Animal Body Systems

Have you ever thought about how hard your body works day and night? Think about all the organs involved every time you breathe! When you breathe in, your lungs are filled with air, and from that air, oxygen travels to your bloodstream, and then carbon dioxide–a waste product–travels from your blood to your lungs and is expelled when you breathe out.

And you might not have thought about it before, but your brain takes part in this process, too: it controls how fast or slow you breathe by sensing how much oxygen your body needs. Just imagine how many organs are involved in more complex tasks!

So, without further ado, let's talk about animal body systems!

• First, we will discuss the definition, parts, and functions of different animal body systems.
• Then, we will discuss how the body maintains stability and how these systems work together to sustain a living organism.

Animal Body Systems Definition

As with any other living organism, an animal’s body is made up of building blocks called cells. Cells are so small that we typically cannot see them with our naked eye, so how do cells form something as complex as a living organism?

Cells are organized into successive layers, each with its own specific form and function (Fig. 1):

• Cells that look similar and perform similar functions come together to form tissues.

• Different types of tissues construct functional organs.

• Organs that work together make up an organ system (or body system).

• Different organ systems work together to perform specific functions to sustain the life of an organism.

With that, we can define the animal body system as follows:

Functions of Animal Organ Systems

What are the different organ systems in animals? What are their main components and functions? As we move along, keep in mind that not all organ systems are the same; not all animals have all of these organ systems and some of them don’t have organs at all!

As we mentioned before, organ systems are composed of organs, which in turn are formed by different tissues. Importantly, organs have different types of specialised cells that coordinate to perform the organ's function.

Each organ system has its own organ composition, with tissues that can be specific to only that organ, or shared among different organs. To understand this better we will go into more detail about some specific organ systems.

Each animal organ system also has its own function. It's of vital importance for the animal's life that all organ systems perform their function well.

Examples of animal organ systems

Each organ system has its own components and function, and we'll touch upon some of them to understand how they may vary.

The digestive system

The main parts of the digestive system are:

• the mouth,
• the stomach,
• the oesophagus that connects the mouth to the stomach,
• a cone-shaped passageway called the pharynx,
• the intestines,
• the liver,
• the pancreas, and
• the rectum and the anus.

Fig. 2. Diagram of the human digestive system.

The digestive system breaks down food (digestion) and absorbs the smaller molecule it generates into the bloodstream as nutrients.

The process of digestion begins in the mouth, where food is mechanically broken down by chewing and mixed with saliva, which contains enzymes that begin the chemical breakdown of carbohydrates. The food then travels down the oesophagus and into the stomach, where it is further broken down by stomach acid and enzymes. From there, the partially digested food moves into the small intestine, where it is mixed with digestive juices from the pancreas and liver, and where the majority of nutrient absorption takes place.

The remaining waste products then move into the large intestine, where water is absorbed and the waste products are formed into faeces for elimination through the rectum and anus.

The skeletal system

The skeletal system is made up of:

• bones,
• tendons,
• ligaments,
• and cartilage.

The most obvious function of the skeletal system is to provide structural support to the body. However, it has many other functions:

• It protects the internal organs
• It enables the body to move
• It produces blood cells in the bone marrow
• It stores minerals like calcium and phosphorus. It can release them to keep the acid-base balance in the body.

The muscular system

The muscular system is mainly made up of a specific cell type called muscle fibres. These are attached to bones, internal organs, and blood vessels. There are three types of muscle fibres, each with their specific functions and location within the body: skeletal, smooth and cardiac muscle fibres.

The muscular system gives the body the ability to move. Like the skeletal system, it also provides support to the body and contributes to regulating blood pressure by changing the diameter of the blood vessel lumen.

The integument system

The integument system is made up of the skin and its derivatives, including hair, nails, and sweat glands. It is the body’s outermost protective layer. It also protects the body from injuries and infections and regulates heat within the body.

The reproductive system

Animals that reproduce sexually have two reproductive systems: the male and the female reproductive system. Each species will have minor or major differences in their male and female reproductive systems, but they all share this basic characteristic: the male system produces sperm and the female system produces eggs.

The female human reproductive system is made up of:

• the vulva
• the vagina and cervix
• the uterus
• the fallopian tubes
• the ovaries

The ovaries are not only the site for egg production, but are also a gland: they produce estrogen and progesterone, essential hormones for female holistic health. Apart from regulating egg maturation, estrogen and progesterone regulate bone density in females, as well as promoting cardiovascular health or a successful pregnancy.

The uterus generates the endometrium (which is released during the period) and is also the site for embryo implantation during pregnancy.

Fig. 5. Human female reproductive system.

The male human reproductive system is made up of:

• the penis and corpus cavernosum
• the epididymis
• the testes
• the prostate gland
• the seminal vesicles
• the urethra
• the vas deferens

As with the ovaries, the testes are not only the site for sperm production but are also a gland that produces testosterone, which is also important for the holistic health of males. Testosterone is not only important for sperm production but also regulates muscle strength and bone density, among other functions.

