The male and female reproductive systems

Illustrative values: a semen sample has a volume of $3.0\mathrm{c}{\mathrm{m}}^{3}3.0\backslash ,\backslash mathrm\{cm^3\}$ and contains $6.0\times {10}^{7}6.0\; \backslash times\; 10^\{7\}$ sperm cells per cm³. Only about 200 sperm reach the oviduct where the oocyte is. (a) How many sperm are in the sample? (b) What fraction reaches the oocyte?

1. (a) $3.0\times 6.0\times {10}^7=1.8\times {10}^{8}3.0\; \backslash times\; 6.0\; \backslash times\; 10^\{7\}\; =\; 1.8\; \backslash times\; 10^\{8\}$ sperm (180 million).
2. (b) $\frac{200}{1.8\times {10}^8}=1.1\times {10}^{-6}\backslash frac\{200\}\{1.8\; \backslash times\; 10^\{8\}\}\; =\; 1.1\; \backslash times\; 10^\{-6\}$: about one sperm in a million.

Interpretation: most sperm are lost in the acidic vagina, at the cervix, or swim into the wrong oviduct. Producing huge numbers makes it likely that a few reach the oocyte, which is why a low sperm count reduces fertility (lesson 5).

Core body temperature is about 37 °C, and the testes in the scrotum are about 2 to 3 °C cooler. Suggest why this matters, and predict the effect of very tight clothing or long periods in hot conditions.

1. The enzymes of sperm production work best a little below core body temperature, so the testes are held outside the body cavity.
2. Muscles in the scrotum adjust its distance from the body: in the cold they pull the testes closer, and in heat they relax.
3. Anything that keeps the testes warm for long periods, such as tight clothing, may reduce the number of healthy sperm made, and so lower fertility. The effect is usually reversed when the testes are kept cooler again.