They include the external oblique, internal oblique, transversus abdominis, and rectus abdominis.

9. Muscles of the pelvic floor and perineum

   1. These muscles form the floor of the pelvic cavity and fill the space of the pubic arch

   2. They include the levator ani, coccygeus, deep transversus perinei, superficial transversus perinei, bulbospongiosus, ischiocavernosus, external urethral sphincter, and external anal sphincter.

10. Muscles that move the thigh 

    1. These muscles are attached to the femur and to some part of the pelvic girdle

    2. They include the psoas major, iliacus, gluteus maximus, gluteus medius, gluteus minimus, piriformis, tensor fasciae latae, pectineus, adductor brevis, adductor longus, adductor magnus, and gracilis

11. Muscles that move the leg

    1. These muscles connect the tibia or fibula to the femur or pelvic girdle

    2. They include the biceps femoris, semitendinosus, semimembranosus, sartorius rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius

12. Muscles that move the foot

    1. These muscles attach the femur, tibia, and fibula to various bones of the foot

    2. They include the tibialis anterior, fibularis tertius, extensor digitorum longus, extensor hallucis longus, gastrocnemius, soleus, plataris, flexor digitorum longus, tibialis posterior, and fibularis longus

    3. Retinacula form sheaths for tendons passing to the foot

9.9 Life-Span Changes

1. Beginning in one’s forties, supplies of ATP, myoglobin, and creatine phosphate begin to decline.

2. By age eighty, muscle mass may be halved. Reflexes slow. Adipose cells and connective tissue replace some muscle tissue

3. Exercise is beneficial in maintaining muscle function.

17.1 General Characteristics of the Digestive System 

Digestion is the process of mechanically and chemically breaking down foods so that they can be absorbed. The digestive system consists of an alimentary canal and several accessory organs that carry out the processes of ingestion, propulsion, digestion, absorption, and defecation. 

1. Structure of the alimentary canal wall

   1. The wall consists of four layers

   2. These layers include the mucosa, submucosa, muscularis, and serosa

2. Movements of the alimentary canal wall

   1. Motor functions include mixing and propelling movements

   2. Peristalsis is responsible for propelling movements

   3. The wall of the tube undergoes receptive relaxation just ahead of a peristaltic wave

3. Innervation of the alimentary canal wall

   1. The tube is innervated by branches of the sympathetic and parasympathetic divisions of the autonomic nervous system

   2. Parasympathetic impulses generally increase digestive activities; sympathetic impulses generally inhibit digestive activities.

   3. Sympathetic impulses contract certain sphincter muscles, controlling movement of digesting food through the alimentary canal.

17.2 Mouth

The mouth is adapted to receive food and begin digestion by mechanically breaking up solid particles (mastication). It also serves as an organ of speech and sensory perception. 

1. Cheeks and lips

   1. Cheeks form the lateral walls of the mouth

   2. Lips are highly mobile and have a variety of sensory receptors useful in judging the characteristics of food.

2. Tongue

   1. The tongue is a thick, muscular organ that mixes food with saliva and moves it toward the pharynx

   2. The rough surface of the tongue handles food and has taste buds

   3. Lingual tonsils are located on the root of the tongue

3. Palate

   1. The palate comprises the roof of the mouth and includes hard and soft portions

   2. The soft palate, including the uvula, closes the opening to the nasal cavity during swallowing

   3. Palatine tonsils are located on either side of the tongue in the back of the mouth

   4. Tonsils consist of lymphatic tissues.

4. Teeth 

   1. Two sets of teeth develop in sockets of the mandibular and maxillary bones

   2. There are 20 primary and 32 secondary teeth

   3. Teeth mechanically break food into smaller pieces, increasing the surface area exposed to digestive actions

   4. Different types of teeth are adapted to handle foods in different ways, such as biting, grasping, or grinding. 

   5. Each tooth consists of a crown and root and is composed of enamel, dentin, pulp, nerves, and blood vessels. 

   6. A tooth is attached to the alveolar process by the periodontal ligament

17.3 Salivary Glands

Salivary glands secrete saliva, which moistens food, helps bind food particles, begins chemical digestion of carbohydrates, makes taste possible, helps cleanse the mouth, and regulates pH in the mouth

