Module 2: The Human Body

Hair, vital signs, bones, senses, chromosomes, and the physical body in numbers

Part A · your hair

Hair strands on a human head

~100,000

Range: 80,000–150,000. Blondes typically have most (~150k); redheads fewest (~80k); brunettes in between (~110k).

Hair growth rate

~1.25 cm/month

About 15 cm per year. To grow shoulder-length hair from bald takes roughly 2½ years.

Hair you shed per day

~50–100 strands

Normal shedding. Each hair lives ~2–7 years before falling out and regrowing.

Hair growth: how long to reach each length?

Buzz cut (1 cm)

~1 mo

Ear-length (10 cm)

8 mo

Chin-length (20 cm)

16 mo

Shoulder (35 cm)

~2.5 yr

Mid-back (55 cm)

~4 yr

Waist-length (70 cm)

~5 yr

Based on 1.25 cm/month average. Individual growth varies ±30%.

Part B · vital signs

Resting heart rate (adult)

60–100 bpm

Elite athletes often sit at 40–60 bpm. Your heart beats ~100,000 times every single day.

Normal body temperature

36.1–37.2 °C

The old "98.6 °F / 37 °C" is a population average, not a fixed target. Individual normals vary by ~0.5 °C.

Normal blood pressure

120 / 80 mmHg

Systolic (top) / Diastolic (bottom). Above 130/80 is considered elevated by current guidelines (AHA 2017).

Breaths per minute (adult at rest)

12–20 /min

You take roughly 17,000–23,000 breaths per day. Each breath moves about 0.5 L of air.

Blood oxygen saturation (SpO₂)

95–100%

Below 90% is clinically significant hypoxia. Most pulse oximeters start alarming below 94%.

Blood volume (adult)

~4.5–6 L

About 7–8% of body weight. Losing >40% (class IV hemorrhage) is immediately life-threatening.

Heart rate zones: interactive explorer

Drag the slider to your age. The zones update based on the 220 − age formula (Karvonen approximation).

Age: 35 Max HR: 185 bpm

Note: 220 − age is a population average formula with ±12 bpm standard deviation. Use a chest strap for precision.

Your heart in numbers

Part C · bones & body composition

Bones in the adult body

206

Newborns have ~270–300, as many bones fuse during childhood. The last fusions (clavicles, wisdom teeth sockets) occur in the mid-20s.

Muscles in the human body

~600–650

Depends on counting method. Skeletal muscles only: ~640. The gluteus maximus is the largest by volume; the stapedius in the ear is the smallest.

Joints in the adult body

~360

Varies by definition. Roughly 80 are freely movable (synovial) joints: the kind you feel when you crack your knuckles.

Where are the 206 bones?

What is your body made of? (average adult, by weight)

Water~60%

Protein (muscle, organs)~16%

Fat (essential + storage)~15%

Minerals (bone matrix)~5%

Carbohydrates + other~4%

Values for a typical lean adult male. Body fat % varies substantially: essential fat is ~3–5% (men) and ~10–13% (women); average adults carry 15–25%.

How much do your organs weigh?

Skin

~4–5 kg: the heaviest organ by far

Liver

~1.5 kg

Brain

~1.3–1.4 kg

Lungs (×2)

~1.0–1.2 kg

Heart

~300 g

Kidneys (×2)

~270 g

Spleen

~150 g

Skin is consistently the largest organ, yet rarely thought of as one. It covers ~1.5–2 m² and regenerates completely about every 2–4 weeks.

Part D · body proportions

The body has a set of remarkably consistent proportions that have been used by artists, tailors, and doctors for centuries. These are averages: individual variation is ±5–10%.

Classic proportion rules

Height = 8 heads. Total body height is approximately 8 times the height of the head. Artists use this as a baseline (heroic figures use 9 heads).

Wingspan ≈ Height. Arm span from fingertip to fingertip roughly equals your total height, a fact Vitruvius noted in 25 BC.

Navel at midpoint. The belly button sits at almost exactly the midpoint of total body height.

Foot ≈ Forearm. The length of your foot is roughly equal to the length of your forearm (elbow to wrist).

👋

Hand span ≈ face width

Your spread hand (pinky to thumb) roughly equals the width of your face. Useful for artists drawing portraits.

👃

Nose length ≈ ear length

The height of the nose (tip to bridge) is approximately equal to the length of the ear. Also, both typically equal the distance between the eyes.

