Human Anatomy: Skeletal, Muscular, Circulatory, Digestive, Respiratory and Nervous Systems
Human anatomy is the branch of biology that deals with the structure of the human body and the arrangement of its organs and tissues. While physiology explains how the body works, anatomy explains what the body is made of and how the parts are placed. For a student preparing for APSC, ADRE, DHS, SSC, Railway or police examinations in Assam, anatomy is important because the general-science paper repeatedly asks about the number of bones, the names of organs, the parts of the heart, the types of joints, the largest and smallest structures in the body, and the functions of glands and sense organs. Most of these questions are factual and can be answered quickly if the structure is remembered as a picture rather than as a list.
This article presents a complete deep-study revision of the human body system by system. It first fixes the basic vocabulary, then takes up the skeletal, muscular, circulatory, digestive, respiratory and nervous systems, and finally discusses the examination trends, common mistakes and a practical method of preparation. The meaning behind each fact is explained so that the numbers and names stay in memory instead of being memorised blindly.
Basic Terms
- Cell: the smallest structural and functional unit of the body.
- Tissue: a group of similar cells performing a particular function.
- Organ: a structure made of two or more tissues that performs a specific task.
- Organ system: a group of organs that work together for a major function.
- Anatomy: the study of the structure of the body.
- Physiology: the study of the functions of the body.
- Skeleton: the framework of bones and cartilage that supports the body.
- Joint: the place where two or more bones meet.
- Ligament: a band of tissue that joins a bone to another bone.
- Tendon: a band of tissue that joins a muscle to a bone.
- Artery: a blood vessel that carries blood away from the heart.
- Vein: a blood vessel that carries blood towards the heart.
- Neuron: the structural and functional unit of the nervous system.
- Gland: a structure that secretes useful substances such as hormones or enzymes.
Core Concepts and Figures
- The adult human skeleton has 206 bones; a newborn baby has about 300 bones that later fuse.
- The axial skeleton has 80 bones and the appendicular skeleton has 126 bones.
- The adult body has 32 teeth; the dental formula is written as 2:1:2:3 for each half of each jaw.
- The heart is a four-chambered structure with two atria and two ventricles.
- Normal resting heart rate is about 72 beats per minute and normal blood pressure is about 120/80 mm Hg.
- An adult human has about 5 litres of blood and roughly 4.5 to 5.5 million red blood cells per cubic millimetre.
- The lungs contain about 300 million alveoli, which give a total surface area of about 70 to 80 square metres.
- The number of chromosomes in a normal human body cell is 46 (23 pairs).
The Skeletal System
The skeleton forms the framework of the body, gives it shape, protects the internal organs, stores minerals such as calcium and phosphate, and produces blood cells in the bone marrow. It is divided into the axial skeleton, which includes the skull, the vertebral column, the ribs and the sternum, and the appendicular skeleton, which includes the limbs and the girdles. The longest bone is the femur, or thigh bone, and the smallest is the stapes, found in the middle ear. The vertebral column has 33 vertebrae in a child, which reduce to 26 in the adult because of fusion. The skull has 22 bones, and the face alone has 14. Bones are living tissues supplied with blood and nerves, and they grow and repair throughout life.
Joints
A joint is the point of contact between bones. Fibrous joints are fixed, as between the bones of the skull, and are called immovable joints. Cartilaginous joints are slightly movable, as between the vertebrae. Synovial joints are freely movable and include the ball-and-socket joint of the shoulder and hip, the hinge joint of the knee and elbow, the pivot joint between the first and second vertebrae, and the gliding joint of the wrist. A hinge joint allows movement in one plane, while a ball-and-socket joint allows movement in all planes. Synovial fluid lubricates the joint and reduces friction, and the knee is the largest joint in the body.
The Muscular System
Muscles move the bones and give the body its shape. There are three types of muscle. Striated or skeletal muscle is voluntary, is attached to bones, and works under our conscious control. Smooth muscle is involuntary and is found in the walls of the stomach, intestine and blood vessels. Cardiac muscle is involuntary and is found only in the heart, where it contracts rhythmically without rest. The strongest and largest muscle in the body is the gluteus maximus, and the smallest is the stapedius in the ear. Muscle contraction depends on two proteins, actin and myosin, and on the energy supplied by ATP. Muscles work in pairs, so that when one contracts the other relaxes, which is why bending and straightening of the arm need different muscles.
The Circulatory System
The circulatory system consists of the heart, blood and blood vessels. The human heart has four chambers: the right atrium and right ventricle carry deoxygenated blood, and the left atrium and left ventricle carry oxygenated blood. The left ventricle has the thickest wall because it pumps blood to the whole body. The heart is protected by the pericardium and is separated internally by the septum. Arteries carry blood away from the heart and have thick elastic walls; veins carry blood back to the heart and have valves to prevent backflow; capillaries are one-cell-thick vessels where the exchange of gases and nutrients takes place. Blood contains plasma, red blood cells, white blood cells and platelets. Red blood cells contain haemoglobin and carry oxygen, white blood cells defend the body, and platelets help in clotting. Blood groups A, B, AB and O are based on antigens on the red blood cells, and AB is called the universal recipient while O is called the universal donor.
