Structural Organisation & Different Levels

Structural Organisation

The most extensive, in-depth studies begin with the basics. The structural organisation in a human organism is studied from the very basic constituents. The smallest unit of life stays the same for any other lifeform at the fundamental level. And what is that similarity? Cells— all life on earth is composed of one or more cells. Moving up the hierarchy, one would come across tissues, organs, organ systems, and organisms at different levels of structural organisation. 

Levels of Structural Organisation

The structural organisation aids in understanding how the human body functions. The complexity of the human body structure makes it easier for scientists to conduct their research and for students to find connections between newly offered concepts. 

Knowledge of structural organisation helps you visualise and comprehend how the human body structure is built and the functions of various parts. The levels of organisation in order of their decreasing complexity are as follows: 

• Organism
• Organ system
• Organ
• Tissue
• Cell 

Organism Level

At the top level is an organism–the life form containing systems, organs, tissues, cells, and molecules. For example, you are an organism.

Organ System Level

The organ system level of structural organisation is immediately below the organism level. Organ systems are large bodily systems that are committed to carrying out particular tasks. An organ system is made up of a number of organs that cooperate to perform a single, essential task for the body. 

The human body structure contains eleven primary organ systems. The eleven distinct organ systems are as follows:

1. Integumentary system: Encloses internal body structures and consists of several sensory receptors. The major constituents of this system are skin, hair, and nails.
2. Skeletal system: Enables movement while supporting the body with the help of the muscular system. Bones are the major components of this system. They also provide a protective covering for delicate organs such as the rib cage that protects the heart and lungs. 
3. Muscular system: Enables movement along with the skeletal system, helps maintain body temperature and gives a form to the body. The muscular system consists of muscles such as skeletal muscles, cardiac muscles, and more. 
4. Nervous system: Detects and processes external and internal stimuli. It perceives sensory information and activates the corresponding bodily responses. The main organs associated with the nervous system are the brain, spinal cord, and nerves.
5. Endocrine system: Responsible for hormonal secretions that regulate bodily functions. Some major endocrine organs include the pancreas, adrenal, pituitary, thyroid glands and glands.
6. Cardiovascular system: Delivers oxygen and blood-carrying nutrients to body tissues. It also removes waste products from the body and helps regulate the body temperature. The functions of the cardiovascular system are performed mainly by the heart and blood vessels.
7. Lymphatic system: Serves to return fluids to the blood. It also defends the body against pathogenic invasions. Major structures associated with the lymphatic system are the thymus, spleen, lymph nodes, and lymphatic vessels.
8. Respiratory system: Delivers oxygen to the body via blood and gets rid of carbon dioxide from the body. The lungs are the central organs that participate in the inhalation and exhalation of oxygen and carbon dioxide. The nasal cavity and trachea work together to carry out the responsibilities of the respiratory system. 
9. Digestive system: Processes food, breaking it down into organic molecules that can be assimilated by the body. It also removes waste as undigested food. The major organs of the digestive system are the oesophagus, stomach, gallbladder, large intestine, and small intestine.
10. Urinary system: Regulates water balance in the body, gets rid of toxins from the blood and excretes them. The kidneys are responsible for urine formation while the bladder holds the urine until it is passed out from the body via the urethra. 
11. Reproductive system: Produces gametes and sex hormones. The male reproductive system participates in transferring the gametes to the female, while the female reproductive system nourishes and helps in embryo development until birth. Male reproductive system organs are the testes, epididymis, and scrotum, while the female reproductive system includes the uterus, mammary glands, and ovaries.

Note: The above-mentioned details about the functions of the major organ systems are just a synopsis. Each of these systems has multiple responsibilities, and they assist one another in carrying out several functions.

Organ Level

Starting with the human organism, then the organ systems, the next level of structural organisation is the organ. An organ is a structure composed of specific tissues that perform a specific function.

