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 Healings


Protein Digestion

The recommended dietary amount of protein is based on a person’s weight.  It is 0.8 grams/kilogram of body weight.  For a girl that weights 140lbs the conversion would be as follows:  

140 lbs (weight)/2.2 =  63.6 kilograms

Now to get her protein needs we simply multiply that number by 0.8

63.6 x 0.8  = 51 grams  (this is the protein need)

For elite level athletes this process changes a little bit, instead of using a factor of 0.8 g/kilogram we typically pick a factor between 1-2 grams per kilogram depending on the athletes sport and goals.  (Note:  It is unlikely there are many athletes exercising hard and long enough at BYUI to justify protein levels in the higher ranges, most will be adequately nourished with intakes around 1 g/kg).

Digestion and absorption of proteins presents some interesting problems for the human body, for one, proteins are an integral part of membrane structure. Thus, how do we digest dietary proteins without digesting our own cell membranes? The answer lies in the structure of protein digesting enzymes or proteases. Proteases are secreted as inactive proteins called zymogens that require conversion to their active forms. For example, stomach cells secrete pepsinogen (an inactive protease) that is converted to pepsin (the active protease) in the presence of acid. Once pepsin is activated in the lumen of the stomach, the enterocytes of the mucosa are protected by a thick mucus layer produced by goblet cells in the epithelium. Pepsin is irreversibly inactivated by the change in pH when it moves from the stomach (pH 1.8 to 3) to the small intestine (pH 6-7). Pepsin is responsible for the digestion of about 10% of dietary proteins.

Once in the small intestine a new set of enzymes is necessary to continue protein digestion. These enzymes come from the pancreas and are secreted in the inactive form. There are 5 proteolytic enzymes from the pancreas. The first of these is an inactive protease called trypsinogen. Trypsinogen is converted to its active form, trypsin, by the brush border enzyme enteropeptidase. Trypsin then activates the other four enzymes (chymotrypsin, elastase, carboxypeptidase A  and carboxypeptidase B). This chain of activation ensures that the enzymes are only activated when needed and when there are plenty of other dietary proteins to keep them busy. Trypsin, elastase and chymotrypsin are endopeptidases capable of cleaving proteins at internal bonds within the peptide chain. They are quite specific and target certain sequences of amino acids. The peptides that result from actions of the endopeptidases are further acted on by the exopeptidases (carboxypeptidases A or B). These enzymes along with a brush border carboxypeptidase cleave off single amino acids from the carboxy-terminus of peptide chains. Meanwhile the brush border enzyme aminopeptidase cleaves single amino acids from the amino-terminus.

Protein Absorption

Similar to carbohydrate absorption, individual amino acids in the lumen are transported across the apical membrane of the enterocytes by a Na+ co-transporter. However, in contrast to carbohydrate absorption in which only monomers can be absorbed, proteins can be absorbed through the apical membrane as amino acids or as di and tripeptides by a H+ co-transporter (PepT1). However, to be transported across the basal membrane the proteins must be broken down to single amino acids. Once inside the cell, the di and tripeptides are subjected to further digestion by intracellular proteases in the lysosomes. In terms of the different sections of the small intestine, although the duodenum is by far the most active for both single amino acids and di and tri peptides, the ileum is more active reabsorbing single amino acids while the jejunum is more active with di and tri peptides.

In young infants, before the age of 6 months, whole proteins can be brought into the cell through the process of endocytosis. This is one of the ways that immunity is transferred from mother to child as antibodies from the mother’s milk are taken into the baby’s blood. Even adults can absorb small amounts of intact proteins and although these proteins are not dietary significant they do play important roles in providing the immune system with a sample of the luminal contents. In fact, many food allergies are caused by an overactive immune system responding to intact proteins that it thinks are bad, examples being celiac sprue (gluten intolerance) or nut allergies.

Absorption of Proteins
Author: New Human Physiology, Paulev-Zubieta 2nd ed., illustrator Kirsten McCord. License: CC BY-NC-ND-4.0 Creative Commons Attribution-Noncommercial-No Derivatives 4.0 International. Link:

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