CoverModule 1.0. Homeostasis, Membranes, Electrophysiology and ANS1.1. Homeostasis1.1.1. Homeostasis Defined1.1.2. Homeostatic Control Systems1.1.3. Feedback Response Loop1.2. Cell Transport; Water & Solutes1.2.1. Fluid Compartments1.2.2. Osmosis1.2.3. Diffusion of Solutes1.2.4. Active Transport1.2.5. Bulk Transport1.3. Electrophysiology1.3.1. Ions and Cell Membranes1.3.2. Membrane Potentials1.3.3. Graded Potential1.3.4. Action Potentials1.3.5. Refractory Periods1.3.6. Propagation of an Action Potential1.4. The Synapse1.5. The Autonomic Nervous System1.5.1. Organization of the Nervous System1.5.2. Structural Organization of the ANS1.5.3. The SNS and the PNS1.5.4. The Enteric Nervous System1.5.5. Physiology of the ANS1.5.6. Neurotransmitters of the ANS1.5.7. Receptors of the ANS1.5.8. Actions of the Autonomic Nervous System1.5.9. Table of Actions for the SNS and PNS and Some Common DrugsModule 2.0. Skeletal Muscle and Special Senses2.1. Structural Organization of Skeletal Muscle2.2.1. Neuromuscular Junction, Excitation-Contraction Coupling2.2.2. Muscle Contractures and Cramps2.3. Whole Muscle Contraction, Fiber Type, Fatigue and Muscle Pharmacology2.3.1. Motor Units2.3.2. Factors that Influence the Force of Contraction2.3.2. Factors that Influence the Force of Muscle Contraction2.3.3. Energy Source for Muscle Contraction2.3.4. Skeletal Muscle Fiber Types2.3.5. Fatigue2.3.6. Muscle Pharmacology2.4. Smooth Muscle2.4.1. Smooth Muscle Contraction2.5. Control of Body Movement2.5.1. Voluntary Control of Muscle2.5.2. Reflexes2.6. Taste and Smell2.6.1. Taste2.6.2. The Sense of Smell2.7. Vision2.7.1. Structure of the Eye2.7.2. Focusing Light on the Retina2.7.3. Converting Light to Action Potentials2.7.4. The Retina2.7.5. Phototransduction2.7.6. Receptive Fields2.8. Hearing and Equilibrium2.8.1. The Nature of Sound2.8.2. The Hearing Apparatus2.8.3. Sound Vibrations to Action Potentials2.8.4. The Sense of Balance and EquilibriumModule 3.0. Cardiovascular System3.1. Structure of the Heart3.1.1. Chambers and Circulation3.2. Cardiac Cell Action Potentials3.2.1. Action Potentials in Cardiac Muscle Cells3.2.2. Action Potentials in Cardiac Autorhythmic cells3.2.3. Cellular Mechanisms of Inotropy and Chronotropy3.3. Electrophysiology of Heart Muscle3.3.1. Heart Conduction System3.3.2. Electrocardiogram (ECG)3.3.3. Abnormal ECG - Current of Injury3.4. The Cardiac Cycle3.4.1. Cardiac cycle3.4.2. Cardiac Measurements and Pressure Volume Loops3.5. Blood vessels and Blood Pressure3.5.1. Arteries and Veins3.5.2. Capillaries3.5.3. Blood Pressure Regulation and Shock3.5.4. Capillary Exchange3.5.5. Myogenic and Paracrine Regulation of Vasoconstriction and Vasodilation3.6. Blood3.6.1. Composition of Blood3.6.2. Hematopoeisis3.6.3. Breaking Down Red Blood Cells3.6.4. HemostasisModule 4.0. Urinary and Respiratory Systems4.1. Function and Structure of the Kidney4.1.1. Urinary System Function4.1.2. Functional Anatomy of the Urinary System4.1.3. The Nephron: Functional Unit of the Kidney4.1.4. The Renal Corpuscle: Bowman's Capsule4.2. Physiology of Urine Production4.2.1. Filtration4.2.2. Renal Clearance4.2.3. Tubular Reabsorption4.2.4. Urine Concentration and Dilution4.2.5. Hormonal Regulation of Urine Production4.3. Acid/Base Balance4.3.1. Buffers4.3.2. Acid/Base Disturbances4.4. The Respiratory System4.4.1. Respiratory System Structure and Function4.4.2. Respiratory Membrane4.4.3. Respiratory pressures and Inspriation/Expiration4.4.4. Alveoli and Surfactant4.4.5. Pneumothorax4.4.6. Pressure-Volume Loops and the Work of Breathing4.5. Gas Exchange and Transport4.5.1. Gas Laws4.5.2. Partial Pressure Gradients in the Lung4.5.3. Alveolar Gas Equation4.5.4. Oxygen and Carbon Dioxide Transport in the Blood4.5.5. Alveolar Ventilation4.5.6. Ventilation/Perfusion Ratio4.6. Chronic Bronchitis and Emphysema4.6.1. Respiratory Control by the Medulla Oblongata4.6.2. Chemicals that Regulate VentilationModule 5.0. Digestive, Endocrine and Reproductive Systems5.1. Functional Anatomy of the Digestive System5.1.1. Layers of the Digestive Tract5.1.2. Enteric Nervous System5.1.3. Organs of the Digestive System5.2. Digestion5.2.1. Carbohydrates5.2.2. Proteins5.2.3. Lipids5.2.4. Lipoproteins5.3. Regulation of Digestive Secretions5.4. Endocrine System5.4.1. Overview of the Endocrine System5.4.2. Hormone Receptors5.4.3. Hormones of the Body5.4.4. Other Hormones: Melatonin and Pheromones5.5. The Hypothalamus and Pituitary Gland5.5.1. Structure and Function of the Hypothalamus and Pituitary Gland5.5.2. The Posterior Pituitary5.5.3. The Anterior Pituitary5.5.4. Growth Hormone5.5.5. Prolactin5.5.6. Thyroid Hormones5.5.7. Adrenal Hormones5.6. Pancreas5.6.1. Insulin and Glucagon5.6.2. Diabetes Mellitus5.7. Reproductive System Anatomy5.7.1. Female Reproductive Anatomy5.7.2. Male Reproductive Anatomy5.7.3. Sexual Development at Puberty5.7.4. Male Reproductive Endocrine Axis5.7.5. Spermatogenesis5.7.6. Female Reproductive System: Oogenesis5.7.7. Ovulation and Fertilization5.7.8. The Ovarian Cycle5.7.9. The Uterine Cycle5.7.10. PregnancyAppendix A. GenderAppendix B. The Placebo EffectB.2.1. The Placebo EffectB.2.2. Examples of the Placebo EffectB.2.3. How do Placebos Work?B.2.4. Are Placebos Ethical?B.2.5. How do we validate actual effectiveness of placebosB.2.6. Tips for evaluating scientific evidenceB.2.7. What about Faith Healingstest chapter


