Biology 2404 A&P Basics

The embryonic development of the heart begins as two tubes that fuse and twist to form a functional pump by four weeks of age. The heart, located in the ventral chest, sits obliquely in a region called the mediastinum. The cone shaped, fist sized organ has a broad base where major vessels enter and leave and a pointed apex tip that lies mainly to the left of midline. Recall that most organs enclosed in a closed body cavity are covered by a serous membrane sac that secretes a watery, serous fluid. The sac that surrounds the heart is called the pericardial sac and is actually folded twice over the heart to create an outer fibrous pericardium, a parietal pericardium that is in contact with the throacic wall, and the visceral pericardium on the heart surface known as the epicardium. The serous fluid secreted by this membrane sac is called pericardial fluid which functions to decrease frictional forces of movement between the heart and lungs.

Cat specimen

Fetal Pig specimen

Pig Pluck Specimen

Normal Heart Size

The heart wall is composed of three layers:

Epicardium simple squamous epithelium that forms the visceral pericardium

Myocardium cardiac muscle that forms the chamber pumps

Endocardium simple squamous epithelial lining of the cardiovascular system

As the heart develops, it forms walls or septums that create the 4 chambers. The upper chambers called atria are divided into right and left sides by the interatrial septum.

The lower chambers called ventricles are divided into right and left sides by the interventricular septum. The septum grows together from top and bottom portions that meet in the middle. Failure to close will create a septal defect between the chambers known as either an ASD (atrial septal defect) or VSD (ventricular septal defect)

Normal Heart Interior Transverse Section

External extensions of the atria form ear flap appendages called auricles which function to help increase the volume of blood that can be received by the atria. The atrial lining is smooth except in the regions of the auriclar appendage where the muscle wall forms bundles called pectinate muscle. Vessel openings in the atria allow blood to be returned from body organs. The right atrium has three vessel openings: the coronary sinus, the superior vena cava, and the inferior vena cava that return blood from all the body organs. The left atrium has four vessel openings: two right and two left pulmonary veins that return oxygenated blood from the lungs back to the heart to be pumped to body organs.

The functions of the atria are:

a) to collect blood returning from the body

b) to pump remaining blood to the ventricles

c) to produce ANP (atrionatriuretic) peptide hormone (by the right atria) that helps regulate blood pressure

Ventricles

The ventricles have several associated structures common to both left and right sides. Their muscle bundles are called trabeculae carnae that help increase surface area and aid in contraction of the chamber. Finger like projections from the ventricle wall are called papillary muscles and are linked to the atrioventricular valves by connective tissue bands called chordae tendonae. The papillary muscle and chordae tendonae help anchor the valve leaflets and prevent them from inverting into the atria. The ventricles also each have a semilunar valve associated with the great vessel that leaves the chamber.

The differences in the right and left ventricular walls of each chamber are associated with their functions as pumps to generate the pressure needed to keep blood moving forward. The right side of the ventricle is thinner since the blood pumped from this region goes to the lungs for gas exchange. The left side of the ventricle is much thicker since blood pumped from this region has to go to all the body organs, above and below the heart.

The functions of the ventricles are:

a) collect blood from the atria

b) to pump blood to body organs

Heart Poster

Heart Drawing

Heart model: Anterior, Posterior

Organman: Thoracic Cage, Thoracic Organs

Heart histology

Cardiac muscle histology

Body Torso: Thoracic Cage, Transverse Sectioned

Heart valves

Heart valves are connective tissue flaps covered by the heart lining called the endocardium. Valves are passive in their movement, that is, they do not actively open and close since no muscle is present in their structure. Valves open and close based on pressure gradients created by the heart muscle pumping to move blood. All valves are designed to prevent backward flow of blood.

