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Body Reps · Physiology

The heart

Your heart is a pump about the size of your fist. It sits in the middle of your chest, between the lungs, tilted slightly to the left. Every rep you do runs on the blood it pushes out.

This page gives you the names: four chambers, four valves, the wall down the middle, the big pipes in and out, and the route blood takes. Then three numbers every instructor should know: heart rate, stroke volume and cardiac output.

Left and right are the owner’s. Heart diagrams are drawn as if the person is facing you in anatomical position. So the heart’s right side is on your left as you look at the picture. Same rule as everywhere else in Body Reps.

Start training Try the interactives

Sources for this intro: [S1], [S3]

Overview

Two sides, two jobs

The heart is really two pumps side by side.

Each side has an upper chamber that receives blood (an atrium) and a lower chamber that pumps it out (a ventricle). A wall, the septum, keeps the two sides apart.

YOUR RIGHT YOUR LEFT (owner facing you · heart’s right is on screen left) Right atrium upper · receive Left atrium upper · receive Right ventricle to lungs Left ventricle thick wall Septum

Schematic only. Left and right are the owner’s: the heart’s right side appears on your left as you look at the picture.

Sources: [S1], [S3]

Names

The four chambers

Right atrium

Say it
right AY-tree-um
Word parts
atrium: entrance hall, the central court of a Roman house. Plural: atria (AY-tree-uh).
Everyday
right upper chamber
What it is
The upper right chamber. It receives low-oxygen blood coming back from the body.
In the gym
When you work hard, more blood comes back to the heart from the working muscles. The right atrium is where it lands first.

Right receives the return: the blood coming home from your body lands on the right.

Right ventricle

Say it
right VEN-trih-kul
Word parts
ventricle: from Latin ventriculus, little belly.
Everyday
right lower chamber
What it is
The lower right chamber. It pumps low-oxygen blood to the lungs.
In the gym
It only has to push blood next door to the lungs, a short trip with low resistance. So its wall is thinner than the left ventricle’s.

Short trip, thin wall. The right ventricle only sends blood next door to the lungs.

Left atrium

Say it
left AY-tree-um
Word parts
atrium: entrance hall.
Everyday
left upper chamber
What it is
The upper left chamber. It receives oxygen-rich blood back from the lungs.
In the gym
This is the first stop for freshly oxygenated blood on its way to your working muscles.

Lungs land on the left. L for L.

Left ventricle

Say it
left VEN-trih-kul
Word parts
ventricle: little belly.
Everyday
left lower chamber, the main pump
What it is
The lower left chamber. It pumps oxygen-rich blood out to the whole body.
In the gym
It has the thickest muscle wall in the heart, because it has to push blood all the way round the body against much more resistance. Both ventricles pump the same amount of blood per beat. The left just works at a higher pressure.

The left ventricle is the long-haul pump: thickest wall, longest trip, all the way to your toes.

Sources for the chambers: [S1], [S3]. Word parts: [S18].

The wall down the middle

Septum

Say it
SEP-tum
Word parts
septum: from Latin saeptum, a fence, enclosure or partition.
Everyday
the dividing wall
What it is
The muscular wall that separates the left side of the heart from the right.
In the gym
It keeps low-oxygen and high-oxygen blood apart, so the blood heading to your muscles is the oxygen-rich blood straight from the lungs.

You have another septum between your nostrils. Same word, same job: a wall down the middle.

Sources: [S1], [S3]. Word parts: [S18]. The nasal septum is in [S5].

The four valves

Valves are one-way doors. They open to let blood through and snap shut to stop it flowing back. The sound of them closing is part of your heartbeat.

Tricuspid valve

Say it
try-KUS-pid valv
Word parts
tri: three. cuspid: from Latin cuspis, point.
Everyday
right atrioventricular valve
What it is
The valve between the right atrium and the right ventricle. It usually has three flaps.
In the gym
It stops blood being pushed back up into the right atrium each time the right ventricle squeezes. Every beat, every rep.

Try before you buy. A pun, not the origin: blood meets the tri-cuspid on the right before the bi-cuspid on the left.

Pulmonary valve

Say it
PUL-muh-nuh-ree valv
Word parts
pulmonary: from Latin pulmo, lung.
Everyday
pulmonic valve
What it is
The valve between the right ventricle and the pulmonary artery (the pulmonary trunk), on the way to the lungs.
In the gym
It stops blood that has just been sent to the lungs falling back into the right ventricle.

Pulmo is Latin for lung. The pulmonary valve is the lungs’ front door.

