A multicellular organism may be divided into many levels of organisation. The smallest unit is the organelle, a specialised structure which performs a specific task within the cell, which is the next level of organisation. Cells then group together based on function into structures known as tissues, which are then grouped together into an organ, which performs a task. Organs often work together to provide a specific function and are grouped together into organ systems. Humans, Animals and Plants are all made of organ systems!
What is an organelle?
As described above, an organelle is a small structure within a cell which is designed to perform a specific function. They may be contained within a membrane, or simply be free-floating functional units within the cytoplasm. Some key examples of organelles are the nucleus , mitochondria and Ribosomes present in our Cells!
Check out the Animal and Plant Cells article to learn more about sub-cellular structures or organelles!
It is generally believed that some organelles, specifically the mitochondria and chloroplast, may have once been free-living organisms which were engulfed by an early cell, but instead of dying, they developed a symbiotic relationship with the cell. Over time they lost components not necessary in their new living arrangement, eventually becoming the organelles we know today. This theory is known as the endosymbiotic theory.
What is a cell?
The cell is the next largest unit of organization. Cells are small, membrane-enclosed spaces which contain organelles, which form the basic units from which larger structures are formed. They may either be the entire organism, as is the case with Bacteria or amoebas (unicellular organisms), or they may be constituents of a larger multicellular organism, like humans.
In multicellular organisms, cells may be specialised in function. Some examples of this are muscle cells or nerve cells, each of which is highly specialised in terms of structure for their specific function. The conversion of non-specialised cells to specialised is referred to as differentiation. Cells of a similar type and function tend to group together, forming larger structures which are known as tissues.
Undifferentiated cells are known as Stem Cells. There are three main sub-types of Stem Cells: totipotent, pluripotent and multipotent, each being more limited in the type of cell it can become. Totipotent cells may become any type of cell within the body, including extra-embryonic tissue (placental cells). Pluripotent cells may become any type of cell within the body, excluding placental cells and multipotent stem cells may become several cell types, but not all.
What is a tissue?
The complex nature of eukaryotic organisms makes it difficult for a single cell alone to perform a function. Therefore, two or more cells with similar structures group together to perform a specific function are named a tissue. There are four main types of tissue:
Epithelial tissue: Epithelial tissues are formed of thin continuous layers of cells and line various internal and external surfaces within the body. The most visible example of epithelial tissue is the skin.
Connective tissue: As the name suggests connective tissue is any tissue that connects and supports other tissues. An example of connective tissue that might not be very obvious is Blood, and a more common example is tendons.
Muscular tissue: Muscular tissue makes up the muscles that move our body and our heart! This includes the Skeletal Muscle, cardiac muscle and smooth muscle.
Nervous tissue: The nervous tissue transmits signals throughout the body and is comprised of neurons, the actual cells which transmit signals and neuroglia, cells which support the nervous system.
Eukaryotes or eukaryotic organisms are organisms with Eukaryotic Cells, meaning cells with membrane-bound organelles like a nucleus. Read more about this in our Eukaryotes and Prokaryotes article!
What is an Organ and an Organ System?
An organ refers to a group of tissues that come together to perform a specific function.
This allows for the formation of things like the pumps that make up our heart, or a tube that is capable of moving food like the small intestine. An organ system is a group of organs also working together to perform a specific function. The organ systems come together to form an organism. There are many organ systems in the human body.
What are the main organ systems in the human body and their functions?
The main organ systems in the human body are the nervous system, Respiratory System, Endocrine System, Circulatory System, digestive system, muscular system, skeletal system, urinary system, Lymphatic System, Excretory System, integumentary system and reproductive systems.
Nervous system: The brain, spinal cord and nerves make up the nervous system. It controls all activities of the other systems.
Respiratory System: Starting from the nostrils to the lungs, the respiratory system controls our breathing.
Endocrine System: The endocrine system secretes hormones, that regulate activities in our bodies. It is made up of the glands like the ovary, testis, thymus and pancreas.
Circulatory System: The circulatory system is responsible for the transport of Blood all around the body. It is made up of The Heart and Blood vessels.
Digestive system: The digestive system is responsible for the digestion of food substances.
Muscular system: The muscular system is responsible for the movement of the body using muscles.
Skeletal system: The skeletal system provides the body structure and support. It is made up of bones.
Urinary system: The urinary system is responsible for excreting metabolic waste and other substances out of the body in the form of urine. It is made up of the kidneys, ureter, bladder and urethra.
Lymphatic System: Made up of the red bone marrow, thymus, lymphatic vessels, thoracic duct, spleen and lymph nodes, the lymphatic system is responsible for protecting the body against infection as well as draining excess fluids from cells and tissues.
Integumentary system: The integumentary system is responsible for protecting the body from the external environment. It is made up of the skin, nails and hair.
