Systole ( ) is the part of the
cardiac cycle
The cardiac cycle is the performance of the human heart from the beginning of one heartbeat to the beginning of the next. It consists of two periods: one during which the heart muscle relaxes and refills with blood, called diastole, following ...
during which some
chambers
Chambers may refer to:
Places
Canada:
*Chambers Township, Ontario
United States:
*Chambers County, Alabama
* Chambers, Arizona, an unincorporated community in Apache County
* Chambers, Nebraska
* Chambers, West Virginia
* Chambers Township, Hol ...
of the
heart
The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide t ...
contract after refilling with blood. The term originates, via
New Latin
New Latin (also called Neo-Latin or Modern Latin) is the revival of Literary Latin used in original, scholarly, and scientific works since about 1500. Modern scholarly and technical nomenclature, such as in zoological and botanical taxonomy ...
, from
Ancient Greek
Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic peri ...
(''sustolē''), from (''sustéllein'' 'to contract'; from ''sun'' 'together' + ''stéllein'' 'to send'), and is similar to the use of the English term ''to squeeze''.
The mammalian
heart
The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide t ...
has four chambers: the left
atrium
Atrium may refer to:
Anatomy
* Atrium (heart), an anatomical structure of the heart
* Atrium, the genital structure next to the genital aperture in the reproductive system of gastropods
* Atrium of the ventricular system of the brain
* Pulmona ...
above the left
ventricle (lighter pink, see graphic), which two are connected through the
mitral (or bicuspid) valve; and the right atrium above the right ventricle (lighter blue), connected through the
tricuspid valve
The tricuspid valve, or right atrioventricular valve, is on the right dorsal side of the mammalian heart, at the superior portion of the right ventricle. The function of the valve is to allow blood to flow from the right atrium to the right vent ...
. The atria are the receiving blood chambers for the circulation of blood and the ventricles are the discharging chambers.
In late ventricular
diastole
Diastole ( ) is the relaxed phase of the cardiac cycle when the chambers of the heart are re-filling with blood. The contrasting phase is systole when the heart chambers are contracting. Atrial diastole is the relaxing of the atria, and ventric ...
, the atrial chambers contract and send blood to the larger, lower ventricle chambers. This flow fills the ventricles with blood, and the resulting pressure closes the valves to the atria. The ventricles now perform
isovolumetric contraction
In cardiac physiology, isometric contraction is an event occurring in early systole during which the ventricles contract with no corresponding volume change ( isometrically). This short-lasting portion of the cardiac cycle takes place while all ...
, which is contraction while all valves are closed. This contraction ends the first stage of systole. The second stage proceeds immediately, pumping oxygenated blood from the left ventricle through the aortic valve and
aorta
The aorta ( ) is the main and largest artery in the human body, originating from the left ventricle of the heart and extending down to the abdomen, where it splits into two smaller arteries (the common iliac arteries). The aorta distributes ...
to all body systems, and simultaneously pumping oxygen-poor blood from the right ventricle through the
pulmonic valve
The pulmonary valve (sometimes referred to as the pulmonic valve) is a valve of the heart that lies between the right ventricle and the pulmonary artery and has three cusps. It is one of the four valves of the heart and one of the two semilunar v ...
and
pulmonary artery
A pulmonary artery is an artery in the pulmonary circulation that carries deoxygenated blood from the right side of the heart to the lungs. The largest pulmonary artery is the ''main pulmonary artery'' or ''pulmonary trunk'' from the heart, and t ...
to the
lungs
The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of th ...
. Thus, the pairs of chambers (upper atria and lower ventricles) contract in alternating sequence to each other. First, atrial contraction feeds blood into the ventricles, then ventricular contraction pumps blood out of the heart to the body systems, including the lungs for resupply of oxygen.
