Page 144 - 84 human physiolofy part-1

Chapter 8

liver and blood cleaned by the liver (from the stomach, pancreas, small intestine and colon) into the
inferior vena cava.

They arise from the substance of the liver, more specifically the central vein of the liver lobule.
They can be differentiated into two groups, the upper group and lower group.

The upper group of three typically arises from the posterior aspect of the liver and drain the
quadrate lobe and left lobe. The lower group rise from the right lobe and caudate lobe, are variable in
number, and are typically smaller than those in the upper group. None of the hepatic veins have valves.

Cardiac Cycle

Cardiac cycle is the term used to describe the relaxation and contraction that occur, as a heart
works to pump blood through the body. Heart rate is a term used to describe the frequency of the
cardiac cycle. It is considered one of the four vital signs. Usually it is calculated as the number of
contractions (heart beats) of the heart in one minute and expressed as "beats per minute" (bpm). When
resting, the adult human heart beats at about 70 bpm (males) and 75 bpm (females), but this rate varies
between people. However, the reference range is nominally between 60 bpm (if less termed
bradycardia) and 100 bpm (if greater, termed tachycardia). Resting heart rates can be significantly
lower in athletes, and significantly higher in the obese.

The body can increase the heart rate in response to a wide variety of conditions in order to increase
the cardiac output (the amount of blood ejected by the heart per unit time). Exercise, environmental
stressors or psychological stress can cause the heart rate to increase above the resting rate.

The pulse is the most straightforward way of measuring the heart rate, but it can be deceptive when
some strokes do not lead to much cardiac output. In these cases (as happens in some arrhythmias), the
heart rate may be considerably higher than the pulse. Every single 'beat' of the heart involves three
major stages: atrial systole, ventricular systole and complete cardiac diastole. Throughout the cardiac
cycle, the blood pressure increases and decreases. As ventricles contract the pressure rise, causing the
AV valves to slam shut.

Systole

The heart in the systole phase. Systole, or contraction, of the heart is initiated by the electrical cells
of the sinoatrial node, which is the heart's natural pacemaker. These cells are activated spontaneously
by depolarization of their membranes beyond a certain threshold for excitation. At this point, voltage-
gated calcium channels on the cell membrane open and allow calcium ions to pass through, into the
sarcoplasm, or interior, of the muscle cell. Some calcium ions bind to receptors on the sarcoplasmic
reticulum causing an influx of calcium ions into the sarcoplasm. The calcium ions bind to the troponin,
causing a conformation change, breaking the bond between the protein tropomyosin, to which the
troponin is attached, and the myosin binding sites. This allows the myosin heads to bind to the myosin
binding sites on the actin protein filament and contraction results as the myosin heads draw the actin
filaments along, are bound by ATP, causing them to release the actin, and return to their original
position, breaking down the ATP into ADP and a phosphate group. The action potential spreads via the
passage of sodium ions through the gap junctions that connect the sarcoplasm of adjacent myocardial
cells.

144 | Human Physiology