# Label the Heart: Diagram and Anatomy Practice
By the end of this guide you will be able to label every major structure on a mammalian heart diagram without hesitation, explain what each structure does, and connect each label to a clinical problem you will actually see in practice. You will also have a blank version of the labeling exercise to test yourself, and a table that pairs each structure with its function and one clinical correlate.
You do not need special software. Have on hand a printed or on-screen heart diagram (lateral or ventral view), a blank outline of the same diagram, colored pens if you like, and this article. A dissected specimen or a plastic model helps enormously because the three-dimensional relationships matter more than the flat drawing suggests. If you are working from a cadaver or a teaching model, keep it beside you while you read.
This article is educational and is not a substitute for veterinary diagnosis or treatment.
## Why Heart Anatomy Matters in Veterinary Practice
A heart diagram labeled correctly is not an academic exercise. Every time you auscultate a murmur, place an ultrasound probe, interpret a radiograph, or explain a congenital defect to an owner, you are working from a mental map of the heart. When that map is wrong, the physical examination is wrong.
The four-chambered heart is the shared architecture of mammals and birds. Mammals and birds independently evolved a complete interventricular septum that fully separates oxygenated from deoxygenated blood, which supports the high metabolic demands of endothermy. Reptiles sit at a different point on that spectrum. Most reptiles have a ventricle that is partially divided rather than fully septated, and blood streams can mix to varying degrees depending on the species and the physiological state.
That comparative point is not trivia. It explains why a lizard tolerates a right-to-left shunt that would rapidly kill a dog, and it explains why the anatomy you learn on a dog heart does not transfer cleanly to a green iguana. In the green iguana (*Iguana iguana*), the ventricle is divided into three subchambers: the cavum arteriosum, the cavum venosum, and the cavum pulmonale. A muscular crest within the ventricle provides additional separation between oxygenated and deoxygenated blood, which improves circulatory efficiency compared with a completely undivided ventricle. The aortic and pulmonary trunk valves in this species are simple bicuspid structures, not the three-cusped semilunar valves of mammals [1].
Learn the mammalian pattern first. It is the reference standard for dogs, cats, horses, cattle, pigs, and birds, and it is the pattern you will be tested on.
## Orientation: How to Tell Right From Left on a Diagram
The single most common labeling error is reversing right and left. Fix the orientation in your head before you label anything else.
Right and left in cardiac anatomy refer to the animal's own right and left, not the viewer's. When you look at a heart diagram drawn from a ventral (cranial) view, the animal's right side appears on your left. When you look at a lateral view, the near side is whichever side the heart is being viewed from.
Three reliable landmarks settle the question:
1. **The apex points left.** In dogs and cats the ventricular apex is directed cranioventrally and to the left. The left ventricle forms the apex.
2. **The right ventricle is the cranial and right-sided chamber.** It wraps around the cranial and right aspect of the heart and does not reach the apex in most domestic mammals.
3. **The aorta emerges from the left ventricle and arches caudally and to the left.** The pulmonary trunk emerges from the right ventricle and crosses ventral to the aorta.
A useful memory device: the left ventricle is the thick-walled pump that drives systemic pressure, and the right ventricle is the thin-walled pump that drives low-pressure pulmonary flow. On a diagram, the chamber with the visibly thicker wall is the left ventricle.
One caution from comparative imaging. The swine heart is frequently used as a translational model because it resembles the human heart in size and coronary arterial pattern, but three-dimensional comparison in the living thorax reveals real differences in cardiac axis, aortic axis, aortic arch plane, the relationship between the aortic arch and pulmonary trunk, atrial appendage extent, and ventricular trabeculation patterns [2]. Do not assume that a diagram drawn from one species maps perfectly onto another. Label the species you are actually studying.
## Step-by-Step Labeling Walkthrough
Use this labeled heart diagram to follow along with each step of the labeling walkthrough. Image: MesserWoland, CC BY-SA 3.0, via Wikimedia Commons.
Work through these steps in order. Each step builds on the last, and the order mirrors the path blood takes through the heart.
### Step 1: Label the Four Chambers
Start with the two atria, then the two ventricles.
**Right atrium.** Receives deoxygenated blood from the cranial vena cava, the caudal vena cava, and the coronary sinus (in species that have one). Its wall is thin. The right auricle (atrial appendage) is a blind pouch that projects from the chamber.
