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Illustration A shows cardiac diastole. The cardiac muscle is relaxed, and blood flows into the heart atria and into the ventricles. Illustration B shows atrial systole; the atria contract, pushing blood into the ventricles, which are relaxed. Illustration C shows atrial diastole; after the atria relax, the ventricles contract, pushing blood out of the heart.
During (a) cardiac diastole, the heart muscle is relaxed and blood flows into the heart. During (b) atrial systole, the atria contract, pushing blood into the ventricles. During (c) atrial diastole, the ventricles contract, forcing blood out of the heart.

The pumping of the heart is a function of the cardiac muscle cells, or cardiomyocytes, that make up the heart muscle. Cardiomyocytes , shown in [link] , are distinctive muscle cells that are striated like skeletal muscle but pump rhythmically and involuntarily like smooth muscle; they are connected by intercalated disks exclusive to cardiac muscle. They are self-stimulated for a period of time and isolated cardiomyocytes will beat if given the correct balance of nutrients and electrolytes.

Micrograph shows cardiac muscle cells, which are oblong and have prominent striations.
Cardiomyocytes are striated muscle cells found in cardiac tissue. (credit: modification of work by Dr. S. Girod, Anton Becker; scale-bar data from Matt Russell)

The autonomous beating of cardiac muscle cells is regulated by the heart’s internal pacemaker that uses electrical signals to time the beating of the heart. The electrical signals and mechanical actions, illustrated in [link] , are intimately intertwined. The internal pacemaker starts at the sinoatrial (SA) node    , which is located near the wall of the right atrium. Electrical charges spontaneously pulse from the SA node causing the two atria to contract in unison. The pulse reaches a second node, called the atrioventricular (AV) node, between the right atrium and right ventricle where it pauses for approximately 0.1 second before spreading to the walls of the ventricles. From the AV node, the electrical impulse enters the bundle of His, then to the left and right bundle branches extending through the interventricular septum. Finally, the Purkinje fibers conduct the impulse from the apex of the heart up the ventricular myocardium, and then the ventricles contract. This pause allows the atria to empty completely into the ventricles before the ventricles pump out the blood. The electrical impulses in the heart produce electrical currents that flow through the body and can be measured on the skin using electrodes. This information can be observed as an electrocardiogram (ECG)    —a recording of the electrical impulses of the cardiac muscle.

 The sinoatrial node is located at the top of the right atrium, and the atrioventricular node is located between the right atrium and right ventricle. The heart beat begins with an electrical impulse at the sinoatrial node, which spreads throughout the walls of the atria, resulting in a bump in the ECG reading. The signal then coalesces at the atrioventricular node, causing the ECG reading to flat-line briefly. Next, the signal passes from the atrioventricular node to the Purkinje fibers, which travel from the atriovenricular node and down the middle of the heart, between the two ventricles, then ups the sides of the ventricles. As the signal passes down the Purkinje fibers the ECG reading falls. The signal then spreads throughout the ventricle walls, and the ventricles contract, resulting in a sharp spike in the ECG. The spike is followed by a flat-line, longer than the first then a bump.
The beating of the heart is regulated by an electrical impulse that causes the characteristic reading of an ECG. The signal is initiated at the sinoatrial valve. The signal then (a) spreads to the atria, causing them to contract. The signal is (b) delayed at the atrioventricular node before it is passed on to the (c) heart apex. The delay allows the atria to relax before the (d) ventricles contract. The final part of the ECG cycle prepares the heart for the next beat.

Visit this site to see the heart’s “pacemaker” in action.

Arteries, veins, and capillaries

The blood from the heart is carried through the body by a complex network of blood vessels ( [link] ). Arteries take blood away from the heart. The main artery is the aorta that branches into major arteries that take blood to different limbs and organs. These major arteries include the carotid artery that takes blood to the brain, the brachial arteries that take blood to the arms, and the thoracic artery that takes blood to the thorax and then into the hepatic, renal, and gastric arteries for the liver, kidney, and stomach, respectively. The iliac artery takes blood to the lower limbs. The major arteries diverge into minor arteries, and then smaller vessels called arterioles , to reach more deeply into the muscles and organs of the body.