The four cardiac valves are:

a) Right atrioventricular (AV) valve or tricuspid valve, between the right atria and ventricle

b) Left atrioventricular (AV) valve or bicuspid or mitral valve, between the left atria and ventricle

c) Pulmonary semilunar valve, between the right ventricle and the pulmonary trunk

d) Aortic semilunar valve, between the left ventricle and the aorta

As seen in the above description, the atrioventricular valves have other names based on their anatomy. The right atrioventricular valve is also called the tricuspid valve since this valve is formed by three flaps or cusps. The left atrioventricular valve is known as the bicuspid valve since this valve is formed by two cusps or flaps. When these two flaps close, they resemble a mitre angle or mitre cap, so the clinical name for this valve is the mitral valve. It is under the greatest pressure and due to its structure and other related anatomical structures, it can fail to open (stenotic) or close (insufficient).

Drawing of Heart and Valves

Heart valve

Model

Histology

Pig specimen: Right Side Heart, Left Side Heart

Blood flow through the heart

The heart muscle pumps blood through the series of vessels that form two major circuits: the Pulmonary circuit and the Systemic Circuit. Blood returning from the body enters the right atrium via the coronary sinus from the heart; itself the superior vena cava from the head, neck, and arms; and the inferior vena cava from the trunk and legs. Blood also is returned to the left atria via the right and left pulmonary veins from the lungs.

Blood is then allowed to enter the ventricles through the open AV valves. The atria contract simultaneously to pump any remaining blood into their respective ventricles.

In the pulmonary circuit, the right ventricle pumps blood through the pulmonary semilunar valve into the pulmonary trunk and pulmonary arteries to be delivered to the lungs for gas exchange. Blood returns to the heart through the pulmonary veins into the left atria. In the systemic circuit, the blood in the left ventricle is pumped through the aortic semilunar valve into the aorta for all the body organs. Blood then returns to the right atria to end the systemic circuit and begin the pulmonary pathways.

Heart model: Posterior Interior, Anterior Interior

Heart Wall Mount

Circulatory System Wall Mount

Circulatory System Web Site

Heart Physiology

The heart has its own intrinsic (internal) electrical cells. These are modified cardiac muscle cells that are specialized to initiate and respond to electrical (ionic) changes. These cells can be regulated and modified by the ANS. The cardiac conduction system consists of the:

A) Sinoatrial (SA) node in the right atria near the SVC

B) Atrioventricular (AV) node in the right atria near the Rt AV valve

C) AV bundle & junction in the interventricular septum

D) Bundle Branches in the interventricular septum

E) Purkinje Fibers in the ventricular free walls

Heart Histology Model

Conduction System Clear Heart

The conduction system of the heart can be represented electrically by recording the changes in ion movement. This recording is called an electrocardiogram or ECG.

Wave deflections on the tracing represent ion movement and reflect how the conduction system is working. The first wave deflection is the p wave and it represents atrial depolarization triggered by the SA node firing. The second deflection wave is a group of three waves called the QRS complex. It represents ventricular depolarization triggered by the firing of the AV nodal system: the AV node, AV bundle-junction, Bundle Branches, and Purkinje fibers. ECG abnormalities are generally termed arrhythmias and can represent problems in the conduction system, ionic concentrations, heart muscle response, and ANS control.

Drawing of Cardiac Conduction System

Heart Model with Conduction System

Clear Heart Model with Conduction System

ECG Heart with Leads

Histology of Purkinje fibers

ECG machines: Field, Lifepack 8, Lifepack 10

ECG tracing: Drawing of wave forms

ECG tracing, Lead II on Machine

ECG tracing, 12 leads

Cardiac Cycle

The heart’s electrical events tie to mechanical events in a sequence marked from the beginning of one heart beat to the next. This cycle, known as the cardiac cycle, involves the contraction and relaxation of the heart. During contraction or systole, the heart muscle pumps blood and during relaxation or diastole, the heart chambers fill with blood. Since the heart muscle contracts and relaxes, it pushes blood creating pressure changes that open and close the heart valves. When each set of heart valves close in relation to the heart movements, there are two heart sounds that are heard.

