Blood Vessels & Circulation
The heart is only the pump. The real work of blood flow happens in the tubes. In this chapter, you will follow one red blood cell on its trip. It is flung out of the heart into arteries that branch into smaller and smaller channels. Then it reaches capillaries, tubes so thin that oxygen and food leak through their walls into your tissues. Last, it climbs back up through the veins, against gravity, to start the loop again.
Along the way, you will learn why an artery wall is thick and a capillary wall is just one cell thick. You will learn what blood pressure really measures. You will see how fluid "decides" whether to leave the blood or go back in. You will also learn the clever tricks your body uses to lift blood up out of your feet. Once you know the vessels, you will know more than how blood moves. You will see why swollen ankles, bulging leg veins, and a stroke all come back to the same set of rules.
By the end of this chapter you can
- Name the three tunics, or layers, of a blood vessel wall. Describe the tissue in each one and its job.
- Compare arteries, veins, and capillaries. Look at how thick the wall is, how wide the inside is, the pressure, and the job.
- Define blood pressure and tell systolic from diastolic pressure. Work out pulse pressure and mean arterial pressure.
- Explain how pressure and resistance shape blood flow. Explain why the arterioles, the tiniest arteries, control most of the body's resistance.
- Draw capillary exchange as a tug-of-war between hydrostatic pressure and colloid osmotic pressure. Explain how this balance can cause swelling, or edema.
- Tell the pulmonary loop from the systemic loop. Describe special routes such as the hepatic portal system.
- List the ways blood gets back to the heart, called venous return. These are the skeletal muscle pump, the breathing pump, and one-way valves. Link their failure to real diseases.
Section 1The wall of a blood vessel
Three layers around a hollow center
Every artery and vein is built the same way. It has a set of layers, one inside the other, called tunics. The word comes from the Latin for "coats." These coats wrap around a hollow channel called the lumen. The lumen is the space the blood flows through.
From the inside out, the three layers are the tunica intima, the tunica media, and the tunica externa. Every vessel has the same layers. What changes from an artery to a vein to a capillary is how thick each layer is. It also changes how much muscle or stretchy tissue each layer has. Learn the three coats once, and you can read the wall of any vessel in the body.
Intima, media, externa: from lining to anchor
The inner coat is the tunica intima. It is mostly one sheet of flat cells called the endothelium. This sheet lines the heart and every blood vessel in one unbroken surface. It is much more than wallpaper. It is so smooth that blood cells glide past without clotting. It also sends out chemical signals, such as nitric oxide. These signals tell the muscle below to relax or tighten. Under the endothelium is a thin base layer. In arteries, there is also a springy sheet called the internal elastic lamina.
The middle coat is the tunica media. It is usually the thickest layer, and it is the vessel's muscle. It is made of rings of smooth muscle mixed with sheets of stretchy fibers. When the smooth muscle tightens, the lumen gets narrower. This is vasoconstriction. When the muscle relaxes, the lumen gets wider. This is vasodilation. Nerves and hormones control this layer. So the tunica media is the dial your body turns to send blood where it is needed. It also helps set your blood pressure.
The outer coat is the tunica externa, also called the tunica adventitia. It is a tough sleeve of collagen fibers. It anchors the vessel to the tissue around it and keeps it from stretching too far. Big vessels have walls too thick to feed themselves from the inside. So their tunica externa carries its own tiny blood vessels. These are called the vasa vasorum, which means "vessels of the vessels."
| Tunic | Main tissue | Job |
|---|---|---|
| Tunica intima | Endothelium + a thin base layer (+ internal elastic lamina in arteries) | Smooth, non-stick lining. Sends signals that relax or tighten the wall. |
| Tunica media | Smooth muscle + stretchy fibers | Vasoconstriction and vasodilation. Sets how wide the vessel is and helps set blood pressure. |
| Tunica externa | Collagen (and stretchy) connective tissue | Anchors and protects the vessel. Carries the vasa vasorum in big vessels. |
Peel the wall apart one layer at a time in the activity below.