Cardiac systole is the contraction of the
cardiac muscle
Cardiac muscle (also called heart muscle, myocardium, cardiomyocytes and cardiac myocytes) is one of three types of vertebrate muscle tissues, with the other two being skeletal muscle and smooth muscle. It is an involuntary, striated muscle th ...
in response to an electrochemical stimulus to the heart's cells (
cardiomyocyte
Cardiac muscle (also called heart muscle, myocardium, cardiomyocytes and cardiac myocytes) is one of three types of vertebrate muscle tissues, with the other two being skeletal muscle and smooth muscle. It is an involuntary, striated muscle th ...
s).
Cardiac output
In cardiac physiology, cardiac output (CO), also known as heart output and often denoted by the symbols Q, \dot Q, or \dot Q_ , edited by Catherine E. Williamson, Phillip Bennett is the volumetric flow rate of the heart's pumping output: t ...
is the volume of blood pumped by the ventricles in one minute. The
ejection fraction
An ejection fraction (EF) is the volumetric fraction (or portion of the total) of fluid (usually blood) ejected from a chamber (usually the heart) with each contraction (or heartbeat). It can refer to the cardiac atrium, ventricle, gall bladder, ...
is the volume of blood pumped divided by the total volume of blood in the left ventricle.
Types of systole
Atrial systole
Atrial systole occurs late in
ventricular diastole
Diastole ( ) is the relaxed phase of the cardiac cycle when the chambers of the heart are re-filling with blood. The contrasting phase is systole when the heart chambers are contracting. Atrial diastole is the relaxing of the atria, and ventric ...
and represents the contraction of
myocardium
Cardiac muscle (also called heart muscle, myocardium, cardiomyocytes and cardiac myocytes) is one of three types of vertebrate muscle tissues, with the other two being skeletal muscle and smooth muscle. It is an involuntary, striated muscle that ...
of the left and right
atria. The sharp decrease in ventricular pressure that occurs during ventricular diastole allows the
atrioventricular valve
A heart valve is a one-way valve that allows blood to flow in one direction through the chambers of the heart. Four valves are usually present in a mammalian heart and together they determine the pathway of blood flow through the heart. A heart v ...
s (or
mitral
The mitral valve (), also known as the bicuspid valve or left atrioventricular valve, is one of the four heart valves. It has two cusps or flaps and lies between the left atrium and the left ventricle of the heart. The heart valves are all one-w ...
and
tricuspid
The tricuspid valve, or right atrioventricular valve, is on the right dorsal side of the mammalian heart, at the superior portion of the right ventricle. The function of the valve is to allow blood to flow from the right atrium to the right ven ...
valves) to open and causes the contents of the atria to empty into the ventricles. The atrioventricular valves remain open while the aortic and pulmonary valves remain closed because the pressure gradient between the atrium and ventricle is preserved during late ventricular diastole. Atrial contraction confers a minor-fraction addition to ventricular filling, but becomes significant in
left ventricular hypertrophy
Left ventricular hypertrophy (LVH) is thickening of the heart muscle of the left ventricle of the heart, that is, left-sided ventricular hypertrophy and resulting increased left ventricular mass.
Causes
While ventricular hypertrophy occurs na ...
, or thickening of the heart wall, as the ventricle does not fully relax during its diastole. Loss of normal electrical conduction in the heart—as seen during
atrial fibrillation,
atrial flutter
Atrial flutter (AFL) is a common abnormal heart rhythm that starts in the atrial chambers of the heart. When it first occurs, it is usually associated with a fast heart rate and is classified as a type of supraventricular tachycardia. Atrial f ...
, and
complete heart block
Third-degree atrioventricular block (AV block) is a medical condition in which the electrical impulse generated in the sinoatrial node (SA node) in the atrium of the heart can not propagate to the ventricles.
Because the impulse is blocked, an a ...
—may eliminate atrial systole completely.
