Venous System of Pigeon – Detailed Guide with Diagrams & Portal System Notes
Detailed Anatomy: The Venous System of the Pigeon
The venous system brings the deoxygenated blood back to the heart from the body for oxygenation and oxygenated blood is transported via arterial system of pigeon back again to the body, completing the circulation in pigeon (birds).
The Venous system consist of caval veins pulmonary veins, hepatic portal system and renal portal system.
Figure: 1
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| A detailed anatomical study of the avian venous system, illustrating how deoxygenated blood returns to the heart via the precaval and postcaval veins, featuring the specialized renal portal system unique to birds. |
(Please refer to the Biology Diagram provided to get a greater understanding of the structure.)
Anatomy of Avian Circulatory system consisting Venous system with hepatic portal system, is similar in structure and function to that of mammals.
I. Caval veins
The Venous blood from the body returns to the heart's right auricle via three large caval veins.
1. Precavals:
Two Precaval (or right and left anterior venae cavae) collects blood from anterior part of the body.
The anterior venae cavae is formed by the union of large common jugular vein and subclavian vein.
The common jugular veins of two sides brings blood from the head region. The right jugular vein is generally larger and they are connected beneath the base of skull under the "chin" by a transverse loop called the jugular anastomosis.
Function:
This loop prevents blocking of momentary blood flow in a vein when the head is twisted.
(Study tip for students)
Did you know? The right jugular vein in a pigeon is typically much larger than the left. This asymmetry is a common adaptation in birds to accommodate their highly mobile necks and specialised respiratory systems.)
A slender brachial vein from wings joins the axillary vessel to form subclavian vein.
In each side the jugular and subclavian vein joins stout pectoral vein from pectoral muscles ( major and minor flight muscles) to form right and left anterior venae cavae (precaval)
Also a small internal mammary vein from ventral thoracic wall joins the pectoral vein.
The precaval empties into right atrium of the heart of the bird.
The left precaval is larger than the right.
2. Postcaval :
Postcaval or posterior venae cavae, collects blood from posterior part of the body via number of different routes to the heart.
Collecting blood from the tail by unpaired caudal vein . It divides anteriorly into right and left renal portal or hypogastric veins. It pass towards the kidneys, recieving the internal iliac vein from the pelvis (legs)and becomes renal portal vessel.
Veins possess valves that prevent blood from flowing backwards. Contraction of general body muscles is also important in keeping the venous flowing.
Renal Portal Valve
Birds have a unique smooth muscle valve at the junction of the renal portal vein and the common iliac vein.
- When the valve is closed, blood is forced through the kidney.
- When the valve is open, blood bypasses the kidney and goes straight to the heart.
( This is also called the inferior mesenteric or caudal mesenteric).
The coccygeo-mesenteric vein then passes forward in the rectal mesentery and receives branches from intestine, rectum, and the cloaca. Anteriorly it connects with the portal system of the liver (hepatic portal vein).
As the renal portal vein passes onward over the kidney, it sends few small branches of afferent renal veins into it.
Delicate sciatic vein joins in between the posterior and middle lobes of kidney and then joins a prominent femoral vein in between middle and anterior lobes.
These veins, sciatic and femoral, brings blood from leg region.
The femoral vein further receives small efferent renal veins from the kidneys and leaves it as the common iliac vein.
The right and left common iliac veins unite anterior to the kidneys to form postcaval vein, which runs forward through the right lobe of liver to open into the right auricle.
The three hepatic vein leave the liver. The larger right vessel receives mainly blood from the mesenteric, pancreatic and coccygeo-mesenteric veins.
The left hepatic vein recieves supply from gastric vein in the fat-laden folds of the peritoneum or great omentum of stomach and pancreatic region and enter hepatic portal system. The two hepatic veins join the posterior vena cava.
II
Pulmonary veins
Two pulmonary veins carry oxygenated blood from each lung and join the corresponding vein from the other side to open by a common opening into the left auricle.
III
Hepatic portal system:
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| Fig: Detailed Portal Systems of the Pigeon. This diagram illustrates the dual portal system: the Hepatic Portal System (liver) and the Renal Portal System (kidneys), which filters blood from the posterior extremities before it returns to the heart via the caudal vena cava. |
| The blood from various organs of digestive system reaches the liver by a hepatic portal vein which is formed by the union of gastro-duodenal, anterior mesenteric, posterior mesenteric and the coccygeomesenteric veins. |
After collecting blood from the rectum, ileum, duodenum and gizzard, it divides into two branches, one entering each liver lobe, where they form capillary networks.