🦷

Thumb = 2 joints wide

The width of your thumb at the nail is a rough estimate for 1 cm, a useful bodily ruler. The hand from palm base to middle fingertip is about 20 cm.

💪

Neck circumference ≈ wrist × 2

A traditional tailor's rule: the neck is roughly twice the wrist circumference, and the waist roughly twice the neck: so waist ≈ 4× wrist.

Part E · blood & genetics

Red blood cells in the body

~25 trillion

By far the most numerous cell type. Each RBC lives ~120 days; your body produces ~2 million new ones per second to keep up.

Total cells in the body

~37 trillion

Classic estimate (Bianconi et al., 2013). Interestingly, your gut alone hosts ~38 trillion bacterial cells, roughly equal to your own cell count.

Length of all DNA uncoiled

~2 metres

Per cell nucleus. Across all ~37 trillion cells, that's ~70 billion km, enough to wrap around Earth 175 million times.

Blood type distribution (global average, ABO system)

Frequencies vary significantly by ethnicity and geography. These are rough global averages.

O+

~38%

A+

~27%

B+

~20%

AB+

~5%

O−

~7%

A−

~6%

B−

~2%

AB−

~1%

O− is the "universal donor" for red blood cells: anyone can receive it in an emergency. AB+ is the universal recipient. AB− is the rarest common type (about 1 in 100 people).

Who can donate to whom? interactive compatibility matrix

Click a donor or recipient type to highlight. ✓ = compatible transfusion. This applies to red blood cells; plasma and platelets follow different rules.

Highlight donor:

Highlight recipient:

Donor ↓ · Recipient →	O−	O+	A−	A+	B−	B+	AB−	AB+

Compatible

Incompatible

Same type

Why does this matter? Red blood cells carry antigens, protein flags on their surface, and your plasma carries antibodies against foreign antigens. If you receive blood your immune system doesn't recognise as "self," it triggers a haemolytic transfusion reaction: the antibodies attack the donated cells, causing them to burst. This can be fatal. The ABO system and the Rhesus factor (the + / − ) are the two most clinically critical of over 30 known blood group systems.

Part F · the senses

Photoreceptors in the eye

~120M rods, 6M cones

Rods handle dim-light vision; cones handle colour and fine detail. The fovea (sharpest point) is packed with ~200,000 cones/mm².

Human hearing range

20 Hz – 20 kHz

The upper limit degrades with age: most adults over 40 can't hear above 15 kHz. Dogs hear up to ~65 kHz; bats to 110 kHz.

Taste buds

~10,000

Mostly on the tongue, also on the palate and throat. Each bud lives ~10 days. Smokers and the elderly have fewer and less sensitive buds.

How sensitive are your senses? (remarkable detection limits)

👁️ Vision

A candle flame visible at 48 km on a clear night

👃 Smell

1 drop of perfume diffused in a 3-room apartment

👂 Hearing

A watch ticking at 6 metres in a quiet room

👅 Taste

1 tsp of sugar in 8 L of water

✋ Touch

A bee wing falling on cheek from 1 cm

From Miller's "Plans and the Structure of Behavior" (1960) and updated sensory research. These are theoretical absolute thresholds under ideal conditions.

🎮 Reaction time tester

Click the button, wait for the green signal, then click as fast as you can. The average human visual reaction time is 200–250 ms.

Click here to begin

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Last (ms)

—

Best (ms)

—

Avg (ms)

Attempts

Reaction time benchmarks

Elite athlete

<150 ms

Young adult avg

200–250 ms

Older adult avg

300–400 ms

Impaired / tired

>500 ms: comparable to drunk driving limit effects

Part G · chromosomes & sex determination

Chromosomes per human cell

46 (23 pairs)

22 pairs are autosomes: identical in males and females. The 23rd pair is the sex chromosomes: XX in females, XY in males.

Genes on the X chromosome

~800–900 genes

The X is large and gene-rich. The Y chromosome carries only ~70 protein-coding genes, most related to male sex development and sperm production.

Total human genes (protein-coding)

~20,000–25,000

Surprisingly modest: far fewer than many plants. The roundworm C. elegans has ~20,000 genes too. Gene count is not a good proxy for biological complexity.

X vs. Y: the sex chromosomes compared

The X and Y chromosomes are dramatically different in size and function. Everyone inherits one sex chromosome from each parent.