Contraction of the atria follows depolarization, represented by the
P wave
A P wave (primary wave or pressure wave) is one of the two main types of elastic body waves, called seismic waves in seismology. P waves travel faster than other seismic waves and hence are the first signal from an earthquake to arrive at any ...
of the ECG. As both atrial chambers contract—from the superior region of the atria toward the atrioventricular septum—pressure rises within the atria and blood is pumped into the ventricles through the open atrioventricular valves. At the start of atrial systole, during ventricular diastole, the ventricles are normally filled to about 70–80 percent of capacity by inflow from the atria. Atrial contraction also referred to as the "atrial kick," contributes the remaining 20–30 percent of ventricular filling. Atrial systole lasts approximately 100 ms and ends prior to ventricular systole, as the atrial muscle returns to diastole.
The two ventricles are isolated electrically and
histological
Histology,
also known as microscopic anatomy or microanatomy, is the branch of biology which studies the microscopic anatomy of biological tissues. Histology is the microscopic counterpart to gross anatomy, which looks at larger structures vis ...
ly (tissue-wise) from the two atrial chambers by electrically impermeable collagen layers of connective tissue known as the
cardiac skeleton
In cardiology, the cardiac skeleton, also known as the fibrous skeleton of the heart, is a high-density homogeneous structure of connective tissue that forms and anchors the valves of the heart, and influences the forces exerted by and through t ...
. The cardiac skeleton is made of dense connective tissue which gives structure to the heart by forming the
atrioventricular septum
The atrioventricular septum is a septum of the heart between the right atrium (RA) and the left ventricle (LV).
Although the name "atrioventricular septum" implies any septum between an atrium and a ventricle, in practice the divisions from RA to ...
—which separates the atria from the ventricles—and the fibrous rings which serve as bases for the four heart valves. Collagen extensions from the valve rings seal and limit electrical activity of the atria from influencing electrical pathways that cross the ventricles. These electrical pathways contain the
sinoatrial node
The sinoatrial node (also known as the sinuatrial node, SA node or sinus node) is an oval shaped region of special cardiac muscle in the upper back wall of the right atrium made up of cells known as pacemaker cells. The sinus node is approximat ...
, the
atrioventricular node
The atrioventricular node or AV node electrically connects the heart's atria and ventricles to coordinate beating in the top of the heart; it is part of the electrical conduction system of the heart. The AV node lies at the lower back section of t ...
, and the
Purkinje fibers
The Purkinje fibers (; often incorrectly ; Purkinje tissue or subendocardial branches) are located in the inner ventricular walls of the heart, just beneath the endocardium in a space called the subendocardium. The Purkinje fibers are specia ...
. (Exceptions such as accessory pathways may occur in this firewall between atrial and ventricular electrical influence but are rare.)
Cardiac rate control via pharmacology is common today; for example, the therapeutic use of digoxin,
beta adrenoceptor antagonists, or
calcium channel blocker
Calcium channel blockers (CCB), calcium channel antagonists or calcium antagonists are a group of medications that disrupt the movement of calcium () through calcium channels. Calcium channel blockers are used as antihypertensive drugs, i.e., as ...
s are important historical interventions in this condition. Notably, individuals prone to
hypercoagulability
Thrombophilia (sometimes called hypercoagulability or a prothrombotic state) is an abnormality of blood coagulation that increases the risk of thrombosis (blood clots in blood vessels). Such abnormalities can be identified in 50% of people who ...
(abnormality of
blood coagulation) are at decided risk of
blood clotting
Coagulation, also known as clotting, is the process by which blood changes from a liquid to a gel, forming a blood clot. It potentially results in hemostasis, the cessation of blood loss from a damaged vessel, followed by repair. The mechanism o ...
, a very serious pathology requiring therapy for life with an
anticoagulant
Anticoagulants, commonly known as blood thinners, are chemical substances that prevent or reduce coagulation of blood, prolonging the clotting time. Some of them occur naturally in blood-eating animals such as leeches and mosquitoes, where the ...
if it cannot be corrected.
