<P> In the human female reproductive system, cervical mucus prevents infection and provides lubrication during sexual intercourse . The consistency of cervical mucus varies depending on the stage of a woman's menstrual cycle . At ovulation cervical mucus is clear, runny, and conducive to sperm; post-ovulation, mucus becomes thicker and is more likely to block sperm . Several Fertility Awareness methods rely on observation of cervical mucus, as one of three primary fertility signs, to identify a woman's fertile time at the mid-point of the cycle . Awareness of the woman's fertile time allows a couple to time intercourse to improve the odds of pregnancy . It is also proposed as a method to avoid pregnancy . </P> <P> Mucus is able to absorb water or dehydrate through pH variations . The swelling capacity of mucus stems from the bottlebrush structure of mucin within which hydrophilic segments provide a large surface area for water absorption . Moreover, the tunability of swelling effect is controlled by polyelectrolyte effect . </P> <P> Polymers with charged molecules are called polyelectrolytes . Mucins, a kind of polyelectrolyte proteoglycans, are the main component of mucus, which provides the polyelectrolyte effect in mucus . The process of inducing this effect comprises two steps: attraction of counter-ions and water compensation . When exposed in physiological ionic solution, the charged groups in the polyelectrolytes attract counter-ions with opposite charges, thereby leading to a solute concentration gradient . An osmotic pressure is introduced to equalize the concentration of solute throughout the system by driving water to flow from the low concentration areas to the high concentration areas . In short, the influx and outflux of water within mucus, managed by the polyelectrolyte effect, contribute to mucus' tunable swelling capacity . </P> <P> The ionic charges of mucin are mainly provided by acidic amino acids including aspartic acid (pKa = 3.9) and glutamic acid (pKa = 4.2). The charges of acidic amino acids will change with environmental pH value due to acid dissociation and association . Aspartic acid, for example, has a negative side chain when the pH value is above 3.9, while a neutrally charged side chain will be introduced as pH value drops below 3.9 . Thus, the number of negative charges in mucus is influenced by the pH value of surrounding environment . That is, the polyelectrolyte effect of mucus is largely affected by the pH value of solution due to the charge variation of acidic amino acid residues on the mucin backbone . For instance, the charged residue on mucin is protonated at a normal pH value of the stomach, approximately pH 2 . In this case, there is scarcely polyelectrolyte effect, thereby causing compact mucus with little swelling capacity . However, a kind of bacteria, Helicobacter pylori, is prone to producing base to elevate the pH value in stomach, leading to the deprotonation of aspartic acids and glutamic acids, i.e., from neutral to negative - charged . The negative charges in the mucus greatly increase, thus inducing the polyeletrolyte effect and the swelling of the mucus . This swelling effect increases the pore size of the mucus and decreases mucus' viscosity, which allows bacteria to penetrate and migrate into the mucus and cause disease . </P>

Purpose of the nose in the respiratory system