Grade by Grade
E. Animals: Structure and Function · Animals: Structure and Function

Blood Vessels, Blood Pressure and Capillary Exchange

E3.3 E2.1 E2.3 A1.2 A1.12About 70 minutes
Learning goals
  • I can relate the structure of arteries, arterioles, capillaries and veins to their functions.
  • I can explain how blood pressure is measured with a sphygmomanometer and interpret a reading such as 118/76 mm Hg.
  • I can explain how blood pressure and osmotic pressure move fluid out of and back into capillaries, and the role of the lymphatic system.
  • I can explain how veins return blood to the heart and how arterioles control blood flow.

Words to know

blood pressure
The force of blood pushing on the walls of blood vessels, measured in millimetres of mercury (mm Hg).
sphygmomanometer
A blood-pressure cuff and gauge used to measure arterial blood pressure.
arteriole
A small artery whose smooth muscle can narrow (vasoconstriction) or widen (vasodilation) to control blood flow to a tissue.
endothelium
The single layer of flat cells lining every blood vessel; a capillary wall is only endothelium.
osmotic pressure
The pull of water toward a region with more dissolved particles; in blood, mostly due to plasma proteins such as albumin.
lymph / lymphatic system
Lymph is the fluid collected from tissues by lymph vessels; the lymphatic system returns it to the blood and helps fight infection.
edema
Swelling caused by excess fluid collecting in the tissues.
pulse
The rhythmic stretching of an artery wall with each heartbeat, felt where an artery is near the surface.

The family doctor's cuff

At a check-up, a nurse wraps a cuff around your upper arm, pumps it tight, and lets it hiss down while listening or watching a screen. Then they say something like "118 over 76." Those two numbers are the systolic and diastolic pressures in your arteries. Today you'll learn what they mean, how they are measured, and why pressure in your blood vessels decides where fluid goes in your body.

Vessels built for their jobs

Blood leaves the heart in arteries, which branch into arterioles, then into capillaries. Capillaries join into venules, which join into veins that return blood to the heart.

  • Arteries have thick walls with layers of smooth muscle and elastic fibres. With each heartbeat they stretch, then recoil, smoothing the flow and keeping pressure up during diastole. The stretch is the pulse you feel at your wrist (radial artery) or neck (carotid artery).
  • Arterioles have relatively more smooth muscle. They are the body's flow-control valves: vasoconstriction (narrowing) reduces flow to a tissue, and vasodilation (widening) increases it. On a cold Canadian winter day, skin arterioles constrict to save heat (your skin goes pale); during exercise, arterioles in your working muscles dilate.
  • Capillaries have walls only one cell thick (endothelium). They are so narrow (about 5 to 10 µm) that red blood cells pass in single file. Blood moves slowest in capillaries, because the total cross-sectional area of all the capillaries together is enormous, which gives time for exchange.
  • Veins have thinner walls and wider lumens, and blood in them is at low pressure. Many veins, especially in the legs and arms, have one-way valves.
lumenarterythick muscle andelastic layer, small lumenwide lumenveinthin wall, wide lumen,valves (not shown here)capillaryone layer of endothelium;just wide enough for one RBCouter connective tissuesmooth muscle + elastic fibres(capillary enlarged far more than the others; not to scale)
Cross-sections of an artery, a vein and a capillary. The artery's thick muscular, elastic wall withstands high pressure; the vein's thin wall and wide lumen carry low-pressure blood; the capillary is just a single layer of cells.

Getting blood back up from your feet

By the time blood reaches the veins, most of the heart's push is used up. Three things return it to the heart:

  1. Valves in the veins let blood move only toward the heart.
  2. The skeletal muscle pump: when leg muscles contract, they squeeze the veins between them, pushing blood up past the valves. (This is why standing very still for a long time can make some people feel faint, and why walking helps.)
  3. The respiratory pump: when you inhale, pressure in the chest falls, drawing blood up into the large veins near the heart.

Measuring blood pressure

Blood pressure is measured in the brachial artery of the upper arm with a sphygmomanometer:

  1. The person sits quietly for a few minutes, feet flat, arm supported at heart level.
  2. The cuff is pumped up until it squeezes the artery completely shut (no blood flows, so no sound).
  3. A stethoscope is placed over the artery just below the cuff, and the cuff is deflated slowly.
  4. When cuff pressure falls just below the systolic pressure, blood spurts through during each systole, making a tapping sound (called Korotkoff sounds). The pressure at the first sound is the systolic reading.
  5. As the cuff keeps deflating, the sounds continue until the artery stays open all the time and blood flows smoothly. The pressure when the sounds disappear is the diastolic reading.

