Human body

The human body is the entire structure of a human being. It is a complex biological machine made of many different parts. These parts work together to maintain homeostasis. Homeostasis is the process of keeping internal conditions stable. This includes keeping safe levels of substances like sugar, iron, and oxygen in the blood.

To understand the body, scientists use different fields of study. Anatomy looks at the physical structure of the body. Physiology focuses on how the organs and systems function. Histology is the study of tissues, often using a microscope. Embryology is the study of how the body develops.

At the most basic level, the body is made of trillions of cells. These cells are the fundamental units of life. An adult human has roughly 30 trillion cells. There are also about 38 trillion bacteria living in the body.

Cells function because of DNA, which is a set of genetic instructions. DNA sits inside a part of the cell called the nucleus. The cell copies parts of the DNA and sends them out via RNA. This RNA is then used to create proteins. Proteins dictate how a cell acts and how it regulates itself. While most cells have a nucleus, some do not. For example, mature red blood cells lose their nucleus as they mature.

Cells group together to form different types of tissues. Histology allows us to see these specialized groups. There are four main types of tissue in the body. The first type is epithelia, which are lining cells. These cells cover surfaces like the skin or the inside of the stomach. They can be flat or shaped like columns. Some have hair-like cilia to help move things. The second type is connective tissue. The third type is nerve tissue. The fourth type is muscle tissue.

These tissues provide the foundation for organs.

Organs are collections of cells that perform specific tasks. Most organs sit inside body cavities. For example, the stomach is in the abdomen. The lungs are in the pleura.

The heart is a vital organ located in the thoracic cavity. It is protected by the pericardium, which helps lubricate its movement. The heart is made of two atria and two ventricles. The atria help blood flow into the ventricles. The ventricles then pump the blood out. The right ventricle sends blood to the lungs. The left ventricle sends blood to the rest of the body.

The heart uses an electrical conduction system to work. It starts at the sinoatrial node. This signal causes the atria to pump blood into the ventricles. The signal then travels to the atrioventricular node. This node slows the signal down slightly. This delay allows the ventricles to fill with blood before they pump. If the coronary arteries become blocked by plaque, it can cause coronary artery disease. This is a leading cause of death worldwide, accounting for 16% of all deaths. A blockage can lead to a myocardial infarction, or heart attack.

Other organs like the gallbladder also play specific roles. The gallbladder is a pear-shaped organ located near the liver. It stores bile, which helps the body digest fats. The liver sends bile to the gallbladder through the cystic duct. The gallbladder then releases it into the small intestine.

Sometimes, stones can form in the gallbladder. This is called a gallstone. If a stone blocks the biliary tract, it can cause a gallbladder attack. This often results in sudden pain in the abdomen or nausea.

Multiple systems work together to keep the body running. The circulatory system uses the heart and blood vessels to transport nutrients and oxygen. It has two circuits: the pulmonary circuit and the systemic circuit.

The digestive system converts food into small molecules. It uses peristalsis, which is the contraction of muscles, to move food. The endocrine system uses glands to release hormones. These hormones act as signals between different body systems.

The immune system uses white blood cells to protect the body from outside threats. Finally, the integumentary system, including the skin, provides protection and structure.

The chemical makeup of the body is also very specific. The body is composed of many elements by mass. Oxygen is the most abundant, making up about 65% of the mass. Carbon makes up about 18.5%. Hydrogen and nitrogen also make up large portions.

Other important elements include calcium and phosphorus. Trace elements make up less than 1% of the body's mass. Water is also a huge part of our composition. An adult male body is about 60% water. This water is found inside cells and in fluids like blood plasma.