10.4: Heart Anatomy - Details
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- 121306
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)The heart’s external and internal structures work together to move blood efficiently through its atria and ventricles, valves, and vessels, ensuring continuous circulation throughout the body.
- Identify the major external structures of the heart and describe how the atria, ventricles, and great vessels function in blood flow.
- Trace the pathway of blood through the heart, lungs, and body, distinguishing between pulmonary and systemic circuits.
- Explain how the septa, valves, chordae tendineae, and papillary muscles maintain one-way blood flow through the heart.
External Structures of the Heart
Atria and Ventricles
The heart contains four hollow chambers: two atria and two ventricles. The right and left atria are the thin-walled, superior chambers that receive blood returning to the heart. Although both atria can be seen from the front, most of the left atrium lies on the posterior side of the heart (see Figures below).
Each atrium has a small, flap-like extension called an auricle, named for its resemblance to a human ear (see Figure B below). The auricles act as expandable “pockets” that can temporarily hold extra blood during times of exertion, helping to relieve high pressure in the atria.
The two ventricles lie below the atria and make up most of the heart’s mass. Their muscular walls are much thicker than those of the atria because they must generate enough force to pump blood into the arteries. The right ventricle forms most of the heart’s anterior surface, while the left ventricle forms much of the posterior and left sides.
The heart is composed of four hollow chambers: two atria and two ventricles. The thin walled right and left atria are the two superior chambers. While both atria are visible from the anterior, the majority of the left atrium is seen on the posterior aspect. Each atrium has a superficial flap-like extension called an auricle because its shape resembles the external ear of a human. You may also hear them referred to as atrial appendages. The auricles can increase the atrial capacity at times of exertion by filling with blood and, thus, relieving high atrial pressure. The two ventricles are inferior to the atria and they form the largest portion of the heart. Their walls are significantly thicker than those of the atria as a large amount of muscle mass is needed to push the blood into the arteries. The right ventricle makes up the right side and much of the anterior portion of the heart, while the left ventricle makes up the left side and much of the posterior portion of the heart.
Figure \(\PageIndex{1}\): Superficial Heart Anatomy. Inside the pericardium, the surface features of the heart are visible. (Caption from OpenStax) (Image credits: "Heart Chambers Anterior View" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, modification of original "Blausen 0451 - Anterior view of the heart - English labels" by Blausen Medical Communications, Inc.; "Heart Chambers Posterior View" by Jennifer Lange is licensed under under CC BY-NC-SA 4.0, modification of original "Blausen 0456 Heart Posterior" by Blausen Medical Communications, Inc.)
Great Vessels of the Heart
Blood enters and exits the heart through the body’s largest arteries and veins, collectively called the great vessels. These major vessels include the inferior vena cava, superior vena cava, pulmonary arteries, pulmonary veins, and the aorta (See figure below).
The pulmonary trunk emerges from the right ventricle and quickly divides into the right and left pulmonary arteries, which carry deoxygenated blood to each lung. After passing through the lung capillaries, the now oxygen-rich blood returns to the heart through the right and left pulmonary veins, which empty into the left atrium. From there, blood flows into the left ventricle, from where it is pumped into the aorta — the largest and thickest artery in the body. The aorta distributes oxygenated blood to every organ through the systemic circuit.
Blood returning from the body completes its journey through two of the great vessels: the superior vena cava, which drains blood from regions above the heart, and the inferior vena cava, which returns blood from regions below the heart. We will explore the structure of these vessels a little further on in this Chapter.
Coronary Arteries: Fueling the Heart
The heart may pump blood to every cell in the body, but it also needs its own dedicated blood supply. The coronary arteries deliver oxygen-rich blood to the myocardium and other tissues of the heart itself.
Immediately after the aorta emerges from the left ventricle, it gives rise to the right and left coronary arteries.
The left coronary artery (LCA) supplies blood to most of the left side of the heart — including the left atrium, left ventricle, and interventricular septum. It passes behind the pulmonary trunk and quickly branches into two major arteries:
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The circumflex artery, which follows the coronary sulcus to the left and eventually connects with small branches of the right coronary artery.
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The anterior interventricular artery, also called the left anterior descending artery (LAD), which travels along the anterior interventricular sulcus toward the apex of the heart. Along its route, it gives rise to smaller branches that form tiny anastomoses (connections) with branches of the posterior interventricular artery. These interconnections can allow limited rerouting of blood flow if a vessel is partially blocked. However, in the heart, these connections are very small — so a coronary artery blockage can still lead to myocardial infarction (heart attack) in the affected area.
The right coronary artery (RCA) travels along the coronary sulcus and supplies the right atrium, portions of both ventricles, and parts of the cardiac conduction system. On the posterior surface of the heart, it curves downward and becomes the posterior interventricular artery (or posterior descending artery, PDA). This artery runs along the posterior interventricular sulcus toward the apex, supplying the back of the heart and the posterior portion of the septum.
Figure \(\PageIndex{3\): Coronary Arteries of the Heart. (Image credits: "Coronary Vessels, Anterior View" by Jennifer Lange is licensed under CC BY-NC-SA 4.0, modification of original "Blausen 0451 - Anterior view of the heart - English labels" by Blausen Medical Communications, Inc.; "Coronary Vessels, Posterior View" by Jennifer Lange is licensed under under CC BY-NC-SA 4.0, modification of original "Blausen 0456 Heart Posterior" by Blausen Medical Communications, Inc.)
Explore a model of the heart in 3D:
Internal Structures of the Heart
Muscular walls called septa divide the heart into right and left sides. The term septum (plural septa) comes from the Latin for “something that encloses.” Each septum is a continuation of the myocardium lined with endocardium.
