SECTION 5.3
Hepatic portal and fetal circulation
Trace the hepatic portal circuit and the fetal shunts, and explain the function each one serves.
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Most of the circulatory system follows one simple pattern: heart, artery, capillary bed, vein, heart. Two special arrangements modify that pattern in useful ways. The hepatic portal system places two capillary beds in series, so blood from the digestive organs passes through the liver before returning to the heart; this lets the liver process absorbed nutrients and remove harmful substances before they reach the rest of the body. Fetal circulation solves a different problem: before birth the lungs and digestive tract are not functional, so oxygen and nutrients come from the placenta. Blood is therefore routed through three shunts that bypass the lungs and the liver. Both arrangements are examples of the same principle you have already met — structure is adapted to function — and both change after birth or in disease.
Key Concepts & Topics
Portal systems: two capillary beds in series
In a standard circuit, blood passes through one capillary bed between an artery and a vein. A portal system is different: blood passes through two capillary beds in series before returning to the heart. The first bed collects substances from one region, and the second bed processes them in an organ before the blood continues. This design is useful when an organ must inspect or modify blood before it reaches the general circulation. The hepatic portal system is the main example in the body, but a smaller portal arrangement also exists in the kidney. Portal systems show that the vascular tree is not a fixed pattern but a set of solutions to delivery problems.
For example: A drug swallowed and absorbed in the intestine passes through the liver before reaching the rest of the body, which is why some drugs are partly inactivated on their first pass.
The hepatic portal circuit
Blood leaving the stomach, intestines, spleen and pancreas is collected by veins that join to form the hepatic portal vein. This vessel enters the liver and divides into sinusoids, the wide, leaky capillaries described in section 5.1. As blood flows slowly through the sinusoids, liver cells absorb glucose and amino acids for storage or processing, remove toxins, drugs and bacteria, and release plasma proteins. The processed blood then leaves the liver through the hepatic veins, which empty into the inferior vena cava and return it to the heart. The liver therefore receives blood from two sources: oxygenated blood through the hepatic artery, and nutrient-rich, deoxygenated blood through the portal vein. This double supply reflects its dual role as a metabolic processor and an oxygen-dependent organ.
For example: In cirrhosis, scar tissue obstructs the sinusoids, portal pressure rises and blood finds alternative routes, which can enlarge veins in the oesophagus.
The fetal problem and the placenta
Before birth, the fetus cannot use its own lungs or digestive tract. Its lungs are fluid-filled and not yet exchanging gas, and its gut receives no food. Oxygen and nutrients therefore come from the mother through the placenta, a temporary organ where the fetal and maternal circulations lie close together and substances cross without the two bloods mixing. Fetal blood travels to the placenta in the umbilical arteries and returns in the umbilical vein. Because the placenta is the source of oxygenated blood, the fetal circulation is arranged to send that blood preferentially to the brain, heart and liver, while sending less to the lungs, which do not yet need it. This preferential routing is achieved by three shunts and by the position of the umbilical vessels.
For example: Fetal haemoglobin binds oxygen more strongly than adult haemoglobin, which helps the fetus extract oxygen from the relatively low-oxygen blood arriving from the placenta.
The three fetal shunts
Three shortcuts route fetal blood around organs that are not yet working. The ductus venosus connects the umbilical vein to the inferior vena cava, allowing most blood to bypass the liver sinusoids and reach the heart quickly. The foramen ovale is an opening between the right and left atria; because pressure in the right atrium is higher before birth, blood passes from right to left, bypassing the pulmonary circuit. The ductus arteriosus connects the pulmonary trunk to the aorta, so any blood that does enter the right ventricle is diverted away from the lungs into the systemic circulation. Together these shunts ensure that oxygenated blood from the placenta reaches the brain and heart with minimal dilution, and that the non-functional lungs are not overloaded.
For example: A baby born with a persistent ductus arteriosus has blood continuing to flow from the aorta into the pulmonary artery, which increases the work of the heart and lungs.
What changes at birth
At birth, two events begin at once: the lungs fill with air and the umbilical cord is clamped. Breathing expands the lungs and lowers resistance in the pulmonary vessels, so more blood flows to the lungs and pressure in the right atrium falls. At the same time, clamping the cord removes the placental circulation, which reduces flow through the ductus venosus and raises pressure in the left atrium. The foramen ovale therefore closes because left atrial pressure now exceeds right atrial pressure. The ductus arteriosus closes in response to higher oxygen levels and falling prostaglandin levels. Over the following days and weeks these openings seal and become ligaments or shallow depressions, leaving the adult pattern of two separate circuits. Failure of a shunt to close produces a congenital heart defect.