Size

~155 Mbp

~57 Mbp

Genes

~800–900

~70

Present in

Everyone

Males only

Key role

Many functions

Sex & sperm

Copies

2 (XX) or 1 (XY)

0 or 1

How does a baby's biological sex get determined?

Every egg cell carries exactly one X chromosome (females are XX, so all eggs get an X). Sperm cells come in two types: X-bearing or Y-bearing, each produced in roughly equal numbers. The sex of the offspring is entirely determined by which sperm fertilises the egg. The father's sperm decides; the mother's contribution is always X.

Punnett square

Sperm → Egg ↓	X (from father)	Y (from father)
X (from mother)	XX Female ♀	XY Male ♂

Each combination has a ~50% chance

XX → female. Two X chromosomes. One X is randomly inactivated in each cell (forming a "Barr body"), so females express a mosaic of genes from both their maternal and paternal X.

XY → male. The Y chromosome carries the SRY gene (Sex-determining Region Y), which triggers the embryo to develop testes at around 6–8 weeks. Without SRY, the default developmental path is female.

Variation exists. Some individuals are XXY (Klinefelter syndrome, ~1 in 600 males), XO (Turner syndrome), or XYY. Sex chromosome variations are among the most common chromosomal differences in humans.

Part H · anchor numbers to memorize

100,000

Hair strands on your head

Think: one per person in a small city

1.25 cm

Hair grows per month

Width of your thumbnail ≈ 1 cm. Hair grows about that per month.

60–100

Resting heart rate (bpm)

Below 60 = athletic or bradycardic. Above 100 = tachycardic at rest.

120/80

Ideal blood pressure (mmHg)

Systolic / Diastolic. "120 over 80": the classic benchmark.

206

Bones in the adult body

You were born with ~270. Fusion reduces the count as you age.

37°C

Core body temperature

± 0.5°C normal variation. Below 35°C = hypothermia. Above 40°C = dangerous fever.

37 trillion

Cells in your body

Roughly the same number as bacteria living in your gut.

200–250 ms

Average visual reaction time

Formula 1 drivers average ~200 ms. Under 150 ms is exceptional.

Part I · Q&A

1. You grow your hair 3 years without cutting. Roughly how long would it be?

About 45 cm: past shoulder length, approaching mid-back. 1.25 cm/month × 36 months = 45 cm. In practice slightly less, because individual hairs shed and restart before reaching full length.

2. A resting heart rate of 52 bpm: is that worrying?

Not necessarily: 52 bpm is below the standard normal range (60–100), but in trained athletes and very fit people, a resting rate of 40–60 bpm is completely normal. The heart is more efficient and pumps more blood per beat. It only becomes worrying if you feel dizzy, faint, or breathless, or if there's no fitness explanation.

3. You're told someone's blood pressure is 145/95. What does that mean and should they be concerned?

145 is the systolic pressure (peak pressure when the heart contracts); 95 is diastolic (pressure when the heart is at rest between beats). By AHA 2017 guidelines, 130/80 and above is "Stage 1 hypertension," and 140/90+ is Stage 2. So 145/95 is Stage 2, persistently elevated and worth medical attention. That said, a single reading doesn't diagnose hypertension; multiple readings over time are needed.

4. A child is born with 270 bones but an adult has 206. Where did the extra 64 go?

They fused together. Bones don't disappear: they merge. The skull, for example, starts as multiple plates that close over childhood (the "fontanelles" you can feel on a newborn's head). The sacrum (base of spine) fuses from 5 separate vertebrae during adolescence. The last bones to fuse are typically the clavicles, completing the process in the early-to-mid 20s.

5. Your reaction time test comes back at 310 ms. Is that good, average, or slow?

Slightly above the average for young adults (200–250 ms), but within the normal range, especially for someone older or who just woke up, or if measured on a touchscreen (which adds ~20–50 ms latency). In context: at 310 ms, while driving at 100 km/h, a car travels about 8.6 metres before you even start braking. That's why following distance matters. Elite Formula 1 drivers average closer to 200 ms.

6. Your body contains about 37 trillion cells, yet 25 trillion of them are a single type. Which type, and why so many?

Red blood cells (erythrocytes). They dominate in count because oxygen delivery is a massive, continuous demand: every cell in the body needs a constant oxygen supply. Red blood cells are also structurally unusual: they have no nucleus (making room for more haemoglobin) and they're tiny (~8 µm), so they can pack densely and squeeze through capillaries narrower than themselves. They live only ~120 days and are replaced at a rate of ~2 million per second.