3. Biology Notes 03

Elaborate Notes

EXCRETORY SYSTEM

The excretory system is a crucial physiological system responsible for osmoregulation—the regulation of water and ionic balance—and the removal of metabolic wastes from the body. The primary metabolic waste in mammals, including humans, is urea, a nitrogenous compound formed in the liver through the urea cycle. This process converts highly toxic ammonia, a byproduct of amino acid metabolism, into the less toxic urea.

• Process of Excretion: The principal organs of the excretory system are the kidneys. The functional unit of the kidney is the nephron. There are approximately one million nephrons in each human kidney. Blood enters the nephron through the glomerulus, where ultrafiltration occurs, forcing water, salts, glucose, urea, and other small molecules from the blood into the Bowman’s capsule. As this filtrate passes through the renal tubule, essential substances like glucose, amino acids, and a significant amount of water are reabsorbed back into the blood. The remaining fluid, containing concentrated waste products, forms urine.
• Urine Composition: Urine is primarily composed of water (around 95%), which acts as a solvent for waste products. The main solute is urea (about 2.5%), a waste product of protein metabolism. The remaining 2.5% consists of other waste products such as uric acid (from nucleic acid breakdown), creatinine (from muscle metabolism), and various ions (e.g., sodium, potassium, chloride). The composition of urine can vary based on diet, hydration levels, and overall health.
• Kidney Stones: Medically termed nephrolithiasis or urolithiasis, kidney stones are hard deposits made of minerals and salts that form inside the kidneys. The most common type is formed from calcium oxalate. This occurs when urine contains more crystal-forming substances—such as calcium, oxalate, and uric acid—than the fluid in the urine can dilute. Factors contributing to their formation include dehydration, certain diets high in protein or sodium, and genetic predispositions. Historical evidence of bladder stones dates back to ancient Egypt, with a stone found in a 7,000-year-old mummy. The Greek physician Hippocrates (c. 460 – c. 370 BC) described the symptoms and even a surgical procedure for their removal.

CIRCULATORY SYSTEM

The circulatory system is the body’s primary transport network, responsible for delivering oxygen and nutrients to tissues, removing carbon dioxide and metabolic wastes, and circulating hormones. This system was first accurately described by the English physician William Harvey in his seminal work Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus (Anatomical Exercise on the Motion of the Heart and Blood in Animals), published in 1628, which laid the foundation for modern physiology by challenging the long-held doctrines of Galen.

• Heart: The heart is a muscular organ that functions as a four-chambered pump.
  • Chambers: The two upper, smaller chambers are the atria (or auricles), which receive blood. The right atrium receives deoxygenated blood from the body, while the left atrium receives oxygenated blood from the lungs. The two lower, larger, and more muscular chambers are the ventricles, which pump blood out. The right ventricle pumps deoxygenated blood to the lungs, and the left ventricle pumps oxygenated blood to the rest of the body.
• Blood Vessels: These are the conduits through which blood flows.
  • Arteries: These vessels carry blood away from the heart. They have thick, muscular, and elastic walls to withstand the high pressure of the blood being pumped by the ventricles. They are generally located deeper within the body tissues. They appear reddish due to the high oxygen content of the blood they typically carry.
  • Veins: These vessels carry blood towards the heart. Their walls are thinner and less elastic than those of arteries, as the blood they carry is at a lower pressure. Many veins, especially in the limbs, contain valves to prevent the backflow of blood. They are often superficial, lying closer to the skin’s surface, and can appear bluish-green because the deoxygenated blood they carry absorbs red light, and the subcutaneous fat absorbs low-frequency light, allowing the more energetic blue light to penetrate to the vein and be reflected back to the eye.
• Double Circulation: Humans exhibit double circulation, meaning blood passes through the heart twice for each complete circuit of the body. This system consists of two loops:
  1. Pulmonary Circulation: Deoxygenated blood is pumped from the right ventricle to the lungs via the pulmonary artery. In the lungs, it releases carbon dioxide and picks up oxygen. The now oxygenated blood returns to the left atrium of the heart via the pulmonary veins.
  2. Systemic Circulation: Oxygenated blood is pumped from the left ventricle to the rest of the body’s organs and tissues via the aorta (the main artery). After delivering oxygen and nutrients and picking up waste products, the deoxygenated blood returns to the right atrium via the superior and inferior vena cava. This cycle then repeats.
• Oxygenated and Deoxygenated Blood:
  • A general rule is that arteries carry oxygenated blood and veins carry deoxygenated blood. However, the critical exceptions are linked to pulmonary circulation.
  • The pulmonary artery is the only artery that carries deoxygenated blood (from the right ventricle to the lungs).
  • The pulmonary veins are the only veins that carry oxygenated blood (from the lungs to the left atrium).