- Every vessel wall has three tunics around a hollow center, the lumen. They are the intima, media, and externa.
- The tunica intima is lined by the endothelium. This smooth, non-stick lining tells the wall when to relax or tighten.
- The tunica media is smooth muscle. It sets how wide the vessel is through vasoconstriction and vasodilation.
- The tunica externa is collagen, and it anchors the vessel. Atherosclerosis starts when the intima is hurt.
Section 2Arteries, veins & capillaries
Arteries: the high-pressure pipes
An artery is any vessel that carries blood away from the heart. This rule is about direction, not about oxygen. That will matter later. Arteries get a hard push of blood with every heartbeat. So they have a thick, muscular tunica media and strong, stretchy walls.
The biggest arteries near the heart, like the aorta, are elastic (conducting) arteries. Their walls stretch as the ventricle pumps blood in. Then they spring back between beats. This turns the pulsing flow into a steadier stream. Farther out, muscular (distributing) arteries bring blood to each organ. The smallest branches are the arterioles. They act like nozzles you can adjust. They control how much blood flows into each bed of capillaries.
Veins: the low-pressure reservoirs
A vein carries blood back toward the heart. By the time blood reaches the veins, it has used up nearly all its pressure in the capillaries. So veins do not need thick, muscular walls. Instead, they have a thin middle layer, a wide and floppy lumen, and a large outer layer.
The key feature of veins is their one-way valves. These are flaps of the inner lining. They snap shut to stop blood from sliding backward. Veins stretch easily, so they act as the body's blood reservoir, or storage tank. At rest, about two-thirds of all your blood sits in your veins. It is ready to be squeezed back into use when your body needs more. For this reason, veins are also called capacitance vessels, which means "holding" vessels.
Capillaries: where the exchange happens
Between the smallest arteriole and the smallest venule are the capillaries. They are the reason the whole system exists. A capillary wall is just one layer of endothelium on a thin base layer. It is the tunica intima with everything else stripped away. That paper-thin wall is the point. Oxygen, carbon dioxide, sugar, and wastes can slip right across it, between the blood and the tissue.
There are so many capillaries that no cell in the body is more than about a tenth of a millimeter from one. Capillaries come in three levels of leakiness. Each type fits how much needs to cross. Continuous capillaries are the tightest. They are found in muscle and the brain. Fenestrated capillaries have small pores, or "windows." They are found in the kidney and intestine. Sinusoidal capillaries have the widest gaps. They are found in the liver and bone marrow.
| Feature | Artery | Capillary | Vein |
|---|---|---|---|
| Direction | Away from heart | Through tissues | Toward heart |
| Wall | Thick, muscular, stretchy | One endothelial cell thick | Thin, wide lumen |
| Pressure | High | Falling | Low |
| Valves | None (the aortic and pulmonary valves are in the heart) | None | Yes, one-way |
| Main job | Carry and spread out blood under pressure | Swap O₂, CO₂, nutrients, and wastes | Bring blood back and store it (reservoir) |
Switch between the three types of vessels and watch their jobs change.
- Arteries carry blood away from the heart under high pressure. They have thick, stretchy, muscular walls.
- Veins carry blood toward the heart. They have thin walls and one-way valves. They hold most of the blood as a reservoir.
- Capillaries are one cell thick. They are the only place where O₂, CO₂, and nutrients are swapped.
- Whether a vessel is an artery or a vein depends on the direction of flow, not on how much oxygen it carries.
Section 3Blood pressure & the pressure gradient
Systolic over diastolic
Blood pressure is the force of moving blood pushing on the vessel walls. It is measured in millimeters of mercury (mm Hg). In the big arteries, this pressure is not steady. It rises and falls with each heartbeat. When the ventricles squeeze and pump out blood, pressure peaks. That high point is the systolic pressure. When the ventricles relax and refill, pressure sinks to its low point. That is the diastolic pressure.
