Animals, despite their diversity in form and function face common challenge and their response to the environment is based on some common principles. Anatomy is the study of structure of an organism and Physiology is the study of the functions of an organism performs. Natural selection can fit structure to function by choosing the best working option over a period of time.

An animal’s size, shape and features significantly affect the way an animal interacts with the environment. Physical laws and the environment constrain animal size and shape and place certain limits on the range of forms they can adopt. The physics of flight direct the evolution of the form taken by flying birds.

Similarly laws of hydrodynamics constrain the shapes that are possible for aquatic animals such as fish, dolphins, seals and whales. Animal form and function are correlated at all levels of organization. Cells are organized into tissues, tissues are combined into functional units called organs and organs that work together form organ systems.

Digestive system consists of mouth, stomach, small intestine, large intestine, liver, pancreas and other organs. Circulatory system consists of heart, blood vessels and blood. Respiratory system consists of lungs and trachea. Immune and lymphatic system consists of bone marrow, lymph nodes, thymus, spleen and white blood cells. Endocrine system has pituitary, thyroid, pancreas and other hormone secreting glands. Reproductive system works with ovaries, testes and other associated organs. Brain, spinal cord, nerves and sensory organs are part of nervous system.

Animals obtain the energy required to sustain their life by way of chemical energy from food, which contains organic molecules synthesized by other organisms. The flow of energy through an animal ultimately limits the animal’s behavior, growth and reproduction.

The energy from molecules is used to generate ATP through cellular respiration and fermentation. ATP powers cellular work, enabling cells, organs and organ systems to perform their functions. The amount of energy an animal uses in a unit of time is called its metabolic rate, which is the sum of all the energy requiring biochemical reactions occurring over a given time interval.

Thermoregulation is the process by which animals maintain an internal temperature which is critical to most of the biochemical and physiological processes that sustain life. Heat exchange takes place through conduction, convection, radiation and evaporation and involves anatomy, physiology and behavior of animals.

This contributes to Homeostasis where in the system tends to maintain a stable, constant condition of properties like temperature or pH by regulating its internal environment. Homeostatic mechanisms manage an animal’s energy budget.

Animal diet should provide for supply of chemical energy, organic raw materials and essential nutrients. Animals require about 20 amino acids and can synthesize about half of them from the other molecules they get from diet. Fatty acids, vitamins and minerals are other essential nutrients. The main stage of food processing is ingestion, digestion, absorption and elimination.

Each organ of the mammalian digestive system has specialized food processing function and they include Oral cavity, pharynx, esophagus, stomach, small intestine and the large intestine. Transport systems functionally connect the organs of exchange with the body cells. Heart and lungs are important organs of the circulatory system. Heart pumps blood to lungs where it loads oxygen and unloads carbon dioxide; oxygen rich blood from the lungs is pumped to the body tissues.

Blood is a connective tissue with cells suspended in plasma and respiratory pigments such as hemoglobin bind and transport gases like oxygen. Osmoregulation balances the uptake and loss of water and solutes. Non essential solutes and wastes are filtered and excreted from the system. Kidneys play a vital role in filtration, water balancing and salt regulation. Nephrons in the kidney control the composition of blood by filtration, secretion and re–absorption. Kidney function is regulated by nervous and hormonal control of water and salt re–absorption.

Innate immunity provides broad defenses against infection. Skin and mucous membranes form physical barriers against micro organisms and viruses. Internal cellular and chemical defenses include phagocytes, antimicrobial proteins, interferons and histamines. Natural killer cells can induce the death of virus infected cells or cancer cells through apoptosis. In acquired immunity, lymphocytes provide specific defenses against infection. Helper T–cells, Cytotoxic T cells and B cells respond to antigens, infected cells and pathogens respectively to provide defense against different types of threats. Exaggerated, self–directed or diminished immune response can cause disease.