Cardiovascular System Notes

BIO 301
Human Physiology
Cardiovascular system

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The Cardiovascular System:

• consists of the heart plus all the blood vessels
• transports blood to all parts of the body in two 'circulations': pulmonary (lungs) & systemic (the rest of the body)

Heart:

• hollow, muscular organ
• 4 chambers: 2 atria (right & left) & 2 ventricles (right & left)

Blood returning from the systemic (body) circulation enters the right atrium (via the inferior & superior vena cavas). From there, blood flows into the right ventricle, which then pumps blood to the lungs (via the pulmonary artery). Blood returning from the lungs enters the left atrium (via pulmonary veins), then the left ventricle. The left ventricle then pumps blood to the rest of the body (systemic circulation) via the aorta.

 
Cardiovascular system
 
Heart anatomy and function
 

Path of a red blood cell
 

Heart walls - 3 distinct layers:

1 - endocardium - innermost layer; epithelial tissue that lines the entire circulatory system 2 - myocardium - thickest layer; consists of cardiac muscle 3 - epicardium - thin, external membrane around the heart

Cardiac muscle tissue:

• striated (see photo below; consists of sarcomeres just like skeletal muscle)
• cells contain numerous mitochondria (up to 40% of cell volume)
• adjacent cells join end-to-end at structures called intercalated discs

Intercalated discs contain two types of specialized junctions:

• desmosomes (which act like rivets & hold the cells tightly together) and
• gap junctions (which permit action potentials to easily spread from one cardiac muscle cell to adjacent cells).

http://en.wikipedia.org/wiki/Gap_junctions

Cardiac muscle tissue forms 2 functional syncytia or units:

• the atria being one &
• the ventricles the other.

Because of the presence of gap junctions, if any cell is stimulated within a syncytium, then the impulse will spread to all cells. In other words, the 2 atria always function as a unit & the 2 ventricles always function as a unit. However, there are no gap junctions between atrial & ventricular contractile cells. In addition, the atria & ventricles are separated by the electrically nonconductive tissue that surrounds the valves. So, as will be discussed later, a special conducting system is needed to permit transmission of impulses from the atria to the ventricles. In cardiac muscle, there are two types of cells:

• contractile cells &
• autorhythmic (or automatic) cells.

Contractile cells, of course, contract when stimulated. Autorhythmic cells, on the other hand, are self-stimulating & contract without any external stimulation. The action potentials that occur in these two types of cells are a bit different:
On the left is the action potential of an autorhythmic cell; on the right, the action potential of a contractile cell. Autorhythmic cells exhibit PACEMAKER POTENTIALS. Depolarization is due to the inward diffusion of calcium (not sodium as in nerve cell membranes). Depolarization begins when:

• the slow calcium channels open (4),
• then concludes (quickly) when the fast calcium channels open (0).
• Repolarization is due to the outward diffusion of potassium (3).

Used with permission: http://mail.bris.ac.uk/~pydml/CVS/Heart/Cells/Electrics/APpmr.htm In Contractile cells:

• depolarization is very rapid & is due to the inward diffusion of sodium (0).
• repolarization begins with a slow outward diffusion of potassium, but that is largely offset by the slow inward diffusion of calcium (1 & 2). So, repolarization begins with a plateau phase. Then, potassium diffuses out much more rapidly as the calcium channels close (3), and the membrane potential quickly reaches the 'resting' potential (4).

Used with permission: http://mail.bris.ac.uk/~pydml/CVS/Heart/Cells/Electrics/APpmr.htm

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Most of the muscle cells in the heart are contractile cells. The autorhythmic cells are located in these areas:

• Sinoatrial (SA), or sinus, node
• Atrioventricular (AV) node
• Atrioventricular (AV) bundle (also sometimes called the bundle of His)
• Right & left bundle branches
• Purkinje fibers

Various automatic cells have different 'rhythms': SA node - 60 - 100 per minute (usually 70 - 80 per minute)
AV node & AV bundle - 40 - 60 per minute
Bundle branches & Purkinje fibers - 20 - 40 per minute
SA node = has the highest or fastest rhythm &, therefore, sets the pace or rate of contraction for the entire heart. As a result, the SA node is commonly referred to as the PACEMAKER.
 
