Cardiovascular function

• The human heart The human heart has four chambers: two upper chambers (atria) and two larger, lower chambers (ventricles). A system of tissue valves in the heart makes it possible to keep blood flowing through the heart in a one-way fashion. Two atrioventricular valves separate the atria and ventricles and prevent back-flow of blood from the ventricles to the atria during ventricular contraction. Two semilunar valves (aortic and pulmonary) prevent back-flow from the aorta and pulmonary arteries to the ventricles during ventricular relaxation.

• Flow of blood The inferior and superior vena cavae carry blood to the right atrium (upper chamber), whcih then empties into the right ventricle (lower chamber). Ventricular contraction pumps the relatively deoxygenated blood to the lungs via the left and right pulmonary arteries. Gas exchange occurs by diffussion at the capillaries surrounding the alveoli in the lungs. Oxygenated blood then leaves the lungs via the left and right pulmonary veins and is returned to the left atrium of the heart. The blood is then emptied into the left ventricle. Contraction of the left ventricle forces the blood into the aorta. Branches of the aorta supply blood to all parts of the body both above and below the heart. Deoxygenated blood then returns from various capillary beds via the veins, eventually merging into the inferior and superior vena cavae.

• Blood Pressure A cardiac contraction cycle consist of first a relatively weak contraction of the two atria, and following a short delay, a stronger contraction of the right and left ventricles. This sequence lasts about one half of a second, and is followed by a relaxation phase that lasts between .3 and .5 seconds. The period of ventricular contraction is called systole, and pressure which is transmitted into the aorta and pulmonary arteries is the systolic pressure. During the peak of systole, the aortic and pulmonary semilunar valves are forced open to allow blood to exit the ventricles and enter these vessels. The relaxation phase is called ventricular diastole, and during this period pressure in the aorta and pulmonary arteries falls to lower values called diastolic pressure. It is important to note that aortic pressure never falls to zero (the elasticity of the large arteries helps to maintain pressure during ventricular relaxation). An individual’s systemic blood pressure is expressed as a ratio of arterial pressure during contraction to arterial pressure during relaxation or as systolic/diastolic. Blood pressure is measured with an instrument called a sphygmomanometer.

• ECG The contraction of cardiac muscle is initiated by a coordinated spread of electrical depolarization throughout the heart muscle mass. This wave of excitation originates in the area of the right atrium called the sinoarial node (S-A node or pacemaker), spreads through the atria, and then to a second node called the atrioventricular node (A-V node), located near the junction of the four chambers. From the A-V node, electrical activity is spread to the ventricles via the Bundle of His and Purkinje Fibers. The Bundle of His consists of specialized conducting fibers in the interventricular septum of the heart that speed up the spread of activity, so that the ventricular muscle mass contracts in synchrony. The sequence of excitation can be reflected in the electrocardiogram (ECG). After the contraction has ended, muscle cells repolarize, returning to the electrical charge on muscle cell membranes to the normal inside-negative state. It is this cycle of depolarization and repolarization of the different regions of the heart that can be recorded as the ECG. Each wave of ECG represents an electrical event in the heart. The P wave corresponds to atrial depolarization, the Q, R, and S waves (known as the QRS Complex) represent ventricular depolarization, and the T wave represents ventricular repolarization.

Respiratory Function

Respiration Inspired air passes through the nasal passages into the pharynx. Air then moves into the trachea, past the larynx and into the two bronchi. Air moves through each of the bronchi, which divide into the bronchioles. The bronchioles ultimately divide into the alveoli, which are the sites of gas exchange. Oxygen moves out of the lungs and into the blood by diffusion, while carbon dioxide moves out of the blood by diffusion.

Total Lung Capacity

• Tidal Volume (TV) is the amount of air that enters and leaves the lungs in a single, normal breath. Inspiratory Reserve Volume (IRV) is that further amount of air (above tidal volume) that can be taken into the lungs during a maximal, forceful inspiration. During a maximal expiration, the additional volume of air that can be forcefully exhaled is the Expiratory Reserve Volume (ERV). The Tidal Volume and Inspiratory and Expiratory Reserve Volumes added together make up the Vital Capacity (VC), and indication of an individual’s ability to maximally fill and empty the lungs. The lungs always contain a remaining volume of air, called the Residual Volume, which ensures that they do not entirely collapse. Vital Capacity plus Residual Volume is considered Total Lung Capacity (TLC).

Pulmonary Ventilation

• Pulmonary Ventilation is the amount of air entering the lungs per minute. It is the respiratory rate multiplied by the Tidal Volume.

Alveolar Ventilation

• Alveolar Ventilation takes into consideration the fraction of Tidal Volume that actually fills the alveolar spaces. Alveolar Ventilation is equal to the difference of Tidal Volume minus dead space times Respiratory Rate.