# AP Biology/Animal Structures and Functions

Animals are complex organisms that are made up of many complex systems of cells that work together to have homeostasis.

## The Organization of Cells
### Tissues

Tissues are made up of groups of similar cells that work together in order to perform a common function. Animal tissues can be split up into six types: 1) epithelial tissue, which consists of the outer layers of skin and internal protective coverings, 2) connective tissue; including bone, cartilage, and blood, 3) nervous tissue that makes up the nervous system, and 4) muscle tissue 5)brain tissue,6)the radius of the bone and the tissue of the nails.

### Organs

Organs are made up of various kinds of tissue that work together to perform a common function. The organs are created by the specialization of stem cells found during the early stages of development. Each of the organs formed serves a specific function or did serve a specific function at some point, but later became a vestigial structure, this makes each organ unique.

### Organ Systems

An organ system is made up of different organs that work together in order to perform a common function. An example of an organ system is the respiratory system which is made up of the nose, pharynx, larynx, trachea, and lungs.

## The Respiratory System

The function of the respiratory system is to provide an animal's body with the oxygen required for aerobic respiration. Respiration itself can be defined as basically being the movement of gas in and out of an organism.

## The Circulatory System

The function of the circulatory system is to distribute the oxygen and nutrients needed by the organism. In humans, the circulatory system consists of the heart, blood, veins, arteries, and capillaries. Also included in the circulatory system is a protein called hemoglobin, which serves a transport vector for carbon dioxide and oxygen. Another component of this system includes the heart, which utilizes corresponding contractions at specific interval.

## The Nervous System

The function of the nervous system is to respond to both the external and internal stimuli of an organism.

## The Digestive System

The function of the digestive system is to break down the food particles an animal eats into smaller molecules that provide energy and nutrients for the organism. The digestive system in humans consists of: the mouth, the stomach, the small intestine, the large intestine, the pancreas, and the liver

## The Excretory System

The excretory system is involved in the re-absorption of water, and the disposal of waste products and other non-useful material.

## The Muscular System

The muscular system functions in locomotion of the body in response to stimuli and in order to counteract environmental pressures. Some pairs of muscles in the muscular system are made in opposing pairs such as the Biceps and Triceps where one contracts as the other relaxes. Another muscle is smooth muscle such as that found in the stomach, which contains tight junctions, which prevents the highly acidic HCl from damaging other sensitive organs. Finally there is cardiac muscle which is found solely in the heart hence why it is called cardiac tissue. This cardiac tissue contains junctions that allow the interconnection of the cytoplasm between cells and therefore allowing quicker contractile signal transfer.