Spread of cardiac excitation (check this animation: Conducting System of the Heart):

• Begins at the SA node & quickly spreads through both atria
• Also travels through the heart's 'conducting system' (AV node > AV bundle > bundle branches > Purkinje fibers) through the ventricles
• For efficient pumping:
• The atria should contract (& finish contracting) before the ventricles contract. This occurs because of AV nodal delay (that is, the impulse travels rather slowly through the AV node & this permits the atria to complete contraction before the ventricles begin contraction).
• The atria should contract as a unit, & the ventricles should contract as a unit. This occurs because the impulse spreads so rapidly that all myocardial cells in the atria and ventricles, respectively, contract at about the same time. The impulse spreads rapidly through the ventricles because of the conducting system.

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Refractory period of contractile cells:

• Lasts about 250 msec (almost as long as contraction period)

The long refractory period means that cardiac muscle cannot be restimulated until contraction is almost over & this makes summation (& tetanus) of cardiac muscle impossible. This is a valuable protective mechanism because pumping requires alternate periods of contraction & relaxation; prolonged tetanus would prove fatal.

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Electrocardiogram (ECG) = record of spread of electrical activity through the heart
P wave = caused by atrial depolarization
QRS complex = caused by ventricular depolarization
T wave = caused by ventricular repolarization
ECG = useful in diagnosing abnormal heart rates, arrhythmias, & damage of heart muscle

Electrocardiogram

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Coronary artery disease (CAD) is a condition in which plaque builds up inside the coronary arteries that supply heart muscle with oxygen-rich blood. Plaque is made up of fat, cholesterol, calcium, and other substances found in the blood. When plaque builds up in the arteries, the condition is called atherosclerosis. Plaque narrows the arteries and reduces blood flow to your heart muscle. It also makes it more likely that blood clots will form and partially or completely block blood flow. When coronary arteries are narrowed or blocked, oxygen-rich blood can't reach the heart muscle. This can cause angina or a heart attack. Angina is chest pain or discomfort that occurs when not enough blood flows to an area of heart muscle. A heart attack occurs when blood flow to an area of heart muscle is completely blocked. This prevents oxygen-rich blood from reaching that area of heart muscle and causes it to die. Without quick treatment, a heart attack can lead to serious problems and even death. Over time, CAD can weaken heart muscle and lead to heart failure and arrhythmias. Heart failure is a condition in which your heart can't pump enough blood throughout your body. Arrhythmias are problems with the speed or rhythm of your heartbeat. CAD is the most common type of heart disease. It's the leading cause of death in the United States for both men and women. Lifestyle changes, medicines, and/or medical procedures can effectively prevent or treat CAD in most people (Source: NHLBI).

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Heart Valves:

• Atrioventricular (AV) valves - prevent backflow of blood from ventricles to atria during ventricular systole (contraction)
• Tricuspid valve - located between right atrium & right ventricle
• Mitral valve - located between left atrium & left ventricle

• Semilunar valves - prevent backflow of blood from arteries (pulmonary artery & the aorta) to ventricles during ventricular diastole (relaxation)
• Aortic valve - located between left ventricle & the aorta
• Pulmonary valve - located between right ventricle & the pulmonary artery (trunk)

All valves consist of connective tissue (not cardiac muscle tissue) and, therefore, open & close passively. Valves open & close in response to changes in pressure:

• AV valves - open when pressure in the atria is greater than pressure in the ventricles (i.e., during ventricular diastole) & closed when pressure in the ventricles is greater than pressure in the atria (i.e., during ventricular systole)

• Semilunar valves - open when pressure in the ventricles is greater than pressure in the arteries (i.e., during ventricular systole) and closed when pressure in the pulmonary trunk & aorta is greater than pressure in the ventricles (i.e., during ventricular diastole)

 
 
Heart valves & function

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Cross-section of a healthy heart, including the four heart valves. The blue arrow shows the direction in which oxygen-poor blood flows from the body to the lungs. The red arrow shows the direction in which oxygen-rich blood flows from the lungs to the rest of the body.

