Two Great Circuits
Blood runs two loops — a short pulmonary circuit to the lungs and a long systemic circuit to the body — through arteries that carry it away from the heart, capillaries where exchange happens, and veins that return it, each vessel's wall built for its task. · 12 min
The heart is the pump; the vessels are the roads. There are only three kinds. Arteries carry blood away from the heart, veins carry it back, and between them lie capillaries — tubes so thin that oxygen and nutrients cross their walls into your tissue. And there are only two trips: a short one to the lungs, a long one to the body. This folio follows a drop of blood around both loops and looks at why each vessel is built the way it is.
Guess before you learn
Which vessel has the thinnest wall — thin enough that a molecule of oxygen can cross it?
The capillary wins: its wall is one cell thick, which is exactly what lets oxygen, food, and waste pass between blood and tissue. If you guessed an artery, that is a common trade — arteries feel important because of the pulse, but the real exchange happens in the thinnest vessels, not the strongest.
Undergrad
3–5
Blood leaves the heart in arteries, thick tubes that can take a strong push. It comes back in veins, thinner tubes with little doors that stop it slipping backward. In between are capillaries, tubes so tiny and thin-walled that oxygen and food pass straight through into your body.
There are two trips. A short trip carries blood to the lungs and back to pick up oxygen. A long trip carries it out to the whole body and back to drop that oxygen off. Every drop takes turns on both.
6–8
Three kinds of vessel carry blood. Arteries carry it away from the heart under high pressure, so their walls are thick, muscular, and elastic. Veins carry it back at low pressure; their walls are thinner and hold valves that stop backflow. Capillaries are only one cell thick — the place where oxygen, food, and waste actually cross between blood and tissue.
Blood runs two circuits in series. The pulmonary circuit is a short loop from the right heart to the lungs and back. The systemic circuit is a long loop from the left heart to the whole body and back. An artery is named for its direction — away from the heart — not for whether its blood is oxygen-rich.
9–12
Vessel walls are a direct record of pressure. Arteries meet the pulse of each heartbeat, so their walls carry elastic sheets and a thick ring of smooth muscle. Capillaries trade wall for thinness: a single layer of cells, the shortest possible distance to cross. Veins run at low pressure and carry valves that keep blood moving toward the heart against gravity.
The two circuits differ in scale. The pulmonary circuit is short and low-resistance, so its pressures stay low; the systemic circuit spans the whole body and runs at high pressure. Pressure falls steadily along any circuit — highest in the arteries, lowest in the veins — because each vessel adds resistance to the flow.
K–2
Blood travels around your body in tubes. Big tubes carry it away from the heart. Tiny tubes let it drop off oxygen and food. Then more tubes carry it back to the heart.
In the tiny tubes the walls are so thin that oxygen can slip right through to your body. That is where all the real work happens.
Undergrad
Read the three vessel classes as solutions to three mechanical problems. Elastic arteries near the heart store energy as they stretch in systole and release it in diastole, smoothing pulsatile output into steadier flow. Muscular arteries and arterioles regulate distribution by adjusting their radius. Capillaries, one endothelial cell thick, minimize the diffusion distance; veins serve as a high-capacitance reservoir holding most of the blood volume at low pressure.
The pulmonary and systemic circuits sit in series yet differ nearly an order of magnitude in resistance and pressure. Because flow equals the pressure difference divided by resistance, the low-resistance pulmonary bed sustains the same flow at roughly a fifth of systemic pressure — which is why the right ventricle that feeds it needs far less muscle than the left.
Postgrad
Along a single systemic path, mean pressure falls from near arterial levels to a few mmHg at the right atrium, and the steepest drop occurs across the arterioles — the principal resistance vessels, whose smooth-muscle tone sets both regional distribution and total peripheral resistance. Total cross-sectional area, meanwhile, rises enormously at the capillaries, so flow velocity is slowest exactly where exchange must occur.
Wall composition maps onto these regimes precisely: elastin dominates where pulsatility is highest, smooth muscle where regulation matters, and a bare endothelium where exchange demands thinness. The venous side, holding the majority of blood volume, trades wall for compliance and relies on valves and the squeeze of surrounding skeletal muscle to return blood against a standing gravitational column.
capillary
The smallest vessel, its wall a single cell thick. Blood gives up oxygen and takes on waste here — the only place exchange with tissue happens.
Look closely at the walls, because each is shaped for its job. An artery meets the full force of the heartbeat, so its wall is thick, with elastic sheets that stretch and recoil and a ring of muscle that can widen or narrow the tube. A vein carries blood back at low pressure, so its wall is thinner — but it holds valves, small flaps that stop blood from sliding backward between beats. A capillary gives up wall almost entirely: one cell thick, it is built for crossing, not for pressure.
Now the two circuits, in order. The right side of the heart sends oxygen-poor blood on the short pulmonary circuit — out to the lungs, where it loads oxygen, and back to the left side. The left side then sends that oxygen-rich blood on the long systemic circuit — out to every tissue, where it unloads oxygen, and back to the right side. The two run in series, so a single drop passes through both, one after the other, again and again.
Trace a red blood cell around both circuits — the steps fade as you master them
left ventricle → aorta
aorta → arteries → body capillaries
body capillaries → veins → right side
right ventricle → lungs → left side
Three vessels, two circuits, one direction. The blood that leaves the left side on the long trip is the same blood the right side just sent to the lungs — the heart and vessels are one closed system. The lungs have been waiting in the background of both this folio and the last. The next folio walks into them: the airways that bring the air to meet the blood.
Why is this true?
Why is an artery still an artery even when its blood is oxygen-poor?
Because the word names direction, not cargo. Any vessel leaving the heart is an artery; the pulmonary artery leaves the right ventricle carrying oxygen-poor blood to the lungs and is an artery all the same.
Practice — new ink and old, interleaved
1.From Unit I: along the arm, the segment of an artery nearer the shoulder is which term, relative to the segment nearer the wrist?
2.The ribs lie between the skin of the chest and the heart. Relative to the heart, the ribs are:
3.From the last folio: which chamber pumps blood into the systemic circuit through the aorta?
4.Without looking back: name the three kinds of vessel and, in a few words, what each one does.
Arteries carry blood away from the heart, capillaries let it exchange oxygen and waste with tissue, and veins carry it back to the heart.
How close were you? Grade yourself honestly — it sets your review date.
5.Without looking back: describe the anatomical position, and name the four main directional pairs with what each means.
Standing erect, feet forward, arms at the sides, palms forward, head level. Superior/inferior (toward head / toward feet), anterior/posterior (front / back), medial/lateral (toward / away from the midline), proximal/distal (nearer / farther from a limb's root).
How close were you? Grade yourself honestly — it sets your review date.
6.Blood has just left the right ventricle through the pulmonary valve. Where is it headed?
7.From Unit I: the heart sits in the thoracic cavity, wrapped in a two-layered serous membrane. What is that membrane called?
8.From Unit I: an artery's wall has a thick middle layer of smooth muscle that widens and narrows the tube. Smooth muscle is which of the four primary tissue types?
9.If pressure in the aorta averages about 100 mmHg and in the vena cava about 5 mmHg, roughly how many mmHg does the pressure fall across the systemic circuit?