A typical healthy reading is about 120/80 mm Hg. That means systolic over diastolic. The gap between the two numbers is the pulse pressure. You find it by taking systolic minus diastolic. Here it is 40 mm Hg. It is the surge you feel as a pulse at your wrist.
Two numbers worth working out
The heart spends more of each beat resting than squeezing. So the true average pressure that pushes blood forward is not just the middle of the two numbers. This average is the mean arterial pressure (MAP). To estimate it, take the diastolic pressure and add one-third of the pulse pressure.
MAP matters to doctors because it is the pressure that actually pushes blood through your organs. If MAP falls much below about 60 mm Hg, the brain and kidneys do not get enough blood. For a reading of 120/80, MAP is about 80 + (40 ÷ 3) ≈ 93 mm Hg.
Pressure falls, and the arterioles set the toll
Follow the blood outward, and its pressure drops the whole way. That steady drop is the pressure gradient. It is exactly what keeps blood flowing, because fluid always moves from high pressure to low pressure. Pressure is high in the aorta and still pulses in the large arteries. Then it drops off a cliff across the arterioles. By the time blood reaches the venae cavae, the pressure is near zero.
Blood flow follows a simple rule: flow = pressure difference ÷ resistance. Resistance is how hard the vessel pushes back against the flow. It depends most on the vessel's radius, or half its width. Resistance goes up as the radius goes down, and it does so very fast. It changes with the fourth power of the radius (R ∝ 1 ÷ r⁴). So if you cut the radius in half, resistance goes up 16 times. If you double the radius, resistance drops to 1/16 of what it was.
Arterioles have muscular walls that can tighten or widen on command. That makes them the body's main resistance vessels. They are like faucets you can adjust. They set how much blood each organ gets. They also set your overall blood pressure.
| Location | About how much pressure | What is happening |
|---|---|---|
| Aorta / large arteries | ~120/80 mm Hg | High and pulsing. Stretchy walls spring back to smooth the flow. |
| Arterioles | Sharp drop | The biggest drop. These are the main resistance vessels. |
| Capillaries | ~35 → 17 mm Hg | Low and gentle. This protects the thin walls and drives the swap. |
| Venules / veins | Low | Little pressure is left. Other pumps take over. |
| Venae cavae | Near 0 mm Hg | Blood arrives back at the heart. |
Tap each part of the trip to see the pressure fall.
- Blood pressure is the force of blood on the vessel wall. It is written as systolic/diastolic, such as 120/80 mm Hg.
- Pulse pressure = systolic − diastolic. MAP ≈ diastolic + ⅓ of pulse pressure. MAP is what pushes blood through your organs.
- Blood flows down a pressure gradient. Flow = ΔP ÷ resistance. Resistance changes with 1 over r⁴ (R ∝ 1 ÷ r⁴), so a narrower vessel resists far more.
- Arterioles are the main resistance vessels. They are the dial for both organ blood flow and blood pressure.
Section 4Capillary exchange
A tug-of-war between two pressures
The whole system is built to bring blood to the capillaries. This is where the key trade happens, across that wall only one cell thick. Small molecules cross easily. The trickier part is how fluid moves in and out. Two pressures pull in opposite directions to decide it.
The first is hydrostatic pressure. This is the blood pressure inside the capillary. It pushes fluid out through the wall and into the spaces in the tissue. The second is colloid osmotic pressure, also called oncotic pressure. It comes mostly from a large protein in the blood called albumin. Albumin is too big to leave the blood. Because these proteins stay behind, they pull water back in by osmosis. Fluid moves in whichever direction the stronger pressure wins.
Out at the start, back in at the end
Here is the clever part. The balance flips along the length of a capillary. At the arteriolar end, where blood comes in, hydrostatic pressure is fairly high. It is about 35 mm Hg. That easily beats the osmotic pull of about 25 mm Hg. So the overall force pushes fluid out. This is filtration. It brings water, oxygen, and nutrients to the cells.
