2.2 Building Blocks of the Animal’s Body

An animal’s body is an amazing machine with many parts working together to allow growth and maintain general health. These parts are the animal’s cells, tissues, and organs, which together form body systems. The body is made up of a number of systems. Each of these systems has specific functions that enable the body to work properly. These systems are adapted to obtain the necessary nutrients and other resources needed by the cells of the body, to remove the wastes those cells produce, to coordinate the activities of the cells, tissues, and organs throughout the body, and to coordinate the many responses of the individual organism to its environment. These systems will be discussed in detail in Chapter 3: Body Systems Part 1 and Chapter 4: Body Systems Part 2. 

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The Levels of Organization

Before we explore the body systems, let’s look at the body starting from a single atom as the basic element from which a structure is formed. Each level of the body builds from atoms to molecules, then to cells, tissues, organs, organ systems, and then the organism itself. Figure 2.1 shows these amazing building blocks.

All matter in the universe is composed of one or more unique pure substances called elements. Some examples of these are hydrogen, oxygen, carbon, nitrogen, calcium, and iron. The smallest unit of any of these elements is the atom, and two or more atoms combine to form a molecule. Examples of molecules include water, proteins, and sugars, which are found in all living things. Molecules are essentially the building blocks of all body structures.

The cell (cyt/o) is the smallest functional unit of an organism. Cells carry out many basic functions like growth, taking in nutrients, secretion, excretion, repair, and reproduction. Cytology is the study of cell origin, structure, function, and pathology.

The cells of an animal can only remain healthy if the conditions are just right. The processes that take place in them are upset if the temperature is too high or too low, or if the fluid around or inside them is too acidic or alkaline. Homeostasis, meaning “staying the same,” is the name given to the processes that help keep the internal conditions constant even when external conditions change. A number of organs in the body play a part in maintaining homeostasis; for example, the skin helps keep the internal temperature of bird and mammals’ bodies within a narrow range even when outside temperatures change, the lungs control the amount of carbon dioxide in the blood,; the liver and pancreas work together to keep the amount of glucose in the blood within narrow limits, and the kidneys regulate the acidity and the concentration of water and salt in the blood.

A tissue (hist/o) is a group of many similar cells (though sometimes composed of a few related cell types) that work together to perform a specific function. Multicellular, complex animals have four primary types of tissue: epithelial, connective, muscle, and nervous. These tissues combine to form organs, like the skin or kidneys. An organ is an anatomically distinct structure of the body composed of two or more tissue types, and each organ performs one or more specific physiological functions. Organs are organized into systems to perform functions; examples include the circulatory system, which consists of the heart and blood vessels, and the digestive system, consisting of several organs, including the stomach, intestines, liver, and pancreas.

An organ system (body system) is a group of organs that work together to perform major functions or meet the physiological needs of the body. Organ systems come together to create an entire organism. You will learn more about these body systems in Chapters 3 and 4, but a brief overview is given below. It is important to note that some organs work in more than one body system.

The organism is the highest level of organization, and an organism can independently perform all the physiological functions necessary for life. In a multicellular organism such as a mammal, all the cells, tissues, organs, and organ systems work together to maintain life.

Body System Overview

Digestive System

• The function of the digestive system is to break down food, release its nutrients, absorb those nutrients into the body, and excrete waste.
• Animals can be classified into different groups depending on their digestive requirements.
• Structures include the stomach, liver, gallbladder, and small and large intestines.

Urinary System

• This system has various functions, but its main responsibility is cleansing the blood and removing wastes from the body.
• It also stores urine until a convenient time for disposal and provides the structures for transporting liquid waste from the body.
• Structures include the kidneys, ureters, urethra, and bladder.

Cardiovascular System

• This system uses blood to deliver nutrients throughout the body and remove wastes from the body.
• The heart, which is the primary organ in this system, pumps blood around the body via a network of blood vessels.
• Blood, blood vessels, and the heart make up this complex system.

Respiratory System

• The primary functions of the respiratory system are to provide oxygen to the body’s tissues for cellular respiration, remove the waste product carbon dioxide from the body, and help maintain the acid-base balance.
• Structures include the nose, trachea, lungs, bronchi, and alveoli.

Integumentary System

• This system is one of the largest body systems.
• The main component of this system is the skin.
• The skin protects the inner organs, controls thermoregulation, functions as a sensory organ, and is necessary for vitamin D synthesis.

Musculoskeletal System

• The function of this system is to support the body, facilitate movement, and protect the internal organs.
• Bones are also vital in the production of red blood cells.
• Structures include all the bones, muscles, joints, tendons, and cartilage in the body.

Eyes, Ears, and Other Special Sense Organs

• The function of this system is to provide sensory input from the animal’s surroundings.
• The eyes and ears are two of the primary sense structures, allowing the animal to see and hear.
• The remaining sensory organs provide input key to survival, including the sense of taste, smell, and balance, to name a few.

Hematologic System

• This system is responsible for the production of blood.
• Blood’s cellular components include red blood cells, white blood cells, and platelets.
• This system consists of the blood and bone marrow.

Lymphatic and Immune Systems

• The major function of the lymphatic system is to drain extra body fluid (lymph) and return it to the bloodstream.
• It is also associated with the immune system and plays an important role in immunity.
• This system consists of a network of lymphatic vessels, as well as the spleen and lymph nodes and ducts.

Endocrine System

• The purpose of the endocrine system is to regulate various organs by releasing hormones (chemical messengers).
• Its main role is maintaining homeostasis.
• The pituitary, thyroid, parathyroid, and adrenal glands make up this system, along with the pancreas, and ovaries/testes.

Reproductive System 

• This system functions to produce gametes (eggs and sperm) and reproductive hormones.
• In the female, its other function is to support a developing fetus and deliver it to the outside world.
• Structures in females include the ovaries, oviducts, uterus, and mammary glands.
• Structures in males include the testes, scrotum, epididymis, vas deferens, accessory sex glands, and penis.

Nervous System

• This system is very complex and is responsible for controlling much of the body, including both voluntary and involuntary functions.
• It is responsible for taking sensory input and integrating it with other sensations, memories, emotional states, and learning.
• The nervous system can be divided into two main components: the central nervous system and the peripheral nervous system.
• Structures include the brain, spinal cord, and nerves.

Image Credits (images are listed in order of appearance)

Figure 1.3 Levels of Structural Organization of the Human Body by OpenStax, CC BY 4.0