The portal veins connects two capillary beds rather than connecting a capillary Bed and heart ( as most of the veins do).
The portal veins permit another organ ( here, like the liver) to process blood and check blood chemistry before returning the blood to the general circulation via heart and lungs.
In the hepatic portal system, nutrient rich blood from the capillary network supplying the microscopic folds of the lining of the small intestine undergo further chemical processing.
The blood filters through liver and via hepatic veins, conveyed to post caval.
IV
Renal portal system:
In birds the renal portal system is greatly reduced such that the portal veins do not break into capillaries in kidney, instead gives off only few small branches (afferent renal veins).
This system collects venous blood from the lower part of the digestive tract via caudal mesenteric vein, and conveys to the venous ring within the kidney. Some of the blood pass through the capillary bed in the kidney before being recollected by renal efferent veins and conveyed to the heart via the caudal vena cava.
In the kidney, both venous and arterial and venous blood mixed.
This circulatory pattern is very peculiar to birds and the reason is still unknown.
Thus, blood from the posterior region of body directly reaches the heart and not through renal capillaries ( as in frogs and lizards).
Physiology:
The exact function of the renal portal system is still unclear and debated, but thought to,
- allow the kidneys to maintain high tubular secretion rates.
- also considered to help in water conservation (osmoregulation) during flight and metabolic stress.
Apart from all these, in long-legged wadding birds like cranes, herons, flamingos, there is found an arteriovenous network of vessels referred to as rete.
These structures are interpreted as vascular heat exchangers serving to prevent body's heat being dissipated through the thin membrane of the foot into the surrounding cold water.
The theory that works is supposed to be that due to the proximity of the arterial and venous blood, the heat is allowed to be transferred from the former to the later.
This way only a less amount of heat is radiated via the foot to the surrounding and the rest is returned back to the body, maintaining the body temperature.
( In penguins, there is presence of arterial rete in the region of the axilla for heat exchange)
Comparison: Avian vs.Mammalian Venous Systems
1. Renal Portal System:
Pigeon (Avian):
Present. Blood from the lower portion (tail/legs) can flow through the kidneys before the heart.
Human (Mammals):
Absent. Blood from the lower limbs bypasses the kidneys and go directly to the heart.
2. Jugular veins:
Pigeon (Avian):
The right and left jugular veins are connected by transverse anastomosis (cross-bridge) for neck rotation without effecting blood flow.
Human (Mammals):
Seperate vessels; no cross-bridge connections in the neck.
3. Venae Cavae:
Pigeon (Avian):
Two precaval veins and one postcaval vein.
Human (Mammals):
One Superior Vena Cava and one Inferior Vena Cava.
4. Symmetry:
Pigeon (Avian):
Asymmetrical; the right jugular is usually significantly larger than the left.
Human (Mammals):
Generally symmetrical; both internal jugulars are of similar size.
5.Caudal vein:
Pigeon (Avian):
Well developed to drain the tail feathers/uropium.
Human (Mammals):
Reduced/Vestigial (represented by the median sacral vein).
Vocabulary Glossary
Anastomosis: A natural connection or bridge between two blood vessels (e.g., the transverse anastomosis between the jugulars).
Afferent Vein: A vein that carries blood toward an organ for filtration (like the Renal Portal Veins).
Efferent Vein: A vein that carries filtered blood away from an organ (like the Efferent Renal Veins).
Portal System: A specialized system where blood passes through a second set of capillaries (in the liver or kidneys) before returning to the heart.
Vena Cava: The large veins that return deoxygenated blood directly into the right auricle of the heart.
Precaval Veins: Also known as the Anterior Venae Cavae; they drain the head, neck, and wings.
Postcaval Vein: Also known as the Posterior Vena Cava; the largest vein that drains the lower body and liver.
Self-Test: Avian Venous Systems
About the Author: This educational content on Zoology is written by Rekha Debnath, M.Sc. & M.Phil. in Zoology, with a focus on university-level academic topics. Read the full Author Credentials and Background here.
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