The bodies of living humans contain many different types of fluids that play an important role in their function. Blood plasma is the liquid part of blood that carries blood cells, hormones, proteins, and other substances throughout the body. Lymphatic fluid and mucus both play a role in protecting the body from disease. The process of digestion is aided by saliva and gastric juice. Other fluids found in the human body include cerebrospinal fluid, sweat, tears, the aqueous and vitreous humors of the eye, and body fluids related to human reproduction, such as semen and vaginal secretions.
Blood cells are suspended in a fluid called plasma, which is mainly composed of water and a mixture of other dissolved substances, or solutes. Plasma is an important transport mechanism within the human body, as it carries substances like hormones, vitamins, amino acids, and antibodies to where they can be stored or put to use. The plasma also contains proteins called clotting factors that help the blood to coagulate, slowing down the rate of blood loss when a person has an injury that causes blood to hemorrhage. When these clotting factors are removed from the blood plasma, it is known as blood serum.
One of the fluids that directly contributes to the body's defense is lymphatic fluid, a fluid that is very similar in composition to blood plasma. This fluid originates in the circulatory system, but passes out of the network of veins and arteries into a space between cells known as the interstitial space. While in the interstitial space, the fluid is known as interstitial fluid, which "bathes" the body's cells and takes microbes and pathogens away with it. The small percentage that enters the lymphatic system is called lymphatic fluid. Mucus is a viscous fluid that also aids in the body's defense by trapping foreign particles that try to enter the body through the respiratory system.
Some body fluids help break down food that is consumed so the body can absorb nutrients and gain energy. Saliva is secreted by the salivary glands found under the tongue. It contains enzymes that begin the digestive process by breaking down starches and fats in food while it is still in the mouth. After this partially digested food is swallowed, it enters the stomach, where the highly acidic gastric juice continues the process of digestion.
Cerebrospinal fluid provides a protective cushion for the brain. Sweat, or perspiration, helps humans maintain a comfortable body temperature. Tears are secreted by the lacrimal glands and help to clean and lubricate the surface of the human eye. Fluids in the inner and outer portions of the eye that maintain pressure in the eye and provide nutrients and immune defense to that organ are called the vitreous and aqueous humors, respectively. Human males produce a fluid called semen that functions as a delivery system for sperm cells, while human females produce vaginal secretions that facilitate sexual intercourse by lubricating the vagina.