Introduction to Human Body: Human Body Organization, Organ Systems and Anatomy Overview
The Human Body:
The human body is the result of millions of years of evolution. Keeping in line with the ‘Theory of Natural Selection’, as envisaged by Charles Darwin, it has evolved with time.
Some organs have been reduced to a vestige (the appendix for one was used for digesting cellulose rich fibers. With the discovery of fire and consequent changes in eating habits, it no longer serves its purpose). Others have developed varied degree of specializations. The human thumb, for example, is freely apposable and the human hand is specialized for grasping, and other dexterous movements.
The erect posture which man acquired has emancipated his upper limbs while the lower limbs became specialized for weight-bearing and support. As a result, the upper limb has more mobility at the cost of stability while the lower limb is less mobile in exchange for increased stability.
Human Body Organization:
The current era man or Homo sapiens is a bipedal animal which has an enormous thinking capacity, courtesy, a well-developed frontal cortex, which is the seat of knowledge and creative thought processes. Like all living organisms, the human body is made of cells.
Cells constitute the basic structural and functional unit of life. Cells comprise od a cell membrane which enclose the cytoplasm or the cellular matrix which has various organelles for its functions. At an elemental level though, the cell is composed of hydrogen, nitrogen, carbon, oxygen and other trace elements. These elements interacted with each other to form the cell, the first living organism. Over time, these cells coalesced to form various organisms.
The longest cells in the human body are the axons which run from the brain and spinal cord to distal most organs and vice-versa. The axons of these neurons can be as long as 1metre, but due to their smaller diameters (in the range of micrometres or 10-6 m, they are seen only under a microscope with appropriate stains). The body is composed of nearly 30 trillion cells, a majority of which are RBCs, which carry oxygen to the tissues. These cells are arranged into tissues. A tissue is a group of cells performing the same function (for e.g. nervous tissue, connective tissue, vascular tissue, etc).
These tissues combine to form organs. The individual cells or tissues do not have any fundamental properties of their own in an organ. It is only when they interact with each other via various signalling pathways, that they function as an organ.
For example, the liver has a connective tissue matrix, it has vessels which carry blood to and from it, specialised Kupffer cells (modified macrophages or those cells which kill invading microbes), and a system of ducts to carry bile to the small intestine. Thus, as an organ, the liver is composed of a diverse set of tissues, which ensures that it functions optimally. The organs combine together to form organ systems.
Each organ system is specialised in its own right. However, they are inextricably linked to each other for their normal functioning. The organ systems include, but are not limited to, the nervous system, the integumentary system (skin and appendages), cardiovascular system, respiratory system, nervous system, endocrine system, the reproductive system and the excretory system.
A system-wise composition includes:
Nervous System- The Brain, spinal cord and the nerves.
Excretory System- Pair of kidneys, ureters, bladder and urethra.
Reproductive system- Male and female gonads (testis and ovaries respectively), duct systems, accessory reproductive organs and the external genitalia in both sexes.
Endocrine System- The pituitary gland, thyroid, pancreas, parathyroid glands, ovaries, testis, etc.
Respiratory System- The external orifices, conducting pathways, lungs and the respiratory centres in the medulla.
Cardiovascular System- The heart, blood vessels and the blood components along with the cardiac centre in the medulla.
Digestive System- It includes the entire tube from the mouth to the anal canal comprising of the esophagus, stomach, small and large intestines, rectum and anus. It is also composed of accessory digestive organs such as the liver, gall bladder and the pancreas.
Integumentary System- The skin, hair, sweat and oil glands.
The nervous system and the endocrine system are responsible for controlling and coordinating various body activities. They ensure that the various processes which go on in the body are regulated. It is this regulation which helps the body to survive in various adverse conditions, which might otherwise have been unfavorable to life.
It is precisely because of this reason that human beings can be found in a diverse set of geographic and climactic conditions: From plains to hill-tops, freezing sub-zero polar areas to the scorching heats of dry deserts, from tribals in forests and islands to city dwellers in urban geographies.
The organ systems work in tandem to make a complete organism, the human being. These processes which help one survive are collectively referred to as life-sustaining processes (breathing, circulation, respiration, reproduction, etc) and the steady state I known as homeostasis. In medical terminology, homeostasis refers to a state of internal constancy despite changes in external surroundings.
Stated otherwise, homeostasis is the maintenance of near constant internal conditions to maintain the life in the wake of external changes. It is a set-point which exemplifies normal functioning. However, it is context specific, gender -specific and individualistic. Thus, we do not have a fixed value of normal values for various processes.
