Anatomy and Physiology 2e · The Cardiovascular System: Blood Vessels and Circulation
Circulatory Pathways
On this page 9 sections
In 30 seconds
Blood travels through two connected circuits, and every drop completes both on each full lap around the body:
- Pulmonary circulation — from the right ventricle through the pulmonary arteries to the lungs (where blood drops off carbon dioxide and picks up oxygen), then back through the pulmonary veins to the left atrium.
- Systemic circulation — from the left ventricle through the Aorta The body's main artery from the left ventricle Full entry → and its branches to all body tissues, then back through the veins and venae cavae to the right atrium.
The two circuits share the heart but serve different masters: the Pulmonary circuit Right heart → lungs → left heart loop Full entry → is short, low-pressure, and exists for gas exchange; the Systemic circuit Left heart → body → right heart loop Full entry → is long, high-pressure, and perfuses every organ. The functional pathways within the systemic circuit — the aorta and its branches, the coronary circulation, the cerebral circulation (including the Circle of Willis Ring of arteries at the brain's base Full entry →), and the hepatic portal system — each have distinctive anatomy worth tracing, because each explains a familiar clinical scenario (heart attack, stroke, liver processing of absorbed nutrients). Fetal circulation is a special variant covered in detail in Topic 6; this topic focuses on the adult pathways.
Why this matters
- The "map" of the circulation is the map of medicine. A heart attack is a blocked coronary artery; a stroke is often a blocked or bleeding cerebral vessel; cirrhosis raises pressure in the portal system; a pulmonary embolus blocks the pulmonary circuit. Knowing the route tells you what is blocked, where it leads, and what tissue is at risk.
- Pulse points are anatomy. Every place you feel a pulse — radial, brachial, carotid, femoral, popliteal, dorsalis pedis — is a named point on the systemic arterial tree.
- Vessels are named by direction, not oxygen content — the pulmonary arteries carry deoxygenated blood and the pulmonary veins carry oxygenated blood. This is one of the most-tested distinctions in the whole book.
- The portal system is a "detour" most students forget: blood from the digestive organs goes first to the liver, not straight back to the heart, so absorbed nutrients are processed before reaching the general circulation.
- Collateral protection varies by organ: the circle of Willis and coronary anastomoses give redundancy, while end arteries (e.g., in the kidney) do not — explaining why ischemia damages some organs so quickly.
The college version
Core Concepts
The pulmonary circuit: gas exchange loop
Trace the route: right ventricle → pulmonary trunk → right and left pulmonary arteries → lung capillaries → pulmonary veins → left atrium. In the lung capillaries, carbon dioxide diffuses out of the blood into the alveoli and oxygen diffuses in. Note the reversal: arteries carry the blood away from the heart to the lungs (deoxygenated), and veins carry it toward the heart (oxygenated). The pulmonary circuit is a low-pressure, low-resistance system — the right ventricle is thinner-walled than the left because it faces far less resistance.
The systemic circuit: the aorta and its branches
Left ventricle → aorta → elastic and muscular arteries → arterioles → capillaries → venules → veins → venae cavae → right atrium.
The aorta is divided into regions that name its branches:
- Ascending aorta — gives off the Coronary arteries Arteries supplying the heart muscle itself Full entry → (right and left) that supply the heart itself.
- Aortic arch — gives off the Brachiocephalic trunk First branch of the aortic arch; splits into right subclavian and right common carotid Full entry → (which divides into the right subclavian and right common carotid arteries), the left common carotid artery, and the left subclavian artery. These supply the head, neck, and upper limbs.
- Descending thoracic aorta — supplies the chest wall (intercostal arteries), esophagus, and other thoracic structures.
- Abdominal aorta — supplies the abdominal organs: the celiac trunk (liver, stomach, spleen), superior mesenteric artery (most of the small intestine and part of the large intestine), renal arteries (kidneys), gonadal arteries (ovaries or testes), inferior mesenteric artery (rest of the large intestine), and terminates by dividing into the common iliac arteries (pelvis and lower limbs).
The carotid sinuses (at the origin of the internal carotids) and aortic arch house the baroreceptors discussed in Topic 4 — the arterial tree is not just plumbing; it is also the sensor network for blood pressure.