Lipid Digestion

The digestion and absorption of lipids presents a whole new set of complexities because most lipids are hydrophobic and the entire digestive tract is full of watery secretions. You could say lipids have a love/hate relationship with our intestinal system. Most of the lipids we eat (> 90%) are in the form of triacyclglycerols (triglycerides). Triacyclglycerols are composed of two molecular building blocks - glycerol and fatty acids.  Fatty acids are chains of carbons that vary in length and the number and type of double bonds between carbons.

Bonding of Glycerol and Three Fatty Acids by Dehydration Synthesis Reaction to Form Triglyceride
Image created by JS at BYU-Idaho 2014

Fatty acid chains with no double bonds are referred to as saturated. This means that every carbon - carbon bond in the chain is a single bond which allows the linking of 2 hydrogen atoms to every carbon in the chain and 3 hydrogen atoms bonded to the last carbon. If a double bond occurs between two carbons in the hydrocarbon chain, then the carbon atoms connected by a double bond will each bond with one less hydrogen atom in order to maintain 4 bonds per carbon atom.  We could say that because of the double bond, the fatty acid hydrocarbon chain is no longer "saturated" with hydrogen atoms at every carbon.  Therefore, an unsaturated fatty acid will contain one or more double bonds.  A fatty acid with one double bond is referred to as a monounsaturated fat and fatty acids with two or more double bonds are polyunsaturated fats. Additionally, some fats are named after the location of the double bond, such as omega 3 or omega 6 fats. This nomenclature refers to the location of the double bond on the carbons counting from the bottom of the chain up. All lipid containing foods have a specific mixture of saturated and unsaturated fatty acids.  Saturated fatty acids tend to be straight and do not have "kinks" or angles that would make "packing" or "stacking" together more difficult.  The more tightly packed molecules of fat are, the more likely to be a solid at room temperature.  Unsaturated fats can have either "cis" or "trans" double bonds in the hydrocarbon chains.  "Cis" bonds allow for a kinked or angled geometry that makes it more difficult to "pack" together.  Unsaturated fats with "cis" bonds include vegetable oils. 

