Layers of Heart Tissue
The heart wall has three main layers: the endocardium, the myocardium, and the epicardium (listed from inside to outside).
Endocardium: The endocardium is the most interior layer of the heart wall. It is simple squamous epithelium and lines the inside of the heart's chambers. It is very smooth and is responsible for keeping the blood
from sticking to the wall and potentially forming blood clots.
Myocardium: The myocardium is the next most interior layer of the heart wall. It is the middle and muscular layer, which contains the cardiac muscle tissue. It is the thickest part of the wall and makes up most of the
mass of the heart wall. The function of the myocardium is to contract and pump blood throughout the heart.
Epicardium: The epicardium is the outermost layer. It is another name for the visceral layer of the pericardium, which is the sac that contains the heart. The epicardium is a layer of serious membrane that lubricates and
protects the outside of the heart.
Pericardium: The pericardium is the sac that the heart is inside of. Pericardium is the name for the walls and lining of the pericardial cavity. The pericardial cavity is a fluid-filled cavity with two walls on each side. The
outer layer of the pericardium is the parietal layer (made of dense, fibrous connective tissue) while the inside layer is the visceral layer. The pericardium is a serous membrane that produces serous fluid to lubricate the
heart. Another purpose of the pericardium is holding the heart in position and maintaining a hollow space (pericardial cavity) for the heart to expand into.
Layers of Heart Wall
Parts of Electrical Conduction in the Heart
The image on the right shows the heart's electrical system with numbers in the approximate order that electricity travels (electricity travels through some parts at the same time). The labels for the numbers are as followed:
Sinoatrial Node: Also known as the SA node, the sinoatrial node is located on the roof of the right atrium. It is the natural pacemaker of the heart.
Intra-atrial Pathway: The intra-atrial pathway carries electricity from the sinoatrial node on the roof of the right atrium to the left atrium. This pathway leads to the Bachmann's bundle, which is a band of cardiac
muscle in the left atrium which controls contraction of the left atrium.
Internodal Pathway: The internodal pathway carries electricity from the sinoatrial node on the roof of the right atrium to the atrioventricular (AV) node located between the atria and the ventricles.
Atrioventricular Node: Also known as the AV node, the atrioventricular node is the backup pacemaker. It is located in the interatrial septum. Its function is to slow conduction of electricity between the atria and the
ventricles so blood can fill up in the ventricles and so that they do not contract at the same time.
Bundle of His: The bundle of His is the last part of atrial conduction. It is made up of myocardial cells and is located in the ventricular wall (interventricular septum) and allows the electricity to move from the AV
node and into the bundle branches in the ventricles.
Right Bundle Branch: The right bundle branch comes off of the Bundle of His. It is one of the two branches that come off of it, with the other branch being the left bundle branch. The right bundle branch carries
electricity to the right ventricle.
Purkinje Fibers: The Purkinje fibers are located in the subendocardium layer of the heart tissue, which is the innermost layer. They distribute electrical energy to the myocardium, which is the layer of muscular
tissue.
Left Bundle Branch: The left bundle branch comes off of the Bundle of His and is the second of the two branches that come off of it. This bundle branch carries electricity to the left ventricle.
Parts of the Hear
Below are the specific parts of the heart in the order that blood travels.
Superior Vena Cava: The superior vena cava is a tube located superior to the right atrium. It brings deoxygenated blood from the upper parts of the body to the heart.
Inferior Vena Cava: The inferior vena cava is a tube located inferior to the right atrium. It brings deoxygenated blood from the lower parts of the body to the heart.
Right Atrium: The right atrium takes in deoxygenated blood from the superior and inferior vena cavae. It then pumps the blood through the tricuspid valve and into the right ventricle. The Sinoatrial (SA) node is
located on the roof of the right atrium.
Tricuspid Valve: The tricuspid valve, located between the right atrium and the right ventricle, prevents blood flow backward from the ventricle to the atrium. It is called the tricuspid valve because it has three leaflets.
Right Ventricle: The right ventricle takes deoxygenated blood that has traveled through the tricuspid valve from the right atrium. It then pumps that blood through the pulmonary valve and into the pulmonary trunk.