**Left atrium.** Receives oxygenated blood from the pulmonary veins. In dogs and cats there are typically several pulmonary vein ostia entering the dorsal wall of the left atrium. The left auricle is smaller and narrower than the right in most domestic mammals.
**Right ventricle.** Receives blood from the right atrium through the right atrioventricular valve and pumps it into the pulmonary trunk. Its wall is roughly one-third the thickness of the left ventricular wall in a normal heart.
**Left ventricle.** Receives blood from the left atrium through the left atrioventricular valve and pumps it into the aorta. Its wall is thick and forms the apex.
A morphometric study of the common pheasant (*Phasianus colchicus*) found the left ventricular wall at the middle and apical regions was four and three times thicker than the right ventricular wall, respectively [3]. That ratio is a useful sanity check when you are examining any heart: if the two ventricular walls look equal in thickness, either you have mislabeled them or the heart is abnormal.
### Step 2: Label the Atrioventricular Valves
The atrioventricular (AV) valves sit between each atrium and its ventricle. They prevent backflow during ventricular contraction.
**Right AV valve (tricuspid valve).** Three leaflets in most mammals. It separates the right atrium from the right ventricle.
**Left AV valve (mitral valve, also called the bicuspid valve).** Two leaflets in most mammals. It separates the left atrium from the left ventricle.
Both valves are anchored by chordae tendineae to papillary muscles that project from the ventricular wall. The chordae prevent the leaflets from prolapsing into the atrium during systole.
Bird hearts differ here in a way worth remembering. In the pheasant, the right muscular atrioventricular valve is attached to the right ventricular free wall by several muscular cords, and the chordae tendineae of the left atrioventricular valve have a branched appearance, with each chorda composed of three to four narrower cords twisted together and attached to a common papillary muscle [3]. Birds also have a muscular right AV valve rather than a purely membranous one, which is a genuine anatomical difference from mammals.
### Step 3: Label the Semilunar Valves
The semilunar valves sit at the outflow of each ventricle and prevent backflow from the great arteries into the ventricles during diastole.
**Pulmonary valve.** Three cusps. Located at the junction of the right ventricle and the pulmonary trunk.
**Aortic valve.** Three cusps. Located at the junction of the left ventricle and the aorta. The coronary arteries arise from the aortic sinuses just above the aortic valve cusps.
Semilunar valve cusp number is a good comparative checkpoint. Mammals have three cusps in each semilunar valve. The green iguana has bicuspid aortic and pulmonary trunk valves [1]. If you are labeling a reptile heart, do not draw three cusps by default.
### Step 4: Label the Great Vessels
**Aorta.** The largest artery. It leaves the left ventricle, arches dorsally and caudally, and supplies the systemic circulation. In mammals the aorta arches to the left. In birds the aortic arch is also present but the branching pattern of the great vessels differs from mammals, and the right aortic arch contributes to the carotid supply in some reptiles such as the green iguana [1].
**Pulmonary trunk.** Leaves the right ventricle and divides into left and right pulmonary arteries. It carries deoxygenated blood to the lungs. In the fetus it is connected to the aorta by the ductus arteriosus, which normally closes after birth.
**Cranial vena cava (precaval vein).** Returns deoxygenated blood from the head, neck, and forelimbs to the right atrium.
**Caudal vena cava (postcaval vein).** Returns deoxygenated blood from the abdomen, pelvis, and hindlimbs to the right atrium. In the green iguana, the ventral abdominal vein connects the pelvic region to the hepatic portal system, which is a reptilian feature with no direct mammalian equivalent [1].
**Pulmonary veins.** Return oxygenated blood from the lungs to the left atrium. In dogs and cats these enter as multiple separate ostia rather than a single vein.
### Step 5: Label the Coronary Circulation
The coronary arteries supply the myocardium itself. They are the first branches off the aorta and they fill during diastole, not systole.
**Left coronary artery.** Typically divides into a paraconal interventricular branch and a circumflex branch in dogs. It supplies the left ventricular free wall, the interventricular septum, and part of the right ventricle.
**Right coronary artery.** Supplies the right ventricular free wall and, in many species, gives off the subsinuosal interventricular branch.
**Coronary sinus or coronary veins.** Drain deoxygenated blood from the myocardium back to the right atrium. Some species have a well-developed coronary sinus, others drain via multiple small veins.
The coronary arterial pattern is one of the features that makes the swine heart a useful translational model, because it resembles the human coronary system. However, the relationship between the coronary arterial orifices and surrounding structures differs between swine and humans in ways that matter for catheter-based procedures [2].