Right and left atrial systoles
The atrial chambers each contains one valve: the
tricuspid valve
The tricuspid valve, or right atrioventricular valve, is on the right dorsal side of the mammalian heart, at the superior portion of the right ventricle. The function of the valve is to allow blood to flow from the right atrium to the right vent ...
in the right atrium opens into the right ventricle, and the
mitral (or bicuspid) valve in the left atrium opens into the left ventricle. Both valves are pressed open during the late stages of ventricular diastole; see Wiggers diagram at the P/QRS phase (at right margin). Then the contractions of atrial systole cause the right ventricle to fill with oxygen-depleted blood through the tricuspid valve. When the right atrium is emptied—or prematurely closed—right atrial systole ends, and this stage signals the end of ''ventricular diastole'' and the beginning of ''ventricular systole'' (see Wiggers diagram). The time variable for the right systolic cycle is measured from (tricuspid) valve-open to valve-closed.
The contractions of atrial systole fill the left ventricle with oxygen-enriched blood through the mitral valve; when the left atrium is emptied or closed, left atrial systole is ended and ventricular systole is about to begin. The time variable for the left systolic cycle is measured from (mitral) valve-open to valve-closed.
Atrial fibrillation
Atrial fibrillation represents a common electrical malady in the heart that appears during the time interval of atrial systole (see figure at right margin). Theory suggests that an
ectopic focus
Ectopia, ectopic, or ectopy may refer to:
*Ectopia (medicine), including a list of medical uses of ectopia or ectopic
**Ectopic pregnancy, a pregnancy that occurs outside the uterus
**Ectopic beat
Ectopic beat is a disturbance of the cardiac r ...
, usually situated within the pulmonary trunks, competes with the
sinoatrial node
The sinoatrial node (also known as the sinuatrial node, SA node or sinus node) is an oval shaped region of special cardiac muscle in the upper back wall of the right atrium made up of cells known as pacemaker cells. The sinus node is approximat ...
for electrical control of the atrial chambers and thereby diminishes the performance of the atrial myocardium, or atrial heart muscle. The ordered,
sinoatrial
The sinoatrial node (also known as the sinuatrial node, SA node or sinus node) is an oval shaped region of special cardiac muscle in the upper back wall of the right atrium made up of cells known as pacemaker cells. The sinus node is approxima ...
control of atrial electrical activity is disrupted, causing the loss of coordinated generation of pressure in the two atrial chambers. Atrial fibrillation represents an electrically-disordered but well perfused atrial mass working (in an uncoordinated fashion) with a (comparatively) electrically-healthy ventricular systole.
The compromised load caused by atrial fibrillation detracts from the overall performance of the heart, but the ventricles continue to work as an effective pump. Given this pathology, the
ejection fraction
An ejection fraction (EF) is the volumetric fraction (or portion of the total) of fluid (usually blood) ejected from a chamber (usually the heart) with each contraction (or heartbeat). It can refer to the cardiac atrium, ventricle, gall bladder, ...
may deteriorate by ten to thirty percent. Uncorrected atrial fibrillation can lead to heart rates approaching 200 beats per minute (bpm). If this rate can be slowed to a normal range, say about 80 bpm, the resultant longer fill-time within the cardiac cycle restores or improves the pumping capability of the heart. The labored breathing, for example, of individuals with uncontrolled atrial fibrillation, can often be returned to normal by (electrical or medical)
cardioversion
Cardioversion is a medical procedure by which an abnormally fast heart rate (tachycardia) or other cardiac arrhythmia is converted to a sinus rhythm, normal rhythm using electricity or pharmaceutical drug, drugs. Synchronized electrical cardiover ...
.
Ventricular systole and Wiggers diagram
![Wiggers Diagram](https://upload.wikimedia.org/wikipedia/commons/f/f4/Wiggers_Diagram.svg)
A
Wiggers diagram
A Wiggers diagram, named after its developer, Carl Wiggers, is a unique diagram that has been used in teaching cardiac physiology for more than a century. In the Wiggers diagram, the X-axis is used to plot time subdivided into the cardiac phases ...
of ventricular systole graphically depicts the sequence of contractions by the myocardium of the two
ventricles. Ventricular systole induces
self-contraction such that pressure in both left and right ventricles rises to a level above that in the two atrial chambers, thereby closing the tricuspid and mitral valves—which are prevented from inverting by the
chordae tendineae
The chordae tendineae (tendinous cords), colloquially known as the heart strings, are inelastic cords of fibrous connective tissue that connect the papillary muscles to the tricuspid valve and the mitral valve in the heart.