Heart valve disease is a condition in which one or more heart valves don't work properly, making the heart work harder and affecting its ability to pump blood. Malfunctioning heart valves can create two basic problems: (1) Regurgitation, or backflow, occurs when a valve doesn’t close tightly. Blood leaks back into the chamber rather than flowing forward through the heart or into an artery. Backflow is most often due to prolapse (the flaps of the valve flop or bulge back into an upper heart chamber during a heartbeat). (2) Stenosis occurs when the flaps of a valve thicken, stiffen, or fuse together. This prevents the heart valve from fully opening, and not enough blood flows through the valve. You can be born with heart valve disease (congenital) or you can acquire it later in life. Although a valve may be normal at first, disease can cause problems to develop over time. Many people have heart valve defects or disease, but don't have symptoms. For some people, the condition will stay largely the same over their lifetime and not cause any problems. For other people, the condition can worsen slowly over time until symptoms develop. If not treated, advanced heart valve disease can cause heart failure, stroke, blood clots, or sudden death due to cardiac arrest. Lifestyle changes and medicines can relieve many of the symptoms and problems linked to heart valve disease, and can also lower the risk of developing a life-threatening condition, such as stroke or sudden cardiac arrest. Eventually, however, faulty heart valves may have to be repaired or replaced (Source: NHLBI).

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Mechanical Events of the Cardiac Cycle:     (also check www-medlib.med.utah.edu/kw/pharm/hyper_heart1.html and McGraw-Hill.com)

• the cardiac cycle has two phases: systole (contraction) & diastole (relaxation)
• 'Electrical' events are correlated with the 'mechanical' events:
• P wave = atrial depolarization = atrial systole
• QRS complex = ventricular depolarization = ventricular systole (& atrial diastole occurs at the same time)
• T wave = ventricular repolarization = ventricular diastole
• What happens in the heart during each 'mechanical' event:
• Atrial systole (labeled AC below):
• no heart sounds (because no heart valves are opening or closing)
• a slight increase in ventricular volume because blood from the atria is pumped into the ventricles
• Ventricular systole:
• the first heart sound (lub) (labeled S1 below) - this sound is generated by the closing of the AV valves (& this occurs because increasing pressure in the ventricles causes the AV valves to close)

• initially there is no change in ventricular volume (called the period of isometric contraction) because ventricular pressure must build to a certain level before the semilunar valves can be forced open & blood ejected. Once that pressure is achieved, & the semilunar valves do open, ventricular volume drops rapidly as blood is ejected.
• Ventricular diastole:
• the second heart sound (dub) (labeled S2 below) - this sound is generated by the closing of the semilunar valves (& this occurs because pressure in the pulmonary trunk & aorta is now greater than in the ventricles & blood in those vessels moves back toward the area of lower pressure which closes the valves)
• ventricular volume increases rapidly (period of rapid inflow) - this occurs because blood that accumulated in the atria during ventricular systole (when the AV valves were closed) now forces open the AV valves (because the pressure in the atria is now greater than the pressure in the ventricles). & flows quickly into the ventricles. After this 'rapid inflow', ventricular volume continues to increase, but at a slower rate (the period of diastasis). This increase in volume occurs as blood returning to the heart via the veins largely flows through the atria & into the ventricles.

Used with permission: http://mail.bris.ac.uk/~pydml/CVS/Heart/Whole/CardCyc/CCprvo.htm
 




Human heart

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Cardiac output:

• volume of blood pumped by each ventricle
• equals heart rate (beats per minute) times stroke volume (milliliters of blood pumped per beat)
• typically about 5,500 milliliters (or 5.5 liters) per minute (which is about equal to total blood volume; so, each ventricle pumps the equivalent of total blood volume each minute under resting conditions) BUT maximum may be as high as 25 - 35 liters per minute

Cardiac reserve:

• the difference between cardiac output at rest & the maximum volume of blood the heart is capable of pumping per minute
• permits cardiac output to increase dramatically during periods of physical activity

What factors permit variation in cardiac output?

