Page 149 - 84 human physiolofy part-1

The Cardiovascular System

P
P wave- indicates that the
atria are electrically
stimulated to pump blood
into the ventricles.
QRS
QRS complex- indicates that
the ventricles are electrically
stimulated to pump blood
out.
ST
ST segment- indicates the
amount of time from the end
of the contraction of the
ventricles to the beginning of
the T wave.
T
T wave- indicates the
recovery period of the
ventricles.

Cardiac Muscle Contraction

After an action potential excites the plasma membrane of the cardiac muscle cell the contraction is
due to an increase in the cytoplasmic concentration of Calcium ions. Similar to skeletal muscle, the
release of Ca+ ions from the sarcoplasmic reticulum binds to troponin which allows actin to bind with
myosin. The difference between skeletal muscle and cardiac muscle is that when the action potential
opens voltage gated calcium ion channels in the T-tubules. The increase in cytosolic calcium causes
calcium ions to bind to receptors on the surface of the sarcoplasmic reticulum. The binding of calcium
ions to these receptors causes the opening of more calcium ion channels in the SR membrane. Calcium
ions then rush out of the SR and bind to troponin and allow the myosin and actin to bind together which
causes contraction. This sequence is called calcium-induced calcium release. Contraction ends when
the level of cytosolic calcium returns to normal resting levels.

Blood Pressure

Blood pressure is the pressure exerted by the blood on the walls of the blood vessels. Unless
indicated otherwise, blood pressure refers to systemic arterial blood pressure, i.e., the pressure in the
large arteries delivering blood to body parts other than the lungs, such as the brachial artery (in the
arm). The pressure of the blood in other vessels is lower than the arterial pressure. Blood pressure
values are universally stated in millimeters of mercury (mm Hg). The systolic pressure is defined as the
peak pressure in the arteries during the cardiac cycle; the diastolic pressure is the lowest pressure (at
the resting phase of the cardiac cycle). The mean arterial pressure and pulse pressure are other
important quantities. Typical values for a resting, healthy adult are approximately 120 mm Hg systolic
and 80 mm Hg diastolic (written as 120/80 mm Hg), with large individual variations. These measures
of blood pressure are not static, but undergo natural variations from one heartbeat to another or
throughout the day (in a circadian rhythm); they also change in response to stress, nutritional factors,

Wikibooks | 149