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Glossary of Physiology Terms
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Definition:
The action potential is a rapid and reversible reversal of the electrical potential difference across the plasma membrane of excitable cells such as neurons, muscle cells and some endocrine cells. In a neuronal action potential, the membrane potential rapidly changes from its resting level of approximately -70 mV to around +50 mV and, subsequently, rapidly returns to the resting level again. The neuronal action potential forms an important basis for information processing, propagation, and transmission. In muscle cells, the action potential precedes, and is necessary to bring about, muscle contraction. Some endocrine cells also exhibit action potentials, where the excitation leads to hormone secretion.

The action potential is also referred to as the electrical impulse or nervous impulse.

Related glossary terms/phrases:
Graded potential

See also:
Neuronal Action Potential



Definition:
The large blood vessel (artery) that receives the output of the left ventricle of the heart. The aorta is the first and largest (in diameter) artery of the systemic circulation.

Following ventricular contraction (systole), hydrostatic pressure within the ventricle exceeds the pressure in the aorta, which forces blood out of the left ventricle, through the aortic semilunar valve, and into the aorta. As the ventricle relaxes during diastole, the pressure within the left ventricle drops to a level below that in the aorta. At this point, the aortic semilunar valve closes, which prevents the backflow of blood from the aorta to the left ventricle.



Abbreviation:
Ca2+

Definition:
Calcium (Ca2+) is a divalent cation. It plays an important role in physiological processes such as muscle contraction and synaptic transmission. Calcium is also an intracellular messenger.

The extracellular concentration of Ca2+ is about 2 mM. The intracellular concentration of Ca2+ is about 70 nM.



Abbreviation:
CO

Definition:
Cardiac output is defined as the volume of blood pumped every minute by each (left or right) ventricle of the heart. It is generally reported in L/min or mL/min.

Cardiac output (CO) is the product of heart rate (HR) and stroke volume (SV). Thus,

CO = HR × SV

Heart rate is defined as the number of heart contractions per minute. Stroke volume is the volume of blood pumped per ventricular contraction.

For a typical resting adult human being at rest, CO is approximately 5 L/min.

CO = 70 contractions/min × 70 mL/contraction = 4,900 mL/min



Definition:
The component of the electrocardiogram (ECG) that corresponds to the depolarization of atrial myocytes. Atrial depolarization precedes atrial contraction.

Related glossary terms/phrases:
QRS complex
T wave



Definition:
The component of the electrocardiogram (ECG) that corresponds to the depolarization of ventricular myocytes. Ventricular depolarization precedes ventricular contraction.

Related glossary terms/phrases:
P wave
T wave









Posted: Sunday, March 31, 2013
Last updated: Friday, August 28, 2015