• Changes in heart rate:
• Parasympathetic stimulation - reduces heart rate
• Sympathetic stimulation - increases heart rate

Effect of parasympathetic stimulation on the heart:
Increased parasympathetic stimulation > release of acetylcholine at the SA node > increased permeability of SA node cell membranes to potassium > 'hyperpolarized' membrane > fewer action potentials (and, therefore, fewer contractions) per minute

a = sympathetic stimulation, b = normal heart rate, & c = parasympathetic stimulation

Effect of sympathetic stimulation on the heart:
Increased sympathetic stimulation > release of norepinephrine at SA node > decreased permeability of SA node cell membranes to potassium > membrane potential becomes less negative (closer to threshold) > more action potentials (and more contractions) per minute

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Regulation of Stroke Volume:

• intrinsic control ==> related to amount of venous return (amount of blood returning to the heart through the veins)
• extrinsic control ==> related to amount of sympathetic stimulation

Intrinsic control:

• Increased end-diastolic volume = increased strength of cardiac contraction = increased stroke volume
• This increase in strength of contraction due to an increase in end-diastolic volume (the volume of blood in the heart just before the ventricles begin to contract) is called the Frank-Starling law of the heart:

• Increased end-diastolic volume = increased stretching of of cardiac muscle = increased strength of contraction = increased stroke volume

Source: http://www.sci.sdsu.edu/Faculty/Paul.Paolini/ppp/lecture21/sld006.htm Extrinsic control:

• Increased sympathetic stimulation > increased strength of contraction of cardiac muscle
• Mechanism = sympathetic stimulation > release of norepinephrine > increased permeability of muscle cell membranes to calcium > calcium diffuses in > more cross-bridges are activated > stronger contraction

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Flow rate through blood vessels

• directly proportional to the pressure gradient
• inversely proportional to vascular resistance

Flow = Difference in pressure/resistance Pressure Gradient = difference in pressure between beginning & end of vessel (pressure = force exerted by blood against vessel wall & measured in millimeters of mercury)
Resistance:

• hindrance to blood flow through a vessel caused by friction between blood & vessel walls
• major determinant = vessel diameter (or radius)
• is inversely proportional to radius to the fourth power (so, for example, doubling the radius of a vessel decreases the resistance 16 times which, in turn, increases flow through the vessel 16 times)

Source: http://www.oucom.ohiou.edu/CVPhysiology/H003.htm

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Arteries:

• serve as passageways for blood from heart to tissues
• act as pressure reservoirs because the elastic walls collapse inward during ventricular diastole (when there is less blood in the arteries):

• blood pressure averages 120 mm Hg during systole (systolic pressure) & 80 mm Hg during diastole (diastolic pressure) (& the difference between systolic & diastolic pressures is called the pulse pressure)

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Arterioles:

• distribute cardiac output among systemic organs (whose needs vary over time)
• Resistance (&, therefore, blood flow) varies as a result of VASODILATION & VASOCONSTRICTION
• Factors that influence radius of arterioles:
• intrinsic (or local) control
• extrinsic control

Intrinsic (local) control:

• changes within a tissue that alter the radius of blood vessels & adjust blood flow
• especially important in skeletal muscles, the heart, & the brain
• increased blood flow in an active tissue results from active hyperemia:

Increased tissue (metabolic) activity > increases levels of carbon dioxide & acid in the tissue & decreases levels of oxygen > these changes in the concentrations of acid, CO2, & O2 cause smooth muscle in the walls of the arterioles to relax & this, in turn, causes vasodilation of the arterioles > vasodilation reduces resistance with the vessel &, as a result, blood flow through the vessel increases So, blood flow increases when a tissue (e.g., skeletal muscle) becomes more active & the increased blood flow delivers the needed oxygen & nutrients.
 
