What are the differences between unicellular and multicellular organisms when it comes to their functions? We already know that three of the basic functions of living organisms are survival, growth, and reproduction.
In order to survive and grow, a cell needs nutrients and oxygen. Moreover, it needs to get rid of carbon dioxide and waste products. If the environment is ideal, then the cell will be able to divide into two and grow in numbers. These functions occur in a quite simple way in unicellular organisms. Everything happens between a single cell and its environment. Nutrients and oxygen enter the cell through the plasma membrane so that they can be used in the metabolic processes and, then, carbon dioxide and waste can end up back to the environment, again through the plasma membrane.
However, things are not that simple in the case of multicellular organisms. Not all multicellular organisms are the same. Τhus, there is not just a unique rule that applies to all of them. We will try though to understand the complexity of the human body as we are more familiar with it compared to other organisms’. We will also come to learn more about what happens inside us every day without us having to think about it or force our bodies to do it.
From molecules to organisms
The human body can exchange nutrients and gases with the environment, but these do not end up directly to the cells where they are needed. The human body consists of trillions of cells, so if for example, a cell in your liver needs oxygen, this molecule will have to travel a very long distance inside you from the moment it enters your mouth or nose until it reaches its destination. That helps us understand that the human body is very complex and in order to function in a coordinated way to cover its needs, it is divided into groups, each one specialized in different functions. Despite their differences and their specializations, there is constant communication between them to ensure that everything works properly and that the human body functions as a whole.
One very important part of our bodies is our heart. Have you ever wondered how the heart takes its shape, and what does it consist of? It’s obviously not just a bunch of cells. We will go through the hierarchical arrangement of the levels that give rise to the whole human body. We have already learned that the cell is the basic unit of life. However, it is not the smallest entity in our bodies. The cells are made up of molecules, which in turn consist of one or more atoms. We won’t concern ourselves with the atoms, though. The molecules can be as simple as oxygen and water or complex and large ones, called macromolecules, such as our DNA or the proteins that are everywhere inside our cells. These molecules make up not only all of the contents of our cells but also substances outside of them, creating the extracellular space.
Cells do not exist individually or in isolation. They are organized into tissues, groups of cells with similar structure and functions. There are only four types of tissues, although they can be divided into smaller and more specialized categories. These are epithelial tissue, connective tissue, muscle tissue, and nervous tissue. Different types of tissue then combine and cooperate to form our organs, such as the heart, the kidneys, or the brain. As we said before, the organs do not function independently. For example, when you eat some food, it has to go through the process of digestion. This doesn’t occur in your stomach at once. There are certain stages the food needs to pass before it reaches the stomach, and after that, it continues its journey until the nutrients are absorbed, and the waste products are removed from your body. Different stages are carried out in different organs such as the esophagus or the small intestine. Therefore, organs that participate in a process to reach a specific goal comprise an organ system, the digestive system in this case.
There are 11 organ systems in the human body. The cardiovascular, respiratory, digestive, nervous, muscular, skeletal, urinary, endocrine, reproductive, lymphatic, and integumentary systems. Some of them have more clear functions than others. Some organs are considered parts of more than one organ system. Others are more difficult to classify in particular systems. However, these 11 systems make up the entire human body.
Organ Systems
The basic functions of the 11 organ systems of the human body are given below along with most of the organs involved in each of them.
Cardiovascular system
Organs: heart, blood, blood vessels
Functions: The heart functions as a pump that sends the blood throughout the body. The blood circulates inside the blood vessels and transports substances. This way, it transports oxygen, nutrients, hormones, and other substances to the cells and also carries carbon dioxide and other waste products away from the cells. The cardiovascular system also plays a role in maintaining body temperature and pH.
Respiratory system
Organs: nose, mouth, larynx, pharynx, trachea, lungs, diaphragm
Functions: The main function of the respiratory system is the exchange of gases: oxygen and carbon dioxide, between the cells and the environment. The air we breathe in through the mouth or nose ends up in the lungs. There, a process occurs during which oxygen diffuses into the blood. Additionally, carbon dioxide is carried away from the blood and leaves our body through exhalation.
Digestive system
Organs: mouth, pharynx, esophagus, stomach, liver, gallbladder, pancreas, small intestine, large intestine
Functions: The digestive system’s role is to breakdown food so that nutrients can be absorbed by our cells while solid waste products are excreted from our bodies. The digestive process begins in the mouth, where we use our teeth and saliva to prepare a mush that will then travel through cavities until it arrives in the stomach. There, the food is broken down with the help of enzymes and acids. In the intestines, useful substances such as proteins, carbohydrates, fats, vitamins, and minerals are absorbed while waste leaves the body in the form of feces.
