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Systemic vs Pulmonary
Circulation

Trace the complete pathway of blood through the heart, lungs, and body: understanding how two distinct circuits work together to sustain life.

Anatomy & Physiology

What Is Circulation?

Circulation is the continuous movement of blood through a closed network of vessels, powered by the heart. It delivers oxygen and nutrients to every cell while removing carbon dioxide and metabolic waste.

Two Circuits, One Heart

Your cardiovascular system isn't a single loop. It's two separate circuits operating simultaneously, both driven by the heart's four chambers.

Pulmonary Circuit
Right side of heart → lungs → left side of heart. Picks up oxygen, drops off carbon dioxide.
Systemic Circuit
Left side of heart → entire body → right side of heart. Delivers oxygen, collects carbon dioxide.

Why Two Circuits?

The lungs are delicate, low-pressure organs. The body requires high-pressure flow to push blood through billions of capillaries in distant tissues. A double circulation system solves both needs.

1
Pressure separation: The right ventricle pumps at lower pressure to protect the lungs' fragile capillaries.
2
Efficiency: The left ventricle pumps at higher pressure to reach the brain, toes, and everything in between.
3
Re-oxygenation: Blood returns to the heart between the two circuits, ensuring fully oxygenated blood enters the systemic loop.

The Heart's Four Chambers

Right Atrium (RA) Right Ventricle (RV) Left Atrium (LA) Left Ventricle (LV) Tricuspid Mitral Deoxy- genated Oxy- genated Septum divides left and right sides

The Pulmonary Circuit

Right Ventricle Pulmonary Arteries Lungs Gas Exchange
\(\text{CO}_2\) out, \(\text{O}_2\) in
 Pulmonary Veins Left Atrium Pulmonary Circulation Pathway Deoxygenated → Oxygenated

The Systemic Circuit

Left Ventricle Aorta Largest artery Body Tissues
\(\text{O}_2\) delivered,
\(\text{CO}_2\) collected
 Venae Cavae (SVC + IVC) Right Atrium Systemic Circulation Pathway Oxygenated → Deoxygenated

Arteries vs Veins: The Rule and the Exception

Arteries
Carry blood away from the heart. Usually oxygenated, except pulmonary arteries.
Veins
Carry blood toward the heart. Usually deoxygenated, except pulmonary veins.

Complete Blood Flow Sequence

1
Venae cavae → Right Atrium: Deoxygenated blood returns from the body.
2
Right Atrium → Right Ventricle: Blood passes through the tricuspid valve.
3
Right Ventricle → Pulmonary Arteries: Blood exits through the pulmonary valve toward the lungs.
4
Lungs (gas exchange): \(\text{CO}_2\) is released and \(\text{O}_2\) is absorbed into the blood.
5
Pulmonary Veins → Left Atrium: Oxygenated blood returns to the heart.
6
Left Atrium → Left Ventricle: Blood passes through the mitral (bicuspid) valve.
7
Left Ventricle → Aorta → Body: Oxygenated blood is pumped through the aortic valve to all tissues.

The Double Circulation Loop

HEART 4 Chambers LUNGS BODY Pulm. Arteries Pulm. Veins Aorta Venae Cavae Deoxygenated Oxygenated

How to Trace a Pathway

When asked to trace blood flow from one point to another, follow these steps:

1
Identify the starting point: Which chamber or vessel does the question begin at?
2
Determine the blood's oxygen status: Is it oxygenated or deoxygenated at this point?
3
Follow the one-way flow: Atrium → Ventricle → Artery → Capillaries → Vein → next Atrium. Never skip a step.
4
Name each valve: Tricuspid (R side), Pulmonary (R exit), Mitral (L side), Aortic (L exit).
5
Note the gas exchange site: Lungs for the pulmonary circuit, body tissues for the systemic circuit.

Example 1: Right Atrium to Aorta

Question: Trace blood from the right atrium to the aorta. Name each structure it passes through.

1
Right Atrium → through the tricuspid valve
2
Right Ventricle → through the pulmonary valve
3
Pulmonary Arteries → Lungs (gas exchange occurs)
4
Pulmonary Veins → Left Atrium
5
Left Atrium → through the mitral valve → Left Ventricle
6
Left Ventricle → through the aortic valve → Aorta

Example 2: Toe to Lung

Question: A red blood cell is in a capillary of the big toe. Trace its path to the lung capillaries.

Toe Cap. Deoxy Veins → IVC RA RV Pulmonary Arteries Lung Capillaries Tricuspid Pulmonary V.

Example 3: Oxygen's Full Journey

Question: An oxygen molecule enters the blood in the lung capillaries. Trace its path to a brain cell and back to the lungs.

1
Lung capillaries: \(\text{O}_2\) binds to hemoglobin on red blood cells.
2
Pulmonary veins → Left atrium → Mitral valve → Left ventricle → Aortic valve → Aorta
3
Aorta → carotid arteries → brain capillaries: \(\text{O}_2\) is released to the brain cell; \(\text{CO}_2\) is picked up.
4
Jugular veins → Superior vena cava → Right atrium → Tricuspid valve → Right ventricle → Pulmonary valve → Pulmonary arteries
5
Lung capillaries: \(\text{CO}_2\) is released, new \(\text{O}_2\) is absorbed. Cycle repeats.

Common Mistakes

✕ "Arteries carry oxygenated blood"
Not always. Pulmonary arteries carry deoxygenated blood. Arteries are defined by direction (away from heart), not oxygen content.
✕ Skipping valves
Students often jump from atrium to artery, forgetting the ventricle and valve between them. Every path goes: atrium → valve → ventricle → valve → artery.
✕ Mixing up sides
Diagrams show the heart from the patient's perspective: the right side appears on your left. Flipping the sides reverses the entire pathway.

Pulmonary vs Systemic Summary

Feature Pulmonary Systemic
Heart side Right Left
Destination Lungs Entire body
Arteries carry Deoxygenated blood Oxygenated blood
Veins carry Oxygenated blood Deoxygenated blood
Pressure Low High
Gas exchange \(\text{CO}_2\) out, \(\text{O}_2\) in \(\text{O}_2\) out, \(\text{CO}_2\) in
Key vessels Pulmonary trunk Aorta, Venae cavae

Question 1 of 5

Which chamber of the heart receives deoxygenated blood from the body via the venae cavae?

Question 2 of 5

What is unique about the pulmonary arteries compared to most other arteries in the body?

Question 3 of 5

Blood flows from the left ventricle into the aorta. Which valve must it pass through?

Question 4 of 5

In the pulmonary circuit, where does gas exchange occur?

Question 5 of 5

A red blood cell is in the right ventricle. Which is the correct sequence of structures it passes through to reach the left atrium?