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

Arteries and Veins

It may help to see a good image of blood vessel anatomy.  Here are a couple of links of images we like.

Three main kinds of blood vessels are found in the human body: arteries, capillaries, and veins. Arteries are systemic vessels which transport blood from the left ventricle to cells and tissues throughout the body. The arteries nearest the left ventricle are massive vessels, having walls composed mostly of elastic tissue and to a lesser extent, smooth muscle. As blood travels further through the system, the arteries become increasingly branched and decrease in size. As this branching occurs, arterial wall composition changes as well, incorporating more smooth muscle and less elastic tissue. Owing to this change in composition, arteries are categorized from largest to smallest as elastic arteries, muscular arteries, and finally arterioles. Arterioles are particularly sensitive to neural and endocrine influences. Arterioles change diameter under these influences and this impacts the amount of resistance to blood flow. Blood within arterioles courses into the smallest type of vessels known as capillaries. Capillary walls are extremely thin which allows the blood to exchange oxygen, nutrients, and waste products with surrounding cells and tissues. The thinness of the walls also reduces their ability to withstand high pressure, thus a major reduction occur at the arterioles before reaching the capillaries. By the time blood hits the capillaries the pressure has been reduced from 120mmHg to around 25mmhg and the flow has gone from a rapid pulsating flow to a slow smooth consistent flow. 

Capillaries connect the arterial blood flow to veins, which transport blood from the cells and tissues of the body back to the right atrium of the heart. Veins are categorized from smallest to largest as venules, small veins, and medium/large veins. Venous wall composition differs from arterial wall composition because they have less smooth muscle and elastic tissue.

Tunica Adventitia

The outermost tunica adventitia is made up of connective tissue which vary in consistency and tissue type based on depth. The deepest layers are composed of dense connective tissue while the outermost layers are composed of loose connective tissue continuous with that of the surrounding tissues.

Tunica Media

The tunica media is the most variable of all the layers between different types of arteries. It is composed of circular bands of smooth muscle cells which can contract or relax and change a vessels diameter. The tunica media also contains various amounts of elastic and collagen fibers. An elastic layer known as the external elastic membrane divides the tunica media from the tunica adventitia. This layer is especially prominent in large vessels like the aorta.

Tunica Intima

The innermost tunica intima is composed of an endothelial layer, a connective tissue basement membrane and a slim layer of connective tissue known as the lamina propria. An elastic layer known as the internal elastic membrane divides the tunic intima from the tunica media.

Vasa Vasorum

Recall that blood vessels exist because all cells within the human body require oxygen, nutrients, and the disposal of waste products in order to survive. Considering that the vessels themselves are composed of living cells many layers thick, they are not exempt from these requirements. The requirements of vessels under 1 mm in diameter are easily satisfied by diffusion between the lumen of the vessel and its surrounding layers. However, vessels larger than this size must be sustained by a capillary network of vessels known as vasa vasorum within the tunica adventitia and tunica media layers.

Vasa Vasorum.
 This image was drawn by BYU-Idaho student Nate Shoemaker Spring 2016


Arterioles are the smallest type of artery and are similar in composition to muscular arteries.  The tunica adventitia is extremely thin and devoid of an external elastic membrane.  The tunica media is composed of smooth muscle tissue, but contains few layers.  The tunica intima does not have an internal elastic membrane.


Veins are thinner than arteries. The tunica media has less smooth muscle in it. The thinner vessels with less smooth muscle makes them have more compliance. Because of the extensibility of the veins, we find that at any moment, there can be around 70 percent of a persons blood in the veins. When the body needs to increase cardiac output, the veins can contribute more blood by some vasoconstriction but also by more skeletal muscle contraction such as occurs during exercise.


Venules are the smallest type of vein.  Venules receiving blood directly from capillaries are very similar to the capillaries themselves.  As the venules increase in size, an increasing number of smooth muscle cells are found surrounding the tunica intima. 

Small Veins

Once venules reach a certain diameter, a tunica media layer of smooth muscle cells completely surrounds them.  At this point they are known as small veins.  A tunica adventitia layer of collagen fibers is also found surrounding the smooth muscle tissue of the tunica media.  All veins tend to be larger than arteries at a similar point in the vascular tree.  This is because veins have thinner walls and Tunica layers.  These thin layers allow veins to stretch a lot.  This ability to stretch is called “compliance”.  Veins have a lot more compliance than arteries.      


Valves are common components of medium and large veins greater than 2 mm in diameter.  A valve is formed by two overlapping sections of tunica intima tissue that converge to close off their respective vein should back flow occur.  Valves are far more prevalent in medium veins and in veins of the legs than in veins of the arms. Sometimes valves can be stretched if there are excessive amounts of pressure in the vein.  This can sometimes happen if a person stands or sits for long periods of time (over many months and years). In these situations, blood can have a hard time moving up the venous vessels because of gravity or compression of lower extremity soft tissue. We tend to find more valves in the veins of the lower extremities. This is likely because we spend so much time upright and gravity pulls on the venous blood. Having more valves will allow more "gates" that blood would have to move past to descend downward toward the feet (under the pull of gravity). 

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