Idea or model
Cells of the human body
An adult has roughly 30 to 37 trillion human cells, and about 90 percent of them belong to the blood-forming lineage.
Human Biology
How the human body is built and kept in balance.
Human biology explores how the body works, from cells to organs like the heart, lungs and kidneys.
It covers anatomy and physiology: how organ systems such as the circulatory, respiratory, digestive and urinary systems work together to keep the body stable.
The field links cell counts, tissue organization and organ physiology with homeostasis, genetics, the microbiome and human evolution.
Big questions
- How do organ systems keep the body in balance?
- How many cells make up a person?
- How has evolution shaped human biology?
Key ideas
Open any idea in the knowledge graph to see what it connects to, here and in other fields.
Structure
Organ systems
Groups of organs that work together, such as the circulatory, respiratory, digestive and urinary systems.
Process
Homeostasis
Keeping internal conditions, such as body temperature and blood chemistry, within a stable range.
Structure
Heart and circulation
The heart pumps blood through blood vessels, carrying oxygen and nutrients around the body.
Structure
Lungs and breathing
The lungs take in oxygen and remove carbon dioxide from the blood.
Process
Digestion
The digestive system breaks food down into nutrients the body can absorb and use.
Structure
Kidneys
The kidneys filter wastes and extra water out of the blood, making urine.
Structure
Brain
The organ that controls thought, movement and many body functions. It holds neurons and other cells in roughly equal numbers.
Molecule
Hemoglobin
The oxygen-carrying protein of red blood cells. Its beta chain is encoded by the HBB gene.
Idea or model
Microbiome
The communities of microbes living in and on a body. A 2016 estimate put a reference adult's bacteria at about 38 trillion, similar to the number of human cells.
Process
Human adaptation
Examples include lactase persistence, which evolved separately in Africa and Europe, and the sickle cell trait, which protects against malaria.
Also in Human Biology
Explore it in the labs
Interactive explorer
Tour an animal or plant cell: what each organelle does, the molecules inside it, related conditions and current research.
- Cell Biology
- Molecular Biology
- Biochemistry
Simulation
Pair bases, follow a real human gene from DNA to protein, make mutations, and predict inheritance with Punnett squares.
- Genetics
- Molecular Biology
- Biochemistry
Current research
Search published papers and preprints in Human Biology, with clear labels for reviews, primary research, clinical trials and preprints.
Search papers in Human BiologySources & further reading
The explanations on this page are our own summaries of these sources. Follow a link to read the original; if anything here disagrees with it, the original wins. How we choose and check sources.
Anatomy and Physiology 2e (opens the original in a new tab)
OpenStax (2022). OpenStax, Rice University.
TextbookEducationalLicence: CC BY-NC-SA 4.0 (linked, not reused)
Revised estimates for the number of human and bacteria cells in the body (opens the original in a new tab)
Sender R, Fuchs S, Milo R (2016). PLOS Biology 14:e1002533.
About 3.0 × 10^13 human cells and 3.8 × 10^13 bacteria in a 70 kg reference adult.
Peer-reviewedPrimary researchAn estimation of the number of cells in the human body (opens the original in a new tab)
Bianconi E, Piovesan A, Facchin F, et al. (2013). Annals of Human Biology 40:463-471.
Organ-by-organ estimate of about 3.72 × 10^13 cells.
Peer-reviewedPrimary researchHow the Heart Works (opens the original in a new tab)
National Heart, Lung, and Blood Institute. nhlbi.nih.gov. National Heart, Lung, and Blood Institute, NIH.
ReferenceGovernment sourceEducationalHow the Lungs Work (opens the original in a new tab)
National Heart, Lung, and Blood Institute. nhlbi.nih.gov. National Heart, Lung, and Blood Institute, NIH.
ReferenceGovernment sourceEducationalYour Digestive System & How It Works (opens the original in a new tab)
National Institute of Diabetes and Digestive and Kidney Diseases. niddk.nih.gov. National Institute of Diabetes and Digestive and Kidney Diseases, NIH.
ReferenceGovernment sourceEducationalYour Kidneys & How They Work (opens the original in a new tab)
National Institute of Diabetes and Digestive and Kidney Diseases. niddk.nih.gov. National Institute of Diabetes and Digestive and Kidney Diseases, NIH.
ReferenceGovernment sourceEducationalBrain Basics: Know Your Brain (opens the original in a new tab)
National Institute of Neurological Disorders and Stroke. ninds.nih.gov. National Institute of Neurological Disorders and Stroke, NIH.
ReferenceGovernment sourceEducationalEqual numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain (opens the original in a new tab)
Azevedo FA, Carvalho LR, Grinberg LT, et al. (2009). Journal of Comparative Neurology 513:532-541.
Counted about 86 billion neurons and a similar number of non-neuronal cells in adult male brains.
Peer-reviewedPrimary researchHBB gene (opens the original in a new tab)
MedlinePlus Genetics. MedlinePlus Genetics. U.S. National Library of Medicine, NIH.
ReferenceGovernment sourceEducationalUniProtKB P68871: Hemoglobin subunit beta (Homo sapiens) (opens the original in a new tab)
UniProt Consortium. UniProt Knowledgebase. UniProt Consortium (EMBL-EBI, SIB, PIR).
Curated entry noting that beta-globin's initiator methionine is removed (mature chain is residues 2 to 147).
DatabaseResearch instituteLicence: CC BY 4.0
Structure, function and diversity of the healthy human microbiome (opens the original in a new tab)
Human Microbiome Project Consortium (2012). Nature 486:207-214.
Peer-reviewedPrimary researchHuman Microbiome Project (HMP) (opens the original in a new tab)
NIH Common Fund. commonfund.nih.gov. National Institutes of Health.
ReferenceGovernment sourceEducationalConvergent adaptation of human lactase persistence in Africa and Europe (opens the original in a new tab)
Tishkoff SA, Reed FA, Ranciaro A, et al. (2007). Nature Genetics 39:31-40.
Peer-reviewedPrimary researchProtection afforded by sickle-cell trait against subtertian malarial infection (opens the original in a new tab)
Allison AC (1954). British Medical Journal 1:290-294.
Early evidence that carrying one sickle cell allele protects against severe malaria.
Historical paperPrimary research