The coronary circulation: feeding the heart
The heart muscle (myocardium) cannot live on the blood inside its chambers — it needs its own supply. The left coronary artery divides into the Left anterior descending (LAD) Main branch of the left coronary artery Full entry → artery (supplying the anterior wall and septum) and the circumflex artery (supplying the lateral and posterior walls). The right coronary artery (RCA) supplies the right ventricle and, in most people, the inferior wall and the sinoatrial (SA) node — the heart's pacemaker — via the SA nodal artery. Venous blood from the myocardium collects in the Coronary sinus Vein collecting most myocardial venous blood Full entry → and drains into the right atrium. Because the heart works continuously and has a high oxygen demand, a sudden coronary blockage quickly starves the myocardium — the basis of myocardial infarction (heart attack). Coronary anastomoses exist but are often inadequate to prevent damage from a sudden complete occlusion.
The cerebral circulation and the circle of Willis
The brain gets blood from two sources: the internal carotid arteries (anterior circulation) and the vertebral arteries, which join to form the Basilar artery Artery formed by the two vertebral arteries Full entry → (posterior circulation). At the base of the brain, these vessels connect in a ring called the circle of Willis, which links the anterior and posterior circulations and the left and right sides. This ring provides collateral routes: if one feeding artery is blocked, blood can be rerouted through the circle to keep the brain perfused. The circle is not always complete in every person, and its protective value is limited, but it is the brain's most important built-in redundancy.
The hepatic portal system: the digestive detour
Most venous blood returns directly toward the heart, but blood from the digestive organs does not. In the hepatic portal system, veins from the stomach, intestines, spleen, and pancreas converge into the Hepatic portal vein Vein carrying gut venous blood to the liver Full entry →, which carries this nutrient-rich blood into the liver. In liver capillaries (sinusoids), hepatocytes process absorbed nutrients, detoxify substances (including alcohol and drugs), and remove old red blood cells; the blood then leaves the liver through the Hepatic veins Veins draining the liver into the inferior vena cava Full entry → into the inferior vena cava. This arrangement means everything absorbed from the gut passes through the liver before reaching the general circulation — which also explains why liver disease (cirrhosis) raises pressure in the portal system and can produce swollen esophageal or abdominal veins.
Venous return: the low-pressure collection side
Systemic venous blood returns through the superior vena cava (head, neck, upper limbs, chest) and inferior vena cava (everything below the diaphragm) into the right atrium. The low-pressure venous side relies on the valves and pumps from Topic 1, and it holds most of the blood volume — the reservoir that sympathetic venoconstriction can tap.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| Pulmonary arteries | Systemic arteries | Pulmonary arteries carry deoxygenated blood to the lungs; vessels are named by direction, not oxygen content |
| Pulmonary veins | Systemic veins | Pulmonary veins carry oxygenated blood to the left atrium |
| Coronary arteries | Coronary veins | Arteries (LAD, circumflex, RCA) supply the myocardium; the coronary sinus and cardiac veins drain it |
| Hepatic portal vein | Hepatic vein | Portal vein brings gut blood into the liver; hepatic veins drain blood out of the liver into the inferior vena cava |
| Circle of Willis guarantees stroke protection | It provides limited collateral | The circle may be incomplete and can still fail under sudden occlusion; protection is partial |
| The right ventricle pumps harder than the left | The left ventricle pumps harder | The left ventricle has the thickest wall because the systemic circuit has high resistance; the pulmonary circuit is low-resistance |
| All systemic venous blood returns via the vena cavae | Portal-system blood detours first | Gut blood goes through the liver (portal system) before reaching the vena cava — the one big exception |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Think of blood as a delivery truck on a loop. First it goes to the lungs to drop off its "trash" (carbon dioxide) and pick up "cargo" (oxygen). Then it drives all over the body delivering the cargo. There are special side routes: one road feeds the heart's own muscle, one protects the brain with a ring of alternate roads, and one takes food from your stomach straight to the liver for sorting before going to the rest of the body.
