Crocodile Heart: Hemodynamic Switching on Land vs. During Diving

Crocodilians possess a distinctive cardiovascular system that combines a fully septated four-chambered heart with the ability to generate regulated extracardiac shunting. The essential anatomic arrangement includes a right aortic arch arising from the left ventricle, a left aortic arch arising from the right ventricle, and a communication between the two aortic roots known as the foramen of Panizza [1,2]. This design allows crocodilians to redistribute ventricular output between the pulmonary and systemic circulations according to physiologic state, particularly during the transition from normal terrestrial respiration to apnea and diving [2,3].

1. Core Anatomical Arrangement

The crocodilian heart is anatomically complete in its ventricular septation, unlike the partially divided ventricles seen in many other reptiles. Nevertheless, its outflow anatomy is unique [1,2].

Key structures

1. Right atrium and left atrium
2. Right ventricle and left ventricle
3. Pulmonary artery arising from the right ventricle
4. Left aortic arch (LAA) arising from the right ventricle
5. Right aortic arch (RAA) arising from the left ventricle
6. Foramen of Panizza connecting the proximal aortic roots [1,2]

This arrangement means that the right ventricle is not confined to pulmonary output alone. Instead, its ejection can be directed preferentially toward either the pulmonary artery or the left aortic arch, depending on downstream resistance and pressure relationships [2,3].

2. The Two Aortas and the Foramen of Panizza

2.1 Right aortic arch

The right aortic arch arises from the left ventricle and functions as the principal systemic outflow tract under normal conditions. It carries oxygen-rich blood at systemic pressure, analogous to the mammalian aorta [1,2].

2.2 Left aortic arch

The left aortic arch arises from the right ventricle, adjacent to the pulmonary outflow tract. This unusual arrangement provides the anatomic substrate for a regulated right-to-left shunt when pulmonary ejection becomes less favorable [1,2].

2.3 Foramen of Panizza

The foramen of Panizza is a small vascular communication between the roots of the two aortas. It is not merely a passive hole; it is a dynamic conduit whose functional role depends on the timing of the cardiac cycle, valve motion, and the relative pressures in the two arches [4,6]. Experimental angioscopy has shown that the foramen is functionally influenced by the aortic valve apparatus: it is obstructed during part of systole by the right aortic valve cusp and remains available for pressure-dependent communication at other phases of the cardiac cycle [6].

3. Hemodynamics on Land

During normal breathing on land, pulmonary vascular resistance is low, so right ventricular ejection is directed predominantly into the pulmonary artery [2,3].

Hemodynamic sequence on land

1. The right ventricle ejects mainly into the pulmonary artery.
2. Pulmonary blood flow increases.
3. Oxygenated pulmonary venous return fills the left atrium and left ventricle.
4. The left ventricle ejects into the right aortic arch.
5. Flow through the foramen of Panizza favors transfer from the right aortic arch toward the left aortic arch [2,4].

Thus, under terrestrial conditions, the circulation is effectively pulmonary-dominant, and the left ventricular outflow supplies the main systemic circulation. Flow measurements in caiman demonstrated that net flow in the left aorta during this state is small relative to the right aorta and is largely explained by foramen-mediated communication rather than major direct right ventricular contribution [4].

This is an important refinement of the older idea of indiscriminate “mixing.” The crocodilian circulation is not simply inefficient admixture of arterial and venous blood; rather, it is a highly regulated flow system in which direction and magnitude of shunting depend on physiologic conditions [1,4,8].

4. Hemodynamics During Diving

During diving, crocodilians enter a period of apnea, and the hemodynamic balance changes substantially. The central physiologic shift is an increase in pulmonary vascular resistance, which reduces right ventricular ejection into the lungs and redirects right ventricular output toward the left aortic arch [2,3,7].

Hemodynamic sequence during diving

1. Apnea raises pulmonary vascular resistance.
2. Right ventricular ejection into the pulmonary artery becomes less favorable.
3. Pulmonary blood flow falls.
4. Pulmonary venous return to the left atrium and left ventricle decreases.
5. The right ventricle ejects more strongly into the left aortic arch.
6. The pressure relationship across the two aortic roots changes, and flow through the foramen of Panizza reverses direction, favoring right ventricular–derived systemic output [2,4,7].

In practical terms, this creates a controlled right-to-left shunt, allowing blood that would otherwise be sent to the lungs to enter the systemic circulation instead [2,3]. This is the physiologic basis for the hemodynamic “switch” illustrated in the teaching material.

5. Mechanistic Basis of the Shunt

The crocodilian right-to-left shunt is generated by the interaction of anatomy and physiology rather than by a simple fixed connection.

5.1 Pressure-dependent flow

The direction of flow across the foramen of Panizza depends on the relative pressures in the two aortic roots [4,6]. On land, the left ventricle and right aortic arch dominate. During diving, increased pulmonary outflow resistance and altered right ventricular afterload favor delivery of right ventricular blood into the left aortic arch [2,7].

5.2 Outflow tract regulation

Experimental work in isolated perfused crocodile hearts demonstrated that changes in pulmonary outflow pressure and right ventricular stroke behavior can initiate or augment right-to-left shunting [5]. These studies support the concept that shunting is not random but rather emerges from regulated outflow resistance and ventricular performance.

5.3 Valve-related control

High-resolution angioscopy provided a particularly important mechanistic insight: the foramen of Panizza is functionally modulated by valve motion, especially during systole [6]. This helps explain how crocodilians maintain a controlled and reversible shunt rather than continuous unrestricted communication.