Pulmonary Valve: The pulmonary valve is one of the four valves of the heart, located between the right ventricle and the pulmonary trunk. Like all valves, it prevents blood from flowing backwards. The pulmonary
valve is a semilunar valve, which has three cusps.
Pulmonary Trunk: The pulmonary trunk takes in deoxygenated blood from the right ventricle and takes it to the pulmonary arteries.
Pulmonary Arteries: The pulmonary arteries are the last part of the pulmonary circuit of the heart. They take deoxygenated blood from the pulmo nary trunk and the right ventricle and take it to the lungs so that gas
exchange can occur. There are two pulmonary arteries, the left and the right, which take the deoxygenated blood to the left and right lungs, respectively.
Pulmonary Capillary Beds: The pulmonary capillary beds are located in the alveoli of the lungs. They are responsible for exchanging the carbon dioxide in the venous blood for oxygen.
Pulmonary Veins: After gas exchange occurs at the capillary beds of the lungs, oxygenated blood goes back to the heart via the left and right pulmonary veins. They then take the blood to the left atrium.
Left Atrium: The left atrium receives oxygenated blood from the left and right pulmonary veins. It then pumps the blood through the mitral valve and into the left ventricle.
Mitral Valve: The mitral valve is another of the four valves of the heart. It allows blood to pass from the left atrium to the left ventricle and prevents backflow back into the atrium. It is also known as the bicuspid
valve because it has two leaflets.
Left Ventricle: The left ventricle takes in oxygenated blood from the left atrium and pumps through the aortic valve and into the aorta and then the whole body. It is the most muscular chamber of the heart due to the
fact that it has to pump blood to the rest of the body.
Aortic Valve: The aortic valve is the last of the four valves of the heart. It allows blood to pass from the left ventricle and into the aorta. It is a semilunar valve and has three cusps.
Aorta: The aorta receives oxygenated blood from the left ventricle to take to the whole body. It is the largest artery of the body, about two centimeters in width. Three smaller arteries branch off of the aortic arch,
which are the brachiocephalic artery, the left common carotid artery, and the left subcla vian artery, which all supply oxygenated blood to the head and arms.
Functions
The lymphatic system functions to:
return tissue fluid to the bloodstream
transport fats from the digestive system to the
bloodstream
return other large molecules such as proteins to blood
provide surveillance and defense against disease
hemopoiesis (production of lymphocytes)
Lymph
Lymph is a clear, watery fluid that is most similar to the
interstitial fluid of the body and contains fewer proteins
than blood plasma. The lymphatic system filters 2500 to
2800 milliliters (mL) of lymph a day, and approximately
half comes from the liver and small intestine alone.
Lymphatic Tissue
The three types of lymphatic tissue are:
diffuse lymphatic tissue
otherwise known as MALT, GALT, SALT,
etc.
found in connective tissue of most organs
no capsule found
lymphatic nodules
no capsule found
oval-shaped masses found singly or in
clusters
lymphatic organs
capsule present
filter tissue fluid
Lymphatic Structures
Lymph Nodes
There are up to 600 lymph nodes spread throughout the
body, most occurring in groups or clusters. The most
common regions are the submental/submaxillary region,
the cervical region, the axillary region, and the inguinal
region.
peyer's Patches, a form of MALT
Submental/submaxillary lymph nodes are found in the
floor of the mouth and drain lymph from the nose, lips,
and teeth. Cervical lymph nodes are in the neck, draining
the neck and head. Axillary lymph nodes are located in
the armpit and upper chest, draining the arm and upper
thorax. The upper thorax includes drainage from the skin
over the breast and deeper portions of the breast.
Inguinal lymph nodes are in the groin and drain the legs
and genitals.
Lymphatic Ducts
Lymphatic ducts are similar to the major vessels of the
cardiovascular systems but are more like veins than
arteries. The two major lymphatic ducts are the right
lymphatic duct and the thoracic duct. The right
lymphatic duct drains the upper right quadrant of the
body into the right subclavian vein and jugular vein. The
thoracic duct drains the remaining 75 percent of the
body: everything below the diaphragm, the left arm, and
the left side of the head, neck, and thorax.