For example, though the normal blood pressure is stated as 120 mm (systolic) by 80 mm (diastolic) of Mercury, there is a normal range. The range for systolic blood pressure is 120-140 mm of Mercury, whereas the same for diastolic blood pressure is 80-90 mm of mercury. Similarly, though the average pulse (which is measured in the forearm radial artery and is an indicator of the heart rate) is taken as 72 beats per minute, the range of pulse is 60-100 beats per minute.
Disease- An outcome of disequilibrium:
Diseases refers to a disruption of homeostasis. There are multiple causes of the same. They may be the result of an insufficient blood supply, termed ischemia, leading to oxygen deprivation (hypoxia) and consequent cell death. They may also be due to infections which can be due to bacteria, viruses, fungi, parasites, etc. Or, they may result from physical and chemical injuries (burns, acids), irradiation, autoimmune diseases and hypersensitivity reactions.
Whatever the initial cause of insult, they lead to cellular injury. A prolonged, severe injury to a susceptible cell leads to cellular death and ultimately organ damage and system dysfunction which leads to signs and symptoms of a disease in a patient.
In the above-mentioned example, if the blood pressure is more than 140/90 mm of Hg, it is termed as hypertension. Hypertension, like most diseases, are multifactorial in origin. It will lead to thickening of heart walls, leading to inadequate pumping of blood to the rest of the critical organs, including the kidney, heart and brain.
Long-standing untreated hypertension can thus lead to chronic renal failure (kidneys), a heart attack or myocardial infarction (heart) or a paralytic attack/ stroke (brain). Similarly, a pulse below 60 is termed as bradycardia (brady-low) or above 100 is labelled as tachycardia (tachy-high) due to various pathologies.
The study of human body in Medicine is along the broad contours of the organ systems. An understanding of the normal functioning is essential before studying about the diseases and its treatments. It can be broadly divided into the three contours: Anatomy, Physiology and Biochemistry.
Anatomy is the structure of the human body as it is. It includes developmental anatomy (embryogenesis and genetics), gross anatomy (specialized subdivisions into the head, neck, upper limbs, thorax, abdomen, pelvis and lower limbs with respective muscles, organs and neuro-vascular structures) and microscopic anatomy (popularly referred to as histology).
The human body is made up of 206 bones which are divided into an axial skeleton comprised of 86 bones (inclusive of the skull bones and the vertebral column) and an appendicular skeleton (the four limbs consist of 120 bones in total, with each limb being composed of 30 bones). This subdivision is true for an adult human body. However, infants have a much larger number of bones, which fuse as the age progresses, resulting in 206 bones in an adult person.
The skeletal framework is supported by muscles, numbering about 640 in an adult person. The number however varies due to anatomical variations in individuals. These muscles have two major functions.
The first is protection of internal organs of the body.
The second is execution of the motor response of the body (pushing, pulling, running, etc).
These skeletal muscles carry out voluntary activity, those which are under our will. Internal organs and viscera composed of smooth muscles which are involuntary in nature. They are not under our command and function with the help of the autonomic nervous system.
The heart is another specialized muscular organ. It is made of cardiac tissue. In anatomy, it shares the features of skeletal muscles. However, like smooth muscles it is involuntary in nature.
The body can also be divided into anatomically distinct compartments. They include the cranial cavity, the thoracic cavity and the abdominal cavity. The cranial cavity houses the brain.
The thoracic cavity has lungs and heart. The abdominal cavity has the stomach, liver, spleen, kidney and intestines, whereas the pelvic cavity (an extension of the abdominal cavity) houses the bladder, rectum and anal canal. In females it has the uterus and the ovaries.
In males, it houses the prostate and the various glands (Bulbourethral glands, seminal vesicles, etc). The thoracic cavity is separated from the abdominal cavity by a diaphragm which moves up and down with respiration.
The vascular system is broadly divided into the arteries, veins and capillaries. Arteries carry oxygenated blood from the heart to all the organs of the body. The veins carry de-oxygenated blood from the cells and tissues back to the heart.
Gaseous exchange occurs across capillaries. In the lungs, there is exchange of CO2 and O2. The superior and inferior vena-cava bring de-oxygenated blood from all the parts of the body to the heart. It is sent to the lungs for oxygenation by the pulmonary arteries.
Oxygen from the air is taken up in the pulmonary arteries which have divided to form pulmonary capillaries. These capillaries unite to form pulmonary veins which bring it to the heart. The aorta pumps the oxygenated blood to all the organs. Again, gaseous exchange takes place across capillaries at the level of organs and tissues. Oxygen is taken up by the cells and the waste products of metabolism and carbon-dioxide diffuse across into veins. Veins coalesce to form the superior and inferior vena cava and thus, the cycle is repeated.