The circulatory system is a complex network of vessels that transport blood throughout the body. It consists of two main circuits: the pulmonary circuit, which oxygenates blood in the lungs, and the systemic circuit, which delivers oxygen and nutrients to tissues.

The heart acts as the central pump, pushing blood through arteries, capillaries, and veins. This intricate system ensures that every cell in the body receives the oxygen and nutrients it needs to function, while also removing waste products and maintaining fluid balance.

Pulmonary Circuit

Blood flow in pulmonary circuit

• Blood exits right ventricle through pulmonary semilunar valve enters pulmonary trunk
• Pulmonary trunk bifurcates into left and right pulmonary arteries
  • Left pulmonary artery carries deoxygenated blood to left lung (alveoli)
  • Right pulmonary artery carries deoxygenated blood to right lung (alveoli)
• In lungs, blood flows through pulmonary capillaries surrounding alveoli
  • Gas exchange occurs: blood picks up oxygen (O₂) and releases carbon dioxide (CO₂)
• Oxygenated blood returns to heart via pulmonary veins
  • Two pulmonary veins from each lung (four total) carry oxygenated blood
• Pulmonary veins empty into left atrium of heart

Systemic Arteries

Major systemic arteries

• Blood exits left ventricle through aortic semilunar valve into ascending aorta
• Ascending aorta branches into aortic arch
  • Brachiocephalic trunk branches off aortic arch
    • Right common carotid artery supplies right side of head and neck (brain, face)
    • Right subclavian artery supplies right upper limb (arm, hand)
  • Left common carotid artery branches directly off aortic arch, supplying left side of head and neck (brain, face)
  • Left subclavian artery branches directly off aortic arch, supplying left upper limb (arm, hand)
• Aortic arch continues as descending aorta
  • Thoracic aorta is portion of descending aorta in thorax (chest)
  • Abdominal aorta is portion of descending aorta in abdomen
    • Celiac trunk, superior mesenteric artery, and inferior mesenteric artery branch off to supply abdominal organs (stomach, intestines, liver, pancreas)
    • Renal arteries branch off to supply kidneys
    • Common iliac arteries branch off to supply pelvic organs and lower limbs (legs, feet)
      • Each common iliac artery divides into internal and external iliac arteries
        • Internal iliac arteries supply pelvic organs (bladder, reproductive organs)
        • External iliac arteries continue as femoral arteries to supply lower limbs (thighs, legs, feet)
• Coronary circulation supplies blood to the heart muscle itself

Venous Return

Venous blood flow to heart

• Venous blood from feet and legs is collected by superficial and deep veins
  • Superficial veins (great saphenous vein) are located close to skin
  • Deep veins (femoral vein) are located deeper within leg muscles
• Venous blood from legs flows into external iliac veins
  • External iliac veins join internal iliac veins to form common iliac veins
• Common iliac veins merge to form inferior vena cava
  • Inferior vena cava also receives blood from abdominal and pelvic organs via various veins (renal veins, hepatic veins)
• Inferior vena cava carries deoxygenated blood into right atrium of heart
• Superior vena cava, which collects blood from head, neck, and upper limbs (jugular veins, subclavian veins), also empties into right atrium
• The hepatic portal system carries nutrient-rich blood from the digestive organs to the liver before it enters the general circulation

Circulatory System Overview

Components and Functions

• The systemic circuit distributes oxygenated blood from the heart to the body and returns deoxygenated blood to the heart
• The lymphatic system works alongside the circulatory system to maintain fluid balance and support immune function
• Blood pressure, regulated by the cardiac cycle, ensures proper blood flow throughout the body