(a) Cis & Trans Double Bond in Monounsaturated Fatty Acid; (b) Cis Double Bond in Unsaturated Fatty Acid creating kink.
 Image created by JS at BYU-Idaho 2014: Modified File: Oleic-acid-3D-ball-&-stick.png; Author: Benjah-bmm27; Site:; License: Public Domain

Trans Double Bonds in Unsaturated Fatty Acid.
  Title: File: Tridecylic-acid-3D-balls.png; Author: Jynto and Ben Mills; Site:; License: public domain

Unsaturated fats with "Trans" bonds contain a geometry that resembles the straight line of a saturated fat.  This geometry allows for trans unsaturated fats to pack together tightly enough that they will be found as a solid at room temperature. Products like Crisco and Margarine often have substantial quantities of "trans fats".  Cis fats are the most common type found in nature, although there are some naturally occurring trans fats. Although trans fats are rare in nature they have appeared in the American diet as a product of oil processing.  Food manufacturers take naturally occurring oils and use high pressures, high temperatures and hydrogen gas to artificially "hydrogenate" unsaturated fats, making them a creamy solid.  A byproduct of this process is the formation fats with rearranged double bonds (trans fats).  This type of fat is usually listed as "partially hydrogenated oil" in the food ingredients list.    Food companies are interested in the "hydrogenation" of oils so that they might get fat that has the texture, flavor and chemistry necessary for many of the food products we enjoy (i.e. many pastries, puddings, sauces, creamers, and confectioneries). Unfortunately, this switch of hydrogen arrangement has been shown to increase the risk of coronary heart disease (discussed further below).

The hydrophobic nature of lipids presents problems for the digestive process. Because lipids do not interact well with water they tend to form large fat droplets. These droplets make it difficult for the enzymes to access the molecular structure of the fat. To alleviate this problem the fats are emulsified (dispersed) by the actions of bile released from the liver and gallbladder into the small intestine. Components of bile act like detergents and help to break up the large masses of fat into smaller more manageable pieces. The emulsified products form small vesicles called micelles. Micelles increase the surface area for digestion but a side effect is that the bile salts inhibit the digestive enzymes (Lipases) need to breakdown the fat.  As a result, the pancreas secretes colipase in addition to various lipases. The colipase helps the lipases overcome the inhibitory action of the bile salts.

Lipid Absorption

Because the partially digested products inside the micelles are lipophilic, absorption across the lipid bilayer of the small intestinal cell occurs primarily through diffusion.

Lipid Absorption
Author: OpenStax License:  License: Creative Commons Attribution License 4.0 Link:

However, once inside the cell, the lipids are again forced to interact with water. This issue is solved through the use of intracellular proteins called fatty acid-binding proteins that bind the digested lipids and transport them to the smooth endoplasmic reticulum. Once inside the smooth endoplasmic reticulum the fats are "reassembled" into triacyclglycerols, phospholipids or cholesterol esters. The newly re-synthesized fats are packaged into new vesicles called chylomicrons that are formed from the smooth endoplasmic reticulum and modified in the Golgi apparatus. The chylomicron vesicles are then exocytosed from the basal portion of the small intestinal cell.  Because of the rather large size of the chylomicrons they cannot be directly absorbed by the capillaries of the lamina propria, therefore they must pass through the larger channels of the associated lymphatic capillaries found in the center of the villi and enter the lymphatic system. The chylomicrons will eventually enter the blood circulation through the left subclavian vein. Once in the blood, chylomicrons will go through a complex regulatory pathway illustrated below.

Absorption of Lipids
Image by BYU-I Student Tabitha D. 2015

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