### Step 6: Note the Conduction System Briefly
You do not need a full electrophysiology course to label a heart diagram, but four structures belong on the map.
**Sinoatrial (SA) node.** The pacemaker. Located at the junction of the cranial vena cava and the right atrium in mammals.
**Atrioventricular (AV) node.** Located at the base of the interatrial septum. It delays the impulse slightly before it passes to the ventricles.
**Bundle of His.** Conducts the impulse from the AV node into the interventricular septum.
**Purkinje fiber network.** Distributes the impulse through the ventricular myocardium. In the pheasant, the Purkinje fiber network is widely distributed in the myocardium and shows strong immunoreactivity for desmin [3].
That is enough for labeling purposes. The clinical relevance of the conduction system is that damage to any of these structures produces a rhythm disturbance, and in congenital heart disease the His-Purkinje system can be identified in real time during surgery using intraoperative conduction mapping, which reduces postoperative atrioventricular block [4].
## The Labeled Heart Diagram: Structure Reference Table
Use this table as your answer key. Cover the right two columns and test yourself from the structure names alone.
| Structure | Function | Clinical correlate |
|--|--|--|
| Right atrium | Receives deoxygenated blood from the cranial and caudal vena cavae | Right atrial enlargement is a common consequence of tricuspid regurgitation and can predispose to atrial arrhythmias |
| Left atrium | Receives oxygenated blood from the pulmonary veins | Left atrial enlargement is a marker of chronic mitral regurgitation and a risk factor for left atrial appendage thrombus |
| Right ventricle | Pumps deoxygenated blood into the pulmonary trunk | Right ventricular dysfunction accompanies severe tricuspid regurgitation, as seen with cardiac extension of intravenous leiomyomatosis [5] |
| Left ventricle | Pumps oxygenated blood into the aorta | Left ventricular wall thickness is roughly four times the right ventricular wall at the mid-ventricular level in the pheasant, and a similar ratio holds in mammals [3] |
| Right AV valve (tricuspid) | Prevents backflow from right ventricle to right atrium | Tricuspid regurgitation can be severe enough to require annuloplasty, as in right-sided cardiac mass resection [5] |
| Left AV valve (mitral) | Prevents backflow from left ventricle to left atrium | Mitral regurgitation is the most common valvular heart disease and can be caused by primary leaflet lesions or annular dilatation from left ventricular dysfunction [6] |
| Pulmonary valve | Prevents backflow from pulmonary trunk to right ventricle | Congenital pulmonary valve stenosis is a component of tetralogy of Fallot and other conotruncal defects |
| Aortic valve | Prevents backflow from aorta to left ventricle | Bicuspid aortic valve is significantly more common in patients with isolated ascending aortic aneurysm than in those with root involvement [7] |
| Aorta | Distributes oxygenated blood to the systemic circulation | Aortic root and ascending aortic dimensions are key measurements in aneurysmal disease, and annular size is significantly larger when the root is involved [7] |
| Pulmonary trunk | Carries deoxygenated blood to the lungs | Patent ductus arteriosus is a persistent fetal connection between the pulmonary trunk and the aorta |
| Cranial vena cava | Returns deoxygenated blood from the head and forelimbs | Obstruction or compression causes cranial caval syndrome with facial and forelimb edema |
| Caudal vena cava | Returns deoxygenated blood from the abdomen and hindlimbs | Tumor thrombus can extend from the abdomen into the right atrium via the caudal vena cava [5] |
| Pulmonary veins | Return oxygenated blood from the lungs to the left atrium | Pulmonary vein ostia are the targets for isolation procedures in atrial fibrillation, and extra-pulmonary triggers complicate treatment [4] |
| Left coronary artery | Supplies the left ventricular free wall and interventricular septum | Coronary revascularization strategies differ in their effects on myocardial perfusion and lymphatic flow in experimental models [8] |
| Right coronary artery | Supplies the right ventricular free wall | Coronary arterial orifice relationships differ between swine and humans in ways that affect catheter procedures [2] |
| Coronary veins | Drain deoxygenated blood from the myocardium to the right atrium | Coronary sinus anatomy is relevant to retrograde cardioplegia delivery |
| Sinoatrial node | Generates the cardiac impulse | Sinus node dysfunction produces inappropriate bradycardia |
| Atrioventricular node | Delays the impulse before ventricular activation | AV block is a risk during congenital heart surgery and is reduced by intraoperative conduction mapping [4] |
| Bundle of His | Conducts the impulse into the interventricular septum | Conduction system pacing is evolving as a physiologic strategy in congenital heart disease [4] |
| Purkinje fibers | Distribute the impulse through the ventricular myocardium | Purkinje fiber distribution is widespread in the avian myocardium and shows strong desmin immunoreactivity [3] |
## Self-Test: Blank Labeling Version
Cover the table above and the walkthrough sections. Use this numbered list as your blank diagram. Write the name of each structure on a separate sheet, then check against the table.
