BIO 301
Human Physiology

Cardiovascular system

The Cardiovascular System:

Drawing showing the various parts of a human heart

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.


How the heart works


How blood flows through the heart


Heart walls - 3 distinct layers:

Drawing showing the different layers in the wall of a heart

Cardiac muscle tissue:

Micrograph of cardiac muscle tissue

Intercalated discs contain two types of specialized junctions:

Drawing of an intercalated disc

Drawing of a gap junction

Cardiac muscle tissue forms 2 functional syncytia or units:

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, 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:

The action potential of an autorhythmic cellThe action potential of a contractile cell

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:

Changing permeabilities of calcium, sodium, and potassium during an action potential of a pacemaker cell membrane
Used with permission:

In Contractile cells:

Changing permeabilities of sodium, calcium, and potassium during an action potential of a contractile cell
Used with permission:

SA node and AV node

Action potentials and contraction in cardiac muscle cells

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

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.

Drawing of a heart showing the various components of the electrical system

Spread of cardiac excitation (check this animation: Conducting System of the Heart):

Animated gif showing the spread of an impulse through the heart's electrical system

Refractory period of contractile cells:

Illustration showing that contractile cells have long refractory periods

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.

An electrocardiogram

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

Drawing of a normal artery and one with plaque

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).

Heart Valves:

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

Drawing of a human heartAnimated gif showing movement of blood through the heart

Heart valves & function


Drawing of a heart showing direction of blood flowCross-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).

Cardiac cycle  

Mechanical Events of the Cardiac Cycle:     (also check and Wiley and and W. H. Freeman)

Diagram showing changes in pressure and blood volume in the ventricles during one cardiac cycle

Used with permission:

Graph showing changes in ventricular volume during a single cardiac cycle
The five 'phases' of ventricular volume

Graph showing relationship between electrical activity and the two heart sounds

Graphs showing an electrocardiogram and changes of aortic, atrial, and ventricular pressure during three cardiac cycles
Illustration showing changes in left ventricular volume and whether heart valves are open or closed during three cardiac cycles

The Cardiac Cycle 

Cardiac output: (Check these animations from

Cardiac reserve:

Cardiac output

What factors permit variation in cardiac output?

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

Three action potentials, including one during normal heart rate, one with sympathetic stimulation, and one with parasympathetic stimulation
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

Regulation of Stroke Volume:

Intrinsic control:

Illustration of the Frank-Starling law of the heart

Extrinsic control:


Flow chart showing how cardiac output is affected by sympathetic stimulation

Flow rate through blood vessels

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)


Illustration of how blood flow through a vessel is determined by the relationship between resistance and the pressure gradient

Graph showing the relation ship between flow rate through a blood vessel and blood vessel radius


Drawing showin how arterial walls expand and collapse during a cardiac cycle


Drawing of an artery, arterioles, and capillaries



Intrinsic (local) control:

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, inDrawing of an arteriole 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:

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.



Capillaries:Drawing of a capillary

Drawing showing how materials and gases are exchanged between the blood and body cells

BULK FLOW: (Check this animation: Fluid exchange across the walls of capillaries and this one)

Drawing showing how blood pressure and osmotic pressure interact to generate bulk flow


Drawings of an artery and a vein

Related links:

NOVA: Cut to the Heart

Valvular Heart Disease

Animated gif of a dog wagging its tailBack to BIO 301 syllabus

Lecture Notes 1 - Cell Structure & Metabolism

Lecture Notes 2 - Neurons & the Nervous System I

Lecture Notes 2b - Neurons & the Nervous System II

Lecture Notes 3 - Muscle

Lecture Notes 4 - Blood & Body Defenses I

Lecture Notes 4b - Blood & Body Defenses II

Lecture Notes 6 - Respiratory System