Organisation of the Human Body (Simple & Detailed)
The human body is organised in a systematic way, from very small units to the whole body.
Levels of Organisation
1. Chemical Level
Made of atoms and molecules (oxygen, calcium, proteins)
These form the basic materials of the body
2. Cellular Level
Cells are the smallest living units
Examples: nerve cell, muscle cell, blood cell
3. Tissue Level
Group of similar cells performing the same function
Types:
Epithelial tissue
Connective tissue
Muscular tissue
Nervous tissue
4. Organ Level
Different tissues combine to form organs
Examples: heart, lungs, kidney
5. System Level
Group of organs working together
Examples: skeletal system, nervous system
6. Organism Level
All systems together form the human body
Skeletal System
The skeletal system is divided into two main parts:
Axial skeleton
Appendicular skeleton
1. Axial Skeleton
Meaning:
“Axial” means central axis of the body.
Total bones: 80 bones
Parts of Axial Skeleton
a) Skull (22 bones)
Cranial bones (8) – protect the brain
Facial bones (14) – form the face
Functions:
Protects brain
Forms face and cavities for eyes, nose, mouth
b) Auditory Ossicles (6 bones)
Malleus, incus, stapes (3 in each ear)
Function:
Helps in hearing
c) Hyoid Bone (1 bone)
Located in neck
Does not articulate with any other bone
Function:
Supports tongue
Helps in swallowing and speech
d) Vertebral Column (26 bones)
Cervical – 7
Thoracic – 12
Lumbar – 5
Sacrum – 1 (fused)
Coccyx – 1 (fused)
Functions:
Supports body weight
Protects spinal cord
Allows movement of trunk
e) Thoracic Cage (25 bones)
Sternum – 1
Ribs – 24 (12 pairs)
Functions:
Protects heart and lungs
Helps in breathing
Functions of Axial Skeleton
Forms central framework
Protects vital organs
Supports posture
2. Appendicular Skeleton
Meaning:
“Appendicular” means limbs and their attachments.
Total bones: 126 bones
Parts of Appendicular Skeleton
a) Pectoral (Shoulder) Girdle
Clavicle (2)
Scapula (2)
Function:
Attaches upper limb to axial skeleton
b) Upper Limbs (60 bones)
Humerus – 2
Radius – 2
Ulna – 2
Carpals – 16
Metacarpals – 10
Phalanges – 28
Function:
Helps in movement, holding, and manipulation
c) Pelvic Girdle
Hip bones (2)
Function:
Attaches lower limbs to axial skeleton
Supports body weight
Protects pelvic organs
d) Lower Limbs (60 bones)
Femur – 2
Patella – 2
Tibia – 2
Fibula – 2
Tarsals – 14
Metatarsals – 10
Phalanges – 28
Function:
Helps in standing, walking, running
Supports body weight
Functions of Appendicular Skeleton
Movement and locomotion
Support and balance
Handling and grasping
Difference Between Axial and Appendicular Skeleton
Axial Skeleton
Appendicular Skeleton
Central axis
Limbs and girdles
80 bones
126 bones
Protection and support
Movement and locomotion
Skull, spine, ribs
Arms, legs, shoulder, pelvis
Below is a simple but detailed explanation of the body cavities, written in easy language and suitable for nursing / medical exams.
Body Cavities (Detailed & Simple Explanation)
What are Body Cavities?
Body cavities are hollow spaces inside the body that:
Protect internal organs
Allow organs to move and grow
Reduce injury by cushioning organs
Main Types of Body Cavities
The body cavities are mainly divided into:
Dorsal cavity
Ventral cavity
1. Dorsal Body Cavity
Located at the back (posterior) side of the body.
It includes:
A. Cranial Cavity
Location:
Inside the skull
Contains:
Brain
Blood vessels
Cranial nerves
Protection:
Surrounded by skull bones
Brain is protected by meninges and cerebrospinal fluid (CSF)
Functions:
Protects brain
Supports brain function
Maintains shape of head
B. Vertebral (Spinal) Cavity
Location:
Inside the vertebral column (spine)
Contains:
Spinal cord
Spinal nerves
CSF
Protection:
Vertebrae
Meninges and CSF
Functions:
Protects spinal cord
Transmits nerve impulses
Supports posture and movement
2. Ventral Body Cavity
Located on the front (anterior) side of the body.
