Your cardiovascular system is like a figure-eight with your heart at the center. Two distinct loops work together to keep you alive:

Pulmonary Circulation (The Lung Loop)

The right side of your heart sends blood to your lungs. This might surprise you, but in pulmonary circulation, arteries carry deoxygenated blood and veins carry oxygenated blood — the opposite of what you'd expect! The pulmonary arteries carry blood FROM the heart TO the lungs (away from heart = artery), and pulmonary veins return it FROM lungs TO heart (toward heart = vein).

Systemic Circulation (The Body Loop)

The left side of your heart sends oxygen-rich blood to every organ and tissue in your body (except the lungs). This is the high-pressure circuit — your left ventricle has walls three times thicker than your right because it needs to generate much more force.

Why two circuits?

• Separates oxygenated and deoxygenated blood
• Allows different pressures for lungs (low) and body (high)
• Maximizes oxygen delivery efficiency

Your heart is about the size of your fist, weighs about 300 grams, and sits slightly left of center in your chest. It's protected by your sternum and rib cage.

Four chambers:

• Right atrium: Receives deoxygenated blood from body (via vena cava)
• Right ventricle: Pumps blood to lungs
• Left atrium: Receives oxygenated blood from lungs (via pulmonary veins)
• Left ventricle: Pumps blood to the body — has the thickest walls!

Four valves ensure one-way flow:

• Tricuspid valve: Between right atrium and right ventricle
• Pulmonary valve: Between right ventricle and pulmonary artery
• Mitral (bicuspid) valve: Between left atrium and left ventricle
• Aortic valve: Between left ventricle and aorta

Your heart beats about 100,000 times per day, pumping 5 liters of blood per minute at rest. During exercise, this can increase to 20-25 liters per minute!

Your kidneys are extraordinary organs that do far more than make urine. Located on either side of your spine, just below your rib cage, each kidney contains about 1 million nephrons — the functional filtering units.

What kidneys do:

• Filter blood: Process about 180 liters daily, producing 1-2 liters of urine
• Regulate blood pressure: Through the renin-angiotensin-aldosterone system
• Maintain electrolyte balance: Sodium, potassium, calcium, phosphate
• Regulate pH: Excrete hydrogen ions, reabsorb bicarbonate
• Produce erythropoietin: Hormone that stimulates red blood cell production
• Activate vitamin D: Convert it to calcitriol for calcium absorption

This week's journey:

• Lesson 2: How blood functions as a delivery system
• Lesson 3: The heart as a pump — anatomy and electrical system
• Lesson 4: Kidney structure and function in detail