Worked example
You eat a meal containing carbohydrate. Glucose is absorbed from the small intestine into blood in the gut capillaries — but that blood does not head straight for the heart. It flows into mesenteric veins that converge to form the hepatic portal vein, which delivers the glucose-rich blood to the liver. In the liver sinusoids, hepatocytes take up much of the glucose (storing some as glycogen) and process other absorbed nutrients and any absorbed toxins or drugs. The blood then exits through the hepatic veins into the inferior vena cava, travels to the right atrium → right ventricle → pulmonary arteries → lungs → pulmonary veins → left atrium → left ventricle → aorta, and from there to working muscles and other tissues. This walk-through shows why the liver sees every meal before the rest of the body does — and why a drug given by mouth may be partially metabolized by the liver before it ever reaches the general circulation (first-pass metabolism). If cirrhosis raises portal pressure, this same route backs up: blood seeks collateral paths, which is why a person with advanced liver disease can develop enlarged veins in the esophagus or abdomen.
Key takeaways
- Two circuits, one heart: pulmonary (right ventricle → lungs → left atrium) and systemic (left ventricle → body → right atrium). Both run continuously.
- Pulmonary arteries carry deoxygenated blood; pulmonary veins carry oxygenated blood — vessels are named by direction, not oxygen content.
- Aortic branch order: ascending (coronary arteries) → arch (brachiocephalic trunk, left common carotid, left subclavian) → thoracic → abdominal (celiac trunk, superior mesenteric, renal, gonadal, inferior mesenteric) → common iliacs.
- Coronary circulation: left coronary → LAD + circumflex; right coronary → right ventricle and, in most people, the SA node. Venous return via the coronary sinus.
- Circle of Willis: ring at the brain's base linking internal carotid and vertebrobasilar systems; provides collateral flow to the brain.
- Hepatic portal system: gut veins → hepatic portal vein → liver sinusoids → hepatic veins → inferior vena cava. Absorbed nutrients pass through the liver first.
- Venous return relies on venous valves, skeletal muscle and respiratory pumps, and venoconstriction (Topic 1); most blood volume sits on the venous side.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
Trace a red blood cell from the right ventricle back to the left ventricle, naming every vessel and chamber.
Show answer
Right ventricle → pulmonary trunk → pulmonary arteries → lung capillaries → pulmonary veins → left atrium → mitral valve → left ventricle.
Why do the pulmonary arteries carry deoxygenated blood while the aorta carries oxygenated blood?
Show answer
Vessels are named by direction, not oxygen content: the pulmonary arteries carry blood away from the heart toward the lungs (deoxygenated), and the aorta carries blood away from the heart to the body after it has been oxygenated in the lungs.
Name the branches of the aortic arch and the regions they supply.
Show answer
Brachiocephalic trunk (divides into right subclavian and right common carotid — right arm, right head/neck), left common carotid (left head/neck), and left subclavian (left arm).
Describe the coronary circulation: main arteries, what each supplies, and how cardiac venous blood returns to the heart.
Show answer
Left coronary artery → LAD (anterior wall, septum) and circumflex (lateral/posterior walls); right coronary artery supplies the right ventricle and, in most people, the SA node. Myocardial venous blood collects in the coronary sinus and drains into the right atrium.
What is the circle of Willis, and what does it protect?
Show answer
A ring of arteries at the base of the brain connecting the internal carotid and vertebrobasilar systems; it provides collateral routes so the brain can be perfused if one feeding artery is compromised.
Explain the hepatic portal system and why absorbed nutrients must pass through the liver before reaching the general circulation.
Show answer
Veins from the digestive organs converge into the hepatic portal vein, which carries blood through the liver before it reaches the vena cava. This lets hepatocytes process nutrients, detoxify absorbed substances, and remove old blood cells before the blood joins the general circulation (the basis of first-pass metabolism).
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Pulmonary circuit
- Right heart → lungs → left heart loop
- Systemic circuit
- Left heart → body → right heart loop
- Aorta
- The body's main artery from the left ventricle
- Brachiocephalic trunk
- First branch of the aortic arch; splits into right subclavian and right common carotid
- Coronary arteries
- Arteries supplying the heart muscle itself
- Left anterior descending (LAD)
- Main branch of the left coronary artery
- Coronary sinus
- Vein collecting most myocardial venous blood
- Circle of Willis
- Ring of arteries at the brain's base
- Basilar artery
- Artery formed by the two vertebral arteries
- Hepatic portal vein
- Vein carrying gut venous blood to the liver
- Hepatic veins
- Veins draining the liver into the inferior vena cava
- Superior/inferior vena cava
- Large veins returning blood to the right atrium
Sources & references
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