Size and Location of the Heart, Coverings of the Heart, and Wall of the Heart
Skeleton of the Heart, Path of Blood Through the Heart, and Blood Supply to the Heart
Cardiac Cycle, Heart Sounds, and Cardiac Muscle Fibers
Cardiac Conduction System, Electrocardiogram, and Regulation of the Cardiac Cycle
Capillaries and Exchanges in Capillaries
Pulmonary Circuit and Systemic Circuit
Principal Branches of the Aorta
Arteries to the Neck, Head, and Brain
Arteries to the Shoulder and Upper Limb
Arteries to the Thoracic and Abdominal Walls
Arteries to the Pelvis and Lower Limb
Characteristics of Venous Pathways
Veins from the Brain, Head, and Neck
Veins from the Upper Limb and Shoulder
Veins from the Abdominal and Thoracic Walls
Veins from the Abdominal Viscera
Veins from the Lower Limb and Pelvis
The heart is a cone-shaped organ about the size of a loose fist. It is within the mediastinum and extends from about the level of the second rib to about the level of the sixth rib.
Click here to see a human heart and the location of the heart
The heart and large blood vessels attached to it are covered by a membrane called the pericardium. The pericardium consists of an outer fibrous layer that covers two inner layers. The innermost layer is called the visceral pericardium and it lies directly on top of the heart. The layer on top of the visceral pericardium is called parietal pericardium. The fibrous pericardium and the parietal pericardium form the pericardial sac. The space between the parietal pericardium and visceral pericardium is called the pericardial cavity. The pericardial cavity contains a slippery fluid called serous fluid. Serous fluid reduces friction between the membranes when the heart contracts.
Click here to see the heart in pericardium
The wall of the heart is made of the following three layers:
1) Epicardium - the outermost layer of the wall of the heart. It is also known at the visceral pericardium and contains fat which helps to cushion the heart.
2) Myocardium - the middle layer of the wall of the heart. This is the thickest layer of the wall and is made primarily of cardiac muscle.
3) Endocardium - the innermost layer of the wall of the heart. This layer is thin and very smooth. It stretches as the heart pumps blood. This layer also contains Purkinje fibers which are described later.
Click here to see the layers of the heart
The upper chambers of the heart are called atria. There is a right atrium and a left atrium. The bottom chambers of the heart are called ventricles. There is a right ventricle and a left ventricle. The atria are smaller than the ventricles.
Valves of the heart keep blood flowing in one direction. The following are the valves found in the heart:
1) Tricuspid valve - A valve with three cusps that is situated between the right atrium and the right ventricle. It prevents blood from flowing back into the right atrium when the right ventricle contracts.
2) Bicuspid valve - A valve with two cusps that is situated between the left atrium and left ventricle. It prevents blood from flowing back into the left atrium when the left ventricle contracts.
3) Pulmonary valve - A valve situated between the right ventricle and pulmonary trunk. It prevents blood from flowing back into the right ventricle.
4) Aortic valve - A valve situated between the left ventricle and aorta. It prevents blood from flowing back into the left ventricle.
Click here to see the a table summarizing the functions of the heart valves
Click here to see an anterior view of the heart
Click here to see a posterior view of the heart
Click here to see a heart in coronal section
The skeleton of the heart is made of rings of fibrous connective tissue that enclose the bases of the aorta and pulmonary trunk.
Here is a summary of the flow of blood through the heart:
vena cava --> right atrium --> over tricuspid valve --> right ventricle --> over pulmonary valve --> pulmonary trunk --> pulmonary arteries --> lungs (here blood picks up oxygen and loses carbon dioxide) --> pulmonary veins --> left atrium --> over bicuspid valve --> left ventricle --> over aortic valve --> aorta --> arteries --> arterioles --> capillaries (here blood loses oxygen and gains carbon dioxide) --> venules --> veins --> vena cava ....and the cycle starts all over again....
Click here to see a schematic of blood flow through the heart
The tissues of the heart receive their blood supply through coronary arteries. Branches of the coronary arteries eventually give rise to very small blood vessels called capillaries. The capillaries are in the myocardium of the heart and allow oxygen to diffuse into the cardiac muscle cells. A heart attack or myocardial infarction often involves the blocking of one of the coronary arteries.