As blood moves along the capillary, its hydrostatic pressure fades. But the osmotic pull stays the same. By the venule end, hydrostatic pressure has dropped below the osmotic pressure. Now the overall force pulls fluid back in. This is reabsorption. It picks up carbon dioxide and wastes on the way. At any point, the difference between the push and the pull is called the net filtration pressure.
The lymphatic safety net, and edema
The two sides do not quite balance. Capillaries push out a little more fluid than they take back. About 85% goes back into the blood. About 15% is left behind in the tissues. That extra fluid is not lost. The lymphatic vessels mop it up and later return it to the blood.
If any part of this system fails, fluid builds up in the tissues. The result is swelling, called edema. Three things can tip the balance this way. One is too much hydrostatic pressure, as in heart failure or a leg you stand on too long. Another is too little albumin to pull fluid back, as in liver disease, kidney disease, or not eating enough protein. The third is blocked lymph drainage.
| Force | Where it comes from | Direction |
|---|---|---|
| Hydrostatic pressure | Blood pressure inside the capillary | Pushes fluid out of the blood |
| Colloid osmotic (oncotic) pressure | Proteins in the blood, mostly albumin | Pulls fluid into the blood |
| Overall result at the arteriolar end | Hydrostatic > osmotic | Filtration (fluid leaves) |
| Overall result at the venule end | Osmotic > hydrostatic | Reabsorption (fluid comes back) |
Tap each end of the capillary to watch the tug-of-war play out.
- Fluid exchange is a contest. Hydrostatic pressure pushes fluid out, and colloid osmotic pressure pulls it in.
- Filtration wins at the arteriolar end. Reabsorption wins at the venule end.
- Capillaries push out a bit more fluid than they take back. The lymphatics return the extra.
- An imbalance causes edema. Causes include high hydrostatic pressure, low albumin, or blocked lymph vessels.
Section 5The circulatory routes
Two great circuits
Blood does not wander. It runs in set loops called circuits. The whole system is really two circuits joined at the heart. The pulmonary circuit is the short loop to the lungs. The right side of the heart pumps blood low in oxygen out through the pulmonary arteries. In the lung capillaries, the blood picks up oxygen and dumps carbon dioxide. Then the pulmonary veins carry the blood, now rich in oxygen, back to the left side of the heart.
The systemic circuit is the long loop to everything else. The left side of the heart pumps blood rich in oxygen out through the aorta. It goes to every organ, arm, and leg. Then the veins bring the blood, now low in oxygen, back to the right side of the heart to start again. The two circuits run one after the other, in series. So the same amount of blood must pass through both. The right and left sides of the heart pump in perfect step.
Portal systems: two capillary beds in a row
Most blood passes through only one bed of capillaries on its way from the heart and back. A portal system is different. Blood flows through two capillary beds, one after the other, joined by a vein. The most important example is the hepatic portal system. "Hepatic" means "of the liver."
Blood leaving the stomach and intestines is full of nutrients it just absorbed. It may also carry drugs or poisons you swallowed. This blood does not go straight back to the heart. Instead, the hepatic portal vein sends it into a second capillary bed inside the liver. The liver stores nutrients, cleans out harmful chemicals, and adjusts blood sugar. Only then does the blood move on. It joins the main flow through the hepatic veins.
Special deliveries
A few organs have their own special setups. The coronary circulation is the heart feeding itself. Its arteries branch off the aorta the moment blood leaves the heart. The cerebral circulation feeds the brain. It has a protective ring of linked arteries called the circle of Willis. If one artery into the ring is blocked, others can send blood around the gap.
