B. Biology: Tissues, Organs, and Systems of Living Things · Tissues, Organs, and Systems of Living Things

The Circulatory System: The Heart, Blood Vessels and Blood

B3.4 B3.3 B2.1 A1.5 A1.6 A1.13 A2.1 A2.2About 75 minutes

Learning goals

I can trace the path of blood through the four chambers of the heart and the pulmonary and systemic circuits.
I can explain how the structures of arteries, veins and capillaries suit their functions.
I can describe the components of blood and what each does.
I can explain how heart tissue is organized and calculate cardiac output from heart rate data.

Words to know

atrium (plural: atria)
An upper chamber of the heart that receives blood.

ventricle
A lower chamber of the heart that pumps blood out of the heart.

valve
A flap that lets blood flow in one direction only.

artery
A blood vessel that carries blood away from the heart.

vein
A blood vessel that carries blood back to the heart.

capillary
A tiny blood vessel with a wall one cell thick, where materials are exchanged with cells.

pulmonary circuit / systemic circuit
The route from the heart to the lungs and back / from the heart to the rest of the body and back.

plasma
The liquid part of blood: mostly water, carrying dissolved nutrients, wastes, hormones and proteins.

cardiac output
The volume of blood the heart pumps per minute: heart rate × stroke volume.

A pump that never takes a day off

Put two fingers on the side of your neck or the inside of your wrist. That beat is your heart pushing blood through about 100 000 km of blood vessels (an often-quoted estimate for an adult, enough to wrap around Earth more than twice). It began beating before you were born and will beat billions of times in your life. The circulatory system is the body's delivery service: it carries oxygen, nutrients, hormones and heat to every cell, and carries carbon dioxide and other wastes away.

The heart: four chambers, two pumps

The heart is a fist-sized organ made mostly of cardiac muscle. A wall called the septum divides it into a right side and a left side, so it works as two pumps side by side:

The right side receives oxygen-poor blood from the body and pumps it to the lungs.
The left side receives oxygen-rich blood from the lungs and pumps it to the body.

Each side has two chambers: an upper atrium that receives blood and a lower, more muscular ventricle that pumps it out. Valves between the chambers, and at the exits of the ventricles, act like one-way doors. The "lub-dub" sound of a heartbeat is the sound of valves closing.

The path of blood:

body → vena cavae → right atrium → tricuspid valve → right ventricle → pulmonary artery → lungs (picks up O₂, drops CO₂) → pulmonary veins → left atrium → mitral valve → left ventricle → aorta → body

The heart drawn as if you are facing the person, so the person's right side is on your left. The left ventricle has the thickest muscular wall because it pumps blood to the whole body. The arrows show the direction of blood flow.

Double circulation. Blood passes through the heart twice on each complete trip: once through the right side to the lungs (pulmonary circuit) and once through the left side to the body (systemic circuit).

Heart tissue: from cells to a system

The heart is a great example of how cells → tissues → organs → systems (B3.3):

Cardiac muscle cells are branched and joined by intercalated discs, forming cardiac muscle tissue that contracts as a unit.
A small cluster of specialized cells in the wall of the right atrium, the sinoatrial (SA) node, is the heart's natural pacemaker. It sends out electrical signals that start each beat. Nervous tissue speeds the rate up or slows it down.
Epithelial tissue lines the chambers and valves; connective tissue forms the valves and holds everything together.
These tissues form one organ, the heart, which works with the blood vessels and blood as the circulatory system.

Canadian contributions

In 1950 in Toronto, surgeon Wilfred Bigelow, surgeon John Callaghan and engineer John Hopps of the National Research Council built one of the first electrical pacemakers, an external device that could restart and steady a heartbeat. Today, tiny implanted pacemakers help many people whose SA node or heart signals don't work properly. Earlier, Montréal physician Maude Abbott became a world expert on heart defects present at birth; her 1936 atlas of these defects helped make heart surgery on children possible.

Blood vessels: structure fits function

Arteries carry blood away from the heart under high pressure. Their walls are thick, with layers of smooth muscle and elastic connective tissue that stretch with each heartbeat and spring back (that's the pulse you feel).
Capillaries are tiny vessels with walls one cell thick. Every cell in the body is close to a capillary. Here, oxygen and nutrients diffuse out to the cells, and carbon dioxide and wastes diffuse in.
Veins carry blood back to the heart under low pressure. Their walls are thinner, with a wider space inside, and many have valves to stop blood flowing backward. Muscles squeezing around veins (for example, in your legs when you walk) help push blood back up to the heart.

Remember: arteries go Away from the heart. The pulmonary artery is the one artery that carries oxygen-poor blood, and the pulmonary veins are the veins that carry oxygen-rich blood, because the names depend on direction, not on oxygen.

Cross-sections of the three kinds of blood vessels. Arteries have thick, muscular walls for high pressure; veins have thin walls and valves; capillaries have walls one cell thick so that materials can diffuse through easily.

Blood: a connective tissue

An adult has about 5 L of blood. If you spin a blood sample in a centrifuge, it separates into:

Plasma (about 55%): a yellowish liquid, mostly water, carrying dissolved glucose, amino acids, salts, hormones, proteins and carbon dioxide.
Red blood cells (most of the rest): carry oxygen using haemoglobin.
A thin layer of white blood cells (fight infection) and platelets (help clotting).

Blood pressure is the force of blood pushing on artery walls. It is written as two numbers in millimetres of mercury, such as 120/80 mm Hg, a typical healthy adult value: the first number is the pressure when the ventricles contract, the second when they relax.