The first heart sound, “lub”, occurs when the left and right AV valves close at the beginning of the ventricular systole. The second heart sound, “dup”, occurs when the pulmonary and aortic semilunar valves close during ventricular diastole. Heart sounds can best be heard by using a stethoscope over the chest wall at specific points between the 3rd and 5th intercostal spaces.

Stethoscope

Cardiac Output

When the heart muscle contracts, a certain amount of blood is pumped.
The amount of blood pumped by a ventricle in one minute is called the cardiac output.
This amount of blood flow can also be represented mathematically as CO = SV x HR.
There are other means to calculate cardiac output such as the Fick principle.

Heart rate (HR) is set by the interaction between the SA node and the AV node. The ANS modifies the response at these nodes to help keep the heart rate at ~ 72 beats per minute. Although the ANS is considered a peripheral control mechanism, it is tied centrally to the brainstem at the medulla. Higher brain centers at the diencephalon and cerebrum can further modify and regulate HR. Other peripheral controls detect blood pressure changes as well as chemical changes by monitoring the respiratory gases O2 and CO2. These receptors are located in large elastic vessels like the aortic arch and carotid arteries. Other chemicals such as hormones like Epinephrine can affect dramatically affect HR.

Stroke volume (SV) is the difference in the volume given to the ventricle to pump and what remains after systole (contraction). Stroke volume is affected by how much blood is returned to the heart, known as venous return, and how hard the heart has to pump to keep blood moving forward, known as afterload. The ANS can also affect SV by changing heart rate and force of contraction.

Another way to represent heart function is to calculate the percentage of blood pumped by the heart. This percentage is called the ejection fraction and should normally be at 60%-70%. The heart muscle health is dependent on its own blood supply. Any changes in blood pressure, vessel size, and vessel lining can affect how efficient the heart can work and how HR, SV and therefore CO are affected.

CO calculation: Heart Rate x Stroke Volume = Cardiac Output

HR = 75 beats per minute and Stroke Volume = 70 mL per beat, the CO = 75 x 70 = 5.25 L / min

Heart Specimen: Right Side, Left Side

Blood Pressure Cuff

Cardiac Drugs: Procainimide, Epinephrine

Medical Terminology

Cardi/o- heart aur/o- ear

Papill/o- fingerlike atri/o- atrium

Peri- around ventricul/o- ventricle

-mural wall tachy- swift

brady- slow cor, coron/o- heart

thorac/o- chest inter- between

trans- across -gram recording

Exercises/Activities

I. ID and Function of heart parts

A Model

B Drawing

C Pig heart specimen

II. Heart sounds: Obtain a stethoscope and listen for the heart sounds of the cardiac cycle. Explain what you are hearing.

III. Name and define the major events of the cardiac cycle.

IV. Label the conduction pathway of the heart

V. ECG tracing

VI. List the pathway of blood through the pulmonary circuit starting at the right ventricle.

VII. Calculate the Cardiac Output for the following parameters

Heart Rate = 80 bpm; EDV = 150ml; ESV = 75 ml
Heart Rate = 60 bpm; EDV = 200ml; ESV = 110 ml
Heart Rate = 100 bpm; EDV = 80ml; ESV = 35 ml

VIII. Label the histology of cardiac muscle

Concept Map: Make a concept map of the heart (histo, gross) anatomy and physiological function of conduction system as well as cardiac muscle in the atria and ventricles.
Include this concept map in your LAR lab report (if selected) as a document insert or as an addtional PDF scan of the map.

Case Study

Applications

Myocardial ischemia Myocardial infarction (MI)

Atherosclerosis Coronary artery disease (CAD)

Valvular stenosis Cardiac Arrest

Bradycardia Tachycardia

Arrhythmias: flutter, fibrillation, heartblock, asystole

Rheumatic heart disease Congenital heart disease

Endocarditis Myocarditis

Pericarditis Angina Pectoris

Congestive Heart Failure (CHF)

Hypertensive Heart Disease Bypass surgery