One heart beat makes up one cardiac cycle. In one cardiac cycle, the top chambers (atria) of the heart contract and relax together then the bottom chambers (ventricles) of the heart contract and relax together.
When the right atrium contracts the tricuspid valve opens and blood flows into the right ventricle. As the left atrium contracts the bicuspid valve opens and blood flows into the left ventricle.
When the right ventricle contracts, the tricuspid valve must close, the pulmonary valve opens and blood is pushed into the pulmonary trunk. When the left ventricle contracts, the bicuspid valve must close, the aortic valve opens and blood is pushed into the aorta.
During one cardiac cycle you can hear two heart sounds. The sounds are called lubb and dupp. These sounds are generated when valves in the heart snap shut.
Lubb- the first heart sound; occurs when the ventricles contract and the tricuspid and bicuspid valves snap shut.
Dupp - the second heart sound; occurs when the atria contract and the pulmonary and aortic valves snap shut.
Cardiac muscle fibers connect together to form groups of cells that contract and relax together. This network of fibers is called a functional syncytium. There is an atrial syncytium located in the atria and a ventricular syncytium located in the ventricles. The presence of these networks allows the atria to contract together and the ventricles to contract together.
The cardiac conduction system consists of a group of structures that send electrical impulses through the heart. When cardiac muscle receives an electrical impulse, it will contract. The components of the cardiac conduction system are as follows:
1) Sinoatrial node (SA node) - located in the wall of the right atrium; it generates an impulse that flows to the atrioventricular node. The SA node is also called the pacemaker of the heart because it generates the heart's rhythmic contractions.
2) Atrioventricular node (AV node) - located between the atria; after the impulse reaches the AV node the atria contract; the impulse is then sent to the atrioventricular bundle.
3) Atrioventricular bundle (AV bundle) - located between the ventricles; sends the impulse to Purkinje fibers.
4) Purkinje fibers - located in the lateral walls of the ventricles; after the impulse flows through the Purkinje fibers, the ventricles contract.
Now the SA node will start the flow of a new impulse.
Click here to see the components of the cardiac conduction system
An electrocardiogram is a recording of the flow of electrical impulses through the cardiac conduction system. A normal electrocardiogram produces three waves.
1) P wave - first wave created when the impulse travels from the SA node to the AV node; signifies that the atria will contract
2) QRS wave - second and largest wave; created when the impulse travels through the AV bundle and through the Purkinje fibers; signifies that the ventricles will contract
3) T wave - third wave created as the ventricles relax
Click here to view a normal EKG
Click here to view some examples of abnormal EKGs
Several factors will influence the cardiac cycle or heart rate as follows:
1) Exercise - strenuous exercise will increase heart rate because skeletal muscles need more oxygen during this time.
2) Parasympathetic Nerves - Generally keep heart rate relatively.
3) Sympathetic Nerves - Increases heart rate.
4) Cardiac Control Center - Located in the medulla oblongata; when blood pressure rises, cardiac control center sends impulses to decrease heart rate; when blood pressure falls, cardiac control center sends impulses to increase heart rate.
5) Body Temperature - An increase in body temperature usually increases heart rate.
6) Potassium ion concentrations in blood - low concentrations of potassium ions will decrease heart rate; a high concentration of potassium ions will cause an arrhythmia (abnormal heart rate).
7) Calcium ion concentrations in blood - low concentrations of calcium depresses heart action; high concentrations of calcium causes longer than normal heart contractions.
Blood vessels form a closed path that carries blood from the heart to cells and back again. These vessels include arteries, arterioles, capillaries, venules, and veins.
Arteries are the strongest of the blood vessels. The muscular layer of this type of vessels contains smooth muscle and is thicker than the muscular layer of other types of blood vessels. Arteries carry blood away from the heart and are under high pressure which is another reason they need to have thick walls.
The muscular wall of an artery can constrict (called vasoconstriction) to increase blood and pressure or it can dilate (called vasodilation) to decrease blood pressure.
Small branches of arteries are called arterioles.
Capillaries are branches off arterioles. They are the smallest type of blood vessel. They connect arterioles to venules. They have very thin walls that are only about one cell layer thick. These thin walls allow substances to pass in and out of capillary. For example, oxygen will pass out of the blood in a capillary to a body cell and carbon dioxide can pass out of a body cell into the blood of a capillary.
