ANSC Physiology - Cardiovascular system

Heart

• Comprised of cardiac muscle tissue

• Found in most but not all animals

• Function: pumps blood through the blood vessels of the circulatory system

4 chambers of the heart

Right atria, right ventricle, left atria, and left ventricle

What species have 4 chambers of the heart

Humans, mammals, and birds

The mediastinum

• Central compartment of the thoracic cavity

• Heart and its vessels are situated together with the esophagus, trachea, thymus, and thyroid gland

• Extends from sternum ventrally to the vertebrae dorsally

Apex of heart

Free tapered caudal/ventral portion of the heart

Base of the heart

The attached wider cranial/dorsal portion of the heart

Pericardium

A fluid filled sack that surrounds and supports the heart within the thoracic cavity

Fibrous (parietal) pericardium

The outer layer of the pericardium that is made up of dense and loose connective tissues

Serous (visceral) pericardium

The inner layer of pericardium that is made up of epithelial tissue

Pericardial Cavity

A fluid filled space that is between the fibrous and serious pericardium

Epicardium

A serous membrane that makes up the innermost layer of the pericardium and also the outermost layer of the heart

Myocardium

The thickest and middlemost muscular (cardiac muscle) layer of the walls of the heart

Endocardium

The innermost layer of epithelial tissue that lines the chambers of the heart

Cardiomyocytes

• They are the cardiac muscle cells that make up most of the cells of the myocardium layer of the wall of the heart

• Small and momonucleated cells (sometimes binucelated)

Intercalated Disks

• Connects the adjacent cardiomyocytes to one another so that they can function as a syncytium (unified)

• Function: Allows synchronized contraction of cardiac muscle so that the heart can pump

Myofibrils

• Bundles of myofilaments that are made of mypsin and actin that are located in each cardiomyocyte

• Make cardiac muscle tissue striated due to them being arranged like sarcomeres

Right atrium

• The deoxygenated blood returns to this chamber by travelling through the cranial and caudal vena cava (large vein)

Right Ventricle

• Travels into this chamber by the atrioventricular valve (AV) from the right atrium (aka triscupid valve)

Pulmonary Circuit

• When the right atrium contracts, the blood pumps through the pulmonary semilunar valve into this

• Blood goes through the lungs to remove CO2 and O2 is replenished

Left atrium

Blood enters this chamber through the pulmonary veins from the pulmonary circuit after it is oxygenated

Left Ventricle

The oxygenated blood uses the AV valve (aka biscupid valve) to enter this chamber after the left atrium

Systemic Circuit

• After contraction, the left ventricle pumps blood through the aortic semilunar valve into the ascending aorta

• Blood carries Oxygen to the brain and peripheral tissue using the systemic areteries of the body

Aortic valve

• Semilunar valve

• Located: between the left ventricle and the ascending aorta

• Closed during ventricular relaxation

• Open during ventricular contraction

Pulmonary Valve

• A seminlunar valve

• Location: Between the right ventricle and the pulmonary arteries

• Closed during ventricular relaxation

• Open during ventricular contraction

Mitral (biscupid) valve

• Left AV valve

• Location: between the left atrium and the left ventricle

• Open during ventricular relaxation

• Closed during ventricular contraction

Triscupid valve

• Right AV valve

• Location: Between the right atrium and the right ventricle

• Open during ventricular relaxation

• Closed during ventricular contraction

Chordae Tendineae

• Tendinous cords colloquially known as the heart strings

• Attach the cusps of the mitral and tricuspid AV valves to the papillary muscles

• Are loose or lax during ventricular relaxation while the ventricles fill with blood from the atria

• Become tight or taught as the AV valves close during ventricular contraction

Papillary Muscles

• Contract to pull on the chordae tendinae

• prevents prolapse of the mitral and trisupid AV valves during ventricular contraction

Cardiac Pacemaker Cells

•  They are specialized cardiomyocytes that spontaneously generate electrical impulses known as cardiac action potentials

• Are about of 1% of heart muscle cells with the rest being contractile cells 

Sinostrial (SA) node

• Primary pacemaker of the heart that determines the frequency of cardiac muscle contraction (heart rate)

• Function: depolarize neighboring cardiomyocytes

Cardiac Conduction System

• Transmits cardiac action potentials generated by the SA node to cause the heart muscle to contract

• Results in the pumping of blood through the circulatory system

Atrial Contraction

1. Spontaneous depolarization of Pacemaker Cells located in the SA node of the right atrium (depolarization is transmitted to neighboring cardiomyocytes via gap junctions located in intercalated disks)

2. The action potential spreads across the atrial surface

3. The left and right atria both contract synchronously (pumps residual blood from the atria into the respective ventricles)

Heart Rate (HR)

• frequency of cardiac cycles usually expressed in beats per minute (BPM)

• Smaller animals have a greater _____

Autonomic Nervous System

• Sympathetic Efferent Neurons - increase HR and stroke volume by releasing norepinephrine to the SA node

• Parasympathetic Efferent Neurons- decrease HR and stroke volume by releasing acetylcholine to the SA node

The Cardiac Cycle

1. Atrial Systole

• Left and Right atria contract simultaneously in response to depolarization of the SA node

• Force residual blood into their respective relaxed ventricles

The Cardaic Cycle

2. Ventricular Systole

• Left and right ventricle contract simultaneously and force the AV valves to close (isovolumetric contraction)

• Pressure rises and exceeds arterial pressure which causes semilunar valves to open. Blood is then ejected into the aorta and pulmonary arteries

The Cardiac Cycle

3. Atrial Diastole

• Left and right atria relax and fill with blood delivered via the vena cava and pulmonary veins, respectively

The Cardiac Cycke

4. Ventricular Diastole

• Left and right ventricles relax and the drop in pressure relative to the aorta and pulmonary arteries closes the semilunar (aortic and pulmonary) valves

• During the late stage of this, all chambers of the heart are relaxed and ventricles passively fill with blood

Electrocardiography

• Recording of the electrical activity of the heart throughout the cardiac cycle made using electrodes placed on the skin

Electrocardiogram (ECG or EXG)

• A graph of voltage (y axis) vs. time (x-axis)

• Depicts small electrical changes associated with cardiac muscle depolarization and repolarization

P wave

• Represents depolarization of the atria

QRS complex

• Represents depolarization of the ventricles

T Wave

• Represents repolarization of the ventricles

Ta Wave

• Represents the repolarization of the atria

• Normally hidden by the QRS complex