Structure
The chordae ...
and the
papillary muscles
The papillary muscles are muscles located in the ventricles of the heart. They attach to the cusps of the atrioventricular valves (also known as the mitral and tricuspid valves) via the chordae tendineae and contract to prevent inversion or pr ...
. Now ventricular pressure continues to rise in
isovolumetric, or fixed-volume, contraction phase until maximal pressure (dP/dt = 0) occurs, causing the pulmonary and aortic valves to open in
ejection phase. In ejection phase, blood flows from the two ventricles down its pressure gradient—that is, 'down' from higher pressure to lower pressure—into (and through) the
aorta
The aorta ( ) is the main and largest artery in the human body, originating from the left ventricle of the heart and extending down to the abdomen, where it splits into two smaller arteries (the common iliac arteries). The aorta distributes ...
and the
pulmonary trunk
A pulmonary artery is an artery in the pulmonary circulation that carries deoxygenated blood from the right side of the heart to the lungs. The largest pulmonary artery is the ''main pulmonary artery'' or ''pulmonary trunk'' from the heart, and ...
respectively. Notably, cardiac muscle
perfusion
Perfusion is the passage of fluid through the circulatory system or lymphatic system to an organ or a tissue, usually referring to the delivery of blood to a capillary bed in tissue. Perfusion is measured as the rate at which blood is deliver ...
through the heart's coronary vessels doesn't happen during ventricular systole; rather, it occurs during ventricular diastole.
Ventricular systole is the origin of the
pulse
In medicine, a pulse represents the tactile arterial palpation of the cardiac cycle (heartbeat) by trained fingertips. The pulse may be palpated in any place that allows an artery to be compressed near the surface of the body, such as at the nec ...
.
Right and left ventricular systoles
The
pulmonary (or pulmonic) valve in the
right ventricle
A ventricle is one of two large chambers toward the bottom of the heart that collect and expel blood towards the peripheral beds within the body and lungs. The blood pumped by a ventricle is supplied by an atrium, an adjacent chamber in the upper ...
opens into the
pulmonary trunk
A pulmonary artery is an artery in the pulmonary circulation that carries deoxygenated blood from the right side of the heart to the lungs. The largest pulmonary artery is the ''main pulmonary artery'' or ''pulmonary trunk'' from the heart, and ...
, also known as the pulmonary artery, which divides twice to connect to each of the left and right lungs. In the
left ventricle
A ventricle is one of two large chambers toward the bottom of the heart that collect and expel blood towards the peripheral beds within the body and lungs. The blood pumped by a ventricle is supplied by an atrium, an adjacent chamber in the upper ...
, the aortic valve opens into the aorta which divides and re-divides into the several branch arteries that connect to all body organs and systems except the lungs.
By its contractions, right ventricular (RV) systole pulses oxygen-depleted blood through the pulmonary valve through the pulmonary arteries to the lungs, providing
pulmonary circulation
The pulmonary circulation is a division of the circulatory system in all vertebrates. The circuit begins with deoxygenated blood returned from the body to the right atrium of the heart where it is pumped out from the right ventricle to the lungs. ...
; simultaneously, left ventricular (LV) systole pumps blood through the aortic valve, the aorta, and all the arteries to provide
systemic circulation
The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...
of oxygenated blood to all body systems. The left ventricular systole enables
blood pressure
Blood pressure (BP) is the pressure of circulating blood against the walls of blood vessels. Most of this pressure results from the heart pumping blood through the circulatory system. When used without qualification, the term "blood pressure" r ...
to be routinely measured in the larger arteries of the left ventricle of the heart.
