Human Body Systems

Overview

The human body is organised as cells → tissues → organs → organ systems. Each system does a specific job, but all work together to maintain homeostasis (a steady internal environment).

Digestive system

Digestion is the breakdown of complex food into simple, absorbable substances.

Alimentary canal: mouth → pharynx → oesophagus → stomach → small intestine (duodenum, jejunum, ileum) → large intestine (colon, rectum) → anus. Associated glands: salivary glands, liver, pancreas.

• Carbohydrates → glucose; proteins → amino acids; fats → fatty acids and glycerol.
• The appendix is a small vestigial organ at the junction of the small and large intestines.
• Enzymes: biological catalysts (proteins) that speed up reactions; work at specific pH and temperature (pepsin: acidic; trypsin: alkaline).
• Ptyalin — mouth; pepsin — stomach; trypsin, lipase, amylase — pancreas; lactase — small intestine (lactose intolerance).

Respiratory system

• Path of air: nose → pharynx → larynx (voice box; contains the vocal cords) → trachea (windpipe, supported by C-shaped cartilage rings) → bronchi → bronchioles → alveoli (tiny air sacs; exchange of gases takes place here; about 300–500 million in the lungs).
• The diaphragm and intercostal muscles help breathing. Inhalation: the diaphragm contracts and moves down, ribs go up and out; exhalation is the reverse.
• Breathing rate: about 12–20 times per minute at rest. Oxygen is carried by haemoglobin; most carbon dioxide is carried as bicarbonate in plasma.
• Air composition: inhaled — about 21 % O₂, 0.04 % CO₂; exhaled — about 16 % O₂ and about 4 % CO₂.
• Respiration at the cellular level (cellular respiration): $\text{glucose}+\text{oxygen}\to {\text{CO}}_2+\text{water}+\text{energy (ATP)}$ occurs in mitochondria. Anaerobic respiration (without oxygen) gives lactic acid in muscles (cramps) and ethanol + CO₂ in yeast.
• Diseases: asthma (narrowing of bronchioles), pneumonia, tuberculosis (TB), bronchitis, emphysema, silicosis and asbestosis (occupational).

Circulatory system

• Heart: a muscular organ, about the size of a fist, located slightly left in the chest; weighs about 300 g; has four chambers — two atria (upper, receive blood) and two ventricles (lower, pump blood). The left ventricle has the thickest wall (pumps blood to the whole body). The right side carries deoxygenated blood to the lungs; the left side carries oxygenated blood to the body. Valves prevent backflow. Normal heart rate is about 72 beats per minute at rest; normal blood pressure is about 120/80 mm Hg (systolic/diastolic), measured with a sphygmomanometer. The natural pacemaker is the sino-atrial (SA) node. ECG records the heart's electrical activity.
• Double circulation: blood passes twice through the heart in one full circuit (pulmonary circulation: heart → lungs → heart; systemic circulation: heart → body → heart).
• Blood vessels: arteries (carry blood away from the heart; thick, elastic walls; the pulmonary artery carries deoxygenated blood), veins (carry blood to the heart; thin walls, valves; the pulmonary vein carries oxygenated blood), capillaries (thin; exchange of materials).
• Blood (about 5–6 litres in an adult; 7–8 % of body weight):

• Blood groups (ABO system, Karl Landsteiner, 1901): A, B, AB, O. O is the universal donor, AB is the universal recipient. The Rh factor (Rhesus) is present (Rh⁺) or absent (Rh⁻). Rh incompatibility can affect a second pregnancy (a mother Rh⁻ and a foetus Rh⁺).
• Haemoglobin low → anaemia (iron deficiency). Normal Hb: about 13–17 g/dL in men, 12–15 g/dL in women. Haemophilia (blood does not clot; a sex-linked genetic disorder).
• Lymphatic system: lymph, lymph nodes, spleen, and thymus; helps in immunity and returns fluid to the blood.
• The spleen is the largest lymphatic organ. Heart attack (myocardial infarction) results from a blocked coronary artery; atherosclerosis is the hardening/blocking of arteries by cholesterol.

Nutrition

Nutrition is the process by which an organism takes in and uses food for energy, growth, repair and protection. Nutrients are the substances in food that the body needs.

• Macronutrients (needed in large amounts): carbohydrates, proteins, fats, water.
• Micronutrients (small amounts): vitamins and minerals.
• Roughage (dietary fibre) is the indigestible plant material (cellulose) that helps bowel movement and prevents constipation.

Carbohydrates

Provide energy (the main source): about 4 kcal (17 kJ) per gram. Sources: rice, wheat, potato, sugar, fruits. Types: monosaccharides (glucose, fructose, galactose), disaccharides (sucrose, maltose, lactose), polysaccharides (starch, glycogen — the animal store, cellulose). Excess carbohydrate is stored as glycogen and fat. Test for starch: iodine turns blue-black.

Proteins

Body-building food for growth and repair; enzymes, hormones, antibodies and haemoglobin are proteins. About 4 kcal/g. Made of amino acids (20 kinds; 9 essential amino acids must be taken in the diet). Sources: milk, eggs, meat, fish, pulses (dal), soya, nuts. Test: Biuret test (violet colour). Deficiency: kwashiorkor (protein deficiency with adequate energy — swollen belly and swollen limbs) and marasmus (deficiency of both protein and energy — very thin and shrunken body) in children.

Fats (lipids)

Concentrated energy source: about 9 kcal/g; also insulate, protect organs, carry fat-soluble vitamins (A, D, E, K). Sources: oil, ghee, butter, nuts. Excess leads to obesity and heart disease; cholesterol (HDL — "good", LDL — "bad").

Water and fibre

Water: about 60–70 % of body weight; solvent, temperature regulation, transport. An adult needs about 2–3 litres a day. Fibre: whole grains, fruits, vegetables.

Balanced diet

A diet containing all nutrients in the right proportions, with enough calories. The energy needs depend on age, sex and activity. Approximate needs: adult man (moderate work) about 2,300–2,700 kcal/day; woman about 1,900–2,200 kcal/day (the exact figure depends on the ICMR tables); a pregnant or lactating woman needs more.

Vitamins

Organic compounds needed in small amounts; most cannot be made by the body.

• Fat-soluble: A, D, E, K (stored in the body).
• Water-soluble: B-complex and C (not stored; needed regularly).

Hints: Vitamin D is called the "sunshine vitamin"; vitamin K helps blood clotting (Koagulation); vitamin C is destroyed by heat and prolonged cooking; B₁₂ contains cobalt; vitamin B₁₂ is found only in animal foods.

Minerals

Malnutrition and health programmes

• Malnutrition is a lack (undernutrition) or excess (overnutrition) of nutrients. Anaemia, stunting and wasting are widespread in India.
• Government programmes: Mid-Day Meal (now PM POSHAN) for school children, ICDS (Anganwadi) for young children and mothers, POSHAN Abhiyaan (National Nutrition Mission), Anaemia Mukt Bharat, and the National Food Security Act, 2013. The FSSAI regulates food safety. (Check the latest numbers before quoting.)
• Obesity is measured by the Body Mass Index (BMI) = weight (kg) / height² (m²); a BMI of 18.5–24.9 is regarded as normal by the WHO (a lower cut-off for obesity is used for Asians).