Membranes are thin sheets of tissue that are composed of cells that line the organs. They mainly cover the visceral organs like the heart, stomach, and lungs. and many other vital organs. The membrane lines the organ in different layers like the Parietal layer and the visceral organ. They form the protective layering to the organs that protect the vitals from friction, trauma, and injuries. The main membranes present in the biological body are:

1. Mucous Membrane
2. Serous Membrane
3. Synovial Membrane

Mucous Membrane 

The membrane lines the major tract of the human body from the mouth to the anal canals like Alimentary tract, Respiratory tract, and Genitourinary tract. This membrane consists of the epithelial cells. Some of these produce a secretion called mucus which is slimy tenacious fluid containing mucin. The secretion is formed within the cytoplasm of the cell. The mucus provides a slippery moist surface to the tract. This mucus plays a role in protecting the lining membrane from different mechanical and chemical injuries. For example, the Nasal mucosa traps the foreign particle and doesn’t allow it to enter the foreign body into the alveoli of the lungs.

Serous Membrane 

The serous membrane usually consists double layer of the epithelium tissue. Two layers are separated by the serous fluid that is secreted by the serous membrane. Serous Membrane consists of two-layer as follows

• Parietal layer 
• Visceral layers 

The parietal layer is the outer lining membrane which lines the space that the organs take whereas the visceral layer is the inner lining of the the organs. The serous fluid present between the parietal and visceral layer supports the organ and protects it from internal friction between the organs. The serous membrane or serosa is present on three main sites of the body. The site is:

1. Lungs 
2. Heart
3. abdomen 

Lung(pleura)

The lungs are one of the important organs of the human body which perform respiration supplying oxygen to the blood cells. The lung is present in the thoracic cavity. The serosa that surrounds the lung is the pleura. It surrounds the thoracic cavity protecting the organ from internal friction and external traumas and injuries and acts as a cushion to the lungs.

The pleura consists of two distinct layers:

• The visceral pleura is a slippery membrane that covers the surface of the lungs and areas separating the different lobes of the lungs (hilum).
• The parietal pleura is the outer membrane that lines the Diaphragm’s inner chest wall.

Diaphragm: The muscle separating the chest and abdominal cavities

Heart (Pericardium)

 The heart is the main organ of the circulatory system. It controls the circulation of the blood in the body and it works continuously. The heart is composed of the cardiac muscle. The Serosa that surrounds the heart is the pericardium. The heart beats about 60 to 100 times a minute. During each beat, the heart is protected by the arrangement of the pericardium and the serous fluid present in it which assists the working action of the heart. The pericardium protects the heart from friction with other organs like the lungs.

• Parietal layer: This is the outer layer that attaches to the fibrous pericardium. There’s no space between them.
• Visceral layer of the serous pericardium: The innermost layer of the pericardium is the visceral pericardium. It directly covers the heart and the roots of great vessels like the Aorta, Pulmonary artery, and superior and inferior vena cava. The layer that covers the heart is the epicardium.

 Abdomen(Peritoneum)

The serosa that lines the abdominal cavity and its organs is known as the peritoneum. The peritoneum insulates the organ and protects the organ from internal friction. The peritoneum is the same as the other serosa which are of 2 types.

• parietal Peritoneum: The parietal peritoneum that lines the internal surface of the abdominal wall. The parietal peritoneum absorbs the pressure pain and temperature.
• Visceral peritoneum: The visceral peritoneum covers the abdominal organs like the stomach, liver, and spleen.

Synovial Membrane 

The membrane is found in the lining of joint cavities and the surrounding tendons which is at risk of rubbing against the bone surface. The synovial membrane consists of fine epithelial cells on the layer of connective tissue. The synovial membrane secretes the fluid known as synovial fluid which is clear, sticky, and oily.

The main function of the synovial fluid secreted by the synovial membrane is the lubrication of the joint and protection of the cartilage that covers the articular surface of the bone.