What are the systems of the human body? 11 Human Body Systems and Functions | Human Body 101

When I am writing this post, thousands of electrical signals are transmitted by nerve cells and all these at a lightning speed and and my brain conveys instruction to my fingers and my fingers just following the instruction. But out of ignorance, I am always after wonders and blunders of the world not knowing that I am the greatest wonder ever and not even getting a chance to appreciate that. Human body is a complex machine, made up of different systems.  Why this post? I  have encountered many questions from this topic in different exams. I think this post will definitely help you in getting maximum information on this topic in less time. Don't forget to appreciate this amazing machine.
What are the systems of the human body? 11 Human Body Systems and Functions | Human Body 101

The important systems are:            
1. Integumentary system
2. Digestive system
3. Circulatory system
4. Muscular system
5. Skeleton system
6. Nervous system
7. Respiratory system
8. Endocrine system
9. Excretory system
10. Reproductive system
11. Immune System
General Facts
  • Average weight of a new bone child: 2.6 Kg
  • Body weight (Average)-70Kg
  • Chromosomes-46
  • Chemical composition of 1 kg fat-free body mass of an adult:720 g water, 210 g protein, 22.4 g Ca, 12 g P, 2.7 g K, 1.8 g Na, 1.8 g Cl, 0.47 g Mg
  • Oxygen: Element is present in maximum amount in human body

Teeth: Humans dentition is described as thecodontheterodont and diphyodont.
  • Thecodont: Teeth are placed in the socket of jaw bones
  • Heterodont : teeth are four different types both in structure and function
  • Diphyodont: Teeth appear twice in the life time of human. They are milk teeth and permanent teeth.
Adult Teeth and Baby Teeth
Adult Teeth and Baby Teeth
Milk teeth vs Permanent teeth
Dental formula:
An adult human being has 16 teeth on each jaw. In each half of jaws starting from the middle and going backwards, there are two incisors, one canine, two premolars and three molars (2+1+2+3).
  • Milk teeth: 20 in number (2+1+0+2)
  • Hardest part: Enamel of tooth
1. Integumentary System
The Integumentary system, consisting of the skin, hair and nails, acts as barrier to protect the body from the outside world.It also functions to retain body fluids, protect against disease, eliminates waste products, and regulate body temperature.
Skin is the largest organ in the body. Healthy skin has a mean pH of 5.5. This natural acidity is responsible for maintaining the skin's natural flora and protecting against bacterial or fungal colonization.
Histology of skin
Histology of skin
  • Structurally, skin consists of two layers: Epidermis and dermis.
  • Accessory organs of Skins: Hair and Nails
  • Glands: Sweat (Sudoriferous) glands and Sebaceous glands
  • Thinnest skin: Conjunctive in Eye

2. The Digestive System/Alimentary system
Digestive system is well developed and consists of alimentary canal and associated glands.
Digestive System
Digestive System

Alimentary canal
  1. Mouth
  2. Buccal cavity
  3. Pharynx
  4. Oesophagus
  5. Stomach
  6. Small intestine
  7. Large intestine
  8. Rectum
  9. Anus
  1. Salivary glands
  2. Gastric glands
  3. Liver
  4. Pancreas
  5. intestinal glands
Enzymes or Secretions
  1. Gastric juice
  2. Pancreatic juice
  3. Bile Juice
Important Points to Remember
  • Duration of digestion in the small intestine: Approximately 12 hours
  • Largest salivary gland: Parotid glands
  • Length of oesophagus: 25 cm
  • Length of small intestine: 6.25 metres
  • Length of large intestine:1.5 metres
  • Fastest enzyme: Trypsin
  • Biggest internal organ in the human body
  • Largest gland, weighs about 1.5 kg in man. It is about 1/40th of the total body weight.
  • Physiological most active organ
  • Largest regeneration power

3. The Respiratory System
Oxygen is necessary for the oxidation of nutrients. Body gets oxygen through respiratory gas exchange. The process by which oxidation of nutrients take place in the living cell is known as respiration.It is the step by step oxidation of food materials to release energy for biological work.
The organs used for breathing, the pharynx, larynx, trachea, bronchi,lungs and diaphragm.
Respiratory system
Respiratory system