Links like this between vessels are called anastomoses. They are natural detours that give a backup blood supply. Where there are few of these links, a blockage is much more dangerous. That is one reason a clot in a coronary artery so often causes a heart attack.
| Feature | Pulmonary circuit | Systemic circuit |
|---|---|---|
| Pumped by | Right side of the heart | Left side of the heart |
| Sends blood to | The lungs | The rest of the body |
| Blood leaving the heart | Low in oxygen (in the pulmonary arteries) | Rich in oxygen (in the aorta) |
| Blood coming back | Rich in oxygen (in the pulmonary veins) | Low in oxygen (in the venae cavae) |
| Main job | Load O₂ and unload CO₂ | Deliver O₂ and nutrients, and collect wastes |
Tap each route to trace where the blood goes.
- The pulmonary circuit (right heart → lungs) loads O₂. The systemic circuit (left heart → body) delivers it.
- In the pulmonary circuit, arteries carry blood low in oxygen, and veins carry blood rich in oxygen.
- The hepatic portal system sends blood from the gut through the liver before it goes back to the heart. It passes two capillary beds in a row.
- Anastomoses, like the circle of Willis, give backup routes. Where they are few, a blockage is dangerous.
Section 6Venous return
An uphill problem
By the time blood reaches the veins, it has almost no pressure left. It is often around 15 mm Hg, and it falls toward zero near the heart. Yet the blood in your feet must somehow climb more than a meter, against gravity, back to the right side of the heart.
The amount of blood coming back to the heart each minute is called venous return. Over time, it must exactly match the cardiac output. Whatever the heart pumps out, the veins must bring back. With so little pressure, the veins cannot do this alone. They rely on outside pumps and clever one-way plumbing.
The muscle pump and the valves
The main helper is the skeletal muscle pump. Deep veins run between your muscles. Every time you tighten a muscle, like with each step you take, it squeezes the veins. This forces blood along. The one-way valves make the squeeze work. When a muscle presses on a vein, the valve above opens to let blood rush upward. The valve below slams shut to stop it from falling back.
Tighten the muscle, and blood is pushed toward the heart. Relax it, and the valves hold onto the ground you gained. When you walk across a room, your calf muscles really are a second heart. They pump blood up your legs.
The breathing pump, and what fails
A second helper is the respiratory pump, or breathing pump. Each time you breathe in, your chest gets bigger and the pressure inside it drops. At the same time, pressure in your belly rises. This squeezes the belly veins. It draws blood up into the chest veins, where the pressure is low, and on toward the heart.
A third helper is venoconstriction. Here, the nervous system tightens the vein walls. This pushes stored blood back into use when the body needs more.
When these helpers fail, the results are familiar. If you stand still for a long time, the muscle pump stops. Blood pools in your legs, and you may faint. Weak or leaky valves let blood collect and stretch the veins. They become bulging varicose veins. Blood that sits still, like on a long flight or after surgery, can clot in a deep vein. This is a dangerous deep vein thrombosis.
| Helper | How it works | When it matters most |
|---|---|---|
| Skeletal muscle pump | Working muscles squeeze deep veins. Valves keep blood flowing one way. | Walking, running, and any leg movement |
| Respiratory pump | Breathing in lowers chest pressure and raises belly pressure. This draws blood upward. | Every breath, and deeper breaths during exercise |
| Venous valves | One-way flaps stop blood from flowing back between pumps | Standing up straight, against gravity |
| Venoconstriction | Nerves tighten the vein walls, pushing stored blood forward | Exercise, blood loss, and standing up |
Watch the muscle pump and valves lift blood against gravity in the activity.
- Venous return is the blood coming back to the heart. Over time, it must equal cardiac output.
- Veins have little pressure left. So they rely on the skeletal muscle pump, the respiratory pump, and one-way valves.
- Venoconstriction pushes stored blood forward during exercise or blood loss.
- When these fail, you can get fainting, varicose veins, and deep vein thrombosis. That is why moving matters.
The chapter in one breath
- Every vessel wall has three tunics (intima, media, externa) around a hollow lumen. Blood vessel disease starts in the endothelium lining.