1. Salivary secretions

   1. Salivary glands include serous cells that secrete salivary amylase and mucous cells that secrete mucus

   2. Parasympathetic impulses stimulate the secretion of a large volume of watery saliva

2. Major salivary glands

   1. The parotid glands are the largest, and they secrete saliva rich in amylase

   2. The submandibular glands in the floor of the mouth produce viscous saliva containing amylase

   3. The sublingual glands in the floor of the mouth primarily secrete mucus

17.4 Pharynx and Esophagus 

The pharynx and esophagus serve as passageways

1. Structure of the pharynx

   1. The pharynx is divided into a nasopharynx, oropharynx, and laryngopharynx

   2. The muscular walls of the pharynx contain fibers in circular and longitudinal groups

2. Swallowing mechanism 

   1. Swallowing (deglutition) occurs in three stages

      1. Food is mixed with saliva and forced into the pharynx

      2. Involuntary reflex actions move the food into the esophagus

      3. Peristalsis transports food in the esophagus to the stomach

   2. Swallowing reflexes momentarily inhibit breathing

3. Esophagus

   1. The esophagus passes through the mediastinum and penetrates the diaphragm

   2. The lower esophageal sphincter, at the distal end of the esophagus, helps prevent regurgitation of food from the stomach.

17.5 Stomach

The stomach receives food, mixes it with gastric juice, carries on a limited amount of absorption, and moves food into the small intestine

1. Parts of the stomach

   1. The stomach is divided into the cardia, fundus, body, and pylorus

   2. The pyloric sphincter serves as a valve between the stomach and the small intestine

2. Gastric secretions

   1. Gastric glands secrete gastric juice

   2. Alkaline mucus protects the inner stomach wall

   3. Gastric juice contains mucus, chemical messengers, pepsin (begins digestion of proteins), hydrochloric acid lipase, and intrinsic factor

3. Regulation of gastric secretion

   1. Parasympathetic impulses and the hormone gastrin enhance gastric secretion

   2. There are 3 stages of gastric secretion

      1. The cephalic phase is the though and sense of foods before they reach the stomach

      2. The gastric phase occurs when food enters the stomach

      3. The intestinal phase occurs when food begins to leave the stomach and enter the small intestine; the small intestine reflexively inhibits gastric secretion;  the hormone cholecystokinin acts on the stomach to decrease gastric activity.  ` 

4. Gastric Absorption

   1. The stomach is not well adapted for absorption

   2. A few substances such as water and other small molecules are absorbed through the stomach wall

5. Mixing and emptying actions

   1. As the stomach fulls, its wall stretches but its internal pressure remains unchanged

   2. Mixing movements aid in producing chyme; peristaltic waves move chyme into the pylorus

   3. The muscular wall of the pylorus regulates chyme movement into the small intestine

   4. The rate of emptying depends on the fluidity of the chyme and the type of food present

   5. The upper part of the small intestine fills, and the enterogastric reflex inhibits peristalsis in the stomach.

6. Vomiting

   1. Vomiting results from a complex reflex that has many stimuli

   2. Motor responses ultimately increase abdominal pressure to force contents upward and out. 

17.6 Pancreas

The pancreas is closely associated with the duodenum

1. Structure of the pancreas

   1. It produces pancreatic juice secreted into a pancreatic duct

   2. The pancreatic duct leads to the duodenum

2. Pancreatic juice

   1. Pancreatic juice contains enzymes that can break down carbohydrates, proteins, fats, and nucleic acids

   2. Pancreatic juice has a high bicarbonate ion concentration that helps neutralize chyme ansd causes the intestinal contents to be alkaline

