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

The Renal Corpuscle: Bowman's Capsule

Juxtaglomerular Apparatus and Glomerulus 
Author: OpenStax College; Site:; License: This file is licensed under the Creative Commons Attribution 3.0 Unported license

Urine formation begins with a process called filtration. Filtration takes place in the renal corpuscle. To describe the structure of the nephron, we will start at Bowman's capsule. Previously we drove the magic school backwards through the nephron, now we will start from where the blood is first filtered. Bowman's capsule is a cup-like sac that participates in the first step of urine formation. Intertwining within the cup is a capillary bed of blood vessels called the glomerulus. The glomerular capillaries contain blood that entered the kidney through the renal artery. Upon entering the kidneys, the renal artery branched into smaller and smaller arteries, eventually into an afferent arteriole and then the glomerular capillary. For its size (<0.5% of total body weight), the kidney receives an impressive amount of blood flow (~20% of cardiac output). For every Bowman's capsule there is an associated glomerular capillary bed (1.2 million/kidney). Together the two structures (Bowman's capsule and the glomerular capillaries) form the renal corpuscle.

As the blood travels to these capillaries, fluids and substances from the blood (plasma) spill out of the glomerulus and are collected in the cup-like portion of Bowman's capsule. Since Bowman's capsule is continuous with the rest of the nephron tubules, the solution that spills out, the filtrate, is the solution that will eventually become urine. The "spilling" out of the solution is a process referred to as filtration (the first of three processes resulting in the formation of urine). Whether a substance will be filtered or not, is determined by two criteria; size and charge. Substances that fit the criteria, and therefore will be filtered, include: water, glucose, NaCl, amino acids, small proteins, metabolites, and urea. Substances that do not fit the criteria, and thus will not be filtered, include medium to large proteins, red and white blood cells, and platelets. The normal rate of filtrate formation is 125ml of plasma per minute and is known as the glomerular filtration rate (GFR). The GFR is a very important diagnostic test of kidney function and will be used in subsequent discussions of kidney function.

In order for substances to pass from the glomerulus through to Bowman's capsule and the rest of the nephron they must be able to cross the filtration barrier. The barrier consists of four potential obstacles, the fenestrated endothelium of the glomerulus, the basement membrane of the endothelium, the visceral layer of Bowman's capsule (podocytes), and the mesangial cells.

Fenestrated Endothelium, Basement Membrane, and Podocytes forming a Visceral Layer.
Image by BYU-Idaho Becky T. F2018

Fenestrated Endothelium: Two features of the glomerular endothelium are responsible for the ability of these capillaries to effectively filter the plasma. First, the glomerulus is composed of a special type of capillary called fenestrated capillaries. The walls of these capillaries contain fenestra (Latin for window) which are pores in the capillary membranes and negatively charged glycoproteins. The fenestra are approximately 70nM in diameter. You are probably thinking, "why in the heck would anyone want to know that, and why DOES anyone know that?!" Well, red blood cells are 7000 nM in diameter so this should solidify the fact that finding blood in the urine is never a good a thing. The smallest of the plasma proteins have diameters about the same size as the fenestra and might be filtered, however, the negative charges on the endothelial cells impair filtration of these negatively charged proteins. Thus, the first filtration barrier is specific to prevent the filtration of blood cells and plasma proteins.

The basement membrane: A basement membrane is always found in association with epithelial cells and, in this case, serves as a porous matrix of anchored, negatively charged proteins that act as a barrier to circulating negatively charged proteins found in the blood. This barrier seems to function as a charge specific barrier that selects against negatively charged substances. However, this does not mean that all negatively charged substances are not filtered as some smaller substances (ions such as Cl-, HCO3-, etc.) are freely filtered. Still, in terms of importance, the barrier seems to be the most important against negative charges.

The podocytes: The podocytes are the specialized cells that form the visceral layer of Bowman's capsule. They contain long, finger-like processes that completely encircle the capillaries of the glomerulus. The finger-like processes interdigitate, forming gaps between them called filtration slits that serve as the third filter (see images below). Each filtration slit is bridged by proteins (nephrin, NEPH-1, podocin) that make up a thin diaphragm which functions as a size and charge selective filter.

Mesangial cells: Mesangial cells are similar to smooth muscle cells and are interspersed throughout the glomerular capillaries as well as between the afferent and efferent arterioles. Mesangial cells can contract which changes the surface area of the glomerular capillaries and thereby influencing filtration. In addition, they have been shown to have phagocytic activity and to secrete local regulatory hormones (paracrines).

Peritubular Capillaries

The kidney capillary bed (glomerulus) is unique in that it is not immediately drained by a venule, instead the glomerular capillaries rejoin to form the efferent arterioles which then divide into another capillary bed called the peritubular capillaries.

Peritubular Capillaries and Vasa Recta.
Author: OpenStax; Site:; License: Creative Commons Attribution 4.0 License.

The peritubular capillary bed surrounds the proximal and distal tubules of the nephron in both cortical and juxtamedullary nephrons. In juxtamedullary nephrons, the peritubular capillaries branch again to form a third capillary bed that surrounds the loop of Henle, where they are known as the vasa recta. The afferent (before the glomerulus) and the efferent (after the glomerulus) arterioles play a very important role in determining the pressure in the glomerular capillaries.

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