Extrinsic control occurs via:

• sympathetic division of the Autonomic Nervous System
• parasympathetic division of the Autonomic Nervous System

The sympathetic division innervates blood vessels throughout the body while the parasympathetic division innervates blood vessels of the external genitals. Varying degrees of stimulation of these two divisions, therefore, can influence arterioles (& blood flow) throughout the body.

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Capillaries

Capillaries:

• site of exchange of materials between blood & tissues
• exchange may occur by simple diffusion
• diffusion enhanced by:
• thin capillary walls (just one cell thick)
• narrow capillaries (so the red blood cells & plasma are close to the walls)
• large numbers (the human body has 10 - 40 billion capillaries!) which translates into a tremendous amount of surface area through which exchange can occur
• relatively slow flow of blood (providing more time for exchange to occur)
• exchange also occurs through pores (located between the cells the form the capillary walls), by vesicular transport (e.g., pinocytosis), & by bulk flow

BULK FLOW:

• protein-free plasma filters out of capillaries, mixes with surrounding interstitial fluid, & is then reabsorbed. Plasma filters out at the arteriole end of capillaries because hydrostatic (blood) pressure (an outward force) exceeds osmotic pressure (an inward force). At the venous end of capillaries, the filtrate tends to move back in because osmotic pressure now exceeds hydrostatic pressure (check this animation at mcgraw-hill.com).
• because the outward force at the arteriole end exceeds the inward force at the venous end, more plasma filters out than moves back in to the capillaries. So, fluid tends to accumulate in the tissues. The lymph vessels pick up this fluid & transport it back to the blood.

BULK FLOW:

1 - not very important in exchange (much more exchange occurs by way of diffusion) 2 - important in regulating the 'distribution' of fluids between the plasma & interstitial fluid (which is important in maintaining normal blood pressure)

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Veins:

• serve as low-resistance passageways to return blood from the tissues to the heart
• serve as a BLOOD RESERVOIR (under resting conditions nearly two-thirds of all your blood in located in the veins) &, therefore, the veins are important in permitting changes in stroke volume

Cardiovascular System Notes

Cardiovascular System Notes in Physiology

Notes on the Cardiovascular System:

Veins and arteries differ in structure

• Left ventricle is thickest because it squeezes the hardest to get blood thoughout the body.

2 types of circulation:

• Pulmonary circulation (involves lung only)
• Systemic circulation (blood leaving the LV dispersed throughout body, and comes back to RA)

Systole-when ventricles are contracted, and atria relaxed and filling with blood (duration is 0.3 second)
Diastole—when ventricles are relaxing and filling, atria are contracting. (duration is 0.5 seconds)

Each complete heartbeat is less than one second (because normal resting is 70bpm)

Three different kinds of heart cells

• Pacemaker cells
  • Autorhythmic myocardia (cause excitation/contraction rhythmically)
  • Found in various heart regions, but typically in the nodes of the heart
    • Sinoatrial node SA node—roof of right atrium
    • Atrioventricular AV node—found at floor of right atrium
      •  (node is cluster of pacemaker cells)
  • Set the pace in terms of heartbeat. SA node starts the signal, sending to AV node, which sends to conduction myofiber

• Conduction myofibers
  • Found in Atrioventricular bundle (Bundle of His) fibers run down between right and left ventricles and throughout the ventricles. Now these fibers are called Purkinje fibers. Purkinje fibers allows signals to be spread/conducted to various parts of the heart

• Contractile Myocardium
  • Actual contractile cells. Millions of cells in the ventricles and atrial walls that contract at same time to perform a heartbeat.

Heart EKG Technique


Three applications of Calmodulin:

1. Calcium calmodulin—promotes Exocytosis
2. Calcium calmodulin in smooth muscle--Activates myosin light chain kinases
1. Small protein associated with myosin head
3. Calcium calmodulin in growth hormone—activates phosphodiesterase
1. Turns cyclic amp (cAMP) into AMP.
2.  GH opposes glucagon because decrease in cAMP decreases glucagon.

