Hi hive people. How are you? I hope you are fine 💕 enjoy your life with pleasure and good health. Yo know human have many system. One of them is cardiovascular system. In cardiovascular system a heart perform it's functions. Heart pumps the blood all over the body convert deoxygenated blood into oxygenated blood here I can explain about heart. 
Heart is a muscular organ. It is present in all living organisms. Heart pump the blood through blood vessels. Blood carry oxygen and also waste like carbon dioxide.
Size :
The size of heart is about our first
Location:
It is located between the lungs.
Chambers :
Heart has four Chambers. Right atria , right ventricle. Left atria , left ventricle. A double membered sac called the pericardium.
Position :
Heart is present between the lungs the left lung is smaller than right one. The heart is cone shaped. A young heart is 250 gram to 350 gram the length of the heart is 12cm.
Valves
The heart has four valves. Valves separate it's chambers . The valves between the atria and ventricles are called the atrioventricular valves. Two additional semilunar valves are also present.
Walls of heart
The heart wall is made up of three layers 1. Endocardium. 2. Myocardium. 3. Epicardium
Heart tissue receives blood from two arteries . Small lymphatic Network called plexuses. Sympathetic stimulation causes the release of the neurotransmitter nor epinephrine ( also known as nor adrenaline) 
The cardiac cycle is the sequence of events in which the heart contracts and relaxed with heart beat the period of time during which the ventricles contract forcing blood out into the aorta and main pulmonary artery , is known as systole the period in which the ventricles relax and refill the blood called diastole
At the beginning of the cardiac cycle the ventricles are relaxing.
Cardiac output
Cardiac output is a measurement of the amount of blood pumped by each ventricles in one minute. This is calculated by multiplying the stroke volume by the beats per minute of the heart rate.
Preload refers to the filling pressure of the atria at the end of the diastole. 
The normal resting heart rate is called the sinus rhythm,created and sustained by the sinoatrial node, a group of peacemaking cells found in the wall of the right atrium cells in the sinoatrial node do this by creating an action potential 