3. Regulation of pancreatic secretion

   1. Secretin from the duodenum stimulates the release of pancreatic juice that contains few digestive enzymes but has a high bicarbonate ion concentration

   2. Cholecystokinin from the intestinal wall stimulates the release of pancreatic juice that has a high concentration of digestive enzymes

17.7 Liver and Gallbladder

The liver is located in the upper-right quadrant of the abdominal cavity

1. Liver structure

   1. The liver is a highly vascular organ, enclosed in a fibrous capsule, and divided into lobes.

   2. Each lobe consists of hepatic lobules, the functional units of the liver

   3. Bile from the lobules is carried by bile ductules to hepatic ducts that unite to form the common hepatic duct

2. Liver functions

   1. The liver has many functions It removes toxic substances from the blood (detoxifies); metabolizes carbohydrates, lipids, and proteins; stores some substances; and secretes bile

   2. Bile is the only liver secretion that directly affects digestion

3. Composition of bile

   1. Bile contains bile salts, bile pigments, cholesterol, and electrolytes

   2. Only the bile salts have digestive functions

   3. Bile pigments are products of red blood cell breakdown

4. Gallbladder

   1. The gallbladder stores bile between meals

   2. A sphincter muscle controls release of bile from the bile duct

   3. Gallstones may form within the gallbladder

5. Regulation of bile release

   1. Cholecystokinin from the small intestine stimulates bile release

   2. The sphincter muscle at the base of the bile duct relaxes as a peristaltic wave as the duodenal wall approaches

6. Functions of bile salts

   1. Bile salts emulsify fats and aid in the absorption of fatty acids, cholesterol, and certain vitamins

   2. Bile salts are reabsorbed in the small intestine

17.8 Small Intestine

The small intestine extends from the pyloric sphincter to the large intestine. It receives secretions from the pancreas and liver, completes digestion of nutrients, absorbs the products of digestion, and transports the residues to the large intestine

1. Parts of the small intestine

   1. The small intestine consists of the duodenum, jejunum, and ileum

   2. The small intestine is suspended from the posterior abdominal wall by mesentery

2. Structure of the small intestinal wall

   1. The wall is lined with villi that greatly increase the surface area of the intestinal lining, aiding absorption of digestive products

   2. Microvilli on the free ends of epithelial cells increase the surface area even more

   3. Intestinal glands are located between the villi

   4. Circular folds in the lining of the intestinal wall also increase its surface area

3. Secretions of the small intestine

   1. Intestinal glands secrete a watery fluid that does not have digestive enzymes but provides a vehicle for moving chyme to the villi

   2. Digestive enzymes embedded in the surfaces of microvilli break down molecules of sugars, proteins, and fats

4. Regulation of small intestinal secretion

   1. Secretion is stimulated by chyme and parasympathetic reflexes stimulated by sitension of the small intestinal wall

5. Absorption in the small intestine

   1. Blood capillaries in the villi absorb monosaccharides and amino acids

   2. Fatty acids diffuse into small intestinal epithelial cells where they are processed to form chylomicrons, which enter the lacteals of the villi

   3. Blood capillaries in the ville also absorb electrolytes and water

6. Movements of the small intestine

   1. Movements include mixing by segmentation and peristalsis

   2. Overdistension or irritation may stimulate a peristaltic rush and result in diarrhea

   3. The ileocecal sphincter controls movement of the intestinal contents from the small intestines into the large intestine.