Cardiac Output

Cardio Output = Heart Rate x Stroke Volume

• The amount of blood the heart pumps in a minute.
• Can vary dramatically depending on activity level.
• At resting 5L per min.
• After exercise (close to aerobic capacity) can be 25 Liters/min.

  Heart Rate: Number of beats per minute.
  
  
  
  Stroke Volume: The amount of blood pumped per heart beat.
  

Brief notes on Venous Return

Cardiac Blood Flow

Definition of high blood pressure (hypertension)

What is hypotension?

 

Other Cardiovascular System Topics

Atheriosclerosis

What is Ischemia?

What is Syncope?

Cause of Edema

cardiovascular

The cardiovascular system is sometimes called the circulatory system. It consists of the heart, which is a muscular pumping device, and a closed system of vessels called arteries, veins, and capillaries. As the name implies, blood contained in the circulatory system is pumped by the heart around a closed circuit of vessels as it passes again and again through the various "circulations" of the body.

The Heart

The heart is enclosed by a sac known as the pericardium. There are three layers of tissues that form the heart wall. The outer layer of the heart wall is the epicardium, the middle layer is the myocardium, and the inner layer is the endocardium. The internal cavity of the heart is divided into four chambers:

• Right atrium
• Right ventricle
• Left atrium
• Left ventricle

The two atria are thin-walled chambers that receive blood from the veins. The two ventricles are thick-walled chambers that forcefully pump blood out of the heart. Differences in thickness of the heart chamber walls are due to variations in the amount of myocardium present, which reflects the amount of force each chamber is required to generate.
The right atrium receives deoxygenated blood from systemic veins; the left atrium receives oxygenated blood from the pulmonary veins.

Valves of the Heart
Pumps need a set of valves to keep the fluid flowing in one direction and the heart is no exception. The heart has two types of valves that keep the blood flowing in the correct direction. The valves between the atria and ventricles are called atrioventricular valves (also called cuspid valves), while those at the bases of the large vessels leaving the ventricles are called semilunar valves.
The right atrioventricular valve is the tricuspid valve. The left atrioventricular valve is the bicuspid, or mitral, valve. The valve between the right ventricle and pulmonary trunk is the pulmonary semilunar valve. The valve between the left ventricle and the aorta is the aortic semilunar valve.
When the ventricles contract, atrioventricular valves close to prevent blood from flowing back into the atria. When the ventricles relax, semilunar valves close to prevent blood from flowing back into the ventricles.
Pathway of Blood through the Heart
While it is convenient to describe the flow of blood through the right side of the heart and then through the left side, it is important to realize that both atria contract at the same time and both ventricles contract at the same time. The heart works as two pumps, one on the right and one on the left, working simultaneously. Blood flows from the right atrium to the right ventricle, and then is pumped to the lungs to receive oxygen. From the lungs, the blood flows to the left atrium, then to the left ventricle. From there it is pumped to the systemic circulation.
Blood Supply to the Myocardium
The myocardium of the heart wall is a working muscle that needs a continuous supply of oxygen and nutrients to function with efficiency. For this reason, cardiac muscle has an extensive network of blood vessels to bring oxygen to the contracting cells and to remove waste products.
The right and left coronary arteries, branches of the ascending aorta, supply blood to the walls of the myocardium. After blood passes through the capillaries in the myocardium, it enters a system of cardiac (coronary) veins. Most of the cardiac veins drain into the coronary sinus, which opens into the right atrium.

Blood Vessels

Blood vessels are the channels or conduits through which blood is distributed to body tissues. The vessels make up two closed systems of tubes that begin and end at the heart. One system, the pulmonary vessels, transports blood from the right ventricle to the lungs and back to the left atrium. The other system, the systemic vessels, carries blood from the left ventricle to the tissues in all parts of the body and then returns the blood to the right atrium. Based on their structure and function, blood vessels are classified as either arteries, capillaries, or veins.