The interatrial septum separates the two atria and contains a shallow depression called the fossa ovalis, a remnant of the fetal foramen ovale that once allowed blood to pass directly from the right to the left atrium, bypassing the lungs. This opening closes shortly after birth as normal circulation begins. The interventricular septum, which separates the ventricles, is thicker because the ventricles generate much higher pressure during contraction.
The atrioventricular septum separates the atria from the ventricles and contains four openings that allow blood flow between chambers and into the major arteries. Each opening is guarded by a valve that ensures one-way flow. The atrioventricular (AV) valves are reinforced by strands of connective tissue called chordae tendineae (“heart strings”), which anchor each valve flap to a papillary muscle on the ventricular wall. At the exits of the ventricles, the semilunar valves—each with three small cusps—control blood flow into the pulmonary trunk and aorta.
Right Atrium
The right atrium receives blood returning from the body through three large veins: the superior vena cava, inferior vena cava, and coronary sinus.
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The superior vena cava drains the head, neck, upper limbs, and thoracic region.
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The inferior vena cava returns blood from below the diaphragm—the lower limbs and abdominopelvic cavity.
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The coronary sinus, a thin-walled vein on the posterior heart, collects blood from the myocardium itself.
The atria receive venous blood almost continuously, so the heart can keep filling even while the ventricles are contracting. Most ventricular filling happens while the atria are relaxed, but they also contract briefly to push the last bit of blood into the ventricles before the latter contract. The opening between the right atrium and right ventricle is controlled by the tricuspid valve.
The atria receive venous blood on a nearly continuous basis, allowing the heart to continue to receive blood even while the ventricles are contracting. While most ventricular filling occurs while the atria are relaxed, they do demonstrate a contractile phase and actively pump blood into the ventricles just prior to ventricular contraction. The opening between the right atrium and right ventricle is guarded by the tricuspid valve.
Right Ventricle
The right ventricle receives blood from the right atrium through the tricuspid valve (also called the right atrioventricular valve). Three papillary muscles in the right ventricle anchor the three flaps of this valve.
When the ventricular myocardium contracts, pressure within the ventricle increases and blood flows toward lower-pressure areas — the pulmonary trunk and the atrium. To prevent backflow into the atrium, the rising pressure pushes the tricuspid valve flaps closed. Simultaneously, the papillary muscles contract, pulling on the chordae tendineae to keep the valve flaps from everting into the atria. This coordination prevents regurgitation of blood during ventricular contraction.
When the right ventricle contracts, it ejects blood into the pulmonary trunk, which branches into the left and right pulmonary arteries that carry deoxygenated blood to each lung. At the base of the pulmonary trunk is the pulmonary semilunar valve that prevents backflow from the pulmonary trunk when the ventricle relaxes.
Left Atrium
After gas exchange occurs in the lungs, oxygenated blood returns to the left atrium through four pulmonary veins. Blood flows almost continuously from the pulmonary veins into the atrium, which serves as a receiving chamber, and then through the open bicuspid (mitral or left atrioventricular) valve into the left ventricle. Most ventricular filling occurs passively while both chambers are relaxed, but near the end of this phase — just as on the right side — the left atrium contracts to push into the ventricle the last 20% of of the blood.
Left Ventricle
Although both ventricles pump the same volume of blood, the left ventricle has a much thicker myocardium because it must generate greater force to propel blood through the systemic circuit and upward to the brain against gravity. The left ventricle contains prominent trabeculae carneae, and its bicuspid valve flaps are anchored to two papillary muscles by the chordae tendineae. Again, this structure ensures the valve closes securely during contraction as blood is ejected into the aorta through the aortic semilunar valve.
Heart Valves
A transverse section through the heart just above the atrioventricular septum reveals all four valves in the same plane (see figure below). These valves ensure that blood flows in only one direction through the heart.
Between the right atrium and right ventricle lies the right atrioventricular (tricuspid) valve. It has three flaps, or leaflets, formed from endocardium strengthened with dense connective tissue. Each flap is anchored by several chordae tendineae to papillary muscles projecting from the ventricular wall.
At the opening between the left atrium and left ventricle is the left atrioventricular valve, commonly called the mitral valve (or bicuspid valve). Clinically, “mitral valve” is preferred since the number of cusps can vary among individuals. The mitral valve’s cusps are also attached by chordae tendineae, this time to two papillary muscles that extend from the ventricular wall.
Pathway of Blood Through the Heart
The right ventricle pumps deoxygenated blood into the pulmonary trunk, which travels upward in front of the aorta and left atrium before splitting into the right and left pulmonary arteries.
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The right pulmonary artery carries blood to the right lung.
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The left pulmonary artery carries blood to the left lung.
Inside the lungs, these arteries branch repeatedly until they reach the pulmonary capillaries, where gas exchange occurs — carbon dioxide leaves the blood and oxygen enters. The pulmonary trunk and arteries are unique because they are the only arteries in the body that carry deoxygenated blood after birth.
Freshly oxygenated blood then returns from the lungs through the pulmonary veins, which are the only veins that carry oxygen-rich blood. These veins empty into the left atrium, which sends the blood into the left ventricle. From there, the left ventricle pumps oxygenated blood into the aorta, which arches over the top of the heart and then descends to deliver blood throughout the body via the systemic circuit.
In the systemic capillaries, oxygen and nutrients move out of the blood and into body cells, while carbon dioxide and waste products move into the blood. The now deoxygenated blood flows into small veins (venules), which merge into larger veins and finally drain into the two main systemic veins: the superior vena cava and inferior vena cava. These return the blood to the right atrium, completing the cycle (See figure below).