17.9 Large Intestine

The large intestine absorbs water and electrolytes and forms and stores feces

1. Parts of the large intestine

   1. The large intestine consists of the cecum, colon, rectum, and anal canal

   2. The colon is divided into ascending, transverse, descending, and sigmoid portions

2. Structure of the large intestinal wall

   1. The large intestinal wall resembles the wall in other parts of the alimentary canal

   2. The large intestinal wall has a unique layer of longitudinal muscle fibers, arranged in district bands, that extend the entire length of the colon

3. Functions of the large intestine

   1. The large intestine has little digestive function, although it secretes mucus

   2. Mechanical stimulation and parasympathetic impulses control the rate of mucus secretion

   3. The large intestine absorbs water and electrolytes

   4. Many bacteria inhabit the large intestine, where they break down some indigestible substances, such as cellulose, and synthesize vitamin K, B12, thiamine, and riboflavin

4. Movements of the large intestine

   1. Movements are similar to those in the small intestine

   2. Mass movements occur two to three times each day

   3. A reflex stimulates defecation

5. Feces

   1. The large intestine forms and stores feces

   2. Feces consists of water, undigested material, mucus, and bacteria

   3. The color of feces is due to bile pigments that have been altered by bacterial action

17.10 Life Span Changes

1. Older people sometimes do not chew food thoroughly because thinning enamel makes teeth more sensitive to hot and cold foods, gums recede, and teeth may loosen

2. Slowing peristalsis in the digestive tract may cause heartburn and constipation

3. Aging affects nutrient absorption in the small intestine

4. Accessory organs to digestion also age, but not necessarily in ways that affect health.

18.1 Overview of Nutrition and Metabolism

Food is broken down into the usable nutrients, amino acids, monosaccharides, and fatty acids. From the extraction of energy, they are used to fuel all cellular processes.

18.2 Carbohydrates

Carbohydrates are organic compounds primarily used to supply cellular energy

1. Carbohydrate Sources

   1. Carbohydrates are ingested in a variety of forms

   2. Polysaccharides, disaccharides, and monosaccharides are carbohydrates.

   3. Cellulose is a polysaccharide that human enzymes cannot digest, but it provides bulk that facilitates movement of intestinal content

2. Carbohydrate Use

   1. Carbohydrates are absorbed as monosaccharides

   2. Enzymes in the liver catalyze reactions that convert fructose and galactose into glucose

   3. Oxidation releases energy from glucose

   4. Excess glucose is stored as glycogen or combined to produce fat

   5. Some cells, such as neurons, require a continuous supply of glucose to survive

   6. If glucose is scarce, amino acids may react to produce glucose

   7. Most carbohydrates supply energy; some are used to produce sugars (ribose, deoxyribose, lactose)

3. Carbohydrate Requirements

   1. Humans survive with a wide range of carbohydrate intakes

   2. Poor nutritional status is usually related to low intake of nutrients other than carbohydrates.

18.3 Lipids

Lipids are organic compounds that supply energy and are used to build cell structures. They include fats, phospholipids, and cholesterol

1. Lipid sources 

   1. Triglycerides are obtained from foods of plant and animal origins

   2. Cholesterol is mostly obtained in foods of animal origin

2. Lipid use

   1. Before fats can be used as an energy source, they must be broken down into glycerol and fatty acids

   2. Beta oxidation decomposes fatty acids

      1. Beta oxidation activates fatty acids and breaks them down into segments of two carbon atoms each

      2. Fatty acid segments are converted into acetyl coenzyme A, which can then be oxidized in the citric acid cycle

   3. The liver and adipose tissue control triglyceride metabolism

   4. Liver enzymes can alter the molecular structures of fatty acids

   5. Linoleic acid and linolenic acid are essential fatty acids which must be obtained through the diet

   6. The liver regulates cholesterol level by synthesizing or excreting it

3. Lipid requirements

   1. Humans survive with a wide range of lipid intakes

   2. The amounts and types of lipids needed for health are unknown

   3. Fat intake must be sufficient to support absorption and transport of fat-soluble vitamins

18.4 Proteins

Proteins are broken down in digestion, the resulting amino acids can be used to form new protein molecules such as enzymes, clotting factors, keratin, elastin, collagen, actin, myosin, hormones, and antibodies, or can be used as energy sources. Before amino acids can be used as energy sources, they must be deaminated, forming the waste urea. During starvation, tissue proteins may be used as energy sources, causing the tissues to waste away. 