The Digestive System
The digestive system breaks food into simple substances that can be absorbed. It begins at the mouth, where saliva contains the enzyme salivary amylase that starts the digestion of starch. Food then passes through the oesophagus to the stomach, where hydrochloric acid and the enzyme pepsin act on proteins. The small intestine is the main site of digestion and absorption; its inner wall has finger-like projections called villi that increase the surface area. The liver secretes bile, which emulsifies fats, and the pancreas secretes digestive enzymes and insulin. The large intestine absorbs water and salts, and the undigested waste is removed through the anus. The total length of the alimentary canal in an adult is about nine metres, and the liver is the largest gland in the body.
The Respiratory System
Respiration is the process by which the body takes in oxygen and gives out carbon dioxide. Air enters through the nose, passes through the pharynx, larynx, trachea and bronchi, and reaches the lungs. The trachea is supported by C-shaped rings of cartilage that prevent it from collapsing. Inside the lungs the bronchi divide into bronchioles, which end in tiny air sacs called alveoli. The alveoli have thin walls and a rich blood supply, so that oxygen diffuses into the blood and carbon dioxide diffuses out. The diaphragm and the intercostal muscles bring about breathing. The lungs are protected by the rib cage, and the right lung has three lobes while the left has two.
The Nervous System and Sense Organs
The nervous system controls and coordinates the working of all organs. It is divided into the central nervous system, made up of the brain and spinal cord, and the peripheral nervous system, made up of nerves. The brain has three main parts: the cerebrum, which controls thinking, memory and voluntary action; the cerebellum, which maintains balance and posture; and the medulla oblongata, which controls involuntary actions such as heartbeat, breathing and blood pressure. The neuron is the structural and functional unit and carries messages as electrical impulses. The five sense organs are the eye, ear, nose, tongue and skin. The eye focuses light on the retina, the ear maintains balance as well as hearing, and the skin is the largest sense organ and the largest organ of the body.
Examination Trends
Questions on anatomy in competitive examinations usually ask direct facts: the number of bones, the longest and smallest bones, the largest organ, the number of chambers in the heart, the universal donor and recipient blood groups, the largest gland, the site of digestion and absorption, the unit of the nervous system, and the master gland. The endocrine glands and their hormones are also frequently asked, especially the pituitary, thyroid, pancreas and adrenal glands. Diagram-based questions ask students to identify the labelled parts of the heart, eye and nephron. A smaller but regular set of questions tests joints and their types and the functions of the blood cells.
Common Mistakes to Avoid
- Do not confuse ligaments with tendons; one joins bone to bone and the other joins muscle to bone.
- Remember that the left ventricle, not the right, has the thickest wall.
- Do not say the liver is the largest organ; the skin is the largest organ, and the liver is the largest gland.
- Distinguish clearly between the axial and appendicular skeleton before counting bones.
- Do not mix up the functions of the cerebrum, cerebellum and medulla.
How to Prepare This Topic
Learn the body system by system rather than as isolated facts. Draw a simple outline of each system, such as the heart, the digestive tract or the nephron, and label it from memory. Maintain a table of record figures, because numbers such as 206 bones, 32 teeth, 46 chromosomes and 5 litres of blood are asked very often. Group the endocrine glands with their hormones in a chart, and write the blood groups with their antigens and antibodies. Practise previous years' question papers to see the exact style of questioning, and revise the record figures once a week so that they are never forgotten.
Frequently Asked Questions
How many bones are there in the adult human body?
An adult human body has 206 bones. A newborn baby has about 300 bones, but many of them fuse together as the child grows, so the number settles at 206 in the adult.
Which is the largest organ and the largest gland in the human body?
The skin is the largest organ of the body and also its largest sense organ. The liver is the largest gland, and it secretes bile and performs many metabolic functions.
What is the difference between an artery and a vein?
An artery carries blood away from the heart, has thick elastic walls and generally carries oxygenated blood. A vein carries blood towards the heart, has thinner walls, and contains valves to stop the backflow of blood.
Why does the left ventricle have the thickest wall?
The left ventricle has to pump oxygenated blood to the entire body against high pressure, so its muscular wall is the thickest of all the four chambers. The right ventricle pumps blood only to the nearby lungs.
Which blood group is called the universal donor and why?
Blood group O is called the universal donor because its red blood cells carry neither A nor B antigens, so it can be given to people of all blood groups without causing an antigen-antibody reaction in most situations.