Capillary openings have precapillary sphincters that control the amount of blood that flows into them. When the sphincter is relaxed, more blood flows into the capillary. Tissues that require a lot of oxygen (muscle tissue, nervous tissue) will have a lot of capillaries.
Oxygen, carbon dioxide, amino acids, glucose, lipids, water, and metabolic wastes are exchanged between the blood in capillaries and body cells. These substances move through capillary walls through diffusion, filtration and osmosis.
When blood first enters a capillary it is has high concentrations of oxygen and nutrients (amino acids, glucose, etc.). The body cells surrounding the capillary usually have low concentrations of oxygen and nutrients; the body cells also have high concentrations of carbon dioxide and other waste products. Remember that substances will diffuse from an area of high concentration to an area of low concentration. Therefore, oxygen and nutrients diffuse out of the capillary and into body cells. At the same time carbon dioxide and waste products diffuse out of the body cells and into the capillary.
Because the blood is under pressure as it enters the capillary, water is forced through the capillary wall through filtration. Now the water can enter a body cell.
By the time the blood is leaving the capillary, it has a high solid concentration and a low water concentration. Therefore water will move back into the capillary through osmosis. Remember water moves towards the greater concentration of solids.
Click here to see blood flow through capillaries
Venules are very small blood vessels that are formed when capillaries merge together. Venules merge together to make larger blood vessels called veins.
Veins carry blood towards the heart. The muscular layer in the walls of veins is thinner than that found in arteries. The blood is under no pressure in veins and therefore does not move very easily. Movement of blood through veins requires skeletal muscle contractions and valves in veins. When skeletal muscle contracts, it squeezes veins and blood is pushed in the vein, much like tooth paste is pushed out of a tube. The valves in the vein prevent blood from flowing backwards.
Click here to see venous valves
Blood pressure is the force blood exerts on the inner walls of blood vessels. In the clinical setting blood pressure refers to the pressure in arteries.
Arterial blood pressure rises and falls as the ventricles of the heart contract and relax. When the ventricles contract, blood pressure is greatest in the arteries. This pressure is called the systolic pressure. When the ventricles relax, blood pressure in arteries is at its lowest. This pressure is called the diastolic pressure. Blood pressure is usually reported as the systolic number over the diastolic number.
You can feel the surge of blood through arteries when you feel a pulse. The pulse is created as the artery expands when pressure increases and then subsequently relaxes as blood pressure decreases. Common places to feel a pulse is in the carotid artery of the neck or the radial artery near the wrist.
Click here to see common places to take a pulse
As cardiac output increases, blood pressure increases. Cardiac output is the total amount of blood pumped out of the heart in one minute. If cardiac output decreases, blood pressure decreases.
As blood volume increases, blood pressure increases. If blood volume decreases (as in hemorrhaging), blood pressure decreases.
Vasoconstriction causes peripheral resistance or a resistance to blood flow. Vasoconstriction will increase blood pressure and vasodilation will decrease blood pressure.
Viscosity refers to the thickness of a fluid. Water is not viscous but syrup is viscous. If the viscosity of blood increases, it will have a harder time flowing through arteries. Therefore, an increase in viscosity will increase blood pressure.
Blood pressure is controlled to a large extent by the amount of blood pumped out of the heart. The amount of blood entering the heart should be equal to the amount of blood pumped out of the heart. The heart has a way to ensure this happens. When blood enters the left ventricle, the wall of the ventricle is stretched. The more the wall is stretched, the harder it will contract, and the more blood it will pump out. This is called Starling's Law of the Heart. If only a small amount of blood enters the left ventricle, it will not be stretched very much and therefore will not contract very forcefully. The result is not much blood is pumped out of the heart.
Baroreceptors in the aorta and carotid arteries messure blood pressure. If pressure increases in these vessels, the information is sent to the cardiac center in the medulla oblongata. The cardiac center then knows to decrease heart rate which will lower blood pressure. If pressure gets too low in the aorta, the baroreceptor picks up this information and relays it to the cardiac center. The cardiac center will then increase heart rate to increase blood pressure.
Click here for a summary of factors that affect blood pressure
Recall that the squeezing action of skeletal muscles and valves in vein control blood flow through veins. The sympathetic nervous system can also influence flow of blood through veins. The sympathetic nervous system can cause the vein walls to constrict thereby forcing blood through the veins. This happens if blood pressure gets too low.
