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 Contraction2.3.3. 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

Chemicals that Regulate Ventilation

Carbon Dioxide

Carbon dioxide is the most influential and tightly controlled chemical regulator of ventilation. Carbon dioxide crosses the blood brain barrier (H+ can't cross the blood brain barrier) and H+ levels elevate by the same reaction catalyzed by carbonic anhydrase in red blood cells as discussed previously and shown below.

Elevated brain extracellular fluid (or Cerebral Spinal Fluid) H+ concentrations are detected by chemoreceptors in the medulla oblongata. About 80% of carbon dioxide homeostasis at rest is maintained by this central detection. There are many neural connections from the chemoreceptors to the medullary regulatory center. High levels of carbon dioxide in the blood (hypercapnia) and consequently H+ ions in the brain, will result in increased rate and depth of ventilation while low levels (hypocapnia) will have the opposing effect. This way, arterial blood traveling from the heart to the tissues is always maintained in the normal PaCO2 range of 37 - 43 mm Hg.  A change of 5 mm Hg in the partial pressure of carbon dioxide will have a profound effect on ventilation - essentially doubling the breathing rate. Peripheral chemoreceptors in the carotid arteries and aortic arch also respond to changes in carbon dioxide levels but are only responsible for about 20% of carbon dioxide response under resting conditions. Peripheral chemoreceptors play a larger role during exercise.

Let’s test your understanding. While sitting in the library reviewing about chemicals that regulate ventilation, you notice your friend Richie across the room. Richie wished he would have studied more and now is stressed about his BIO 461 respiratory exam coming up. Becoming more anxious, he starts to hyperventilate. As he does so, you consider what will happen to the carbon dioxide blood levels in his blood and brain. You have studied diligently and know that hyperventilation may lead to hypocapnia which can cause vasoconstriction of cerebral vessels, cutting off some blood supply, possibly leading to dizziness and fainting. As he complains of dizziness you tell him to breath into a paper bag. You understand that this will increase the amount of carbon dioxide in his inhaled air and help bring his blood carbon dioxide levels back up. Congratulations! You saved the day. Now back to work.


There are also chemoreceptors for oxygen in the carotid arteries and aortic arch. The oxygen partial pressure in arterial blood under normal conditions is 95 mm Hg. Low levels of oxygen in the blood is known as hypoxemia. As discussed previously, the manner in which hemoglobin was created to bind to oxygen is truly magnificent. Remember the oxygen-hemoglobin dissociation curve. The top of the curve is quite flat. Even if oxygen partial pressures drop to 80,70 or even 65, hemoglobin is still highly saturated (see the table 1 below). It is not until levels drop to around 60 mm Hg, that oxygen on its own really starts to stimulate an increased breathing rate. Supplemental oxygen for patients (i.e. COPD patients) doesn't generally need to be given until partial pressure levels go below 65 mm Hg. Due to the unique way hemoglobin binds to oxygen, 70-100 mmHg can be considered "normal".

Oxygen partial pressure (PO2) Oxygen Saturation
100 mm Hg 98%
80 mm Hg 95%
60 mm Hg 89%

Table:  Percent saturation of hemoglobin with oxygen at certain arterial blood oxygen partial pressures.

Oxygen Saturation Moniter
By Thinkpaul (Own work) [CC BY-SA 3.0 (], via Wikimedia Commons

pulse oximeter (Pictured above) is a noninvasive method to quickly determine oxygen saturation (Note: this device does NOT measure PaO2). The device is placed on a translucent part of the body like a fingertip or earlobe and utilizes differences in light absorbance between oxyhemoglobin and deoxygenated hemoglobin to determine percent oxygen saturation of hemoglobin. A saturation of 90-95% is considered the normal range for patients without a pulmonary disorder or disease.