Accessory Lymphatic Organs
Tonsils
The tonsils are clusters of lymph nodes embedded at the
base of the pharynx or throat. Tonsils contain deep pits
called crypts which hold food debris, bacteria, and white
blood cells. The three main sets of tonsils are: the
pharyngeal tonsils, or adenoids; the palatine tonsils; and
the lingual tonsils.
The adenoids can be found on the wall of the
nasopharynx. The palatine tonsils are located at the edge
of the oral cavity, or on the palate of the mouth. The
palatine tonsils are the largest and most susceptible to
infection (tonsilitis). Lingual tonsils are present on each
side of the root of the tongue.
Anatomy of the tonsils
Spleen
The spleen is the largest of the lymphatic organs, and it
is located below the diaphragm on the left side of the
abdomen. The spleen detects antigens and provides
defense against pathogens, produces monocytes and
lymphocytes as well as fetal erythrocytes, destroys red
blood cells and platelets, and filters and stores blood.
The spleen's sinuses can store approximately 350 mL of
blood and pump blood into the cardiovascular system in
case of damage. It also transfers excess plasma from the
blood to the lymphatic system.
Thymus
The thymus is located in the mediastinum (area between
the lungs) and neck region. It is largest in infants and
children and can reach a mass of 30 to 50 grams during
puberty, later degenerating with age. The thymus aids in
the production of lymphocytes, and it is the site of
maturation for T-cells.
Peyer's Patches
Peyer's Patches are small masses of lymphatic tissue
found in the ileum of the small intestine. They analyze
and respond to pathogens in the intestine.
Appendix
The appendix has a minor role in immunity because it
stores good bacteria. Blockage can lead to appendicitis,
which can be fatal if the appendix ruptures.
Relation to Cardiovascular System
Connection between vascular and lymphatic vessels
The lymphatic system is directly connected to the
circulatory system, but there are a few notable
differences. Unlike the cardiovascular system, the
lymphatic system is a one-way system. The lymph
vessels do not form a complete circuit between the
lymph organs, and lymph is not "pumped" like blood.
The lymphatic system also lacks arteries. However,
lymphatic and blood vessels both have the same three
tunics, and valves to prevent backflow. Lymph is moved
by contractions of skeletal muscle and other body
movements, and leakage from lymphatic vessels enters
cardiac vessels to increase blood volume. Likewise,
lymph vessels act as a reservoir for cells and fluid that
escape the cardiovascular system.
Disorders
Swollen Glands
Swollen glands, also known as swollen lymph nodes or
lymphadenitis, occur as a result of infection. Symptoms
may include tenderness and pain and swelling the size of
a pea or kidney bean. Other symptoms, such as
respiratory symptoms, fever, or night sweats, depending
on the cause of inflammation. Generalized swelling
throughout the body may indicate an autoimmune
disorder such as systemic lupus erythematosus or HIV.
LAM
Lymphangioleiomyomatosis (LAM) is a rare disease that
causes muscle-like cells to grow in certain organs or
tissues, especially the lungs, lymph nodes, and kidneys.
The two main forms are sporadic LAM or TSC-LAM,
which is LAM that occurs along with tuberous sclerosis
complex (a rare disease causing tumors to develop
throughout the body).
Edema
Edema is an excessive accumulation of interstitial fluid.
It can result from any disruption of lymphatic flow, such
as injury, inflammation, surgery, or parasitic infections.
Hodgkin's Lymphoma
Hodgkin's disease is a malignancy of the lymph nodes.
The nodes become enlarged and painful, especially the
cervical nodes.
Non-Hodgkin's Lymphoma
Non-Hodgkin's Lymphoma is a more common form of
Hodgkin's Disease, with more widespread malignancy
and a higher death rate.
Ruptured Spleen
A ruptured spleen occurs as a result of trauma to the left
thoracic or abdominal wall. This rupture may cause fatal
hemorrhaging, and the spleen must be removed.
However, removing the spleen does not hinder lymphatic
function.
Elephantitis
Lymphatic filariasis, or elephantitis/elephantiasis, is a
vectorborne disease in which the patient is bitten by a
mosquito infected with a filarial worm, and the resulting
edema leads to fibrosis and severe thickening of the skin.
There are other forms of elephantiasis that can be caused
by chronic lymphatic obstructions, such as nonfilarial
elephantiasis (podoconiosis), elephantiasis nostras, and
elephantiasis in cancer patients