20.5 Circulatory Pathways

Pulmonary Circuit

The pulmonary circuit has one job: send deoxygenated blood to the lungs, pick up oxygen, dump carbon dioxide, and return freshly oxygenated blood to the heart. It's the shorter of the two circuits, and the only one where arteries carry deoxygenated blood and veins carry oxygenated blood. That distinction trips people up on exams, so keep it in mind.

Blood Flow Through the Pulmonary Circuit

1. Deoxygenated blood exits the right ventricle through the pulmonary semilunar valve and enters the pulmonary trunk.
2. The pulmonary trunk splits (bifurcates) into the left pulmonary artery and the right pulmonary artery, each carrying deoxygenated blood to its respective lung.
3. Inside the lungs, blood flows through pulmonary capillaries that surround the alveoli (tiny air sacs).
4. Gas exchange occurs at the capillary-alveolar interface: $O_2$ diffuses into the blood, and $CO_2$ diffuses out into the alveoli to be exhaled.
5. Now oxygenated, blood drains into the pulmonary veins. There are two pulmonary veins from each lung (four total).
6. All four pulmonary veins empty into the left atrium, completing the pulmonary circuit.

Systemic Arteries

The systemic circuit is the larger loop. It begins when oxygenated blood leaves the left ventricle and branches out to supply every tissue in the body. Knowing the major arterial branches off the aorta is essential for this unit.

Aorta and Its Major Branches

Blood exits the left ventricle through the aortic semilunar valve into the ascending aorta. From there, the aorta curves into the aortic arch, then descends through the thorax and abdomen.

Three branches arise from the aortic arch (right to left):

1. Brachiocephalic trunk — the first and largest branch. It splits into:

   • Right common carotid artery → supplies the right side of the head and neck (brain, face)
   • Right subclavian artery → supplies the right upper limb

2. Left common carotid artery — branches directly off the arch; supplies the left side of the head and neck

3. Left subclavian artery — branches directly off the arch; supplies the left upper limb

Notice the asymmetry: on the right side, the carotid and subclavian share a common trunk (brachiocephalic), while on the left side they each branch independently from the arch.

Descending Aorta

The aortic arch continues downward as the descending aorta, which is divided by location:

• Thoracic aorta — the portion within the chest cavity
• Abdominal aorta — the portion within the abdomen, giving off several major branches:
  • Celiac trunk → supplies the stomach, liver, and spleen
  • Superior mesenteric artery → supplies the small intestine and first portion of the large intestine
  • Inferior mesenteric artery → supplies the remainder of the large intestine
  • Renal arteries → supply the kidneys
  • Common iliac arteries → the abdominal aorta terminates by splitting into these paired vessels

Each common iliac artery divides into:

• Internal iliac artery → supplies pelvic organs (bladder, reproductive organs)
• External iliac artery → continues into the thigh as the femoral artery, supplying the lower limb

Coronary Circulation

The heart muscle doesn't get its oxygen from the blood passing through its chambers. Instead, the coronary arteries branch off the ascending aorta just above the aortic semilunar valve and supply the myocardium directly.

Venous Return

Veins carry blood back toward the heart, working against gravity in many areas of the body. Two large veins, the superior and inferior venae cavae, serve as the final collection points before blood re-enters the heart.

Lower Body Venous Drainage

• Blood from the feet and legs is collected by two sets of veins:
  • Superficial veins (e.g., great saphenous vein) — located just beneath the skin
  • Deep veins (e.g., femoral vein) — run deeper, within the leg muscles
• Leg veins drain into the external iliac veins, which join the internal iliac veins (from the pelvis) to form the common iliac veins.
• The two common iliac veins merge to form the inferior vena cava (IVC).
• As the IVC ascends, it also receives blood from abdominal organs via the renal veins (kidneys) and hepatic veins (liver).
• The IVC empties deoxygenated blood into the right atrium.

Upper Body Venous Drainage

• Blood from the head and neck drains through the jugular veins.
• Blood from the upper limbs drains through the subclavian veins.
• These veins converge to form the superior vena cava (SVC), which also empties into the right atrium.

Hepatic Portal System

The hepatic portal system is a unique venous arrangement. Instead of returning blood directly to the heart, veins from the digestive organs (stomach, intestines, spleen, pancreas) drain into the hepatic portal vein, which carries nutrient-rich blood to the liver first. The liver processes nutrients, detoxifies substances, and stores glucose as glycogen. Only after passing through liver sinusoids does this blood enter the hepatic veins and then the IVC. This "portal" design means blood passes through two capillary beds before returning to the heart.

Circulatory System Overview

Putting it all together, blood follows a continuous figure-eight loop:

• The pulmonary circuit (right side of the heart → lungs → left side of the heart) handles gas exchange.
• The systemic circuit (left side of the heart → body tissues → right side of the heart) delivers oxygen and nutrients while removing metabolic waste.
• The lymphatic system works alongside the circulatory system by draining excess interstitial fluid back into the bloodstream and supporting immune function.
• Blood pressure, generated by ventricular contraction during the cardiac cycle, is what drives blood flow through both circuits. Pressure is highest in the aorta and drops progressively through arterioles, capillaries, and veins.