The male reproductive system shares components and ducts with the excretory system, i.e. the urine and semen exit the male body through the same channel, the urethra. In the female reproductive system, though, urine and vaginal discharge exit the body through different channels (the urethra and the vagina, respectively).

Fig. 6. Human male reproductive system.

The respiratory system

The respiratory system is composed of:

• the nose
• the lungs,
• the larynx,
• the trachea,
• the lungs, and
• the bronchi.

The diaphragm and intercostal muscles are also essential for the respiratory system, as they expand and contract the lungs, which is where gas exchange takes place.

Fig. 7. Human respiratory system.

The respiratory system is responsible for gas exchange or the uptake of oxygen and the removal of carbon dioxide from the body.

The circulatory system

The circulatory system is made up of:

• the heart,
• the blood vessels, and
• the blood itself.

Fig. 8. Diagram of the human circulatory system.

The circulatory system is in charge of distributing materials such as oxygen, carbon dioxide, nutrients, hormones, and wastes within the body, as well as (in part) maintaining homeostasis in the body.

The immune and lymphatic systems

The immune and lymphatic systems are made up of:

• lymph vessels, in charge of distributing lymphatic fluid with white blood cells throughout the body,
• lymph nodes,
• the bone marrow, where the immune cells are produced,
• a small gland called the thymus, and
• the spleen.

Fig. 9. Diagram of the human lymphatic system.

The immune and lymphatic systems are responsible for fighting against infections, viruses, and even cancerous cell growth.

The endocrine system

The endocrine system is a complex interconnected system composed of:

• the pituitary gland,
• the thyroid gland,
• the pancreas,
• the adrenal gland, and
• other hormone-secreting glands.

The endocrine system works with the nervous system to control the body’s internal organs and coordinate responses to stimuli from the external environment.

Fig. 10. The main organs of the human endocrine system.

The nervous system

The nervous system is made up of:

• the brain,
• the spinal cord,
• nerves, and
• sensory organs (eyes, ears, nose, tongue, and skin).

The nervous system (along with the endocrine system) coordinates the body’s activities. It also detects stimuli from the environment and comes up with responses to them.

Fig. 11. Diagram of the human nervous system (central and peripheral).

The excretory system

The main components of the excretory system are:

• the kidneys,
• the ureters,
• the urinary bladder, and
• the urethra.

It is responsible for disposing of organic wastes and regulating the volume of internal body fluids.

Animal Body Systems Characteristics

Some organs perform more than one role in the body and can thus belong to more than one organ system.

Animal Body System and Homeostasis

The different organ systems work together to maintain homeostasis, the ability of a living system to maintain dynamic equilibrium while responding to changing external conditions. What we mean by dynamic equilibrium is that there is a fixed point that the body tries to maintain; when there are deviations from the set point, the body makes adjustments to get back to that point.

How do the different parts of the animal body system work together to maintain homeostasis?

So how does the body maintain homeostasis? When there is a change in the animal’s external environment, cellular receptors sense these changes and send signals to the control center (usually the brain), which in turn, generates a response that is sent to an effector, a muscle that contracts or relaxes or a gland that secretes.

The body uses negative and positive feedback mechanisms to maintain homeostasis.

Various body systems may be involved in homeostasis, but all negative and positive feedback mechanisms that maintain homeostasis are controlled by the body's nervous and endocrine systems.

Example of how an animal organ system maintains homeostasis

Let’s look at how different organ systems work together to maintain blood glucose levels in animals. When an animal eats, the following changes take place:

• The digestive system breaks down food into glucose and other energy-rich molecules.

• Cells in the intestine absorb glucose and release it into the bloodstream.

• Blood glucose levels rise.

• The nervous system senses this change and sends signals to specialized cells in the pancreas.

• The pancreas produces and releases a hormone called insulin into the bloodstream.

• Insulin lowers blood glucose levels.

On the other hand, when an animal has not eaten, its blood glucose levels will decrease, so the pancreas releases a different hormone–glucagon–to increase blood sugar levels. This process of glucose regulation is just one example of the body's various negative feedback loop mechanisms.

Now, let’s discuss an example of positive feedback. When a blood vessel is damaged, the following changes take place:

• The damaged blood vessel initiates the clotting process.

• Platelets in the blood start to stick to the damaged area.

• These platelets release chemicals that recruit even more platelets.

• As the platelets accumulate, more chemicals are produced, and more platelets are drawn to the location of the clot, forming a platelet plug.

• Small molecules called clotting factors bring together components in the blood known as fibrin, sealing the inside of the wound.

In this case, we can see how the positive feedback loop speeds up the clotting process until the clot is big enough to stop the bleeding.