Arteries
Arteries carry blood away from the heart. Pulmonary arteries transport blood that has a low oxygen content from the right ventricle to the lungs. Systemic arteries transport oxygenated blood from the left ventricle to the body tissues. Blood is pumped from the ventricles into large elastic arteries that branch repeatedly into smaller and smaller arteries until the branching results in microscopic arteries called arterioles. The arterioles play a key role in regulating blood flow into the tissue capillaries. About 10 percent of the total blood volume is in the systemic arterial system at any given time.
The wall of an artery consists of three layers. The innermost layer, the tunica intima (or just intima), contains simple squamous epithelium, basement membrane and connective tissues. The epithelium is in direct contact with the blood flow. The middle layer, the tunica media, is primarily smooth muscle and is usually the thickest layer. It not only provides support for the vessel but also changes vessel diameter to regulate blood flow and blood pressure. The outermost layer, which attaches the vessel to the surrounding tissue, is the tunica externa or tunica adventitia. This layer is connective tissue with varying amounts of elastic and collagenous fibers. The connective tissue in this layer is quite dense where it is adjacent to the tunic media, but it changes to loose connective tissue near the periphery of the vessel.

Veins
Veins carry blood toward the heart. After blood passes through the capillaries, it enters the smallest veins, called venules. From the venules, it flows into progressively larger and larger veins until it reaches the heart. In the pulmonary circuit, the pulmonary veins transport blood from the lungs to the left atrium of the heart. This blood has a high oxygen content because it has just been oxygenated in the lungs. Systemic veins transport blood from the body tissue to the right atrium of the heart. This blood has a reduced oxygen content because the oxygen has been used for metabolic activities in the tissue cells.
The walls of veins have the same three layers as the arteries. Although all the layers are present, there is less smooth muscle and connective tissue. This makes the walls of veins thinner than those of arteries, which is related to the fact that blood in the veins has less pressure than in the arteries. Because the walls of the veins are thinner and less rigid than arteries, veins can hold more blood. Almost 70 percent of the total blood volume is in the veins at any given time. Medium and large veins have venous valves, similar to the semilunar valves associated with the heart, that help keep the blood flowing toward the heart. Venous valves are especially important in the arms and legs, where they prevent the backflow of blood in response to the pull of gravity.

Capillaries

Capillaries, the smallest and most numerous of the blood vessels, form the connection between the vessels that carry blood away from the heart (arteries) and the vessels that return blood to the heart (veins). The primary function of capillaries is the exchange of materials between the blood and tissue cells.
Capillary distribution varies with the metabolic activity of body tissues. Tissues such as skeletal muscle, liver, and kidney have extensive capillary networks because they are metabolically active and require an abundant supply of oxygen and nutrients. Other tissues, such as connective tissue, have a less abundant supply of capillaries. The epidermis of the skin and the lens and cornea of the eye completely lack a capillary network. About 5 percent of the total blood volume is in the systemic capillaries at any given time. Another 10 percent is in the lungs.
Smooth muscle cells in the arterioles where they branch to form capillaries regulate blood flow from the arterioles into the capillaries.

Role of the Capillaries

In addition to forming the connection between the arteries and veins, capillaries have a vital role in the exchange of gases, nutrients, and metabolic waste products between the blood and the tissue cells. Substances pass through the capillaries wall by diffusion, filtration, and osmosis. Oxygen and carbon dioxide move across the capillary wall by diffusion. Fluid movement across a capillary wall is determined by a combination of hydrostatic and osmotic pressure. The net result of the capillary microcirculation created by hydrostatic and osmotic pressure is that substances leave the blood at one end of the capillary and return at the other end.