It is divided by the diaphragm into:
A. Thoracic Cavity (In Detail)
Location:
Chest region
Above diaphragm
Boundaries:
Front → Sternum
Back → Thoracic vertebrae
Sides → Ribs
Below → Diaphragm
Divisions of Thoracic Cavity
1. Pleural Cavities (2)
One for right lung
One for left lung
Lining:
Pleura (visceral and parietal)
Function:
Allows smooth movement of lungs during breathing
2. Mediastinum
Central space between lungs
Contains:
Heart (in pericardial cavity)
Trachea
Esophagus
Major blood vessels
Thymus gland
3. Pericardial Cavity
Located within mediastinum
Contains:
Heart
Function:
Protects heart
Reduces friction during heart beating
Functions of Thoracic Cavity
Protects heart and lungs
Helps in respiration
Allows expansion of lungs
B. Abdominopelvic Cavity (In Detail)
Location:
Below diaphragm
Extends to pelvic floor
Divided into:
Abdominal cavity
Pelvic cavity
1. Abdominal Cavity
Location:
Upper part of abdominopelvic cavity
Contains:
Stomach
Liver
Gall bladder
Pancreas
Spleen
Small intestine
Part of large intestine
Kidneys (retroperitoneal)
Lining:
Peritoneum
Functions:
Digestion and absorption
Stores and processes nutrients
Protects abdominal organs
2. Pelvic Cavity
Location:
Lower part of abdominopelvic cavity
Within pelvic bones
Contains:
Urinary bladder
Rectum
Reproductive organs
Functions:
Supports excretion
Supports reproduction
Protects pelvic organs
Functions of Abdominopelvic Cavity
Protects digestive and reproductive organs
Allows expansion of organs (stomach, bladder)
Helps in digestion, urination, and reproduction
Summary Table (Exam-Oriented)
Cavity
Location
Main Organs
Cranial
Skull
Brain
Vertebral
Spine
Spinal cord
Thoracic
Chest
Heart, lungs
Abdominal
Upper abdomen
Digestive organs
Pelvic
Lower abdomen
Bladder, reproductive organs
Very Short Exam Notes
Cranial cavity → Brain protection
Vertebral cavity → Spinal cord protection
Thoracic cavity → Heart and lungs
Abdominopelvic cavity → Digestive, urinary , reproductive
Abdominal Regions and Quadrants (Simple Explanation)
Doctors and nurses divide the abdomen into regions and quadrants to locate organs, describe pain, and make diagnosis easier.
I. Abdominal Regions (9 Regions Method)
The abdomen is divided into 9 regions by:
2 vertical lines (mid-clavicular lines)
2 horizontal lines (subcostal and transtubercular lines)
Names of Abdominal Regions
Upper Row
Right Hypochondriac Region
Organs: Liver, gall bladder
Epigastric Region
Organs: Stomach, pancreas
Left Hypochondriac Region
Organs: Spleen
Middle Row
Right Lumbar Region
Organs: Ascending colon, right kidney
Umbilical Region
Organs: Small intestine
Left Lumbar Region
Organs: Descending colon, left kidney
Lower Row
Right Iliac (Inguinal) Region
Organs: Appendix, cecum
Hypogastric (Pubic) Region
Organs: Urinary bladder, uterus
Left Iliac (Inguinal) Region
Organs: Sigmoid colon
Easy Memory Trick
“Right Hypo – Epi – Left Hypo
Right Lumbar – Umbilical – Left Lumbar
Right Iliac – Hypogastric – Left Iliac”
II. Abdominal Quadrants (4 Quadrants Method)
The abdomen is divided into 4 quadrants by:
One vertical line
One horizontal line crossing at the umbilicus
Names of Abdominal Quadrants
Right Upper Quadrant (RUQ)
Liver
Gall bladder
Right kidney
Left Upper Quadrant (LUQ)
Stomach
Spleen
Left kidney
Right Lower Quadrant (RLQ)
Appendix
Cecum
Right ovary (female)
Left Lower Quadrant (LLQ)
Sigmoid colon
Left ovary (female)
Homeostasis
What is Homeostasis?
Homeostasis is the ability of the body to maintain a stable internal environment even when the external environment changes.
Homeostasis means keeping the body condition normal and balanced.
Examples of Homeostasis
Maintaining normal body temperature (≈ 37°C)
Maintaining blood sugar level
Maintaining water and electrolyte balance
Maintaining blood pressure
Importance of Homeostasis
Keeps body cells healthy
Helps organs work properly
Prevents disease
Maintains life
Mechanism of Homeostasis
The mechanism of homeostasis works mainly by feedback systems.
Components of Homeostatic Mechanism
Every homeostatic mechanism has three main parts:
1. Receptor (Sensor)
Detects change in body condition
Example: Temperature receptors in skin
2. Control Center
Receives information from receptor
Decides what action to take
Usually brain (hypothalamus) or endocrine gland
3. Effector
Produces response to correct the change
Example: Muscles, glands, organs
Types of Feedback Mechanism
1. Negative Feedback Mechanism (Most Common)
Meaning:
The response reverses the change and brings the body back to normal.
Example: Body Temperature Regulation
Increase in body temperature
→ Receptors detect heat
→ Control center (hypothalamus) activated
→ Effector: Sweat glands produce sweat
→ Body cools down
Decrease in body temperature
→ Shivering occurs
→ Body warms up
Result: Temperature returns to normal