Reproductive system: The reproductive system enables us to produce offspring. It is made up of the penis, testis, prostate gland and scrotum in males and the ovary, uterus, vagina, and fallopian tube in females.
Diagram of Human Organ Systems
Here is a diagram showing an overview of many of the main organ systems of the body discussed above.
Examples of Organ Systems
Two main systems of relevance, the digestive system and the circulatory system, are explored below, along with non-communicable diseases which often affect human organ systems.
Overview Of The Digestive System
The digestive system, like all organ systems, is formed of a variety of organs working together to achieve a certain function. In the case of the digestive system, it is to process and extract nutrients and water from the food and liquids we consume. It does this by breaking down large molecules into smaller molecules and then absorbing these small molecules into the body via diffusion, Osmosis and Active Transport.
The organs that make up the digestive system are the organs of the digestive tract, a series of hollow organs, whose lumen is technically outside of the body! The digestive tract is comprised of the mouth, oesophagus, stomach, small intestine, large intestine and anus. These are supported by the liver, pancreas and gallbladder, which produce and store substances that support digestion. The various organs of the digestive system all coordinate their actions to work together and efficiently extract nutrients and water from the food and fluids consumed.
The mouth begins chemical digestion by secreting Enzymes, as well as physically mashing up the food by chewing. The partially digested food then flows down the oesophagus into the stomach, where acid and enzymes continue to break it down. It then flows into the small intestine, where additional enzymes and substances are added by the pancreas and gall bladder to absorb nutrients. Finally, it travels through the large intestine where Bacteria digest the last remnants and water is absorbed before the waste is released in faeces.
Overview Of The Circulatory System
The circulatory system is responsible for, as the name suggests, circulating blood around the body. It is comprised of the heart and Blood vessels, along with the blood itself. It is responsible for feeding cells with nutrients and oxygen, as well as removing waste products. It also carries components of the Immune System, regulates water in the body and, by way of the endocrine system, acts as a communication system within the body.
The Heart, as you know, pumps blood around the body, through the blood vessels. These blood vessels consist of the arteries, veins and capillaries. Arteries carry high pressure, oxygenated blood away from The Heart around the body. Veins carry deoxygenated, relatively low-pressure blood back to The Heart. Capillaries bridge between smaller versions of the previous two types, known as arterioles and venules, and penetrate into Tissues and Organs. Capillaries are very small and have thin walls, making them the site of the majority of entry to and exit from the blood.
Read our article Circulatory System to learn more about how blood travels around the body!
Non-Communicable Diseases in Organ Systems
While the body's organ systems are affected by many infectious diseases, meaning diseases caused by Microorganisms like bacteria or viruses, they can also suffer from diseases which are not caused by infectious pathogens. These are termed non-communicable diseases. Two of the main non-communicable diseases affecting humans are coronary heart disease and Cancer, each of which has its own set of Risk Factors.
Coronary heart disease is a disease resulting from a build-up of Fatty Acids in the arteries supplying the heart with blood. It causes limited or no blood supply to areas of the heart, causing symptoms ranging from mild chest pain to death.
Cancer is a disease characterised by the uncontrolled division of cells within the body, sometimes forming a Tumour, usually stemming from damage or mutation to the Genes that control these processes within cells. A key characteristic of cancer is that the cells can spread around the body, whereas a benign tumour stems from the same division of cells but doesn't spread to new areas. The symptoms of cancer vary significantly and depend on the cells and tissues affected.
Risk Factors are anything which increases the likelihood of a disease occurring. Some examples are exposure to radiation or carcinogenic chemicals increasing the chance of cancer, or consumption of lots of fatty foods increases the risk of coronary heart disease.
Check out the articles Non-Communicable Diseases and Communicable Diseases to learn the differences between them!
Plant Organs
Just like humans, Plants also have organ systems. They function in the same way as in any other organism, however, tend to be quite simpler. Plants have two organ systems, the root and shoot systems. The root system acts somewhat like a digestive system in humans, except instead of absorbing resources from consumed foods, it absorbs resources from the environment. The shoot system consists of stems and leaves, along with the reproductive organs of the plant.
Check out our article Plant Organs to learn more about these systems!
Organ Systems - Key takeaways
- Organisms can be broken down into several organisation levels (organelles, cells, tissues, organs, organ systems)
- Organ systems consist of several organs all working together to achieve a common purpose, such as the digestion and absorbance of substances from food and liquids consumed in the digestive system.
- The key organ systems of the body are the: nervous system, respiratory system, endocrine system, circulatory system, digestive system, muscular system, skeletal system, Urinary system, lymphatic system, Excretory System, integumentary system and reproductive system.
- Organ systems may be impacted by communicable and non-communicable diseases.
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