The Heart's Structures: The Chambers
Right Atrium
Right Ventricle
Left Atrium
Left Ventricle
The Septum
"It isn't what you have in your pocket that makes you thankful, but what
you have in your heart." Anonymous
The chambers of the heart provide an intricate multiple step pathway for blood to be first sent to the lungs for respiration and then dispensed to the body's cells to sustain life. How this occurs can be explained through the perspective of: 1) differing chamber volumes; and by 2) looking at the differing pressure gradients achieved by contracting and relaxing chambers.
The heart consists of 4 chambers - 2 atria and 2 ventricles. The smaller atria are about 1/3 the size and volume of the ventricles. The left ventricle is the largest chamber of the heart, with about 3 times more muscle mass than the right ventricle - both ventricles share a similar volume capacity. Due to the predominant size of the left ventricle, it is not surprising that 70% of all myocardial infarctions occur within the left ventricle.
While the heart's chambers' primary purpose is to fulfill the mechanical function of pumping blood, other functions include influencing heart rate and serving several endocrine roles.
The right atrium receives oxygen-depleted blood from the inferior and superior vena cava. The right atrium also receives blood from the coronary sinus, the venous outlet from the coronary circulation (a one-way valve covers the coronary sinus, called the Thebesian valve). While the superior vena cava has no valve, the inferior vena cava has a partial valve called the eustachian valve. Note that the patent inferior vena cava will receive retrograde pulsations of blood (observed as pulsations to the jugular veins - called cannon 'a' waves) when the right atrium contracts against a closed tricuspid valve. This may occur with such cardiac rhythms as junctional rhythm, ventricular tachycardia and 3rd degree AV block.
The right atrium also effects heart rate through barroreceptors present on its inside surface. The barroreceptors respond to falling blood supply (preload) by increasing the heart rate (in other words, by not stimulating the Vagus nerve, allowing the sympathetic nervous system to have the upper hand). An increased blood supply results in stimulation of the Vagus nerve which results in the slowing of the heart rate (and thus, cardiac output). This effect is seen with the cardiac rhythm sinus arrhythmia, seen as a rhythm that originate from the sinus node but is slightly irregular, speeding up during inspiration (increased thoracic cage and less venous return) and slowing down during expiration (decreased size of thoracic cage and more venous compression = more venous return).
The right and left atria also perform an endocrine function with the
release of atrial natriuretic peptide (ANP), blunting the effects of epinephrine,
endothelin and the renin-angiotension-aldosterone cascade. ANP is released
when the atria is stretched i.e. increased preload to the heart. The effects
of ANP include:
decreased heart rate;
reduced preload through vasodilation and increased urinary output;
stimulation of the Vagus nerve; and
inhibited ventricular hypertrophy.
The right atrium's wall is approximately only 2mm in thickness due to the combined influence of the low pressure of this chamber and the ease of pumping to low pressure areas (right ventricle).
The right ventricle is separated from the left ventricle by the septum. The right ventricle ejects blood through the pulmonic valve into the pulmonary arteries. While an aortic blood pressure might be 120/80, a pulmonary artery pressure is commonly 26/10. In other words, while the left ventricle has to pump against a diastolic pressure (pressure before contraction) of '80', the right ventricle pumps against only 1/8th of the left, at only '10' mm of Hg.
The right ventricle's wall thickness for an adult is 4-5 mm. It's right ventricle's myocardial blood supply is primarily from the right coronary artery. An inferior myocardial infarction (MI) usually involves the right ventricle.
Similar to the right atrium, the left atrium fulfills several functions: 1) atrial kick for the left heart; 2) influences heart rate; and 3) has endocrine qualities (see the right atrium). The left atrium wall is a little larger than the right atrium at 3mm.
The left ventricle is the largest chamber of the heart, accounting for the majority of the anterior and left lateral surfaces of the heart. The left ventricle also occupies the vast majority of the heart's apex. The wall of the left ventricle is 8-15 mm thick although the tip of the apex can be as little as 2mm thick.
The blood ejected by the left ventricle (or the right ventricle for that matter) is dependent on several factors:
Starling's Law - the more the muscle wall is stretched, the more forceful the contraction. Therefore, more blood is ejected.
Heart Rate - the amount of blood ejected by the left ventricle increases as the heart increases to a limit. Note that as the heart rate approaches 150+/minute, time is limited for the heart to fill between each beat (filling time) resulting in less volume in the left ventricle, less stretch, less force of contraction and as a result...less blood ejected.
Preload - the supply of blood to the heart effects how much blood makes it to the ventricle. This supply is refered to as preload. (Technically preload is defined as left ventricular end diastolic pressure - does that help at all - perhaps but unlikely). As preload or supply increases, blood ejected (stroke volume) increases and visa versa.
Afterload - the pressure within the aorta prior to ventricular contraction (diastolic BP) and the systemic vascular resistance (SVR) determines afterload. Afterload is the pressure that the left ventricle must overcome to eject blood into the aorta. The more afterload, the more difficult for the left ventricle to pump to the body and the blood ejected (known as stroke volume) is reduced. As afterload is decreased, stroke volume increases.
Atrial kick - the contraction of the atria prior to the contraction of the ventricle accounts for more volume (1/3 more) to the ventricle and more muscle stretch. With Starling's Law at play, atrial kick accounts for 15-30% of blood ejected. Note that the older one becomes, the higher the percentage owed to atrial kick. For example, for the elderly with only quivering atria - atrial fibrillation - they are often more effected because as much as a third of their blood supply to ALL of their cells has been eliminated.
Coordinated Heart - the heart's contraction follows a specialized pathway that optimizes the amount of blood ejected. When the electrical pathways followed are aberrant or unusual, the efficiency of contraction is often hampered resulting in a reduced stroke volume. For example, impulses originating from the ventricles most often result in a less efficient pumping action for the ventricles than impulses that follow the ideal pathway that begins with a supraventricular impulse.
Together, all of these factors must be considered when assessing a patient. An ECG, a cardiac history and a set of vital signs should point you in the right direction on how effective the left ventricle is in pumping sufficient blood to the tissues.
The heart is often discussed as if there are two hearts - the right and left heart. This is because a thick layer of connective tissue called the septum separates the left and right heart. Note that the top of the heart (the atria) is also separated by a layer of connective tissue from the bottom of the heart (the ventricles). These layers of connective tissue provide structure to the heart. Connective tissue does not conduct electrical activity and serves as an electrical barrier - even an electrical insulator. This is an important factor to understand the heart's electrical system (i.e. AV heart blocks).
The septum serves a role as well in ventricular contraction, aiding with
the amount of blood ejected - the left ventricle is effected more than
the right.
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