1. Protein sources

   1. Proteins are mainly obtained from eating meats, dairy products, cereals, and legumes

   2. 8 amino acids are essential for adults, whereas 10 are essential for growing children

   3. All essential amino acids must be present at the same time for growth and repair of tissues to take place

   4. Complete proteins contain adequate amounts of all the essential amino acids needed to maintain the tissues and promote growth

   5. Incomplete proteins do not contain adequate amounts of one or more essential amino acids

2. Nitrogen balance 

   1. In healthy adults, the gain of protein equals the loss of protein, and a nitrogen balance exists

   2. A starving person has a negative nitrogen balance; a growing child, a pregnant woman, or an athlete in training usually has a positive nitrogen balance

3. Protein Requirements

   1. Proteins and amino acids are needed to supply essential amino acids and nitrogen for the synthesis of nitrogen-containing molecules

   2. The consequences of protein deficiencies are particularly severe among growing children

18.5 Energy Expenditures

Energy is of prime importance to survival and may be obtained from carbohydrates, fats, or proteins.

1. Energy values of food

   1. The potential energy values of foods are expressed in calories

   2. When energy losses due to incomplete absorption and incomplete oxidation are taken into account, 1 gram of carbohydrates or 1 gram of proteins yields about 4 calories, whereas 1 gram of fat yields about 9 calories

2. Energy requirements 

   1. The amount of energy required varies from person to person

   2. Factors that influence energy requirements include basal metabolic rate, muscular activity, body temperature, and nitrogen balance. 

3. Energy balance

   1. Energy balance exists when caloric intake equals caloric output

   2. If energy balance is positive, body weight increases; if energy balance is negative, body weight decreases.

4. Desirable Weight

   1. The most common nutritional disorders involve caloric imbalances

   2. Average weights of persons 25-35 years old are desirable for older persons as well

   3. Body mass index (BMI) assesses weight taking height into account

   4. A BMI between 25 and 30 indicated overweight, and above 30, obesity. 

18.6 Appetite Control

1. Appetite is the drive that compels us to eat. Food powers the activities of life

2. Hormones control appetite by affecting the arcuate nucleus, a part of the hypothalamus

3. Leptin and ghrelin are hormones that affect appetite

18.7 Vitamins

Vitamins are organic compounds that cannot be synthesized by body cells in adequate amounts and are essential for normal metabolic processes.

1. Fat-soluble vitamins

   1. General characteristic 

      1. Fat soluble vitamins are carried in lipids and are influenced by the same factors that affect lipid absorption

      2. They resist the effects of heat; thus, they are not destroyed by cooking or food processing

   2. Vitamin A

      1. Vitamin A exists in several forms, is synthesized from carotenes, and is stored in the liver

      2. It is an antioxidant required for production of visual pigments

   3. Vitamin D

      1. Vitamin D is a group of related steroids.

      2. It is found in certain foods and is produced commercially; it can also be synthesized in the skin

      3. When needed, vitamin D is converted by the kidneys to an active form that functions as a hormone and promotes the intestine’s absorption of calcium and phosphorus

