What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Venae Pulmonales: Pulmonary Vein Anatomy

Name: Venae Pulmonales: Pulmonary Vein Anatomy
Creator: Dr. Zubair Khalid

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-20

The venae pulmonales (pulmonary veins) are the great vessels that return oxygenated blood from the pulmonary capillary bed to the left atrium. They are the only veins in the body that carry oxygen-rich blood, which makes them functionally the mirror image of the pulmonary arteries rather than of the systemic veins.

Pulmonary vein anatomy matters because it sits at the intersection of three practical problems. First, the number and arrangement of the veins varies between species and between individuals, which affects how a clinician reads an echocardiogram or a CT scan. Second, the veins are the target of pulmonary vein isolation, the cornerstone procedure for atrial fibrillation in human cardiology, and the same anatomical substrate exists in animals. Third, congenital and acquired diseases of the pulmonary veins, including anomalous connections and stenosis, are recognized in dogs and cats and can be missed on routine imaging.

What the Venae Pulmonales Are

A pulmonary vein is a large-caliber vessel with a thin wall and a wide lumen. It begins as small venules in the interalveolar septa, where it collects blood that has just been oxygenated across the alveolar-capillary membrane. Those venules merge into progressively larger tributaries that follow the bronchi and the branches of the pulmonary artery. The largest tributaries converge near the hilus of the lung and drain into the left atrium through openings called ostia (singular: ostium).

The term venae pulmonales is the Latin plural. In English usage, "pulmonary veins" is standard, and the two forms are interchangeable in anatomy texts.

Three structural features distinguish these vessels from systemic veins.

They carry oxygenated blood. Systemic veins carry deoxygenated blood to the right atrium. The pulmonary veins carry oxygenated blood to the left atrium, completing the pulmonary circuit.
They have no valves. Systemic veins of the limbs contain valves that prevent backflow. Pulmonary veins lack valves, and the left atrial pressure and the left ventricular suction during diastole drive forward flow.
They have myocardial sleeves. The wall of each pulmonary vein near its ostium contains a cuff of atrial myocardium that extends variable distances along the vein. These sleeves are electrically active and are the anatomical basis for arrhythmogenesis.

Why the Myocardial Sleeves Matter

The myocardial sleeve is not decorative tissue. It is a contractile and electrically excitable extension of the left atrial myocardium onto the vein wall. In the horse, researchers dissected 23 Warmblood hearts and found that the sleeve of the vein draining through the third ostium showed full antral coverage with extensions onto its branches, while sleeves on the other veins were shorter and more variable [1]. That study also identified myocardial continuity between the sleeve of that vein and the sleeve of the caudal vena cava in a subset of hearts, which suggests an anatomical pathway that could allow electrical activity to cross between structures [1].

In Thoroughbred horses with paroxysmal atrial fibrillation, RNA sequencing of tissue taken from the left superior pulmonary vein near the left atrium showed upregulation of SCN5A, a gene encoding a cardiac sodium channel, and MYH7, a gene encoding a myosin heavy chain [2]. The same study reported enrichment of gene sets related to muscle and endothelial cell development, cell shape, and extracellular matrix components in affected horses [2]. These findings support the idea that the pulmonary vein sleeve is not a passive conduit but a site of active remodeling in animals with atrial arrhythmias.

Comparative Anatomy Across Species

The number and arrangement of pulmonary vein ostia differ across species. This is not a trivial textbook detail. It determines how many openings a surgeon, imager, or electrophysiologist must account for.

Humans

The classic description is four pulmonary veins: two right and two left, each pair consisting of a superior and an inferior vein. Accessory veins and conjoined veins are common variants. A left common pulmonary vein, in which the two left veins share a single ostium, is a well-recognized pattern in people with atrial fibrillation [3]. Variant anatomy was observed in 32% of patients in one cryoballoon ablation series and 40% in a laser ablation series [4].

Dogs

Dogs typically have multiple separate ostia rather than a fixed count of four. The clinical literature on canine pulmonary vein anatomy is dominated by anomalous connections rather than by normal variation. In a series of 10 dogs with sinus venosus atrial septal defect, CT angiography confirmed partial anomalous pulmonary venous connection in every case, with drainage into the right atrium, the junction with the cranial vena cava, or the cranial vena cava itself [5]. That study also showed that transthoracic echocardiography missed the septal defect in 3 of 10 dogs, which is a reminder that the pulmonary veins are best assessed with cross-sectional imaging when an anomalous connection is suspected [5].

