Human Physiology: Organ Systems, Their Functions and Coordination in the Human Body
Human physiology is the study of how the human body works, how its different organs and organ systems carry out their functions, and how they work together to keep the body in a balanced state called homeostasis. From the moment we wake up and take a breath to the moment we digest our food, pump blood, remove waste and think, a remarkable set of coordinated processes is running inside the body. Understanding these processes is the very heart of biology and medicine.
For candidates preparing for APSC, ADRE, DHS, DME, nursing entrance, SSC, Railways, NEET and similar examinations in Assam, human physiology carries a large number of marks because it overlaps with both general science and health-related papers. Questions are commonly asked on the digestive system, the respiratory system, the structure of the heart, blood and blood groups, the nervous system, hormones and their functions, and the excretory system. This article presents a complete deep-study revision of the important organ systems with the reasons behind each fact.
Basic Terms
- Physiology: the branch of biology that deals with the normal functions of living organisms and their parts.
- Tissue: a group of similar cells that performs a particular function.
- Organ: a structure made of two or more tissues that performs a specific function.
- Homeostasis: the tendency of the body to maintain a stable internal environment, such as constant temperature and pH.
- Enzyme: a biological catalyst, usually a protein, that speeds up a chemical reaction without being consumed.
- Digestion: the breakdown of complex food into simple, absorbable substances.
- Respiration: the process of exchange of gases and the release of energy from food.
- Blood pressure: the pressure exerted by blood on the walls of the arteries.
- Hormone: a chemical messenger secreted by an endocrine gland into the blood.
- Neuron: the structural and functional unit of the nervous system.
- Excretion: the removal of harmful metabolic wastes from the body.
Core Concepts and Formulas
- Cardiac output = heart rate x stroke volume. In a resting adult this is about 5 litres per minute.
- Normal blood pressure is about 120/80 mm Hg; the upper value is systolic and the lower is diastolic.
- Number of bones in an adult human body is 206; the number of muscles is about 600.
- Normal body temperature is about 37 degrees Celsius, or 98.6 degrees Fahrenheit.
- Number of chromosomes in a human body cell is 46, or 23 pairs.
- Nephron is the functional unit of the kidney; each kidney has about one million nephrons.
- Alveoli provide the surface for gas exchange in the lungs; the lungs contain millions of alveoli.
The Digestive System
The digestive system consists of the alimentary canal, which runs from the mouth to the anus, together with associated glands such as the salivary glands, the liver and the pancreas. Food is broken down mechanically by chewing and chemically by enzymes. Saliva contains the enzyme salivary amylase, which begins the digestion of starch. In the stomach, hydrochloric acid provides an acidic medium and the enzyme pepsin digests proteins. The small intestine is the main site of digestion and absorption, and its inner surface is covered with finger-like projections called villi, which greatly increase the surface area. The liver secretes bile, which emulsifies fats, and the pancreas secretes enzymes for starch, protein and fat. The large intestine absorbs water and mineral salts and forms the faeces.
The Respiratory System
Respiration involves taking in oxygen and giving out carbon dioxide. Air enters through the nostrils, passes through the nasal cavity, pharynx, larynx, trachea and bronchi, and finally reaches the lungs. The trachea is supported by C-shaped rings of cartilage so that it does not collapse. Inside the lungs the bronchi branch into bronchioles, which end in tiny air sacs called alveoli. The alveoli are thin-walled, richly supplied with capillaries and provide a very large surface area for the exchange of gases. Oxygen diffuses into the blood and combines with haemoglobin in red blood cells, while carbon dioxide diffuses out. The diaphragm and the intercostal muscles bring about the movements of breathing.
The Circulatory System and Blood
The circulatory system consists of the heart, blood vessels and blood. The human heart has four chambers, two upper atria and two lower ventricles. It is a double pump; the right side sends deoxygenated blood to the lungs for purification and the left side pumps oxygenated blood to the whole body. Blood consists of plasma, red blood cells, white blood cells and platelets. Red blood cells contain haemoglobin, which carries oxygen, and they are formed in the bone marrow. White blood cells defend the body against disease, and platelets help in the clotting of blood. The ABO blood group system has four groups, A, B, AB and O, based on the antigens present on the red blood cells, while the Rh factor is another important antigen that makes blood positive or negative.