   4. Vitamin E

      1. Vitamin E is an antioxidant

      2. It is stored in muscles and adipose tissue

      3. It prevents breakdown of polyunsaturated fatty acids and stabilizes cell membranes.

   5. Vitamin K

      1. Vitamin K is in foods and is produced by intestinal bacteria

      2. Some vitamin K is stored in the liver

      3. It is used to produce prothrombin, required for blood clotting

2. Water-soluble vitamins

   1. General characteristics

      1. Water-soluble vitamin s include the B vitamins and vitamin C

      2. Cooking or processing food destroys some water-soluble vitamins.

      3. B vitamins make up a group called the vitamin B complex and oxidize carbohydrates, lipids, and proteins

   2. Vitamin B Complex

      1. Thiamine (vitamin B1)

         1. Thiamine functions as part of coenzymes that oxidize carbohydrates and synthesize ribose

         2. Small amounts are stored in the tissues; excess is excreted in the urine

         3. Quantities needed vary with caloric intake

      2. RIboflavin (vitamin B2)

         1. Riboflavin functions as part of several enzymes and coenzymes essential to the oxidation of glucose and fatty acids

         2. Its absorption is regulated by an active transport system; excess is excreted in the urine

         3. Quantities required vary with caloric intake

      3. Niacin (vitamin B3/nicotinic acid)

         1. Niacin functions as part of coenzymes required for the oxidation of glucose and for the synthesis of proteins and fats

         2. It can be synthesized from tryptophan; daily requirement varies within the tryptophan intake

      4. Pantothenic acid (vitamin B5)

         1. Pantothenic acid functions as part of coenzyme A; thus, it is essential for energy-releasing mechanisms

         2. Most diets provide sufficient amounts; deficiencies are rare

      5. Vitamin B6

         1. Vitamin B6 is a group of compounds that function as coenzymes in metabolic pathways that synthesize proteins, certain amino acids, antibodies, and nucleic acids

         2. Its requirement varies with protein intake

      6. Biotin (vitamin B7)

         1. Biotin is a coenzyme required for the metabolism of amino acids and fatty acids, and for nucleic acid synthesis

         2. It is stored in metabolically active organs, including the brain, liver, and kidneys.

      7. Folacin (vitamin B9 or folic acid)

         1. Liver enzymes catalyzed reactions that convert folacin to physiologically active folinic acid

         2. It is a coenzyme needed for the metabolism of certain amino acids, DNA synthesis, and the production of normal red blood cells

      8. Cyanocobalamin (vitamin B12)

         1. The cyanocobalamin molecule contains cobalt.

         2. Its absorption is regulated by the secretion of intrinsic factor from the gastric glands

         3. It functions as part of coenzymes needed for nucleic acid synthesis and for the metabolism of carbohydrates and fats

         4. It is important to erythrocyte production and myelin formation in the central nervous system

   3. Ascorbic acid (vitamin C)

      1. Vitamin C is similar chemically to monosaccharides

      2. It is required for collagen production, the metabolism of certain amino acids, and iron absorption.

      3. It is not stored in large amounts; excess is excreted in the urine.

18.8 Minerals

1. Characteristics of minerals

   1. Minerals account for about 4% of body weight

   2. Minerals are usually incorporated into organic molecules, although some are in inorganic compounds or are free ions

   3. They compose structural materials, function in enzymes, and play vital roles in various metabolic processes

   4. Homeostatic mechanisms regulate mineral concentrations

   5. The physiologically active form of minerals is the ionized form

2. Major minerals

   1. Calcium

      1. Calcium is essential for forming bones and teeth, neurotransmitter release, contracting muscle fibers, the cardiac action potential, clotting blood, and activating various enzymes

      2. Existing calcium concentration, vitamin D, protein intake, and motility of the digestive tract affect calcium absorption