Cats

Cats have been studied with electrocardiography-gated multidetector CT, which allows the ostia to be measured at defined points in the cardiac cycle. In a study of six cats, the pulmonary veins drained into the left atrium through three ostia: a right cranial ostium, a left cranial ostium, and a caudodorsal ostium [6]. The same study found that ostial diameter varied with the cardiac cycle, with the maximal diameter at end-systole and the minimal diameter at end-diastole [6]. That cyclic variation is a practical point for anyone measuring ostia on CT. A single measurement taken at an arbitrary phase of the cycle will not match a measurement taken at a different phase.

Horses

The horse has four principal ostia in most hearts. In the dissection study of 23 Warmblood hearts, four principal ostia were present in 20 hearts and five in 3 hearts, with variable accessory vein openings [1]. Ostia II and III were the largest, and ostium I was the smallest [1]. The myocardial sleeve of the vein draining through ostium III showed full antral coverage, which is a regional difference within the same heart [1].

Ruminants

Ruminant pulmonary vein anatomy is variable in number and arrangement. The general pattern follows the lobar organization of the ruminant lung, with veins draining the cranial, middle, caudal, and accessory lobes and converging on the left atrium. The number of separate ostia is not fixed, and the arrangement differs between cattle, sheep, and goats.

Birds

Birds have a distinct pulmonary venous anatomy that reflects their unique respiratory system. The avian lung is rigid and receives air through a system of air sacs and parabronchi rather than through tidal alveolar ventilation. The pulmonary veins of birds collect blood from the exchange surface and return it to the left atrium, but the number and course of the vessels differ from the mammalian pattern. The avian heart also differs in the arrangement of its great vessels, so a mammalian template should not be applied directly.

Summary Table

Species	Typical number of pulmonary veins	Drainage pattern	Notable variant
Human	Four (two right, two left)	Superior and inferior veins on each side drain separately into the left atrium	Left common pulmonary vein, accessory veins
Dog	Multiple separate ostia	Separate openings into the left atrium	Partial anomalous pulmonary venous connection with sinus venosus atrial septal defect
Cat	Three ostia	Right cranial, left cranial, and caudodorsal ostia drain into the left atrium	Ostial diameter varies with the cardiac cycle
Horse	Four principal ostia	Ostia I through IV drain into the left atrium	A fifth ostium and accessory openings occur, and the ostium III sleeve shows full antral coverage
Ruminant	Variable	Lobar veins converge on the left atrium	Number and arrangement differ between species
Bird	Distinct from mammals	Pulmonary veins return blood to the left atrium through a species-specific pattern	Avian lung architecture differs fundamentally from the mammalian lung

How Pulmonary Venous Blood Returns to the Heart

The return path can be traced in five steps.

Oxygenated blood enters the pulmonary venules in the interalveolar septa.
Venules merge into intrapulmonary veins that run alongside the bronchi and pulmonary artery branches.
The intrapulmonary veins converge at the hilus into the largest extrapulmonary veins.
Each large vein opens into the left atrium through an ostium.
Left atrial contraction and left ventricular diastole move blood through the mitral valve into the left ventricle.

The ostium is the boundary between vein and atrium. Its anatomy is not a simple circle. In people, the anatomical ostium defined on MRI or CT geometry and the electrical ostium defined by intracardiac signals differ. A study of 20 patients with paroxysmal atrial fibrillation found that the electrically defined ostium was 8.4 ± 4.7 mm more distal in the vein than the anatomically defined ostium, and the distance varied between the four veins [7]. That distinction matters because it shows that the visible junction on imaging is not the same as the functional boundary used for clinical decision-making.

The Myocardial Sleeve in Detail

The sleeve is a cuff of atrial myocardium that wraps the vein wall near the ostium. Its length and thickness vary by species, by individual, and by vein. The equine study measured sleeve length, coverage, distal edge morphology (sharp or blurred), and myocardial thickness at standardized locations [1]. The finding that the ostium III sleeve had full antral coverage with branch extensions, while other sleeves were shorter, shows that variation within a single heart can be as important as variation between species [1].

