The journey to life · chapter 3
Sperm in the Uterus: The Race and the White Blood Cells
Inside the womb the space is enormous for a cell this small, like a person swimming across a lake. Waves of muscle in the womb's wall help carry the sperm upward. But the body also treats sperm as strangers: white blood cells pour in and swallow thousands of them. At the top the road splits in two. Only one tube holds an egg this month, so about half the sperm go the wrong way.
- Where
- Uterus (womb)
- How long
- Minutes to hours
In numbers
- MeasuredUterus length: about 7–8 cm
- MeasuredA neutrophil (white blood cell): about 8–10 µm across
- EstimateReaching the tubes: hundreds
Every number is labelled measured, an estimate or drawn: how we know.
The science
The uterus is about 7–8 cm long, including the cervix. Peristaltic contractions of its muscle, strongest around ovulation, push fluid towards the tube on the side of the ovary that is about to ovulate. Semen sets off an influx of neutrophils (about 8–10 µm across) within hours; in women this has been measured in the cervix, while inside the womb the evidence comes mostly from animals. They engulf sperm by phagocytosis and trap them in nets of DNA. The first sperm can reach the tubes within minutes, but most of the useful transport takes hours. The one direct count in women found only a few hundred sperm in both tubes together.
In Endo-View
This is chapter 3 of the journey to life, an interactive 3D simulation at true scale: watch it, slow it down or run it at real speed, and read every number with its label.
The other chapters
What Happens to Sperm After Ejaculation How Sperm Get Through the Cervix: Cervical Mucus Sperm in the Fallopian Tube: Waiting for the Egg The Human Egg Cell: Its Size and Its Layers How Fertilisation Works: Sperm Meets Egg The Zygote and the First Cell Divisions What Is a Blastocyst? Days 3 to 6 How Implantation Works: Days 6 to 12 The Fetus from Week 10 to Week 20 Pregnancy Week by Week: The Fetus to Week 40 How Birth Works: Labour and the First Breath The Embryo Week by Week: Weeks 4 to 10
More of the body
The heart and blood vessels The lungs and airways The digestive system The kidneys and the urinary system The brain and the nervous system