The result is written systolic/diastolic, such as 118/76 mm Hg. Automatic home monitors detect the pressure pulses electronically instead of by sound.

  • A typical healthy young adult reading is around 120/80 mm Hg or a bit lower.
  • Pulse pressure = systolic − diastolic (about 40 mm Hg).
  • Mean arterial pressure (the average pressure pushing blood to the organs) ≈ diastolic + ⅓ × pulse pressure, because the heart spends longer in diastole than in systole.
  • Blood pressure that stays high is hypertension. In Canada, readings that are consistently at or above about 140/90 mm Hg in a clinic are generally treated as high for most adults (lower targets apply for some people). High pressure often has no symptoms, which is why it is checked regularly.
6080100120140160180pressure (mm Hg)time (cuff slowly deflating) →first tapping sound: systolic 120sounds stop: diastolic 80cuff squeezes artery shut: silentthen silent (smooth flow)
As the cuff deflates, tapping sounds start at the systolic pressure (120) and stop at the diastolic pressure (80), giving a reading of 120/80 mm Hg. (Schematic.)
Worked example

Example 1: Interpreting a blood-pressure reading

A student's reading is 112/70 mm Hg. (a) Which number is systolic? (b) Find the pulse pressure. (c) Estimate the mean arterial pressure.

  1. (a) The first (higher) number, 112 mm Hg, is systolic: the pressure during ventricular contraction.
  2. (b) Pulse pressure = 112 − 70 = 42 mm Hg.
  3. (c) MAP ≈ 70 + ⅓ × 42 = 70 + 14 = 84 mm Hg.

(a) 112 mm Hg; (b) 42 mm Hg; (c) about 84 mm Hg. The MAP is closer to the diastolic value because diastole lasts longer.

Capillary exchange

Capillaries are where the circulatory system does its real work. Oxygen, glucose, amino acids and other small molecules leave the blood; CO₂ and wastes enter. Gases and small molecules diffuse across the thin wall. But fluid also moves, driven by two opposing pressures:

  • Blood pressure (hydrostatic pressure) pushes fluid out of the capillary through small gaps in the wall.
  • Osmotic pressure pulls water in, because blood contains proteins (mostly albumin, made by the liver) that are too big to leave, so the blood has more dissolved particles than the tissue fluid.

Typical values:

  • Arterial end: blood pressure ≈ 35 mm Hg; osmotic pull ≈ 25 mm Hg. Net ≈ 10 mm Hg outward: fluid, carrying nutrients, leaves.
  • Venous end: blood pressure has fallen to ≈ 15 mm Hg; osmotic pull is still ≈ 25 mm Hg. Net ≈ 10 mm Hg inward: fluid, carrying wastes, returns.

A little more fluid leaves than returns. The extra is collected by tiny lymph capillaries, becomes lymph, and flows through lymph vessels and lymph nodes (where white blood cells check it for microbes) before emptying back into the blood in large veins near the collarbones. If fluid builds up in the tissues, edema (swelling) results.

capillary: blood flows this way →fromarterioletovenulefluid, O₂, glucose OUTblood pressure 35 > osmotic 25net 10 mm Hg pushes fluid outfluid, CO₂, wastes INblood pressure 15 < osmotic 25net 10 mm Hg draws fluid inleftover fluid → lymph vessels → back to blood
Capillary exchange. At the arterial end, blood pressure is greater than the osmotic pull, so fluid moves out; at the venous end, blood pressure has dropped below the osmotic pull, so fluid moves back in. Lymph vessels collect the rest. (Typical values in mm Hg.)
Worked example

Example 2: Predicting edema

In some kinds of severe liver disease, the liver makes much less albumin. Use capillary exchange to predict and explain the effect on the tissues.

  1. Albumin is the main source of the blood's osmotic pull. Less albumin → lower osmotic pressure (say 15 mm Hg instead of 25).
  2. Arterial end: 35 − 15 = 20 mm Hg outward (more fluid leaves than before).
  3. Venous end: 15 − 15 = 0: little or no fluid is drawn back in.
  4. The lymphatic system cannot keep up, so fluid collects in the tissues and abdomen: edema.

Low albumin lowers osmotic pressure, so more fluid leaves the capillaries and less returns, causing swelling (edema).