1. Chamber that receives deoxygenated blood from the vena cavae.
2. Chamber that receives oxygenated blood from the pulmonary veins.
3. Chamber that pumps into the pulmonary trunk.
4. Chamber that pumps into the aorta.
5. Valve between the right atrium and right ventricle.
6. Valve between the left atrium and left ventricle.
7. Valve at the outflow of the right ventricle.
8. Valve at the outflow of the left ventricle.
9. Largest artery, leaving the left ventricle.
10. Artery leaving the right ventricle.
11. Vein returning blood from the head and forelimbs.
12. Vein returning blood from the abdomen and hindlimbs.
13. Vessels returning oxygenated blood to the left atrium.
14. Artery supplying the left ventricular free wall.
15. Artery supplying the right ventricular free wall.
16. Veins draining the myocardium.
17. Pacemaker of the heart.
18. Structure that delays the impulse before ventricular activation.
19. Structure that conducts the impulse into the interventricular septum.
20. Network that distributes the impulse through the ventricular myocardium.
For a harder version, label a diagram with no list at all. For an easier version, use the table and cover only the function and clinical correlate columns.
## Species Comparison: Mammal, Bird, and Reptile
The four-chambered heart is the mammalian and avian pattern. Both groups have two atria, two ventricles, and a complete interventricular septum. The differences between mammals and birds are in valve structure and great vessel branching, not in chamber number.
Birds have a muscular right atrioventricular valve rather than a membranous one, and the chordae tendineae of the left AV valve are branched and twisted [3]. The right AV valve in the pheasant is attached to the right ventricular free wall by muscular cords [3]. This is a real functional difference, not a labeling detail, because it changes how the valve behaves under pressure.
Reptiles have a partially divided ventricle. The green iguana has three ventricular subchambers with a muscular crest providing additional separation, and its semilunar valves are bicuspid rather than tricuspid [1]. The sinus venosus is positioned dorsally, lacks internal septa, and drains into the right atrium via a bicuspid sinoatrial valve [1]. Mammals retain only a remnant of the sinus venosus as part of the right atrial wall.
If you are labeling a reptile heart, you need to add the sinus venosus, the cavum arteriosum, the cavum venosum, and the cavum pulmonale to your diagram. Those structures have no mammalian equivalent.
## Clinical Relevance, Limitations and Common Mistakes
### Clinical Relevance of Accurate Labeling
Anatomical knowledge translates directly into diagnostic accuracy. Cardiac computed tomography in dogs can visualize the mitral and tricuspid valves with excellent spatial and temporal resolution, overcoming the acoustic window limitations of echocardiography, and it is used to evaluate congenital malformations such as dysplasia and stenosis as well as acquired conditions like degenerative valve disease and neoplasia [9]. You cannot interpret those images without knowing which valve is which.
Three-dimensional visualization techniques, including volume rendering, virtual reality, and three-dimensional printing, enhance anatomical understanding and procedural planning [9]. The same principle applies to fetal cardiac imaging, where virtual navigation provides realistic views of cardiac anatomy and enables exploration of ventricles, valves, and outflow tracts [10]. All of these tools assume the user already knows the normal anatomy.
### Common Labeling Mistakes
**Reversing right and left.** The most frequent error. Use the apex and ventricular wall thickness to orient yourself before labeling anything.
**Confusing the pulmonary trunk with the aorta.** The pulmonary trunk arises from the right ventricle and crosses ventral to the aorta. The aorta arises from the left ventricle and arches caudally. On a lateral view they overlap, which is why the ventral view is often easier for beginners.
**Forgetting that the pulmonary veins enter as multiple ostia.** Students often draw a single pulmonary vein. In dogs and cats there are typically several separate ostia in the dorsal left atrial wall.
**Omitting the coronary circulation.** The coronary arteries are the first branches off the aorta and are frequently left off student diagrams. They are essential for understanding myocardial infarction and for interpreting coronary imaging.
