Biology, Eighth Edition (Raven)

Chapter 49: The Circulatory and Respiratory Systems

Mechanical Events of the Cardiac Cycle

Why does the atrium not experience large changes in pressure? The hearts of mammals and birds have four chambers consisting of two pairs – a left ventricle and atrium and a right ventricle and atrium. In both cases the atrium collects blood entering the heart and then transfers it to the ventricle. The force moving blood from atrium to ventricle is largely generated by the expansion of the ventricle although there is also some contraction of the atrium. As blood leaves the atrium more blood can enter from the vena cava. Once the ventricle is full it will contract which closes the valve between the two chambers. Thus the amount of blood in the atrium does not dramatically change during the cycle. Movement of Oxygen and Carbon Dioxide This animation appears to be identical to the one entitled “Changes in the Partial Pressure of Oxygen and Carbon Dioxide” earlier on this list.

View the animation below, then complete the quiz to test your knowledge of the concept.


1.	During which phase of the cardiac cycle does aortic pressure reach its maximum?

		A)	ventricular ejection.
		B)	ventricular filling.
		C)	isovolumetric – ventricles contract.
		D)	isovolumetric – ventricles relax.
		E)	both C and D.

2.	During which complete phase of the cardiac cycle do the atria both relax and contract?

		A)	ventricular ejection.
		B)	ventricular filling.
		C)	isovolumetric – ventricles contract.
		D)	isovolumetric – ventricles relax.
		E)	both C and D.

3.	During which complete phase of the cardiac cycle do both the atria and ventricles relax?

		A)	ventricular ejection.
		B)	ventricular filling.
		C)	isovolumetric – ventricles contract.
		D)	isovolumetric – ventricles relax.
		E)	both C and D.

4.	The audible heart sounds are caused by the contraction of the atria and ventricles.

		A)	True
		B)	False

5.	The P wave of the ECG coincides with ventricular filling.

		A)	True
		B)	False