There are five types of membranes found within the body. Membranes are flat sheets of tissue that cover or line parts of the body and are typically composed of epithelial cells and connective tissue. Epithelial cells cover the inner and outer layers of surfaces and form glands that secrete fluids. Connective tissue is the most abundant type of tissue in the body; it binds and supports the structures of the body.
Mucous membranes, also called mucosa, line the inside of cavities that open directly to the exterior environment. Mucous membranes line the gastrointestinal tract, respiratory tract, reproductive tracts and the urinary tract. This type of membrane is composed of an epithelial cell layer and an underlying connective tissue layer. The epithelial layer is an important defense mechanism that prevents the entry of pathogens and microbes into the body. The cells are tightly packed together, so fluid cannot leak through the epithelial layer. Specialized cells secrete mucous to keep the membrane moist. Mucous also traps dust particles in the respiratory, or lung passage ways and lubricates food as it travels through the intestinal tract. The connective tissue component of a mucous membrane stabilizes the membrane against the structure it is protecting; it also holds blood vessels that supply blood and nutrients to the epithelial layer in place.
Serous membranes, or serosa, line cavities of the body that do not open directly to the external environment; it also covers the organs within the cavities. For example, this type of membrane lines the chest and abdominal cavities, in addition to covering the liver, spleen, kidneys, heart etc. Serous membranes are made of two layers: a layer to line a cavity, called the parietal membrane, and a layer to cover an organ, called the visceral layer. Serous membranes secrete a lubricant called serous fluid that allows the organs to glide against other structures without causing friction, according to "Principles of Human Anatomy".
The cutaneous membrane, also known as the skin, covers the entire body. It is composed of many layers of epithelial cells to protect the body from invading microbes or pathogens, in addition to light, heat and injury. The skin is the largest organ of the body that also stores fat, vitamin D and water and houses the sensory receptors for touch and pain. It regulates body temperature by secreting sweat to dissipate heat, according to UMMC.
The junction where two bones meet is called a joint. Surrounding freely movable joints like the shoulder, elbow, or knee is a synovial membrane. The synovial membrane secretes synovial fluid to lubricate the joint space, making motion much easier. The synovial fluid also nourishes the cartilage attached to the ends of bones and contains immune cells called macrophages that rid the joint space of invading microbes and debris, according to "Principles of Human Anatomy".
Covering the brain is a dense connective tissue membrane, composed of three layers, called the meninges. The outer most layer is called the dura mater; it is a thick connective tissue that prevents the brain from moving too much in the skull. The second layer is the arachnoid layer; it is a loose connective tissue layer that resembles the web of a spider. The inner most layer is the pia mater; it is a thin layer that adheres directly onto the brain, according to California State University.