Nervous system
Organs: brain, spinal cord, nerves, sensory organs
Functions: The nervous system senses, processes and transforms information so that it can coordinate the other systems’ activities accordingly. It uses the sensory organs like our eyes and ears to respond to stimuli like light and smells. The brain is the control center of our body that commands the other organs to carry out specific tasks. For example, the nervous system makes our heart beat faster when we are afraid or sends a signal for the digestive process to begin after consuming food. The nervous system is also responsible for our memories, emotions, and behavior.
Muscular system
Organs: skeletal muscles, cardiac muscle, smooth muscles
Functions: The muscular system consists of all the muscles in our body. When we hear the word muscle, we often think of the skeletal muscles that help us move our arms and legs. But there are three types of muscles in total. The cardiac muscle is only found in the heart and it contracts involuntarily to keep the blood pumping. The smooth muscle covers internal organs such as the stomach or the uterus, and it is controlled involuntarily as well. However, the skeletal muscles that are attached to the skeleton, contract or relax according to our will, whether to move or not.
Skeletal system
Organs: bones, tendons, ligaments, cartilage, bone marrow
Functions: The skeleton provides structure and support to our body giving us our shape. It takes part in our movement with the association of the skeletal muscles attached to them. Another important function is protection, as some bones encase vital organs that would be very vulnerable if they were exposed. For example, the rib cage consists of bones, the ribs, that enclose the heart and lungs, protecting them from damage. The brain is also shielded inside the cranium, a series of connected bones. Haemopoiesis, which means blood production, also occurs inside the bones. The bone marrow is a tissue found inside the bones, where most blood cells are formed.
Urinary system
Organs: kidneys, ureters, urinary bladder, urethra
Functions: All of our blood is filtered through our two kidneys, which are parts of the urinary system. Dangerous products as well as substances that are found in higher concentrations than what’s needed are extracted from the blood so that they can be removed from the body. They move through the ureters and end up in the bladder. When there is enough fluid gathered there, it is excreted from the urethra in the form of urine. By filtering the blood, the urinary system plays a very important role in maintaining blood homeostasis since it regulates blood volume and pH.
Endocrine system
Organs: hypothalamus, pituitary, pineal gland, thyroid, parathyroids, adrenals, pancreas, ovaries, testes
Functions: The endocrine system comprises of glands throughout the body, whose cells have the ability to produce hormones. Hormones are substances that act as messengers. They are released into the bloodstream and are taken up by cells in specific target-organs. These organs respond to the messages they receive by altering their functions in a certain way. The production of hormones by the endocrine system helps in the communication between different organ systems. This way, hormones allow the body to react to changes that occur either inside or outside the organism. For example, when we are in danger, the adrenal glands produce cortisol to cause heart rate elevation. Another case is the production of high estrogen concentration for a specific time window during the menstrual cycle.
Reproductive system
Organs: mammary glands, ovaries, fallopian tubes, uterus, vagina, prostate, testes, penis
Functions: The reproductive system consists of the sex organs that are required for reproduction. The reproductive system differs between a male and a female. However, they share an important, common function; the production of gametes: sperm in the male and eggs in the female that can give rise to an offspring. The female system is also responsible for pregnancy, childbirth, and breastfeeding. During each menstrual cycle, the woman’s body prepares to host an embryo in case a pregnancy occurs.
Lymphatic system
Organs: lymph, lymph nodes, lymph vessels, spleen, thymus
Functions: The lymphatic system carries fluid from tissues back to the bloodstream. This fluid is called lymph and it flows inside the lymph vessels. The lymphatic system plays an important role in the body’s response against pathogens. White blood cells, which are part of the immune system, circulate in the blood but also in the lymph, and they can find and attack invaders. Lymph nodes can be the areas of the body where the fight against pathogens takes place since immune cells are able to cluster there.
Integumentary system
Organs: skin, hair, nails
Functions: The integumentary system constitutes a barrier between the inside of our body and the external environment. A very important component of this system is the skin, which is considered the largest organ in the body and the first line of defense against dangerous substances, microorganisms, or injuries. It is also involved in the regulation of the body’s temperature. More precisely, the sweat that’s produced by glands inside the skin helps in the removal of water and heat.