LV systole is volumetrically defined as the
left ventricular ejection fraction (LVEF). Similarly, RV systole is defined as the
right ventricular ejection fraction (RVEF). Higher than normal RVEF is indicative of
pulmonary hypertension. The time variables of the ventricular systoles are: right ventricle, pulmonary valve-open to valve-closed; left ventricle, aortic valve-open to valve-closed.
Electrical systole
The
sinoatrial node
The sinoatrial node (also known as the sinuatrial node, SA node or sinus node) is an oval shaped region of special cardiac muscle in the upper back wall of the right atrium made up of cells known as pacemaker cells. The sinus node is approximat ...
(S-A Node) is the heart's natural
pacemaker
An artificial cardiac pacemaker (or artificial pacemaker, so as not to be confused with the natural cardiac pacemaker) or pacemaker is a medical device that generates electrical impulses delivered by electrodes to the chambers of the heart eith ...
, issuing electrical signaling that travels through the heart muscle, causing it to contract repeatedly in cycle. It is situated at the top of the
right atrium
The atrium ( la, ātrium, , entry hall) is one of two upper chambers in the heart that receives blood from the circulatory system. The blood in the atria is pumped into the heart ventricles through the atrioventricular valves.
There are two at ...
adjacent to the junction with the superior vena cava. The S-A Node is a pale yellow structure. For humans, it is approximately 25 mm long, 3–4 mm wide and 2 mm thick. It contains two types of cells: (a) the small, round P ''cells'' which have very few ''organelles and myofibrils,'' and (b'')'' the slender elongated ''transitional cells'', which are intermediate in appearance between the P and the ordinary myocardial cells. Intact, the SA node provides continual electrical discharge known as
sinus rhythm
A sinus rhythm is any cardiac rhythm in which depolarisation of the cardiac muscle begins at the sinus node. It is characterised by the presence of correctly oriented P waves on the electrocardiogram (ECG). Sinus rhythm is necessary, but not su ...
through the atrial mass, the signals of which then coalesce at the
atrioventricular node
The atrioventricular node or AV node electrically connects the heart's atria and ventricles to coordinate beating in the top of the heart; it is part of the electrical conduction system of the heart. The AV node lies at the lower back section of t ...
, there to be organized to provide a rhythmic electrical pulse into and across the ventricles through sodium-, potassium- or calcium-gated
ion channel
Ion channels are pore-forming membrane proteins that allow ions to pass through the channel pore. Their functions include establishing a resting membrane potential, shaping action potentials and other electrical signals by gating the flow of io ...
s.
The continual rhythmic discharge generates a wavelike movement of electrical ripples that stimulate the smooth muscles of the myocardium and cause rhythmic contractions to progress from top to bottom of the heart. As the pulse moves out of the (upper) atria into the (lower) ventricles, it is distributed throughout a muscular network to cause systolic contraction of both ventricular chambers simultaneously. The actual pace of the cycle—just how fast or slowly the heart beats—is cued by messages from the brain, reflecting the brain's responses to conditions of the body, such as pain, emotional stress, level of activity, and to ambient conditions including external temperature, time of day, etc.
Mechanical systole
Electrical systole opens voltage-gated sodium, potassium and calcium channels in cells of myocardium tissue. Subsequently, a rise in intracellular calcium triggers the interaction of
actin
Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of over ...
and
myosin
Myosins () are a superfamily of motor proteins best known for their roles in muscle contraction and in a wide range of other motility processes in eukaryotes. They are ATP-dependent and responsible for actin-based motility.
The first myosin ...
in the presence of
ATP which generates mechanical force in the cells in the form of muscular contraction, or mechanical systole. The contractions generate intra-ventricular pressure, which is increased until it exceeds the external, residual pressures in the adjacent trunks of both the
pulmonary artery
A pulmonary artery is an artery in the pulmonary circulation that carries deoxygenated blood from the right side of the heart to the lungs. The largest pulmonary artery is the ''main pulmonary artery'' or ''pulmonary trunk'' from the heart, and t ...
and the
aorta
The aorta ( ) is the main and largest artery in the human body, originating from the left ventricle of the heart and extending down to the abdomen, where it splits into two smaller arteries (the common iliac arteries). The aorta distributes ...
