About blood pathways

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I now have excellent material from multiple authoritative textbooks. Here is a thorough overview of blood pathways.

Blood Pathways - A Complete Overview

The circulatory system has three main circuits: the pulmonary circulation, the systemic circulation, and the special portal circulation. A completely separate set of pathways exists in fetal life that closes at birth.

1. The Two Main Circuits

Schematic illustration of the circulatory system

Fig. 2.1 - Blood circulation and lymph vessels. Color atlas of Human Anatomy, Vol. 2

In a resting adult, roughly 70% of blood is in the systemic circulation, 18% in the pulmonary circulation, and 12% in the heart itself. - Junqueira's Basic Histology

Pulmonary Circulation (Right Heart - Lungs - Left Heart)

This is the short, low-pressure circuit for gas exchange.

Pathway: Deoxygenated blood from the body → Right Atrium → (tricuspid valve) → Right Ventricle → (pulmonary valve) → Pulmonary Trunk → splits into Right & Left Pulmonary Arteries → Lung Capillaries (O₂ in, CO₂ out) → Pulmonary Veins (x4) → Left Atrium

Key points:

The pulmonary circulation is the only circuit where arteries carry deoxygenated blood and veins carry oxygenated blood.
The driving pressure is only ~15 mmHg (vs. ~95 mmHg in the systemic circuit), making it a low-resistance, high-flow system. - Medical Physiology

Systemic Circulation (Left Heart - Body - Right Heart)

This is the long, high-pressure circuit supplying all organs.

Pathway: Oxygenated blood → Left Atrium → (mitral valve) → Left Ventricle → (aortic valve) → Aorta → Arteries → Arterioles → Capillaries (gas and nutrient exchange) → Venules → Veins → Inferior Vena Cava (lower body) + Superior Vena Cava (head, arms) → Right Atrium

Color Atlas of Human Anatomy, Vol. 2

Portal Circulation (Special Systemic Circuit)

Venous blood from the stomach, intestine, pancreas, and spleen does NOT drain directly to the inferior vena cava. Instead:

Gut capillaries → Portal Vein → Liver sinusoids (metabolic processing) → Hepatic Veins → Inferior Vena Cava → Right Atrium

This allows the liver to process absorbed nutrients and detoxify substances before they reach the systemic circulation.

2. The Heart as a Pump - Step by Step

Step	Location	Valve Opens	Valve Closes
1	Systemic venous blood fills Right Atrium via SVC + IVC	-	-
2	Tricuspid valve opens → blood enters Right Ventricle	Tricuspid	-
3	RV contracts → Pulmonary valve opens → blood enters pulmonary trunk	Pulmonary	Tricuspid
4	Blood oxygenated in lungs → returns via Pulmonary Veins → Left Atrium	-	-
5	Mitral valve opens → blood enters Left Ventricle	Mitral	-
6	LV contracts → Aortic valve opens → blood enters aorta	Aortic	Mitral

3. Fetal Circulation

Fetal blood pathways are fundamentally different because the lungs are non-functional and the placenta acts as the gas-exchange organ. Three special shunts exist that close at birth.

Fetal circulatory system showing ductus venosus and umbilical vessels

FIG. 13.48 - Course of the umbilical vein to the liver and ductus venosus. The Developing Human

The Three Fetal Shunts

Shunt	Connects	Purpose	Adult Remnant
Ductus Venosus	Umbilical vein → IVC	Bypasses liver (~50% of umbilical blood)	Ligamentum venosum
Foramen Ovale	Right Atrium → Left Atrium	Bypasses pulmonary circuit	Fossa ovalis
Ductus Arteriosus	Pulmonary trunk → Descending Aorta	Bypasses non-functional lungs	Ligamentum arteriosum

Fetal Blood Flow Step by Step

Oxygenated blood from the placenta travels via the umbilical vein.
~50% bypasses the liver via the ductus venosus → IVC → Right Atrium.
The crista dividens (lower edge of septum secundum) directs most IVC blood through the foramen ovale → Left Atrium → Left Ventricle → Ascending Aorta → head and upper limbs (best-oxygenated blood goes to the brain).
The remaining blood in the Right Atrium mixes with deoxygenated blood from the SVC → Right Ventricle → Pulmonary Trunk.
Only ~10% of that blood goes to the (fluid-filled) lungs; the remaining ~90% takes the ductus arteriosus → Descending Aorta → body + umbilical arteries back to the placenta.

Fetal atrial blood flow showing crista dividens and foramen ovale

FIG. 13.50 - Blood flow through fetal atria showing how crista dividens separates IVC blood. The Developing Human

The Developing Human: Clinically Oriented Embryology

4. Circulatory Changes at Birth

When the baby takes its first breath, lung resistance drops dramatically and four simultaneous changes occur:

Foramen ovale closes - increased left atrial pressure (from pulmonary blood flow) pushes the valve flap shut → becomes the fossa ovalis.
Ductus arteriosus closes - rising O₂ tension triggers smooth muscle contraction → becomes the ligamentum arteriosum.
Ductus venosus closes - loss of umbilical flow triggers sphincter contraction → becomes the ligamentum venosum.
Umbilical vessels obliterate - cord clamping causes thrombosis → umbilical vein becomes the round ligament of the liver; umbilical arteries become the medial umbilical ligaments.

Color Atlas of Human Anatomy, Vol. 2

Summary Comparison

Feature	Systemic	Pulmonary	Fetal (before birth)
Driving pressure	~95 mmHg	~15 mmHg	Low (placenta-driven)
Arteries carry	Oxygenated blood	Deoxygenated blood	Mixed (shunts active)
Gas exchange site	Tissue capillaries	Alveolar capillaries	Placenta
Resistance	High	Low	High pulmonary resistance

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