Hands-on activity

Harvey's vein experiment (and a home blood-pressure reading)

Part A (no equipment). In 1628 the English physician William Harvey used this simple test to argue that blood flows in a circuit, returning to the heart through veins.

  1. Let one arm hang down for a minute and make a gentle fist, so the veins on the back of the hand or the inside of the forearm stand out.
  2. Find a straight vein about 4 to 5 cm long. Press one fingertip firmly on the end of it farther from the heart (toward the hand).
  3. With a finger of your other hand, press on the vein just next to the first finger and slide it toward the heart, emptying the vein, then hold it there.
  4. Lift the sliding finger. Does the vein refill from the heart side? Now lift the first finger instead. What happens?

Part B (only if your household has an automatic blood-pressure monitor). Sit quietly for 5 minutes, feet flat, arm resting at heart level, and take a reading. Record systolic, diastolic and pulse, then calculate pulse pressure and mean arterial pressure.

What to look for: in Part A, the emptied stretch stays flat as long as the hand-side finger presses; blood cannot flow back down from the heart side because a valve blocks it. When you release the hand-side finger, it fills quickly from the hand side: veins carry blood toward the heart only. In Part B, compare with about 120/80 mm Hg; readings vary with activity, stress and how you sit. One reading tells you little: doctors use several readings on different days.

Just for fun

An artery complains to a vein: "I'm under so much pressure!" The vein says: "Relax. Some of us get by on valves, a little help from our muscles, and good breathing."

Your turn

1

Give one structural feature of each vessel and explain how it fits its function: artery, capillary, vein.

2

In the reading 124/82 mm Hg, what do the two numbers mean?

3

When measuring blood pressure by hand, what tells you the systolic pressure? The diastolic?

4

Name the three things that help return blood through the veins.

5

What two pressures control fluid movement across capillary walls?

6

What happens to the extra fluid that leaves capillaries but doesn't return?

7

Why do your hands turn pale in the cold?

More practice: warm-up

1

Which artery is used to measure blood pressure?

2

What is the unit of blood pressure?

3

What is vasodilation?

4

Where is blood flow slowest?

5

What protein provides most of the blood's osmotic pressure?

More practice: on your own

1

Calculate the pulse pressure and mean arterial pressure for 130/85 mm Hg.

2

Explain why blood moves slowest in capillaries even though each capillary is so narrow.

3

Why might a person who stands still for a long time (for example, a soldier at attention) faint?

4

A lymph node is removed during surgery. Predict a possible effect on the nearby limb.

5

Why is high blood pressure called a "silent" condition, and why does it matter?

Challenge

1

At the arterial end of a capillary, blood pressure is 32 mm Hg and osmotic pressure 25 mm Hg. At the venous end, blood pressure is 12 mm Hg. Find the net pressure and direction at each end. If blood pressure rose by 10 mm Hg along the whole capillary (for example, from high venous pressure), what would happen?

2

Explain, using arterioles, how the body sends more blood to your leg muscles during a run while sending less to your digestive organs, even though cardiac output is limited.

3

A manual blood-pressure reading by a student gave 150/96 mm Hg, but a nurse's reading 10 minutes later was 118/74. Suggest at least three reasons, related to technique or the person, that could explain the difference.

Check your understanding

Quiz

1.

Which blood vessels control how much blood flows to each tissue?

2.

In the reading 120/80 mm Hg, 80 is the:

3.

During manual measurement, the first tapping sound marks the:

4.

At the arterial end of a capillary, fluid moves out because:

5.

Which feature of veins prevents blood from flowing backward?

6.

The extra tissue fluid left after capillary exchange is returned by the:

For parents and teachers

Big idea: E3.3 covers the anatomy of blood vessels and the transport of substances, and E2.3 asks students to use medical equipment such as a sphygmomanometer. This lesson explains how blood pressure is measured and what it means (E2.1 terms systolic, diastolic), and how fluid moves at capillaries.

How to help: Try Harvey's vein experiment together; it's quick and convincing. If you have a home blood-pressure monitor, take readings together under proper conditions (seated, rested, arm at heart level).

Common difficulties:

  • Which number is which? Systolic (higher) is first.
  • Capillary exchange direction. Out at the arterial end, in at the venous end; lymph collects the rest.
  • Thinking veins carry only oxygen-poor blood. Pulmonary veins carry oxygen-rich blood.

Note: a single high reading in a teen is usually due to activity or nerves. If readings are repeatedly high, talk to your family doctor.