; this stage, in turn, causes the
pulmonary
The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of th ...
and
aortic valves to open. Blood is then ejected from the two ventricles, pulsing into both the
pulmonic
In phonetics, the airstream mechanism is the method by which airflow is created in the vocal tract. Along with phonation and articulation, it is one of three main components of speech production. The airstream mechanism is mandatory for sound p ...
and
aortic circulation systems.
Mechanical systole causes the
pulse
In medicine, a pulse represents the tactile arterial palpation of the cardiac cycle (heartbeat) by trained fingertips. The pulse may be palpated in any place that allows an artery to be compressed near the surface of the body, such as at the nec ...
, which itself is readily palpated (felt) or seen at several points on the body, enabling universally adopted methods—by touch or by eye—for observing systolic
blood pressure
Blood pressure (BP) is the pressure of circulating blood against the walls of blood vessels. Most of this pressure results from the heart pumping blood through the circulatory system. When used without qualification, the term "blood pressure" r ...
.
The mechanical forces of systole cause rotation of the muscle mass around the long and short axes, a process that can be observed as a "wringing" of the ventricles.
Physiological mechanism
Systole of the heart is initiated by
electrically excitable cell
Membrane potential (also transmembrane potential or membrane voltage) is the difference in electric potential between the interior and the exterior of a biological cell. That is, there is a difference in the energy required for electric charges ...
s situated in the
sinoatrial node
The sinoatrial node (also known as the sinuatrial node, SA node or sinus node) is an oval shaped region of special cardiac muscle in the upper back wall of the right atrium made up of cells known as pacemaker cells. The sinus node is approximat ...
. These cells are activated spontaneously by
depolarization
In biology, depolarization or hypopolarization is a change within a cell, during which the cell undergoes a shift in electric charge distribution, resulting in less negative charge inside the cell compared to the outside. Depolarization is esse ...
of the electrical potential across their cell membranes, which causes
voltage-gated calcium channels
Voltage-gated calcium channels (VGCCs), also known as voltage-dependent calcium channels (VDCCs), are a group of voltage-gated ion channels found in the membrane of excitable cells (''e.g.'', muscle, glial cells, neurons, etc.) with a permeabi ...
on the cell membrane to open and allow
calcium ions
Calcium ions (Ca2+) contribute to the physiology and biochemistry of organisms' cells. They play an important role in signal transduction pathways, where they act as a second messenger, in neurotransmitter release from neurons, in contractio ...
to pass through into the
sarcoplasm
Sarcoplasm is the cytoplasm of a muscle cell. It is comparable to the cytoplasm of other cells, but it contains unusually large amounts of glycogen (a polymer of glucose), myoglobin, a red-colored protein necessary for binding oxygen molecules tha ...
(cytoplasm) of cardiac muscle cells. Calcium ions bind to
molecular receptors on the
sarcoplasmic reticulum (see graphic), which causes a
flux
Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications to physics. For transport ph ...
(flow) of calcium ions into the
sarcoplasm
Sarcoplasm is the cytoplasm of a muscle cell. It is comparable to the cytoplasm of other cells, but it contains unusually large amounts of glycogen (a polymer of glucose), myoglobin, a red-colored protein necessary for binding oxygen molecules tha ...
.
Calcium ions bind to
troponin C
Troponin C is a protein which is part of the troponin complex. It contains four calcium-binding EF hands, although different isoforms may have fewer than four functional calcium-binding subdomains. It is a component of thin filaments, along wit ...
, causing a
conformational (i.e., structural) change in the troponin-tropomyosin
protein complex
A protein complex or multiprotein complex is a group of two or more associated polypeptide chains. Protein complexes are distinct from multienzyme complexes, in which multiple catalytic domains are found in a single polypeptide chain.
