By the end of this lesson
- Distinguish pulmonary and systemic circuits.
- Trace blood through major named vessels.
- Explain liver and kidney vessel connections.
BIG QUESTION
How can one red blood cell visit the lungs, gut, liver and kidneys without losing its route home?
A double circulation uses named vessels and organ-specific branches to maintain directed flow.
Pulmonary circulation exchanges gases
The right ventricle pumps deoxygenated blood through the pulmonary arteryThe artery that takes deoxygenated blood from the right ventricle to the lungs to be oxygenated. to lung capillaries. Oxygen enters and carbon dioxide leaves.
Oxygenated blood returns through pulmonary veinA vein that carries oxygenated blood from the lungs to the left atrium of the heart. to the left atrium.
Systemic circulation supplies the body
The left ventricle pumps through the aortaThe main artery, carrying oxygenated blood from the left ventricle to the body. to organ arteries and capillaries. Systemic veins ultimately return blood through the vena cavaThe main vein, carrying deoxygenated blood from the body to the right atrium of the heart. to the right atrium.
This is a double circulation because blood passes through the heart twice per complete circuit.
The liver receives two inputs
The hepatic arteryThe artery that carries oxygenated blood to the liver. supplies oxygenated blood. The hepatic portal veinThe vein that carries blood rich in absorbed food molecules from the small intestine to the liver. carries nutrient-rich blood from the gut, allowing the liver to process absorbed substances before they reach general circulation.
The hepatic veinThe vein that carries blood from the liver towards the heart, in the vena cava. drains liver blood to the vena cava.
Kidneys filter systemic blood
Each renal arteryThe artery, branching from the aorta, that carries blood containing urea to the kidney. branches from the aorta and brings blood containing urea and other solutes. A renal veinThe vein that carries filtered blood, with less urea, away from the kidney. returns blood after filtration and regulation to the vena cava.
Kidneys remove some substances into urine but retain cells and most useful plasma components.
Major circulation routes
Oxygenation is useful but direction and organ connection define vessels.
| Vessel | Direction | Typical blood feature |
|---|---|---|
| Pulmonary artery | Heart → lungs. | Deoxygenated, carbon-dioxide-rich. |
| Pulmonary vein | Lungs → heart. | Oxygenated. |
| Aorta | Heart → systemic organs. | Oxygenated at departure. |
| Vena cava | Systemic organs → heart. | Deoxygenated at return. |
| Hepatic portal vein | Gut → liver. | Nutrient-rich after absorption. |
| Hepatic artery/vein | Aorta → liver / liver → vena cava. | Oxygen supply / mixed drained blood. |
| Renal artery/vein | Aorta → kidney / kidney → vena cava. | Urea higher before kidney than after. |
Language that earns precision
- pulmonary circulation
- Flow between heart and lungs.
- systemic circulation
- Flow between heart and body organs.
- hepatic portal vein
- Vessel carrying blood from gut to liver.
- renal artery
- Artery carrying blood from aorta to kidney.
- double circulation
- System in which blood passes through the heart twice per full circuit.
Common misconceptions
Claim: All veins carry deoxygenated blood.
Correction: Pulmonary veins carry oxygenated blood; direction defines veins.
Claim: The hepatic portal vein carries blood directly to the heart.
Correction: It carries gut blood to the liver first.
Claim: Renal veins contain no urea.
Correction: Kidneys remove some urea, not necessarily every molecule.
Match the answer to the command word
Use arrows and include the heart between pulmonary and systemic circuits.
Use pulmonary, hepatic and renal vessel names rather than “a vessel”.
Recall, apply, then justify
Trace blood from the right ventricle to a kidney and back to the right atrium.
Hint
Lungs and left heart come before the kidney.
Suggested marking points
- Pulmonary artery carries blood to lungs.
- Pulmonary vein returns it to left atrium.
- It passes to left ventricle and aorta.
- Renal artery carries it to kidney.
- Renal vein returns it toward the vena cava.
- Vena cava delivers it to right atrium.
Explain the importance of the hepatic portal vein.
Hint
Gut first, liver second.
Suggested marking points
- It carries absorbed nutrient-rich blood from the gut to liver.
- The liver can process/store/regulate absorbed substances.
- Blood then enters general circulation through hepatic vein/vena cava.
Compare pulmonary, hepatic portal and renal routes in terms of purpose and vessel sequence.
Planning hint
Gas exchange, processing and excretion.
What a strong design includes
- Pulmonary route is heart → pulmonary artery → lungs → pulmonary vein → heart.
- Its main purpose is gas exchange.
- Hepatic portal route is gut capillaries → portal vein → liver capillaries → hepatic vein.
- It allows liver processing before general circulation.
- Renal route is aorta → renal artery → kidney capillaries → renal vein → vena cava.
- It supports excretion and blood composition regulation.
- All use capillary beds for exchange but differ in connection and transported materials.