The average heart pumps the equivalent of five 1-L drinks each minute.
The average amount of blood in a person is also 5 L—every minute, all the blood has circulated through the body.
During heavy exercise this output can increase five fold, i.e. it will circulate much more often.
What Causes the Heart to Beat?
Sinoatrial node (SA node or pacemaker)
A collection of nervous tissue located in the wall of the right atrium that spontaneously creates a periodic electrical signal.
This signal travels through both the right and left atria, causing the atria to contract.
Word derivation:
Sinus: cavity or depression
Atrial: in the atrium
Node: a collection of tissue
Atrioventricular node (AV node)
A collection of nervous tissue in the wall between the atria and ventricles.
Delays the signal so the atria can fully contract before sending the electrical signal into the ventricles (through the Bundle branches and Purkinje fibers), causing the ventricles to contract.
No signal from the brain is necessary to tell the pacemaker to produce its regular signal, 70 times every minute.
The brain and various chemicals in the body can influence the pace of the pacemaker.
Much of this is autonomic (not under conscious control).
A stressful situation can cause the pace to increase: the heart rate is used in polygraph tests (lie detectors).
A dangerous situation can cause the pace to increase: the “fight or flight response.”
An attractive member of the opposite sex can cause the pace to increase.
Blood Vessels and Circulation
Five different kinds of vessels exist in the circulatory system:
Arteries
Carry blood away from heart
The thickest-walled vessels, since they experience the greatest pressure in the system.
Generally not found near the skin surface.
Arterioles: arteries divide into these smaller vessels that take blood to the capillaries.
Capillaries
Oxygen, carbon dioxide, nutrients, and wastes diffuse through the capillary walls.
They touch every cell in the body.
The smallest vessels in the circulatory system.
The thinnest-walled vessels, since molecules must diffuse across it: they only have an endothelial cell layer and a basement membrane.
Venules: capillaries merge into these larger vessels
Veins
Return blood to the heart.
Venules merge into these even larger vessels.
Have valves to prevent backflow of blood because of the low pressure.
Thicker-walled than capillaries, but thinner than arteries, since this is a low pressure system.
Can be found near skin surface, because of their low pressure.
Via Domenic Denicola's mad Paint skills; Figures 42.8 and 42.9, page 818, Campbell's Biology, 5th Edition
Blood Pressure
The Basics
Blood pressure: pressure the blood exerts against the vessel walls due to…
Force of heart contraction
Size of the arterioles which provide peripheral resistance
Smooth muscles in the wall of the arterioles can relax or tighten and in turn alter blood pressure.
Arteries are surrounded by walls that are very elastic, which help maintain pressure in the system during the phase of the cardiac cycle in which the heart is not contracting (diastole).
As the cross-sectional area of the system increases due to the increasing number of vessels (arterioles and capillaries), the pressure drops (P = F∕A).