Questions & Answers

What's the main function of the CELL
Victor Reply
Is the smallest unit of an living things
Chanda Reply
Okay
Jason
What is a cell
Jason Reply
A cell is the basic unit of life.
Bernard
what is the photosynthesis
Brian Reply
describe cellular event during meiosis
Ruth Reply
what is reproduction
Ruth
reproduction is the process by which living organisms give birth to new offsprings of the same kinds
Bernard
Reproduction is a process by which organisms give rise to new members of their species
Fru
Reproduction is the process whereby living organisms produces the young ones of their own kind to ensure continuity of life
Eunice
what are the example of photosynthesis
Gamshe Reply
Is how plant covert sugar and energy, air and sunlight into energy to grow
Timileyin
An example of photosynthesis is how plants convert sugar and energy from water, air and sunlight into energy to grow.
Bernard
Pls how fertilization occurs in the womb
Rebecca
Pls am asking how does fertilization occurs in the womb
Rebecca
An egg is released in the ovary of a woman during ovulation. in the presence of a sperm this egg cell fuses with the sperm cell to form a zygote ( fertilization) this zygote now moves thru the fallopian tube down to the womb where implantation takes place. the zygote develops thru to a baby
Fru
how are you everyone in here
Brian
what is biology
Odion
My cordial salam to everybody. I have a question to all. What do you mean by plasma membrane?
Mahmud Reply
Plasma membrane or Cell membrane is the outer layer of tissue surrounding the whole or part of an organ. in addition, it is the outer flexible or semi-flexible covering or waterproofing whose primary function is to exclude water.(usually in plants and animals).
Job
I have a question .... Why cell wall is not present in animals cells and why it is present in plants cells?
ShAmy
what is glucose
Sisay
how alkali metal form
Puskal Reply
what is a cytosol
Siddeeqah Reply
cytosol is the internal fluid of the cell and a large part of cell were metabolism occur
Bernard
u are right dear.
Job
what is cell
Prince Reply
I think that cell is structural unit of our body. Because I have come to know that cell is the structure and biological function of an organism in the eye of scientists.
Mahmud
Cell is the fundamental, structural and functional unit of life
Winifred
Absolutely you are right. But what do you mean by life?
Mahmud
cell is the basic unit of life
Siddeeqah
life is the study of living organisms
Bernard
Cell is the basic, structural and functional unit of life. therefor no living organisms exist without a cell or a healthy cell.
Job
synthesis of 1 molecules of glucose requires
Purvesh Reply
what is the chemical composition of water
Abigail Reply
h20
Rita
h20
Asad
H20
Mathews
H2o
Bernard
H2O
Michelle
H2O
Emzzy
What is skeleton
Emzzy
skeleton is an internal or external framework of bones
Michelle
Skeleton is the structural frame work of the body
Niimat
Skeleton is the internal bony framework of the body of living organisms.
Job
what is ecology
Iyiola
What is a lymph?
Karisto
Oh, how's is it going..
Brian Reply
not too good
Monique
hy
Adeola
hi
Imamkasim
hi
Veronica
Any one else taking Bio 1406 with Stephanie Martin?
Veronica
where is it ?
ShAmy
am here Veronica
iyota
Not bad
Winifred
Part of compound microscope
Bakish Reply
a. body b. stage clip c. adjacent knob d. arm e. eye piece
Kpodo
E
Rita
give five difference between worker and queen bee.
Imamkasim
How do u know when you want to urinate
Akpo Reply
how do you know when you want to urinate
Akpo
I don't know please explain
Coded
As the bladder fills up .. the signals are sent to the brain specifying that its filling up and should be emptied and the fuller it gets, the more signals/ alerts are sent to brain ...leading to the urge to urinate .... to go pee
Khalida
OK thank you
Coded
hello
Issiya
hi
Abigail
wat
Gamshe

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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