Worked example

Let's calculate cardiac output

Maya's resting heart rate is 70 beats per minute. Each beat, her left ventricle pumps about 70 mL of blood (her stroke volume). (a) What is her cardiac output in litres per minute? (b) She has about 5 L of blood. About how long does it take for a volume equal to all her blood to pass through her heart's left side?

(a) Cardiac output = heart rate × stroke volume = 70 beats/min × 70 mL/beat = 4900 mL/min.
Convert: 4900 mL ÷ 1000 = 4.9 L/min.
(b) Time = total volume ÷ rate = 5 L ÷ 4.9 L/min ≈ 1.0 min.
So at rest, a volume equal to all your blood is pumped around the body about once every minute.

(a) About 4.9 L/min. (b) About 1 minute.

Worked example

Let's analyse pulse data

Sam counts his pulse for 15 seconds at rest (18 beats) and right after 2 minutes of skipping (35 beats). Find both heart rates and the percent increase. Explain the change.

Resting: 18 beats in 15 s → 18 × 4 = 72 beats/min.
After skipping: 35 × 4 = 140 beats/min.
Percent increase = (140 − 72) ÷ 72 × 100% = 68 ÷ 72 × 100% ≈ 94%.
Explanation: working muscles do more cellular respiration, so they need more oxygen and glucose and make more CO₂. The heart beats faster to increase cardiac output and deliver blood faster.

Resting 72 bpm; after skipping 140 bpm: an increase of about 94%.

Just for fun

Funny but true: the first stethoscope was a rolled-up tube of paper. In 1816, French doctor René Laennec was too embarrassed to press his ear against a patient's chest, so he rolled up some paper, listened through it, and was amazed at how clearly he could hear the heart.

Hands-on activity

Pulse lab and paper stethoscope

You need: a timer, a cardboard paper-towel tube (or a sheet of paper rolled into a tube), a partner, and paper for a data table.

Find a pulse: press your index and middle fingers (not your thumb, which has its own pulse) gently on the inside of the wrist below the thumb, or on the side of the neck. Count beats for 15 s and multiply by 4. Take 3 resting readings and find the mean.
Listen: with your partner's permission, place one end of the tube on the left side of their chest (over their shirt) and your ear on the other end. Can you hear "lub-dub"? That's the valves closing.
Investigate: predict how the heart rate will change after 1 minute of brisk marching, then after 1 minute of jumping jacks. Measure right after each, resting between activities until the pulse returns to normal. Record in a table and make a bar graph.
Recovery: after the hardest activity, measure every minute until the rate is back near resting. Graph pulse against time.

What to look for: resting rates for teens are often roughly 60 to 100 bpm; rates rise with harder activity and then fall during recovery. Fitter people often recover faster. Safety: keep the activity moderate; anyone with a heart or breathing condition should check with a grown-up first and stop if they feel dizzy or unwell.

Your turn

Trace a drop of blood from the vena cavae to the aorta, naming each chamber and the lungs.

Why is the wall of the left ventricle thicker than the wall of the right ventricle?

What is the job of heart valves?

Give one structural difference between arteries and veins and explain it.

Why do capillaries have walls only one cell thick?

Name the four components of blood and one job of each.

A heart beats 80 times per minute with a stroke volume of 65 mL. What is the cardiac output in L/min?

More practice: warm-up

Which chambers receive blood?

Which blood vessel carries blood from the heart to the body?

What is the heart's natural pacemaker called?

What kind of muscle tissue makes up most of the heart?

A pulse count is 20 beats in 15 seconds. What is the heart rate?

More practice: on your own

The pulmonary artery carries oxygen-poor blood. Why is it still called an artery?

Explain why the circulatory system is called a double circulation.

What would happen if the mitral valve did not close properly?

Explain how leg muscles help blood in leg veins return to the heart.

Resting heart rate 64 bpm; after exercise 112 bpm. Find the percent increase.

Challenge

During hard exercise, an athlete's heart rate is 180 bpm and stroke volume is 110 mL. Compare her cardiac output with a resting output of 5.0 L/min.

Some babies are born with a hole in the septum between the ventricles. Predict how this affects the oxygen content of blood going to the body, and why.

A capillary bed has a total cross-sectional area much greater than the aorta. Blood therefore moves very slowly through capillaries. Explain why slow flow is useful there.

Check your understanding

Quiz

Which chamber pumps oxygen-rich blood to the body?

Which vessels carry blood from the lungs to the heart?

Which feature belongs to veins?

Where are materials exchanged between blood and body cells?

A heart beats 75 times per minute with a stroke volume of 60 mL. What is the cardiac output?

What starts each heartbeat?

For parents and teachers

Big idea: expectation B3.4 uses the circulatory system as its main example ("the circulatory system transports materials through the organism"); B3.3 uses heart tissue as an example of cells forming tissues, organs and systems. Students should be able to trace blood through the heart, compare vessel types and use data such as pulse rates.

How to help: Take each other's pulses before and after a walk or climbing stairs. The paper-tube stethoscope works surprisingly well in a quiet room. Ask your teen to draw the heart from memory and trace blood through it.

Common difficulties:

Left/right confusion in diagrams (the person's right is on the viewer's left).
Thinking arteries always carry oxygen-rich blood.
Forgetting that blood passes through the heart twice per full circuit.
Multiplying a 15-second pulse count by 15 instead of 4.