The Nervous System
The nervous system is divided into the central nervous system, which includes the brain and spinal cord, and the peripheral nervous system, which includes the nerves. The brain has three main parts: the cerebrum, which controls thinking, memory and voluntary actions; the cerebellum, which controls balance and coordination; and the medulla oblongata, which controls involuntary actions such as heartbeat, breathing and blood pressure. The structural and functional unit of the nervous system is the neuron. A nerve impulse travels along the neuron and is transmitted to the next neuron or to a muscle across a junction called a synapse with the help of chemical messengers called neurotransmitters. The spinal cord is the centre for many reflex actions that protect the body from harm.
The Endocrine System and Hormones
The endocrine system consists of ductless glands that secrete hormones directly into the blood. The pituitary gland is called the master gland because it controls other endocrine glands. The thyroid gland secretes thyroxine, which controls the rate of metabolism, and a deficiency of iodine causes goitre. The pancreas secretes insulin, which lowers blood sugar, and a deficiency of insulin causes diabetes mellitus. The adrenal glands secrete adrenaline, which prepares the body for emergency situations by increasing heartbeat and blood supply to the muscles. The testes in males secrete testosterone, and the ovaries in females secrete oestrogen and progesterone, which control the reproductive functions.
The Excretory System
The excretory system removes metabolic wastes from the body. The main organs are the kidneys, which filter the blood and produce urine. The functional unit of the kidney is the nephron, and each kidney contains about one million nephrons. The kidney removes urea, excess salts and excess water, and it also helps to maintain the water balance and the pH of the blood. The urine formed in the kidneys passes through the ureters to the urinary bladder, where it is stored, and is finally expelled through the urethra. Besides the kidneys, the skin, the lungs and the liver also help in excretion to some extent.
How Numerical Problems Are Solved
Physiology numericals are usually straightforward and depend on a few standard values and formulas. To find the cardiac output, multiply the heart rate by the stroke volume; for example, a heart rate of 70 beats per minute and a stroke volume of 70 millilitres gives about 4.9 litres per minute. For blood pressure, remember that the normal value is about 120/80 mm Hg and that the systolic pressure is always the higher value. Questions on the number of bones, muscles, chromosomes and nephrons are direct factual questions and are best answered from memory. When a problem gives the number of heartbeats and the time, first convert the time to minutes and then apply the formula.
Examination Trends
In competitive examinations, the most frequently asked areas are the parts and functions of the digestive system, the structure of the heart and the path of blood, blood groups and the Rh factor, the parts of the brain, the names of hormones and the glands that secrete them, and the organs of excretion. Objective questions commonly ask the numbers of bones and muscles, the normal blood pressure, the master gland, the hormone that controls blood sugar, and the site of maximum absorption in the digestive tract. Health-related examinations also ask about deficiency diseases such as goitre, diabetes and anaemia.
Common Mistakes to Avoid
- Do not confuse arteries with veins; arteries carry blood away from the heart, veins carry it towards the heart.
- Remember that the pulmonary artery carries deoxygenated blood, which is an exception to the general rule.
- Do not mix up the hormones of different glands, such as insulin and thyroxine.
- Always write the systolic value before the diastolic value in blood pressure.
- Do not forget that the liver, skin and lungs also take part in excretion.
How to Prepare This Topic
Study one organ system at a time and draw a simple labelled diagram for each, because diagrams make the relationship between parts and functions clear. Prepare a single table of hormones with their gland, function and related deficiency disease, and revise it often. Compare similar structures, such as arteries and veins, and the three types of muscles, so that differences become easy to recall. Use the numerical formulas only after the concepts are clear, and finally solve previous years' questions to get used to the examination language.
Frequently Asked Questions
Which gland is called the master gland and why?
The pituitary gland is called the master gland because it secretes hormones that control the activity of several other endocrine glands, such as the thyroid, the adrenal cortex and the gonads, and thereby regulates many body functions.
Why is the small intestine the main site of absorption?
The small intestine is very long, and its inner wall has numerous finger-like projections called villi, which are richly supplied with blood capillaries. These features give it a very large surface area, making it the main site of digestion and absorption of nutrients.
What is the function of haemoglobin in the blood?
Haemoglobin is the red, iron-containing pigment present in red blood cells. It combines with oxygen in the lungs and carries it to the tissues of the body, and it also helps to transport some carbon dioxide back to the lungs.
Which hormone controls blood sugar and what happens if it is deficient?
Insulin, secreted by the beta cells of the pancreas, lowers blood sugar. If insulin is deficient or does not act properly, blood sugar rises and the condition known as diabetes mellitus results.
What is the normal blood pressure of a healthy adult?
The normal blood pressure of a healthy adult is about 120/80 mm Hg, where 120 mm Hg is the systolic pressure measured when the heart contracts and 80 mm Hg is the diastolic pressure measured when the heart relaxes.