GLANDS

Glands are organs that synthesize substances (such as hormones) for release into the bloodstream (endocrine gland) or into cavities inside the body or its outer surface (exocrine gland).

• Exocrine Glands: These glands possess ducts or tubes through which their secretions are released to a specific location. Examples include salivary glands (secreting saliva into the mouth), sweat glands (secreting sweat onto the skin), and the liver (secreting bile into the small intestine via the bile duct).
• Endocrine Glands: These are ductless glands that release their chemical products, called hormones, directly into the interstitial fluid, from where they diffuse into the bloodstream. The circulatory system then transports these hormones to target cells or organs throughout the body. The pancreas is a unique example of a heterocrine (or mixed) gland, having both exocrine functions (secreting digestive enzymes into the duodenum) and endocrine functions (secreting insulin and glucagon into the blood).

ENDOCRINE SYSTEM

The endocrine system, along with the nervous system, is a major communication system of the body. It regulates a wide range of physiological processes, including growth, metabolism, and reproduction, through the action of hormones. The term “hormone” was coined by English physiologist Ernest Starling in 1905, derived from the Greek word horman, meaning “to set in motion.”

• Hypothalamus: Located in the brain, the hypothalamus is the primary link between the nervous and endocrine systems. It controls the pituitary gland by producing:
  • Releasing Hormones: These stimulate the anterior pituitary to secrete its hormones. For example, Gonadotropin-releasing Hormone (GnRH) stimulates the release of FSH and LH.
  • Inhibiting Hormones: These prevent the anterior pituitary from secreting its hormones. For example, Somatostatin inhibits the release of Growth Hormone.
• Pituitary Gland: Often called the “master gland” because it controls the functions of many other endocrine glands. It is divided into two main parts:
  • Anterior Pituitary: Produces and secretes several key hormones:
    • Thyroid-Stimulating Hormone (TSH): Stimulates the thyroid gland.
    • Prolactin (PRL): Stimulates milk production in mammary glands after childbirth.
    • Adrenocorticotropic Hormone (ACTH): Stimulates the adrenal cortex to release its hormones, particularly glucocorticoids.
    • Follicle-Stimulating Hormone (FSH): In females, it stimulates the growth and maturation of ovarian follicles (eggs). In males, it is crucial for spermatogenesis (sperm formation).
    • Growth Hormone (GH): Stimulates growth, cell reproduction, and regeneration. Its over-secretion can lead to gigantism, and under-secretion to dwarfism.
    • Luteinizing Hormone (LH): In females, it triggers ovulation and stimulates the production of progesterone. In males, it stimulates the testes to produce testosterone.
  • Posterior Pituitary: It does not produce hormones but stores and releases two hormones produced by the hypothalamus:
    • Antidiuretic Hormone (ADH or Vasopressin): Regulates water balance by increasing water reabsorption in the kidney nephrons, thus concentrating urine and reducing water loss.
    • Oxytocin: Stimulates uterine contractions during childbirth and the release of milk during breastfeeding.
• Thyroid and Parathyroid Glands:
  • Thyroid Gland: Located in the neck, it produces Thyroxine (T4) and Triiodothyronine (T3). These hormones are essential for regulating the body’s basal metabolic rate (BMR), affecting the metabolism of carbohydrates, proteins, and fats. Iodine is a crucial component of these hormones; its deficiency leads to goitre.
  • Parathyroid Glands: Four small glands located on the posterior surface of the thyroid gland. They secrete Parathyroid Hormone (PTH), which is the primary regulator of calcium and phosphate levels in the blood. It increases blood calcium levels by stimulating its release from bones, increasing absorption in the intestine, and reducing its excretion by the kidneys. It works antagonistically with Calcitonin (produced by the thyroid gland).
• Adrenal Gland: Located on top of each kidney, each gland consists of two parts:
  • Adrenal Medulla (inner part): Produces catecholamines, primarily Adrenaline (epinephrine) and Noradrenaline (norepinephrine). These are the “fight-or-flight” hormones, released during stress or excitement, leading to increased heart rate, blood pressure, and glucose levels, preparing the body for emergency situations.
  • Adrenal Cortex (outer part): Produces corticosteroids.
    • Mineralocorticoids (e.g., Aldosterone): Regulate the balance of water and electrolytes (minerals like sodium and potassium) by acting on the kidneys. They are vital for maintaining blood volume and pressure.
    • Glucocorticoids (e.g., Cortisol): Primarily involved in metabolism, particularly increasing blood glucose levels. They also possess anti-inflammatory and immunosuppressive properties.
• Pancreas: An organ in the abdomen with both exocrine and endocrine functions. Its endocrine component consists of the Islets of Langerhans, which contain different cell types secreting different hormones. The discovery of insulin in 1921 by Frederick Banting and Charles Best at the University of Toronto was a landmark achievement in medicine, revolutionizing the treatment of diabetes.
  • Insulin: Secreted by beta cells, it lowers blood glucose levels by promoting the uptake of glucose by cells and its conversion into glycogen in the liver and muscles.
  • Glucagon: Secreted by alpha cells, it raises blood glucose levels by stimulating the liver to break down glycogen (glycogenolysis) and synthesize glucose from other sources (gluconeogenesis).