6. Functional Significance

The crocodilian shunt is a physiologic adaptation, not a pathologic lesion. During prolonged submergence, sending a large volume of blood to unventilated lungs is inefficient. By increasing pulmonary vascular resistance and redirecting right ventricular output into the systemic circulation, crocodilians can [2,3,7]:

1. Reduce unnecessary pulmonary perfusion during apnea
2. Maintain systemic perfusion during diving
3. Redistribute ventricular output according to metabolic state
4. Limit wasted flow to non-ventilated lungs
5. Sustain prolonged submergence behavior

This differs fundamentally from congenital right-to-left shunting in human cardiovascular disease, where cyanosis usually reflects structural pathology. In crocodilians, the shunt is a normal, reversible, and regulated feature of their physiology [2,3].

7. Evidence Base Supporting the Hemodynamic Switch

The concept of crocodilian hemodynamic switching is supported by several complementary lines of evidence.

7.1 Foundational anatomical and physiologic studies

Early work by White established the basic anatomic arrangement of the crocodilian heart and identified the importance of the left aortic arch and foramen of Panizza in shaping blood flow patterns [1]. White later framed these findings in functional terms, emphasizing how pulmonary vascular resistance can shift the balance between pulmonary and systemic output, especially during diving [2].

7.2 Flow recordings

Direct flow measurements demonstrated a complex biphasic pattern in the left aortic arch, including mid-systolic reversal of flow, consistent with foramen-mediated shunting and time-varying interactions between the aortic roots [4].

7.3 Isolated perfused heart experiments

Perfused-heart studies showed that right-to-left shunting can be reproduced experimentally by altering loading conditions, especially pulmonary outflow pressure, confirming that the shunt is a controlled hemodynamic phenomenon rather than an incidental anatomic curiosity [5].

7.4 Angioscopy

Angioscopic studies directly visualized the behavior of the foramen of Panizza and its relation to the right aortic valve, strongly supporting its role as an active regulator of shunt direction and timing [6].

7.5 Later syntheses and functional experiments

Subsequent reviews emphasized that the crocodilian heart is more precisely regulated than previously assumed and that older descriptions of simple blood mixing are incomplete [7,8]. Experimental surgical elimination of the right-to-left shunt in American alligators altered ventricular morphology but did not abolish diving apnea or metabolism, suggesting that the shunt has important physiologic roles but is not the sole determinant of diving performance [9].

8. Comparison With Human Cardiovascular Physiology

For surgeons and cardiologists, the crocodilian circulation offers a valuable comparative model. It demonstrates that blood flow follows pressure gradients and resistance fields, not anatomy alone. In humans, complete separation of pulmonary and systemic circulations is necessary for efficient postnatal oxygen delivery. In crocodilians, by contrast, a fully divided heart coexists with a strategically retained arterial communication that permits reversible shunting [2,3,8].

This makes the crocodile heart a useful teaching model for understanding:

• Pressure-dependent shunting
• Parallel versus series circulatory behavior
• The relationship between outflow resistance and ventricular ejection
• The difference between physiologic and pathologic right-to-left flow

9. Key Teaching Points

• Crocodilians have a four-chambered heart with two systemic aortas [1,2].
• The right aortic arch arises from the left ventricle, whereas the left aortic arch arises from the right ventricle [1,2].
• The foramen of Panizza connects the aortic roots and enables pressure-dependent communication between them [1,4,6].
• On land, low pulmonary vascular resistance directs right ventricular output mainly to the lungs, and flow across the foramen favors RAA-to-LAA transfer [2,4].
• During diving, pulmonary vascular resistance rises, right ventricular output shifts toward the left aortic arch, and the shunt becomes systemic right-to-left [2,3,7].
• The diving shunt is a regulated physiologic adaptation, not an anatomic defect [2,3].
• The crocodilian heart illustrates a fundamental physiologic principle: circulatory behavior is determined by anatomy, valve dynamics, pressure gradients, and downstream resistance acting together [4-8].

References

[1] White FN. Circulation in the reptilian heart (Caiman sclerops). Anat Rec. 1956;125(3):417-431.

[2] White FN. Functional anatomy of the heart of reptiles. Am Zool. 1968;8(2):211-219.

[3] Hicks JW, Wang T. Functional role of cardiac shunts in reptiles. J Exp Zool. 1996;275(2-3):204-216.

[4] Axelsson M, Holm S, Nilsson S. Flow dynamics of the crocodilian heart. Am J Physiol. 1989;256(4 Pt 2):R875-R879.

[5] Franklin CE, Axelsson M. The intrinsic properties of an in situ perfused crocodile heart. J Exp Biol. 1994;186(1):269-288.

[6] Axelsson M, Franklin CE, Löfman CO, Nilsson S, Grigg GC. Dynamic anatomical study of cardiac shunting in crocodiles using high-resolution angioscopy. J Exp Biol. 1996;199(2):359-365.

[7] Nilsson S. The crocodilian heart and central hemodynamics. Cardioscience. 1994;5(3):163-166.

[8] Axelsson M. The crocodilian heart; more controlled than we thought? Exp Physiol. 2001;86(6):785-789.

[9] Eme J, Gwalthney J, Blank JM, Owerkowicz T, Barron G, Hicks JW. Surgical removal of right-to-left cardiac shunt in the American alligator (Alligator mississippiensis) causes ventricular enlargement but does not alter apnoea or metabolism during diving. J Exp Biol. 2009;212(21):3553-3563.