**Assuming all hearts have three-cusped semilunar valves.** Mammals do. The green iguana does not [1]. Check the species before you assume.
**Treating the conduction system as optional.** The SA node, AV node, bundle of His, and Purkinje network belong on the diagram. They explain why congenital surgery carries a risk of AV block and why conduction system pacing is being adopted more widely [4].
**Confusing atrial appendage with atrium.** The auricle is a blind pouch, not the whole chamber. The left atrial appendage is a specific site of thrombus formation and a target for imaging before pulmonary vein isolation [11].
### Limitations
Individual animals vary. A heart diagram is a generalization, and real anatomy differs with breed, body condition, age, and disease. Congenital anomalies such as mirror-image dextrocardia reverse the normal positions of chambers and great vessels entirely, which poses significant challenges for both imaging and intervention [6]. When you are working with a live patient, confirm your anatomical assumptions with imaging rather than relying on a diagram alone. Individual cases need a veterinarian.
## Frequently Asked Questions
### What are the four chambers of the heart?
The right atrium, left atrium, right ventricle, and left ventricle. The atria receive blood and the ventricles pump it out.
### Which valve is the mitral valve?
The left atrioventricular valve, also called the bicuspid valve because it has two leaflets. It sits between the left atrium and left ventricle.
### How do I tell the aorta from the pulmonary trunk on a diagram?
The aorta arises from the left ventricle and arches caudally. The pulmonary trunk arises from the right ventricle and crosses ventral to the aorta.
### Do birds have a four-chambered heart?
Yes. Birds and mammals both have two atria and two ventricles with a complete interventricular septum. Birds differ in valve structure, not chamber number.
### How many chambers does a reptile heart have?
Three main chambers with a partially divided ventricle. The green iguana has three ventricular subchambers rather than a single undivided cavity [1].
### What is the pacemaker of the heart?
The sinoatrial node. It sits at the junction of the cranial vena cava and the right atrium in mammals.
### What does the coronary circulation do?
It supplies blood to the heart muscle itself. The coronary arteries are the first branches off the aorta and they fill during diastole.
### What is patent ductus arteriosus?
A persistent fetal connection between the pulmonary trunk and the aorta that normally closes after birth. It is one of the most common congenital cardiac defects in dogs.
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## Sources
1. [The Cardiovascular Anatomy of the Green Iguana (Iguana iguana): Heart Structure, Major Vessels, and Coelomic Circulation.](https://pubmed.ncbi.nlm.nih.gov/41472398/)
2. [Comprehensive macroscopic living anatomy of the swine heart: comparative visual approach with virtual dissection.](https://pubmed.ncbi.nlm.nih.gov/42156813/)
3. [Morphologic and Morphometric Study of the Heart and Its Great Arteries in the Common Pheasant (Phasianus colchicus).](https://pubmed.ncbi.nlm.nih.gov/41533221/)
4. [Innovative Management of Rhythm Disorders in Adults with Congenital Heart Disease.](https://pubmed.ncbi.nlm.nih.gov/42403553/)
5. [A rare case of cardiac metastatic uterine intravenous leiomyomatosis: A case report.](https://pubmed.ncbi.nlm.nih.gov/41731856/)
6. [Comprehensive Nursing Management of Anticoagulation and Heart Failure Surveillance in a Chinese Patient With Mirror-Image Dextrocardia Post-MitraClip Surgery: A First Case Report.](https://pubmed.ncbi.nlm.nih.gov/41857799/)
7. [Features of aortic anatomy in patients with proximal thoracic aortic aneurysmal disease.](https://pubmed.ncbi.nlm.nih.gov/42732596/)
8. [Comparative analysis of bypass vs. stent for coronary revascularization using an ex-vivo organ care system in an animal model.](https://pubmed.ncbi.nlm.nih.gov/42120519/)
9. [Cardiac computed tomography of the canine mitral and tricuspid valves.](https://pubmed.ncbi.nlm.nih.gov/42270470/)
10. [Hypoplastic Left Heart Syndrome: Three-Dimensional Virtual Navigation of Fetal Heart Chambers and Great Vessels.](https://pubmed.ncbi.nlm.nih.gov/41427877/)
11. [Comparison of cardiac computed tomography and transesophageal echocardiography for left atrial appendage thrombus detection.](https://pubmed.ncbi.nlm.nih.gov/42216113/)