The sleeve is the substrate for triggered activity that can initiate atrial fibrillation. In horses with paroxysmal atrial fibrillation, the transcriptomic changes in the sleeve included upregulation of genes involved in transport and extracellular matrix components, and cell deconvolution showed a trend toward increased fibroblast numbers [2]. Fibrosis in the sleeve can slow conduction and create the conditions for reentry.

How Pulmonary Vein Anatomy Is Examined

Several imaging methods are used to define pulmonary vein anatomy in animals and in research settings.

Echocardiography

Transthoracic echocardiography is the first-line tool for cardiac assessment in dogs and cats. It can show the left atrium, the pulmonary veins near the ostia, and flow velocity in the veins. In a cat with pulmonary vein stenosis caused by a lymphoma compressing the left atrium, echocardiography showed a peak pulmonary venous flow velocity of 1.5 m/sec, and the velocity fell to 0.9 m/sec after treatment reduced the mass [8]. In another cat with a mediastinal mass, echocardiography identified a turbulent jet-like flow from the pulmonary veins at 1.6 m/sec, and CT showed dilation of the distal caudal pulmonary veins with stenosis of the caudodorsal ostium [9]. These cases show that Doppler velocity is a practical marker of pulmonary venous obstruction.

Echocardiography has limits. In the canine sinus venosus atrial septal defect series, transthoracic echocardiography suspected the defect in seven dogs but missed it in three [5]. When an anomalous pulmonary venous connection is suspected, CT angiography is the more reliable test.

Computed Tomography

CT angiography provides high diagnostic accuracy for anomalous pulmonary venous connections in dogs [5]. In cats, electrocardiography-gated multidetector CT allows the ostia to be measured at defined phases of the cardiac cycle and has shown that ostial diameter is largest at end-systole and smallest at end-diastole [6]. The same study found no significant correlation between heart or left atrial size and the maximal or minimal diameter of the ostia [6], which means that ostial size cannot be predicted from chamber size alone.

Magnetic Resonance Imaging

In human cardiology, magnetic resonance angiography is used to map pulmonary venous anatomy before ablation. Steady-state MRA with a blood pool contrast agent compared favorably with time-resolved MRA for quantitative assessment of pulmonary venous anatomy, with strong interobserver correlation for diameter and area measurements [10]. Accelerated free-breathing ECG-triggered contrast-enhanced pulmonary vein MRA with compressed sensing improved quantitative vein sharpness compared with conventional first-pass MRA [11]. These techniques are not yet standard in veterinary practice, but they illustrate the imaging principles that apply across species.

Gross Dissection

The equine study used post-mortem dissection with calibrated photographs to measure ostial and branch diameters, antral length, sleeve length and coverage, distal edge morphology, and myocardial thickness [1]. Histology with Masson's trichrome staining was used to examine myocardial continuity between the ostium III sleeve and the caudal vena cava sleeve [1]. Gross dissection remains the reference standard for defining normal anatomy and for validating imaging findings.

Clinical Relevance, Limitations and Common Mistakes

This article is educational and is not a substitute for veterinary diagnosis or treatment.

Pulmonary Vein Isolation and Atrial Fibrillation

Pulmonary vein isolation is the mainstay of catheter ablation for paroxysmal atrial fibrillation in people [12]. The procedure targets the myocardial sleeves at the vein ostia, because those sleeves contain the triggers that initiate the arrhythmia. The same anatomical substrate exists in animals. The equine dissection study was designed to provide equine-specific data on sleeve distribution and wall thickness because such data are needed to develop ablation strategies for horses [1]. The Thoroughbred transcriptomic study showed that the sleeves in horses with paroxysmal atrial fibrillation differ from those in healthy horses at the level of gene expression [2].

Anatomical variants affect the procedure in people. A left common pulmonary vein has been associated with less recurrence after repeat ablation [3]. Vein orientation has also been linked to outcomes, with left lower pulmonary vein orientation associated with recurrence in one study [13]. These findings are from human cardiology, but they establish the general principle that pulmonary vein anatomy is not a fixed template and that variant anatomy can change procedural results.

