Classification & Human Body Systems

Two topics, one lesson

The CBE covers classification (how we organize living things) and human body systems (how organs work together). Each gets only a handful of questions, so they fit nicely in one lesson.

The taxonomic hierarchy

Linnaeus organized life into a nested hierarchy. Eight levels, from broadest to most specific:

Humans, for example: Eukarya / Animalia / Chordata / Mammalia / Primates / Hominidae / Homo / sapiens.

The deeper you go, the fewer organisms share that category — and the more similar they are. Two organisms in the same Species are nearly identical; two in the same Domain might share little beyond cell type.

The three domains of life

• Bacteria — prokaryotic (no nucleus, no membrane-bound organelles). Found everywhere.
• Archaea — prokaryotic, but biochemically distinct. Often live in extreme environments (hot springs, salt lakes, deep sea vents).
• Eukarya — eukaryotic (have a true nucleus and membrane-bound organelles). Includes animals, plants, fungi, and protists.

Karyotype: counting chromosomes

A karyotype is a photograph of all chromosomes from one cell, arranged in pairs by size. For humans, normal = 23 pairs (46 total).

• 22 autosome pairs (numbered 1-22).
• 1 sex chromosome pair: XX = female, XY = male.

Common conditions tested:

• Down syndrome — trisomy 21 (three copies of chromosome 21 instead of two).
• Turner syndrome — XO (one X chromosome only).
• Klinefelter syndrome — XXY (extra X in males).

Respiratory system

The respiratory system delivers oxygen to the blood and removes carbon dioxide. Air follows this path:

Nasal cavity → Trachea → Bronchi → Bronchioles → Alveoli

• Trachea — the main windpipe.
• Bronchi — two main branches, one to each lung.
• Bronchioles — smaller branches inside each lung.
• Alveoli — tiny air sacs at the end of bronchioles. This is where gas exchange happens.
• Diaphragm — muscle below the lungs that contracts to inhale.

Circulatory system basics

The heart has four chambers and pumps blood in two circuits:

• Right atrium → Right ventricle → lungs (pulmonary circuit).
• Left atrium → Left ventricle → body (systemic circuit).

Key fact: the left ventricle pumps oxygenated blood to the entire body — it’s the chamber with the thickest muscular wall.

Check yourself

Practice with real CBE questions

• The three domains of life — which has no nucleus and lives in extreme environments?
• The taxonomic hierarchy — which level contains organisms most similar to each other?
• The respiratory system — which structure is the primary site of gas exchange?
• A karyotype — what condition is shown?
• Which heart chamber pumps oxygenated blood to the body?