   2. Phosphorus

      1. Phosphorus is incorporated into the salts of bones and teeth

      2. It participates in nearly all metabolic reactions as a constituent of nucleic acids, proteins, and some vitamins.

      3. It also is in the phospholipids of cell membranes, in ATP, and in phosphates of body fluids.

   3. Potassium

      1. Potassium is concentrated inside cells

      2. It maintains osmotic pressure, regulates pH, and plays a role in impulse conduction in neurons

   4. Sulfur

      1. Sulfur is incorporated into two of the 22 amino acids

      2. It is also in thiamine, insulin, biotin, and mucopolysaccharides

   5. Sodium

      1. Most sodium is in extracellular fluids or is bound to the inorganic salts of bone

      2. The kidneys, under the influence of aldosterone, regulate the blood concentration of sodium 

      3. Sodium helps maintain solute concentration and regulates water balance

      4. It is essential for impulse conduction in neurons and moving substances through cell membranes

   6. Chlorine

      1. Chlorine is closely associated with sodium as chloride ions

      2. It acts with sodium to help maintain osmotic pressure, regulate pH, and maintain electrolyte balance.

      3. Chlorine is essential for hydrochloric acid formation and for carbon dioxide transport by red blood cells

   7. Magnesium

      1. Magnesium is abundant in the bones as phosphates and carbonates

      2. It functions in ATP production and in the breakdown of ATP to ADP

      3. A reserve supply of magnesium is stored in the bones; excesses are excreted in the urine

3. Trace Elements

   1. Iron

      1. Iron is part of hemoglobin in red blood cells and myoglobin in muscles

      2. A reserve supply of iron is stored in the liver, spleen, and bone marrow

      3. It is required to catalyze vitamin A formation; it is also incorporated into various enzymes and the cytochrome molecules

   2. Manganese

      1. Most manganese is concentrated in the liver, kidneys, and pancreas

      2. It is necessary for normal growth and development of skeletal structures and other connective tissues; it is essential for the synthesis of fatty acids, cholesterol, and urea.

   3. Copper

      1. Most copper is concentrated in the liver, heart, and brain.

      2. It is required for hemoglobin synthesis, bone development, melanin production, and myelin formation.

   4. Iodine

      1. Iodine is most highly concentrated in the thyroid gland

      2. It is an essential component of thyroid hormones

      3. It is often added to foods as iodized table salt

   5. Cobalt

      1. Cobalt is widely distributed throughout the body

      2. It is an essential part of cyanocobalamin and is required for the synthesis of several enzymes

   6. Zinc

      1. Zinc is most concentrated in the liver, kidneys, and brain.

      2. It is a component of several enzymes that take part in digestion, respiration, and metabolism

      3. It is necessary for normal wound healing.

   7. Fluorine

      1. The teeth concentrate fluorine

      2. It is incorporated into enamel and prevents dental caries

   8. Selenium

      1. The liver and kidneys store selenium

      2. It is a component of certain enzymes

   9. Chromium

      1. Chromium is widely distributed throughout the body

      2. It regulates glucose use

18.9 Healthy Eating

1. An adequate diet provides sufficient energy and essential nutrients to support optimal growth, as well as maintenance and repair, of tissues.

2. Individual needs vary so greatly that it is not possible to design a diet adequate for everyone

3. Devices to help consumers make healthy food choices include recommended daily allowances, recommended dietary allowances, food group plans such as MyPlate, and food labels.

4. Malnutrition 

   1. Poor nutrition is due to lack of foods or failure to wisely use available foods

   2. Primary malnutrition is due to poor diet

   3. Secondary malnutrition is due to poor diet

   4. Secondary malnutrition is due to an individual characteristic that makes a normal diet inadequate

5. Starvation

   1. A person can survive 50-70 days without food

   2. A starving body digests itself, starting with carbohydrates, then fats, then proteins.

   3. Symptoms include low blood pressure, slow pulse, chills, dry skin, hair loss, and poor immunity. Finally, vital organs cease to function

   4. Marasmus is undernutrition involving a lack of calories and protein

   5. Kwashiorkor is protein starvation

   6. Anorexia nervosa is a self-starvation eating disorder

   7. Bulimia is an eating disorder characterized by bingeing and purging

18.10 Life-Span Changes

1. Basal metabolic rate rises in early childhood, declines, then peaks again in adolescence, with decreasing activity during adulthood

2. Weight gain, at any age, occurs when energy in exceeds energy out, and weight loss occurs when energy out exceeds energy in

3. Changing nutrition with age reflects medical conditions and social and economic circumstances.