For example: If the ductus arteriosus fails to close, a drug such as indomethacin may be used to encourage closure; if it must stay open, prostaglandins are given instead.
Key facts to remember
- A portal system passes blood through two capillary beds in series before it returns to the heart.
- The hepatic portal vein carries nutrient-rich blood from the digestive organs and spleen to the liver sinusoids for processing.
- The placenta supplies fetal oxygen and nutrients; the umbilical vein carries oxygenated blood to the fetus and the umbilical arteries carry deoxygenated blood to the placenta.
- Three fetal shunts bypass non-functional organs: ductus venosus (liver), foramen ovale and ductus arteriosus (lungs).
- At birth, lung expansion and cord clamping reverse atrial pressures and raise oxygen, closing the shunts.
Example · Trace fetal blood from the placenta. Oxygenated blood returns in the umbilical vein. Most of it passes through the ductus venosus, bypassing the liver, and enters the inferior vena cava. In the right atrium, the higher pressure pushes blood through the foramen ovale into the left atrium, so it reaches the left ventricle and the aorta without passing through the lungs. That blood supplies the brain and heart first. Blood returning from the upper body enters the right ventricle and is diverted through the ductus arteriosus into the aorta, bypassing the lungs. Deoxygenated blood returns to the placenta through the umbilical arteries. At birth, breathing and cord clamping close these routes.
Read full notes
Read one topic at a time. These notes explain the mechanisms and relationships behind the key facts.
Portal systems: two capillary beds in series
A portal system is defined by having two capillary beds in series, with the second bed processing blood that has already passed through the first. The hepatic portal system is the body’s main example: the first capillary bed lies in the walls of the digestive organs, where nutrients and toxins are absorbed, and the second lies in the liver, where they are processed. This arrangement is efficient because it inspects everything absorbed before it reaches the general circulation. It also explains why the liver receives both a nutrient-rich supply and an arterial supply: it needs the nutrients for its work and oxygen to carry that work out. When liver tissue is damaged, the resistance of the second bed rises, pressure backs up into the portal vein and alternative routes develop.
The hepatic portal circuit
The hepatic portal vein is formed by the union of veins draining the stomach, small and large intestine, spleen and pancreas. It carries blood that is rich in the products of digestion but low in oxygen, because the blood has already passed through the capillary beds of those organs. In the liver the portal vein divides into sinusoids, wide capillaries with gaps that allow liver cells direct access to the blood. Here the liver performs several tasks at once: storing glucose as glycogen, converting amino acids, storing vitamins and iron, breaking down drugs and toxins, and removing bacteria that have entered from the gut. The processed blood leaves by the hepatic veins into the inferior vena cava. Because the liver sits between the gut and the heart, it is the first organ to see everything absorbed.
The fetal problem and the placenta
Fetal circulation is a solution to a temporary problem: the organs that will later oxygenate blood and absorb nutrients are not yet in use. The lungs are collapsed and filled with fluid, and the digestive tract has nothing to digest. The placenta takes over both roles, acting as lung and gut together. Fetal blood reaches it through the umbilical arteries and returns through the umbilical vein. Because oxygen is obtained there, the oxygen content of fetal blood is highest in the umbilical vein and falls as blood passes through the fetal tissues. The circulation is arranged to send the best-oxygenated blood to the brain and heart first, because those organs cannot tolerate oxygen lack. Fetal haemoglobin, with a higher affinity for oxygen, helps extract oxygen from maternal blood across the placenta.
The three shunts
Each shunt solves a specific problem. The ductus venosus protects the liver from a flow it does not need and speeds oxygenated blood to the heart. The foramen ovale lets blood pass directly from the right atrium to the left atrium, so the left ventricle is filled and the systemic circulation, especially the brain and heart, receives oxygenated blood. The ductus arteriosus provides a second bypass for blood that does enter the right ventricle, sending it into the aorta rather than into the non-functional lungs. The three work together as a routing system, and their combined effect is that the fetal lungs receive only enough blood to keep their tissue alive. This is why the fetal pulmonary circulation is a low-flow system, and why the pulmonary vessels are constricted before birth.