Protein c ...
, causing the
myosin head
The myosin head is the part of the thick myofilament made up of myosin that acts in muscle contraction, by sliding over thin myofilaments of actin. Myosin is the major component of the thick filaments and most myosin molecules are composed of a he ...
(binding) sites on
F-actin filamentous proteins to be exposed, which causes
muscle contraction
Muscle contraction is the activation of tension-generating sites within muscle cells. In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in muscle length, such as ...
to occur.
The
cardiac action potential
The cardiac action potential is a brief change in voltage ( membrane potential) across the cell membrane of heart cells. This is caused by the movement of charged atoms (called ions) between the inside and outside of the cell, through proteins c ...
spreads
distally
Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...
(or outwardly) to the small branches of the Purkinje tree via the flux of cations through
gap junction
Gap junctions are specialized intercellular connections between a multitude of animal cell-types. They directly connect the cytoplasm of two cells, which allows various molecules, ions and electrical impulses to directly pass through a regulate ...
s that connect the sarcoplasms of adjacent myocytes.
The electrical activity of ventricular systole is coordinated by the
atrioventricular node
The atrioventricular node or AV node electrically connects the heart's atria and ventricles to coordinate beating in the top of the heart; it is part of the electrical conduction system of the heart. The AV node lies at the lower back section of t ...
, which is a discrete collection of cells that receives electrical stimulation from the left and right atria and can provide an intrinsic (albeit slower) heart pacemaker activity. The cardiac action potential is propagated down electrical pathways through the
bundle of His
The bundle of His (BH) or His bundle (HB) ( "hiss"Medical Terminology for Health Professions, Spiral bound Version'. Cengage Learning; 2016. . pp. 129–.) is a collection of heart muscle cells specialized for electrical conduction. As part of t ...
to the
Purkinje fibres
The Purkinje fibers (; often incorrectly ; Purkinje tissue or subendocardial branches) are located in the inner ventricular walls of the heart, just beneath the endocardium in a space called the subendocardium. The Purkinje fibers are specia ...
; this electrical flux causes coordinated depolarisation and excitation-contraction coupling from the
apex of the heart
The heart is a muscular organ in most animals. This organ pumps blood through the blood vessels of the circulatory system. The pumped blood carries oxygen and nutrients to the body, while carrying metabolic waste such as carbon dioxide to t ...
up to the roots of the great vessels.
Clinical notation
When blood pressure is stated for medical purposes, it is usually written with the systolic and diastolic pressures separated by a
slash
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* Nash ...
, for example, 120/80
mmHg. This clinical notation is not a mathematical figure for a fraction or ratio, nor a display of a numerator over a denominator. Rather, it is a medical notation showing the two clinically significant pressures involved (systole followed by diastole). It is often shown followed by a third number, the value of the
heart rate
Heart rate (or pulse rate) is the frequency of the heartbeat measured by the number of contractions (beats) of the heart per minute (bpm). The heart rate can vary according to the body's physical needs, including the need to absorb oxygen and excr ...
(in beats per minute), which typically is measured jointly with blood pressure readings.
See also
*
Wiggers diagram
A Wiggers diagram, named after its developer, Carl Wiggers, is a unique diagram that has been used in teaching cardiac physiology for more than a century. In the Wiggers diagram, the X-axis is used to plot time subdivided into the cardiac phases ...
*
Systole (mathematics)
*
Diastole
Diastole ( ) is the relaxed phase of the cardiac cycle when the chambers of the heart are re-filling with blood. The contrasting phase is systole when the heart chambers are contracting. Atrial diastole is the relaxing of the atria, and ventric ...
*
Blood pressure
Blood pressure (BP) is the pressure of circulating blood against the walls of blood vessels. Most of this pressure results from the heart pumping blood through the circulatory system. When used without qualification, the term "blood pressure" r ...
References
External links
*
Essential Hypertension Treatment{{Cardiovascular physiology
Blood pressure