Click here to see blood flow through veins
The pulmonary circuit is the route blood takes from the heart to the lungs and back to the heart again. The function of this circuit is to oxygenate the blood. It also allows carbon dioxide to leave the blood and enter the lungs. The pulmonary circuit can be summarized as follows:
right atrium --> right ventricle --> pulmonary trunk --> pulmonary arteries --> lungs --> pulmonary veins --> heart (left atrium)
The systemic circuit is the route blood takes from the heart through the body and back to the heart. The function of this circuit is to deliver oxygen and nutrients to body cells. It also picks up carbon dioxide and waste products from body cells. The systemic circuit can be summarized as follows:
left atrium --> left ventricle --> aorta --> arteries --> arterioles --> capillaries --> venules --> veins --> vena cava --> heart (right atrium)
Click here for a figure showing the pulmonary and systemic circuit
The aorta comes directly off the left ventricle. It is the largest artery in the body with respect to diameter. The divisions of the aorta are as follows:
1) ascending aorta - very short; first part of aorta; blood ascends in it
2) arch of aorta - second portion of aorta; also short
3) descending aorta - third portion of aorta; longest portion; this portion goes descends through the thoracic cavity and into the abdominal cavity. The portion of the descending aorta in the thoracic cavity is called the thoracic aorta; the portion of the descending aorta in the abdominal cavity is called the abdominal aorta.
These are the major arteries that come off the aorta:
1) Coronary arteries - supply heart
2) Brachiocephalic artery - supplies right arm and right side of head
3) Left common carotid artery - supplies left side of head
4) Left subclavian artery - supplies left arm
5) Bronchial artery - supplies bronchi of lungs
6) Pericardial artery - supplies pericardium
7) Esophageal artery - supplies esophagus
8) Celiac artery - supplies upper digestive organs
9) Phrenic artery - supplies diaphragm
10) Superior mesenteric artery - supplies intestines
11) Renal artery - supplies kidneys
12) Gonadal artery - supplies ovaries or testes
13) Inferior mesenteric artery - supplies large intestine
14) Lumbar artery - supplies posterior abdominal wall
15 ) Common iliac artery - supplies abdominal wall, pelvic organs, and legs
1) Lingual arteries - supply tongue
2) Facial arteries - supply face
3) Occipital arteries - supply back of scalp, neck muscles and meninges
4) Maxillary arteries - supply teeth, jaw and eyelids
5) Opthalmic arteries - supply eye and eye muscles
1) Axillary artery - supplies armpit area
2) Brachial artery - supplies upper arm
3) Ulnar artery - supplies forearm and hand
4) Radial artery - supplies forearm and hand
1) Intercostal arteries - supply rib area
2 ) Lumbar arteries - supply posterior abdominal wall
3) External iliac arteries - supply anterior abdominal wall
1) Common iliac arteries - supply legs, gluteal region, and pelvic organs
2) Femoral arteries - supply thigh area
3) Popliteal arteries - supply area behind the knee
4) Tibia arteries - supply lower leg and feet
Click here to see major branches of the aorta
Click here to see major arteries
The names of large veins are often the same as the names of the arteries they run next to. However, there are exceptions.
1) Jugular veins - return blood from head and neck areas
2) Brachiocephalic veins - return blood from head and neck
1) Axillary veins - return blood from armpit area
2) Brachial veins - return blood from upper arm
3) Ulnar veins - return blood from lower arm and hand
4) Radial veins - return blood from lower arm and hand
1) Intercostal veins - return blood from thoracic area
2) Azygos veins - return blood from thoracic and abdominal areas
Most veins of digestive organs deliver their nutrient rich blood to the liver. The liver will then process the nutrients. Eventually the blood of the liver is returned to the general circulation through hepatic veins.
1) Gastric veins - carry blood from stomach to liver
2) Splenic veins - carry blood from spleen, pancreas and stomach to liver
3) Mesenteric veins - carry blood from intestines to liver
4) Hepatic portal veins - carry blood from gastric, splenic and mesenteric veins to liver
5) Hepatic veins - returns blood from liver to the vena cava
1) Iliac veins - return blood from pelvic organs, legs, and gluteal regions
2) Femoral veins - return blood from thigh area
3) Popliteal veins - return blood from knee area
4) Saphenous veins - return blood from legs