Mitral valve (bicuspid valve)

Say it
MY-trul valv. Also: by-KUS-pid valv
Word parts
mitral: like a mitre, a bishop’s tall hat. bi: two. cuspid: point.
Everyday
left atrioventricular valve. In a clinic it is called the mitral valve.
What it is
The valve between the left atrium and the left ventricle. It has two flaps.
In the gym
It stops oxygen-rich blood being forced back into the left atrium when the left ventricle squeezes hard.

Picture a bishop’s mitre, the tall hat with two peaks. Two peaks, two flaps.

Aortic valve

Say it
ay-OR-tik valv
Word parts
aortic: of the aorta (see below).
Everyday
none in common use
What it is
The valve between the left ventricle and the aorta. It normally has three flaps.
In the gym
It is the last door out of the heart. Blood passes it on the way to your working muscles.

Last door out. The aortic valve opens straight into the aorta.

Sources for the valves: [S1], [S3]. Word parts: [S18].

The big pipes

First, one rule that saves a lot of confusion: arteries carry blood away from the heart. Veins carry blood back to it. It’s about direction, not oxygen.

Vena cava

Say it
VEE-nuh KAY-vuh. Plural: venae cavae (VEE-nee KAY-vee)
Word parts
vena: vein. cava: from Latin cavus, hollow.
Everyday
the hollow vein. There are two: the superior vena cava (from above the diaphragm) and the inferior vena cava (from below it).
What it is
The two big veins that bring low-oxygen blood from the body back into the right atrium.
In the gym
The superior vena cava drains your head, neck and arms. The inferior vena cava drains your legs and abdomen. After a leg session, that’s the return route.

Cava sounds like cave, and both come from the Latin for hollow. Two big hollow veins carry the blood home.

Pulmonary artery

Say it
PUL-muh-nuh-ree AR-tuh-ree
Word parts
pulmonary: of the lungs. artery: from Greek arteria.
Everyday
none in common use. It starts as one pulmonary trunk and splits into left and right pulmonary arteries.
What it is
The vessel that carries low-oxygen blood from the right ventricle to the lungs.
In the gym
It is the odd one out: an artery that carries low-oxygen blood. Get this right and the route makes sense.

Arteries go Away from the heart. A for A, whatever the oxygen.

Pulmonary veins

Say it
PUL-muh-nuh-ree vaynz
Word parts
pulmonary: of the lungs. vein: from Latin vena, a blood vessel.
Everyday
none in common use
What it is
The four veins that bring oxygen-rich blood from the lungs into the left atrium.
In the gym
The other odd one out: veins carrying oxygen-rich blood. This is the blood that will feed your muscles next.

Veins head home to the heart. These four come home from the lungs, full of oxygen.

Aorta

Say it
ay-OR-tuh
Word parts
aorta: from Greek aortē, probably from aeirein, to lift or raise. Aristotle used it for the great artery of the heart.
Everyday
the main artery
What it is
The body’s main artery. It carries oxygen-rich blood from the left ventricle out to the body.
In the gym
Every working muscle is fed by branches off the aorta. The coronary arteries that feed the heart muscle itself branch off it too.

Aorta probably comes from the Greek for lift: it rises up out of the heart before heading off to feed the whole body.

Sources for the vessels: [S1], [S3]. Word parts: [S18].

Follow the drop: the route

Follow one drop of blood round one full lap. Start in a working thigh muscle.

  1. Body tissues. In the capillaries of the muscle, oxygen leaves the blood and carbon dioxide comes in.
  2. Veins → vena cava. The blood, now low in oxygen, travels back through bigger and bigger veins into the superior or inferior vena cava.
  3. Right atrium. It arrives in the right atrium.
  4. Tricuspid valve → right ventricle. It passes the tricuspid valve into the right ventricle.
  5. Pulmonary valve → pulmonary artery. The right ventricle pumps it through the pulmonary valve into the pulmonary trunk, which splits into the pulmonary arteries.
  6. Lungs. In the lung capillaries, carbon dioxide leaves the blood and oxygen enters.
  7. Pulmonary veins → left atrium. Oxygen-rich blood returns through the pulmonary veins into the left atrium.
  8. Mitral valve → left ventricle. It passes the mitral (bicuspid) valve into the left ventricle.
  9. Aortic valve → aorta. The left ventricle pumps it through the aortic valve into the aorta.
  10. Body. Branches of the aorta carry it back to the thigh muscle. Lap complete.