The human body has several organs that are constituents of the large organ systems. The human body has five vital organs that are essential for survival. A slight impairment in these organs can have adverse issues. The vital organs in the human body are as follows:

1. Brain: It is the body’s master control centre that forms the core of the central nervous system. It is responsible for receiving, processing, sending, and creating nerve impulses, emotions, thoughts, physical sensations, and more. The delicate organ is enclosed in the cranium/skull to protect it from injury.
2. Heart: It is the most important organ of the circulatory system that pumps blood to the body. The heart and the lungs together function to add oxygen to the blood and pump the oxygenated blood to the body via blood vessels. It also receives deoxygenated blood from a separate group of blood vessels. 
3. Lungs: A pair of lungs participate in the process of oxygenation of the blood. They are responsible for removing excess carbon dioxide in exchange for oxygen.
4. Liver: It is another important organ that regulates the body’s metabolism. It enables the conversion of nutrients into usable substances. The liver is also responsible for detoxifying substances. It filters the incoming blood from the digestive tract. 
5. Kidneys: A pair of kidneys located on either side of the back helps filter blood and get rid of all the waste from the body.

Tissue Level

The organs are composed of tissues. A tissue is a group of similar cells that work together to carry out a specific function. The human body consists of four basic types of tissue, including connective tissue, epithelial tissue, muscle tissue, and nervous tissue. 

1. Epithelial Tissue: Characterised by the absence of blood vessels, these tissues receive nutrition via diffusion. The epithelial tissues form the outer covering of the body and the inner lining of cavities and many organs. They have densely packed cells with little spaces between them. Epithelial tissue is classified as follows:

• Simple epithelium: One-cell thick epithelium that has the following five types:
  • Squamous 
  • Cuboidal
  • Columnar
  • Ciliated 
  • Pseudostratified 
• Compound epithelium: More than one-cell thick epithelium that has the following two types:
  • Transitional
  • Stratified squamous and stratified cubical

2. Connective Tissue: The most abundant tissue in animals. The connective tissues are so because they link and support other body tissues. The three different types of connective tissues are as follows:  

• Connective tissue proper
  • Adipose 
  • Areolar
  • White
  • Tendon 
  • Ligament
• Skeletal tissue 
  • Cartilage 
    • Hyaline cartilage
    • White fibro-cartilage
    • Yellow fibro-cartilage
  • Bone
    • Compact bone
    • Spongy bone
• Vascular tissue
  • Lymph 
  • Blood

3. Muscular Tissue: Composed of long fibres that run in parallel arrays and are composed of fine myofibrils. Muscular tissues help in contraction, relaxation, and other body movements. The different types of muscular tissues are as follows: 

• Skeletal/ Striated/ Voluntary/ Striped muscles
• Smooth/ Visceral/ Involuntary/ Non-striated muscles
• Cardiac muscles

4. Nervous Tissue: Comprises neurons/nerve cells and neuroglial cells. The neuron is the structural and functional unit of the nervous system. It is an excitable cell that carries nerve impulses. Nervous tissues have maximum control over the body’s reaction to changing conditions, both internal and external. 

Cellular Level

The smallest independently functioning unit of an organism is a cell. While some organisms are single-celled such as E.coli, the others are multicellular. 

Cells can be divided into two main categories:

• Prokaryotic cells that do not have a membrane-bound nucleus are found in bacteria. 
• Eukaryotic cells have membrane-bound nuclei found in humans.

Humans consist of eukaryotic cells. These cells carry specialised structures called cell organelles that perform specific functions. A typical eukaryotic cell in humans consists of the following components:

• Cell membrane 
• Nucleus
• Golgi body
• Lysosome
• Ribosome
• Mitochondria
• Endoplasmic reticulum
• Cytoplasm
• DNA

Conclusion

The structural organisation comprises cells that combine to form tissues. Similar tissues form organs. The organs performing similar functions form an organ system, and several organ systems give rise to a human organism. 

Frequently Asked Questions

Q1. What is the function of epithelial tissues?

The functions of the epithelial tissue are as follows:

• Protection of underlying tissue from injury and dehydration. 
• Secretion of mucus, hormones, and gastric juices.
• Excretion of nitrogenous waste. 
• Absorption of digested food materials. 
• Conduction of mucus and fluids. 
• Sensation
• Respiration
• Reproduction
• Exoskeleton formation 

Q2. What is an organ?

A differentiated structure comprising cells and tissues and performing some specific functions in an organism is called an organ. Some examples of organs in the human body are the heart, liver, pancreas, brain, kidney, lungs, spleen, ovaries, testes, and small and large intestines. 

Q3. What is an organ system?

Organs of a system function in a coordinated manner to carry out major life processes and form an organ system. Some important organ systems are the digestive system, nervous system, circulatory system, and more.