  • The lung is the essential respiration organ in higher vertebrates (man).
  • The organ without muscle is lungs.
  • The main function of the lungs is the oxygenation of blood and to supply oxygen to it.
  • The lung is the essential respiration organ in higher vertebrates (man).
  • The organ without muscle is lungs.
  • The main function of the lungs is the oxygenation of blood and to supply oxygen to it.
  • Total lung capacity (TLC): 7 L (Male), 6.2 L (Female)
  • Vital capacity (VC): 5.6 L (Male), 5 L (Female).
  • Tidal volume (VT) at rest: 0.6 L (Male), 0.5 L (Female).
  • Inspiratory reserve volume: 3.2 L (Male), 2.9 L (Female).
  • Expiratory reserve volume: 1.8 L (Male), 1.6 L (Female).
  • Residual volume: 1.4 L (Male), 1.2 L (Female).
  • Max. breathing capacity in 30 breaths/min: 110 L (Male), 100 L (Female).
  • Partial pressure of O2 Air: 21.17 kPa (159mmHg)
  • Partial pressure of CO2 Air: 0.03 kPa (0.23mmHg)
  • Respiratory rate (at rest) 16 breaths/min
  • Oxygen capacity of blood 180–200mLO2/L blood = 8–9mmolO2/L blood
  • Respiratory quotient 0.84 (0.7–1.0)
  • Weight of lung-right: 700 gms
  • Weight of lung-Left:600 gms
  • Functional units of the lungs: Alveoli
Normal breathing rate:
  • At birth: 40-60/minute
  • 5 years age: 25-26/minute
  • 15 years age:20-22/minute
  • Adult:14-18/minute
4. The Circulatory System
It consists of the following components
The Circulatory System
1. Plasma
2. Blood cells
  • White blood cells
  • Red blood cells
  • Blood platelets
Blood Groups

  • Blood: Fluid connective tissue
  • Blood pH:7.4 (slightly alkaline)
  • Total amount of blood in body: 5.5 litres (in 70 kg man)
  • Average amount of Hemoglobin-Male: 13-16 g/dl
  • Average amount of Hemoglobin-Female: 11.5-14 g/dl
  • Total Hemoglobin content in body: 1000 gms
Composition of Blood
Composition of Blood
  • RBC: (Red blood cell or erythrocyte)- blood cell containing the pigment hemoglobin , the principal function of which it the transport of oxygen.
  • Number of RBC-In male: 5000, 000/cu mm+ 3, 00, 000
  • Number of RBC-In Female: 5000, 000/cu mm+ 3, 00,000
  • Life span of RBC: 120 days
  • WBC (White Blood Corpuscles or Lekocytes) which are involved in protecting the body against foreign substances and in antibody production.
  • Number of WBCs: 5,000-10,000/cu mm
  • Smallest WBC: Lymphocyte
  • Largest WBC: Monocyte
  • Average number of WBCs: 6000/cu mm
  • Life span of WBC: 2-5 days
Blood group
  • Common Blood groups: A, B, AB,O, RH+,RH-
  • Universal blood receipt: Blood group AB
  • Universal Blood donar: Blood group O
  • Bleeding time: 2-6 minutes
  • Blood clotting time: 3-6 minutes
  • Blood discharge from heart: 4-5 litres/minute
  • Normal blood pressure: 120/80 mm Hg
  • Weight of heart: 300 gms (Male)
  • Weight of heart : 250 gms (Female)
  • Heart beat: 72 per minute
Normal pulse rate:
  • At birth: 140/minute
  • 1 year: 120/minute
  • 10 years age: 90/minute
  • Adult:70/minute
  • Adult Male: 62-72/minute
  • Adult Female: 72-80/minute
  • Largest vein: Inferior vena cava
  • Largest artery: Abdominal aorta
5. Muscular System
Muscles, the organs of the muscular system are the specialised cells which are responsible for the movement and locomotion. They are characterized by properties such as excitability, contractility, extensibility and elasticity.
Muscular system
Muscular system
Three types of muscles: Voluntary, involuntary and cardiac
  • Total number of muscles in body:639 (constitutes 43% of the body weight)
  • Smallest muscle: Stapedius
  • Largest muscle: Gluteus maximus (Buttok muscle)
  • Longest muscle: Sartorius
6. The Skeletal System
The human skeletal system consists of bones and cartilages of various types. They provide support, surface for the attachment of muscles and protection to many vital organs like brain, heart, eyes etc, also give a definite form and shape to the body.
The Skeletal System
Skeletal system
  • Total number of bones in body: 206
  • Smallest bone: Stapes (in the middle ear)
  • Longest bone: Femur (Thigh bone)
In an infant, there are 33 vertebrae in the vertebral column. Five of these bones eventually fuse to form the sacrum, and four other join to become the coccyx. As a result, an adult vertebral column has 26 vertebrae.