Endocrine system

I Endocrine System

 A) Function
  1) control systems, maintain homeostasis
   a) control chemical and water balance in body
   b) control growth and metabolism
   c) control embryonic development and preparation for nurturing
    a newborn
   d) influence sexual behavior, stimulate growth and 
    maturation of the gonads 
   e) feedback to the nervous system
   f) early studies on insect molt
 B) How does it work?  - Basic Overview of the Feedback System
  1) Nervous systems monitors environment
  2) If aberrations are detected - nervous system sends message to endocrine glands
  3) Endocrine glands produce and secrete hormones into blood
  4) Hormones hit target organ
  5) Target organ produces substance
  6) Substance is picked up by receptors and brain stops sending message to
   endocrine glands
 
II Endocrine System - Components

 A) Components
  1) Endocrine glands
  2) Chemical messages = hormones
  3) works intimately in conjunction with the nervous and circulatory systems
   Example: Hypothalamus - Anterior Pituitary
   Example: Hypothalamus - Posterior Pituitary
  4) Disorders - homeostatic imbalances
    Hyposecretion of hormones by glands - low levels of 
     hormones being produced
    Hypersecretion of hormones by glands - high levels of 
     hormones being produced
 B) Humans as an example of typical vertebrate endocrine system
  1) Anterior pituitary gland and its hormones (base of the brain)
   a) GH - growth hormone, stimulates muscle and skeletal growth,
    1) Target organ - liver, stimulates liver to produce growth factors
     that stimulate bone and cartilage growth
    2) disorders
      Hypo (pituitary dwarfism)
      Hyper (early development leads to gigantism; abnormal growth 
        of hands and feet - acromegaly in adulthood)
   b) TSH - Thyroid stimulating hormone, stimulates Thyroid Gland to 
    produce hormones
    Disorder: Hyper - Goiter (overstimulation of thyroid)
   c) FSH - follicle stimulating hormone, stimulates 
    development of follicles
   d) LH - luteinizing hormone, stimulates development of testes and 
    ovaries
   e) ACTH - stimulates adrenal glands to produce hormones
   f) PRL (prolactin) - stimulates mammary gland growth and milk production
  2) Posterior Pituitary
   a) Oxytocin - stimulates uterine contractions, milk ejection reflex
    in the mammary glands
   b) ADH - antidiuretic hormone, stimulates retention of water
    action blocked by alcohol and caffeine
  3) Thyroid Gland (neck)
   a) Thyroxine
     Target organs - primarily skeletal, muscle tissue
     stimulates metabolic rate for growth
     increases oxygen consumption and cell metabolism
    Disorders
     Hyper - hyperthyroidism (increases temperature and BP, weight loss, irritability)
     Hypo - lethargy, weight gain, cretinism in children (abnormal skeletal growth and retardation)
   b) Calcitonin - regulates Ca++ levels in the blood (decreases concentration)
    Target organs - kidney, bone, digestive system 
  4) Adrenal Glands (on top of kidneys) - hormones in the 
    fight/fright response
   a) Glucocorticoids - raises glucose levels in the 
    blood, stimulates glucose production by cells
   b) Epinephrine - complement supply from the sympathetic system
    function is to give the body an energetic boost, increase metabolic rate,
    dilates bronchioles in the lungs, increases heart rate, etc.
    Target - most systems of the body
   c) Mineralocorticoids - retention of ions
  5) Pancreas (at junction of stomach and small intestine)
   a) Insulin - lowers blood sugar levels (secreted by beta cells)
    Target organs - liver, skeletal, adipose (fat) tissues
    Diabetes - Hypo
     Type I - autoimmune disorder (destroys ability to produce insulin)
     Type II - due to aging, lifestyle, heredity and other lifestyle (diet) factors
      decreased responsiveness by cells of target organs to insulin
   b) Glucagon - increases blood sugar levels (secreted by alpha cells)
    Target organs - liver, skeletal, adipose (fat) tissues
  6) Male gonads - testes (pelvic region on men)
   a) testosterone, maintain secondary sexual characteristics,  
    stimulate sperm production
  7) Female gonads - ovaries (pelvic region of women)
   a) estrogen and progesterone, maintain secondary 
    sexual characteristics,  
    regulate reproductive cycle
 C) How do hormones work?
  1) 2nd messenger system - most hormones attach to 
   receptors on the cell 
   membrane, they never enter the cell, the receptor 
   delivers the message, 
   usually in the form of cAMP
  2) Direct system - only steroids (sex hormones) can pass through
   the cell membrane and directly affect DNA or chemical