Short version: body → vena cava → right atrium → tricuspid → right ventricle → pulmonary valve → pulmonary artery → lungs → pulmonary veins → left atrium → mitral → left ventricle → aortic valve → aorta → body.

Pulmonary circuit

Say it
PUL-muh-nuh-ree SUR-kit
Word parts
pulmonary: of the lungs.
Everyday
the lung loop
What it is
The loop from the right ventricle to the lungs and back to the left atrium.
In the gym
This is where your blood drops off carbon dioxide and picks up oxygen. Breathe harder in a set and this loop is busier.

The short loop: right side out, lungs, back to the left.

Systemic circuit

Say it
sis-TEM-ik SUR-kit
Word parts
systemic: of the body as a whole (from system).
Everyday
the body loop
What it is
The loop from the left ventricle out through the aorta to the body’s tissues and back to the right atrium.
In the gym
This is the loop that delivers oxygen and fuel to your working muscles and carries carbon dioxide and waste away.

Systemic means the whole system: the loop that reaches everything from your scalp to your calves.

Sources: [S1], [S3]. Word parts: [S18].

Three numbers: HR, SV, Q

Heart rate

Say it
HART rayt
Word parts
plain English. Shortened to HR.
Everyday
pulse, beats per minute (bpm)
What it is
The number of times your heart beats in one minute.
In the gym
It rises when you exercise, because your muscles need more oxygen and the heart has to pump faster. You can count it at the wrist with two fingers. Smart watches can misread it, so check by hand if a number looks odd.

Rate means how often. Heart rate: how many beats in a minute.

Stroke volume

Say it
STROHK VOL-yoom
Word parts
stroke: a beat. English has used stroke for the heart’s beating since the 1400s. Shortened to SV.
Everyday
blood per beat
What it is
The amount of blood one ventricle pumps out in one beat.
In the gym
It goes up during exercise as the heart contracts more strongly. Aerobic training can make the heart a more efficient pump.

Like one stroke of a rowing machine: one pull moves one lot. One beat pushes out one stroke volume.

Cardiac output

Say it
KAR-dee-ak OWT-put
Word parts
cardiac: from Greek kardia, heart. Shortened to Q (sometimes CO).
Everyday
blood pumped per minute
What it is
The amount of blood one ventricle pumps in one minute. Heart rate × stroke volume.
In the gym
It is the heart’s total work rate. During exercise both heart rate and stroke volume rise, so cardiac output rises a lot.

Q = HR × SV. Like volume in the gym: how many reps times how much each rep moves.

The formula

Q = HR × SV

Cardiac output = heart rate × stroke volume

Worked example: made-up numbers to show the maths. This is not a normal value or a target. Say a heart is beating 60 times a minute and pumping 80 mL per beat. Q = 60 × 80 mL = 4,800 mL/min. Divide by 1,000: 4.8 litres per minute.

Real values vary a lot from person to person and from moment to moment. They depend on heart size, fitness, sex, stress and emotion, and much more. That figure is for one ventricle. The left and right pump the same amount, so don’t double it to get “the heart’s” output.

Sources: [S2], [S3], [S4]. Word parts: [S18].

Try it

Interactives

Follow the drop

Tap the next stop in order. Wrong taps get a gentle hint. Right taps build the route.

Your route so far

Tap the next stop

Tap the next stop in order.

Cardiac output slider

Move either slider and watch the output change. Same idea as volume in the gym: reps × load. Maths only, not a norm.

Q = 70 × 70 = 4900 mL/min 4.90 L/min

Slider ranges are just the edges of the tool, not suggested values. No health interpretation.

Standard guidance, not medical advice. Anyone with a heart condition, anyone taking medication that affects heart rate, and anyone with symptoms such as chest pain, dizziness or palpitations when exercising should follow their GP’s advice on exercise, including what heart rate (if any) to aim for.

Recall

Quick check

Type each answer from memory. No list.

Sources

  1. [S1] OpenStax, Anatomy and Physiology 2e, 19.1 Heart Anatomy — openstax.org
  2. [S2] OpenStax, 19.4 Cardiac Physiology — openstax.org
  3. [S3] British Heart Foundation, How your heart works — bhf.org.uk
  4. [S4] British Heart Foundation, What should my heart rate be when I exercise? — bhf.org.uk
  5. [S5] OpenStax, 22.1 Organs and Structures of the Respiratory System (nasal septum) — openstax.org
  6. [S18] Online Etymology Dictionary (Etymonline) — etymonline.com

All accessed 6 October 2026. Full list in the physiology notes.

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