7. The Nervous System
Nervous system consists of central nervous system ( includes brain and spinal cord), peripheral nervous system ( includes cranial nerves and spinal nerves) and autonomic nervous system( Nerve chains on either sides of the vertebral column).
Functions of Nervous System 
Neurons : The structural and functional units of the nervous system.It is the longest cell in the body
The Nervous System
Nervous system

The brain and spinal cord constitute the central nervous system (CNS). The brain is the largest and the most complex part of the nervous system. The brain is enclosed in the cranium and the spinal cord in the vertebral column.
  • Weight of brain: 1220-1400 gms
  • Largest part of the human brain: Cerebrum
  • Length of spinal cord: Average 42-45 cm
The peripheral nervous system(PNS) consists of the nerves that arise from the central nervous system and connect it to various body parts.The PNS includes cranial nerves and spinal nerves. There are12 pairs of cranial nerves and 31 pairs of spinal nerves.
The cranial nerves (12) include sensory (3), motor (5) and mixed (4) nerves.
The spinal nerves are named according to the level from they arise. There are 8 pairs of cranial nerves, 12 pairs of thoracic nerves, 5 pairs of lumbar nerves, 5 pairs sacral nerves and a pair of coccygeal nerve.
  • Longest nerve: sciatic
  • Thinnest and smallest nerve: Trochlear (Pathetic)
  • Largest cranial nerve: Trigeminal
8. Endocrine System
Endocrine system are those systems which generally control long term activities of target organs as well as physiological process such as digestion, metabolism, growth, development and reproduction in contrast to more rapid activities under the control of nervous system either directly or indirectly.
Endocrine System
Endocrine system

The endocrine system carries out a wide variety of physiological process through chemical messengers called "hormones".
Supreme commander of endocrine regulation: Hypothalamus

9. Reproductive System
The reproductive system includes reproductive organs, reproductive ducts and accessory structures. The organs which produce sex cells and secrete hormones are called primary sex organs. The development of sex organs (gonads) is influenced by gonadotropins of the anterior pituitary. The organs which perform important functions in reproduction but neither produce gametes nor hormones are called secondary sex organs
Male Reproductive system
male reproductive system
Male reproductive System
  • Primary sex organ: Testes
  • Reproductive ducts: Epididymis, vas deferens and penis
  • Accessory or secondary glands include a prostrate, two seminal vesicles and two Cowper's gland
  • Volume of semen: 2-4 ml
  • Sperms: 300 million sperms
Female reproductive system
  • Primary sex organ: Ovaries
  • Reproductive ducts: Fallopian tube (Oviduct), uterus and vagina
Female reproductive system
Female reproductive System
  • Accessory or secondary glands: Bartholin's glands or vesticular glands
  • Time of puberty in girl: 13 years of Age
  • Duration menstrual cycle in woman: 28 days
  • Time of ovulation: About 14 days before the onset of next menstruation
  • Egg produced at one time: One
  • Age of Menopause: 45-55 years
  • Gestation period: Approximately 270 days
  • Foetal membranes: chorion, amnion, allantois, yolk sac
  • Placenta: Haemochorial

10. Excretory System
The organs and tissues participating in the removal of waste products constitute the excretory system. It includes mainly the urinary system. The urinary system is concerned with the production, storage and elimination of urine. In addition to the urinary system some other organs such as liver, lungs and skin also function as excretory organs. These organs are called accessory excretory organs.
The excretory system of man consists of a pair of kidneys, a pair of ureters,urinary bladder and urethra
Excretory System
The two kidneys are not in same level. The right kidney is slightly lower the left kidney, because the right side of the abdominal cavity is occupied by the liver.
  • Size of kidney: 11 cm long, 6cm wide and 5 cm in thickness
  • Weight of kidney: 150 gms
Nephrons : The structural and functional units of kidneys.
A cross section of the kidney and Nephron
A cross section of the kidney (left) and Nephron (right).
  • Nephrons in each kidney: 1 million (10 lakhs)
  • Length of nephrons: 35-50 mm
  • Urine pH 4.5–8.2
  • The volume of urine formed in 24 hours in 1-1.8 litres.
It is a collection of mechanism within an organism that protects aginst diseases by identifying and kiiling pathogens and tumor cells. It detects a wide variety of agents from viruses to parasitic worms, and needs to distinguish them from the organism’s own healthy cells and tissues in order to function properly.The immune system of vertebraes such as human consists of many types of proteins (immunoglobulins), cells, ogans and tissues, which interact in an elaborate and dynamic network .
Two types of lymphoid organs :
Immune System
Immune System
Primary lympoid organs (Central Lympoid organs) : Thymus and bone marrow
Thymus is the site of T lymphocyte maturation and bone marrow is the site of B lymphocytes maturation and are responsible for cellular and humoral immune response respectively.
Secondary Lymphoid organs(Peripheral Lymphoid organs) : Lymphnodes, spleen, tonsils, Peyer's patches of the small intestine, appendix and mucousal associated lymphoid tissue (MALT).

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