Anomalous Pulmonary Venous Connections

A partial anomalous pulmonary venous connection is a congenital defect in which one or more pulmonary veins drain into the right atrium or a systemic vein instead of the left atrium. In dogs, this defect is frequently associated with a sinus venosus atrial septal defect, a defect in the interatrial septum near the cranial vena cava [5]. The combination produces a left-to-right shunt. In people, cardiovascular magnetic resonance studies have shown that isolated atrial septal defect, isolated partial anomalous pulmonary venous connection, and the two combined produce different degrees of right ventricular enlargement and different shunt fractions [14]. The canine series reported right atrial and ventricular dilation in all 10 dogs, and nine underwent surgical repair [5].

Pulmonary Vein Stenosis

Pulmonary vein stenosis is rare in cats but has been reported in several forms. One cat developed stenosis because a pulmonary lymphoma compressed the left atrium, and the stenosis improved when the mass shrank with treatment [8]. Another cat had unilateral pulmonary vein stenosis diagnosed during life, in association with cor triatriatum sinister and pulmonary hypertension [15]. A third cat had acquired pulmonary vein stenosis secondary to a mediastinal mass, with stenosis of the caudodorsal ostium and dilation of the distal caudal pulmonary veins [9]. These cases share a pattern: an external or intrinsic process narrows the vein, Doppler velocity rises, and the distal vein dilates.

Common Mistakes

The first mistake is assuming that the pulmonary veins are structurally similar to systemic veins. They lack valves, they carry oxygenated blood, and they have myocardial sleeves. Each of these differences has functional consequences.

The second mistake is assuming that four veins is the normal count in every species. Dogs and cats have multiple separate ostia, and the cat pattern described on CT is three ostia [6]. Horses usually have four principal ostia but can have five, and accessory openings occur [1].

The third mistake is measuring an ostium once and treating the value as fixed. In cats, ostial diameter changes with the cardiac cycle, with the maximum at end-systole and the minimum at end-diastole [6]. A measurement taken at one phase cannot be compared directly with a measurement taken at another.

The fourth mistake is relying on echocardiography alone when an anomalous connection is suspected. In the canine series, echocardiography missed the septal defect in 3 of 10 dogs, and CT angiography was needed to confirm the diagnosis [5].

The fifth mistake is confusing the anatomical ostium with the electrical ostium. In people, the electrically defined ostium is on average 8.4 mm more distal in the vein than the anatomically defined ostium, and the distance varies between veins [7].

Quick Review

The venae pulmonales carry oxygenated blood from the lungs to the left atrium and are the only veins in the body to do so.
They have no valves and possess myocardial sleeves that extend from the left atrium onto the vein wall.
Humans typically have four pulmonary veins, but accessory and conjoined veins are common variants.
Dogs and cats have multiple separate ostia. The cat pattern described on CT is three ostia: right cranial, left cranial, and caudodorsal.
Horses usually have four principal ostia, sometimes five, and the ostium III sleeve shows full antral coverage.
Pulmonary vein ostial diameter varies with the cardiac cycle, with the maximum at end-systole and the minimum at end-diastole.
Pulmonary vein isolation targets the myocardial sleeves and is the mainstay of atrial fibrillation ablation in human cardiology.

Frequently Asked Questions

What are the venae pulmonales?

The venae pulmonales are the pulmonary veins, the vessels that return oxygenated blood from the lungs to the left atrium. They are the only veins in the body that carry oxygen-rich blood.

How many pulmonary veins does a dog have?

Dogs typically have multiple separate pulmonary vein ostia rather than a fixed count of four. The exact number varies between individuals, and anomalous connections are recognized in dogs with sinus venosus atrial septal defect.

How many pulmonary veins does a cat have?

In a CT study of six cats, the pulmonary veins drained into the left atrium through three ostia: a right cranial ostium, a left cranial ostium, and a caudodorsal ostium [6].

Why do pulmonary veins carry oxygenated blood?

Because they drain the pulmonary capillary bed, where gas exchange has already added oxygen to the blood. Systemic veins drain tissues that have consumed oxygen, so they carry deoxygenated blood.

What is a pulmonary vein myocardial sleeve?

It is a cuff of atrial myocardium that extends from the left atrium onto the wall of the pulmonary vein near its ostium. The sleeve is electrically active and is the target of pulmonary vein isolation for atrial fibrillation.

What is pulmonary vein stenosis?

Pulmonary vein stenosis is a narrowing of one or more pulmonary veins. In cats it has been reported from compression by a lymphoma, from a mediastinal mass, and in association with cor triatriatum sinister [8][15][9].