What changes at birth
Birth is a sudden change in the whole circulatory plan. The first breath expands the lungs, oxygen enters them and the pulmonary vessels dilate, so resistance to flow through the lungs falls sharply. More blood therefore flows through the lungs and returns to the left atrium, where pressure rises. At the same time, clamping the umbilical cord stops placental blood flow, so less blood reaches the right atrium and right atrial pressure falls. Because the foramen ovale is a flap, the reversed pressure difference closes it. The ductus arteriosus constricts in response to higher oxygen and falling prostaglandins and usually seals within days. The ductus venosus closes as flow through it stops. These closures convert the fetal routing into the adult series arrangement, and failure of closure is a common congenital heart problem.
Explore the diagram
Start with the whole view, then select a numbered structure. Compare the additional illustrations to connect what a structure looks like with what it does.
Two capillary beds in sequence
Reduced motion · use the slider to explore each state.
Usually formed by superior mesenteric and splenic veins.
Educational schematic · selected major pathways, not to scale. Motion shows a slowed trace or comparison, not real-time physiology or disease progression.
Connect the illustrations to the key ideas
PORTAL SYSTEM · two capillary beds in series
Gut capillaries absorb, liver sinusoids process, then blood returns to the heart.
FETAL SHUNTS · ductus venosus, foramen ovale, ductus arteriosus
They bypass the liver and lungs and send oxygenated blood to the brain and heart first.
CHANGES AT BIRTH · breathing and cord clamping close the shunts
Pulmonary resistance falls and atrial pressures reverse, converting to the adult series plan.
Core Concepts Defined
- Portal system
- A vascular arrangement with two capillary beds in series before blood returns to the heart.
- Hepatic portal vein
- The vein carrying nutrient-rich blood from digestive organs and spleen to the liver.
- Hepatic vein
- A vein carrying processed blood from the liver to the inferior vena cava.
- Sinusoid
- A wide, leaky capillary allowing proteins and cells to pass, found in the liver.
- Placenta
- The temporary organ where fetal and maternal circulations exchange substances without mixing.
- Umbilical vein
- The vessel carrying oxygenated blood from the placenta to the fetus.
- Umbilical arteries
- The vessels carrying deoxygenated blood from the fetus to the placenta.
- Ductus venosus
- A fetal shunt connecting the umbilical vein to the inferior vena cava, bypassing the liver.
- Foramen ovale
- A fetal opening between the atria that lets blood bypass the lungs.
- Ductus arteriosus
- A fetal shunt connecting the pulmonary trunk to the aorta, bypassing the lungs.
- Fetal haemoglobin
- A form of haemoglobin with a higher oxygen affinity than adult haemoglobin.
- Ligamentum venosum
- The remnant of the ductus venosus after it closes.
- Fossa ovalis
- The shallow depression left by the closed foramen ovale.
- Ligamentum arteriosum
- The remnant of the ductus arteriosus after it closes.
Go deeper · worked example
Routing oxygenated blood in the fetus
- Receive oxygen at the placenta. Maternal and fetal blood lie close in the placenta, so oxygen and nutrients cross into fetal blood without the two bloods mixing.
- Return in the umbilical vein. Oxygenated blood travels from the placenta to the fetus in the umbilical vein, the only fetal vessel carrying well-oxygenated blood.
- Bypass the liver. Most of this blood passes through the ductus venosus into the inferior vena cava, avoiding the liver sinusoids, which are not yet needed for processing.
- Bypass the lungs. In the right atrium, higher pressure directs blood through the foramen ovale to the left atrium, so it reaches the left ventricle and aorta without passing through the lungs.
- Deliver to the brain and heart, and close the routes at birth. The aorta sends this blood to the brain and heart first. Blood returning from the upper body is diverted through the ductus arteriosus. At birth, breathing and cord clamping reverse atrial pressures and close the shunts.
Explain it yourself · Explain why the fetus needs shunts and why they are no longer useful after birth.
Reveal a model explanation
Before birth the lungs are fluid-filled and do not exchange gas, and the gut receives no food, so oxygen and nutrients come from the placenta. Blood must therefore be routed away from the lungs and quickly to the brain and heart. The ductus venosus bypasses the liver, the foramen ovale and ductus arteriosus bypass the lungs, and fetal haemoglobin extracts oxygen efficiently. At birth, air enters the lungs, pulmonary resistance falls, and clamping the cord removes the placental supply. Blood now flows to the lungs for gas exchange and the shunts close. Keeping them open would send blood the wrong way and overload the heart and lungs.
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OpenStax, Anatomy and Physiology 2e, 20.5 Circulatory Pathways (hepatic portal and fetal circulation); Tortora & Derrickson, Principles of Anatomy and Physiology, 12e, ch. 21. Author review only; medical and French-language expert review pending.
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