- Arteries carry blood away under high pressure and have thick, stretchy walls. Veins bring it back under low pressure, have valves, and hold most of the blood. Capillaries are one cell thick and do all the swapping.
- Blood pressure is written as systolic/diastolic. Pulse pressure is the difference between them. MAP is what pushes blood through the organs.
- Blood flows down a pressure gradient (flow = ΔP ÷ resistance). The muscular arterioles are the main resistance vessels, because resistance changes with 1 ÷ r⁴.
- Capillary exchange is a tug-of-war. Hydrostatic pressure pushes out, and colloid osmotic pressure pulls in. Filtration happens at the arteriolar end and reabsorption at the venule end. An imbalance causes edema.
- Blood runs in a pulmonary circuit (to the lungs) and a systemic circuit (to the body). Special routes like the hepatic portal system send blood through two capillary beds.
- Venous return against gravity relies on the skeletal muscle pump, the respiratory pump, and one-way valves. When they fail, you can get fainting, varicose veins, and clots.
ReferenceWords worth knowing
Here is a quick list of the terms in this chapter. Skim it now, or come back any time a word trips you up.
- Arteriole
- A very small artery with a muscular wall. It controls how much blood flows into a capillary bed. It is the body's main resistance vessel.
- Artery
- A vessel that carries blood away from the heart. It is defined by direction, not by how much oxygen it carries.
- Blood pressure
- The force of moving blood pushing on the vessel wall. It is measured in millimeters of mercury (mm Hg).
- Capillary
- The smallest vessel, just one endothelial cell thick. Here oxygen, carbon dioxide, nutrients, and wastes are swapped with the tissues.
- Colloid osmotic (oncotic) pressure
- The inward pull made by proteins in the blood, mostly albumin. It draws water back into the capillary.
- Diastolic pressure
- The lower pressure in the arteries. It is measured while the ventricles relax and refill.
- Edema
- Swelling from extra fluid building up in the tissues. It happens when filtration and reabsorption in the capillaries get out of balance.
- Endothelium
- The single layer of flat cells lining the heart and all blood vessels. It is the inner surface of the tunica intima.
- Filtration
- Fluid moving out of a capillary into the tissues. It wins at the arteriolar end, where hydrostatic pressure is highest.
- Hepatic portal system
- A route that sends blood full of nutrients from the gut through the liver’s capillaries before it goes back to the heart.
- Hydrostatic pressure
- The blood pressure inside a capillary. It pushes fluid out through the wall.
- Mean arterial pressure (MAP)
- The average pressure pushing blood through the body loop. It is about the diastolic pressure plus one-third of the pulse pressure.
- Pulmonary circuit
- The loop from the right heart to the lungs and back. The blood picks up oxygen and drops off carbon dioxide.
- Pulse pressure
- The difference between systolic and diastolic pressure. It is the surge you feel as a pulse.
- Reabsorption
- Fluid moving back into a capillary from the tissues. It wins at the venule end, where osmotic pressure is higher than hydrostatic pressure.
- Resistance
- How much a vessel pushes back against blood flow. It depends mostly on the vessel’s radius. It changes with the fourth power of the radius (R ∝ 1 ÷ r⁴), so it rises steeply as the vessel narrows.
- Skeletal muscle pump
- Working muscles squeezing the deep veins. Together with one-way valves, this pushes blood back toward the heart.
- Systemic circuit
- The loop from the left heart to the whole body and back. It delivers oxygen and collects wastes.
- Systolic pressure
- The higher pressure in the arteries. It is measured while the ventricles squeeze and pump out blood.
- Tunica media
- The middle layer of a vessel, made of smooth muscle and stretchy fibers. It sets how wide the vessel is through vasoconstriction and vasodilation.
- Vein
- A vessel that carries blood toward the heart. It has thin walls and valves. It is the body’s main blood reservoir.
- Venous return
- The amount of blood coming back to the heart each minute. Over time, it must equal cardiac output.