The Circulatory System

Blood travels throughout out body through arteries, veins, and capillaries.  Arteries are firm conduits made of several layers, able to withstand pressure.  They carry blood rich in oxygen (O₂) and nutrients.  The Heart pumps blood to our largest artery, the Aorta:

As Arteries constantly divide and branch off, spreading throughout the body into tissue, they get progressively smaller; the tiniest are called Arterioles.  Finally, these vessels become so small that they’re only about the width of a single blood cell, when they’re called Capillaries. 

It’s in the capillaries that O₂ & nutrients ooze out to enter all our body’s cells, which need them to make the energy and proteins necessary to survive. As cells utilize the oxygen and nutrients, they give off carbon dioxide (CO₂) & wastes.  These diffuse out of the cells and into the capillaries, which turn into veins. 

Venous blood carries these wastes; veins keep getting larger and larger en route back to the heart.  The largest vein is called the Vena Cava (superior vena cava from the arms & head, inferior vena cava from abdomen & legs).  In contrast to arteries, veins are somewhat flimsy conduits, not so complex.  The heart’s pumping pressure has phased away by the time arteries reach the capillaries, so blood in veins has to eke its way along.  It gets boosted by constant muscle movement.

When venous blood reaches the heart, it gets pumped into the lungs to give off its stale CO₂, and pick up fresh oxygen, brought back to the heart to get pumped, and we start over.  The wastes from nutrients get filtered out when arteries carry them into the kidneys, and they’re excreted as urine (see The Urinary System).

Trivia – which artery carries carbon dioxide, which vein carries oxygen?  The Pulmonary Artery leaving the right side of the heart to enter the lungs, is called an “artery” because it leaves the heart, but the blood still has CO₂ from the veins.  The Pulmonary Vein is a “vein” because it heads back to the heart, but it carries the O₂ just picked up in the lungs.  See Diagram — The Heart.

Diseases of the Circulatory System

Arteries can develop Aneurysms, weaknesses which cause them to balloon out bit by bit over the years.  Eventually they can rupture, causing massive bleeding.  In the chest there are Thoracic Aortic Aneurysms, usually as the Aorta leaves the heart heading upward (“ascending”), less commonly sometimes as it curves around heading downward (descending).  Abdominal Aortic Aneurysms (“AAA”) occur once the aorta passes through the diaphragm into the abdomen.  They’re usually due to Atherosclerosis – gradual arterial damage due to Hypertension, Smoking, and especially Age.

Cerebral Aneurysms occur in arteries in the brain.  If they burst (Subarachnoid Hemorrhage), the bleeding is less in quantity, but causes devastating & often fatal Strokes.  Strokes can also occur from plaque (fats and blood clots) which build up inside the Carotid or Vertebral arteries in the neck, if pieces break off, to be carried downstream up into the brain.  A rare injury, often from vigorous exercise, is Cervical Artery Dissection, bleeding into the wall of the artery itself.

Plaque can also accumulate in the femoral arteries of the leg.  Then, dislodged pieces can wind up in a foot artery, blocking circulation there, killing tissue, requiring eventual amputation.  This is Peripheral Vascular Disease, usually due to smoking; the arms are affected much less often.  We say Ischemia when the blood supply is temporarily disrupted and salvageable; Infarction when tissue permanently dies.

Arteries are active structures with different layers, providing blood pressure to keep circulation moving.  As such, their complexity has potential to be disrupted by a variety of rare diseases usually auto-immune in nature.  The broad term for arterial inflammation is Vasculitis; Lupus is the most well-known.  See a small comment under Rheumatological Conditions.

Veins are too simple to be similarly affected.  But they can sometimes wear out, leading blood to pool in the legs as it tries to return to the heart.  This can result in troublesome varicosities, and chronic leg ulcers that are quite hard to cure.  Worse, stagnant blood within larger veins can clot (Thrombosis).  If a piece of clot breaks off, it gets swept into the lung to cause a Pulmonary Embolism, occasionally fatal.  The biggest risk for thrombosis is immobility (long-legged cast, general anesthesia), other risks include inherited diseases, active cancer, and now Covid.

Prevention  —  The best way to keep the circulatory system healthy is exercise.  Everything helps, but the most optimal exercise is active cardio, even simple walking if it leads you to huff and puff a little.  Aim for 150 minutes per week.  The beneficial effects of exercise were first realized through fascinating studies shortly after World War II.