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


As mentioned above, capillaries are the smallest vessels in the human body, with an interior diameter roughly equivalent to the width of a single erythrocyte at 5-10 µm.  Widespread branching creates a capillary network capable of carrying blood to within 1 mm of almost every human cell.  There are estimated 10 billion capillaries in the body, each 1.1-millimeter-long which if stretch end to end would equal 25,000 miles. Accordingly, capillaries lie very close to one another arranged in complexes throughout the body known as capillary beds.

Blood Pressure Changes From Arteries to Veins
Image created at BYU-Idaho by Nate Shoemaker Spring 2016

Owing to this widespread branching, the total area of vasculature within the capillary network greatly exceeds that of the arterial network that supplies them. Consequently, as the total area of vasculature increases, the velocity and pressure of the blood decreases. This decreased velocity allows more time for the diffusion of oxygen, nutrients, and waste products to occur between capillaries and the tissues they support.

Three main types of capillaries exist within the human body based on their permeability to substances departing and entering the blood stream. These three types are known as continuous, fenestrated, and sinusoidal capillaries.

Continuous, Fenestrated and Sinusoidal Capillaries
Image created at BYU-I Fall 2013

Continuous Capillaries

The endothelial tissue of continuous capillaries is arranged with virtually no gaps between individual cells.  This allows continuous capillaries to be impermeable to polar molecules and in some cases even water.  Continuous capillaries are the most common type in the body and are located in kidneys, nervous system, muscle, fat, heart and a number of other tissues throughout the body.   

Fenestrated Capillaries

The endothelial tissue of fenestrated capillaries is also arranged in a tight weave, but comparatively large pores known as fenestrae occur within the individual cells. Fenestrated capillaries primarily supply tissues which require a high level of permeability for accelerated diffusion such as the intestines, and kidney glomeruli. Fenestrae size varies depending upon tissue type. The size of the fenestrae determines the size of the substances that can pass in and out of the bloodstream. 

Sinusoidal Capillaries

Sinusoidal capillaries are similar to fenestrated capillaries but are larger in diameter and are arranged in a looser weave to allow the presence of gaps between individual cells. These capillaries are found in tissues such as endocrine glands, which require permeability to large molecules. 

Sinusoid is a special type of capillary bed, found in the bone marrow, spleen and liver. These capillaries possess very large gaps between individual cells. The gaps are large enough that all the components of blood freely pass.

Pre-capillary Sphincters
Image created at BYU-Idaho Fall 2013

As blood flows from an arteriole into a capillary bed, it passes by a concentration of smooth muscle cells known as the precapillary sphincter, located at the arteriole end of each arteriole. This sphincter controls the amount of blood that enters the capillary bed. In this way, the precapillary sphincters control the local blood flow in the tissues.

When the sympathetic nervous system is activated during times of “fight and flight” responses, muscular arteries and arterioles constrict. This greatly decreases the amount of blood flow to an area of capillary beds. However, the sympathetic nervous system does not constrict precapillary sphincters. This is because pre-capillary sphincters are not innervated by sympathetic nerves but instead rely on products of increased metabolism like CO2, acid, and adenosine to stimulate sphincter relaxation. In fact, through mechanisms not completely understood it can be shown that when precapillary sphincters are stimulated by waste products to relax, vessels upstream from these metabolically active tissues also dilate. The effects of metabolic waste products on precapillary sphincters overrides the sympathetic response to constrict arterioles in the area. This is advantageous during exercise because the circulation will increase blood flow to the metabolically active tissue while decreasing blood flow almost everywhere else. This ability for tissues to regulate their blood flow based off of metabolic need is called autoregulation.

Capillaries and Temperature Regulation

Capillaries play an important role in temperature regulation as well as in capillary exchange. There is a structure of special vessels known as arteriovenous anastomoses that shunts blood flow directly from arterioles into small veins, avoiding capillaries entirely. This becomes especially important in the skin, when the body needs to shunt a lot of blood away from the skin quickly to control heat loss. Arteriovenous anastomoses may also develop pathologically due to genetic or developmental errors or from tissue damage or as a result of tumor growth. These vessels allow large quantities of blood to flow directly from arteries into veins, greatly increasing venous return and thereby, the workload of the heart. Heart failure may result if these vessels are allowed to grow to extremely large sizes. Also, these malformed anastomoses can break and cause unwanted bleeds, especially if they are in the brain.

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