HEALTH

The World Health Organization (WHO), in the preamble to its constitution of 1948, defined health as “a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity.” This holistic definition emphasizes that health is a multidimensional concept.

• Disease Classification:
  • Congenital Diseases: These are present at birth and can be caused by genetic mutations, chromosomal abnormalities, or environmental factors affecting the fetus during pregnancy. Examples include Down syndrome (a chromosomal disorder), sickle-cell anemia (a genetic disorder), and congenital heart defects.
  • Acquired Diseases: These develop after birth. They are broadly categorized into:
    • Communicable/Infectious Diseases: Caused by pathogenic microorganisms, such as bacteria, viruses, parasites, or fungi. These diseases can be spread, directly or indirectly, from one person to another. Examples: Malaria, Tuberculosis (TB), COVID-19.
    • Non-communicable/Non-infectious Diseases (NCDs): These are not transmitted from person to person. They are typically of long duration and are the result of a combination of genetic, physiological, environmental, and behavioral factors. Examples include:
      • Cardiovascular diseases (e.g., heart attacks, stroke).
      • Chronic respiratory diseases (e.g., asthma).
      • Deficiency diseases due to malnutrition (e.g., scurvy from vitamin C deficiency, rickets from vitamin D deficiency).
      • Mental disorders (e.g., depression).
      • Environmental diseases caused by exposure to toxins, such as Minamata disease, a neurological syndrome caused by severe mercury poisoning. It was first discovered in Minamata city, Japan, in 1956, resulting from the release of industrial wastewater.
      • Occupational diseases resulting from exposure to risk factors at the workplace, such as silicosis, a lung disease caused by inhaling silica dust, common among miners and quarry workers.
      • Cancer.

COMMUNICABLE DISEASES

These diseases spread through various modes of transmission. Understanding these modes is fundamental to public health and epidemiology.

• Modes of Transmission:
  • Airborne: Pathogens are transmitted via droplets or dust particles in the air (e.g., COVID-19, Tuberculosis, influenza).
  • Waterborne: Pathogens are transmitted through contaminated water (e.g., Cholera, Typhoid).
  • Foodborne: Pathogens are transmitted through contaminated food (e.g., Typhoid, Amoebiasis).
  • Soil-transmitted: Infection occurs through contact with contaminated soil (e.g., Hookworm, Tetanus).
  • Blood and Body Fluids: Direct transmission through contact with infected fluids (e.g., HIV/AIDS, Hepatitis B).
  • Direct Contact: Transmission through physical contact with an infected person (e.g., common cold, chickenpox).
  • Fomite-borne: Transmission through contact with contaminated inanimate objects (fomites) like doorknobs, utensils, or clothing (e.g., chickenpox, influenza).
  • Zoonotic: Diseases transmitted from animals to humans (e.g., Rabies from dogs, Swine Flu from pigs).
  • Vector-borne: Transmission by an intermediate organism, or vector, which transfers the pathogen.
    • Mechanical Vector: The vector carries the pathogen on its body parts (e.g., a housefly carrying cholera bacteria on its legs from feces to food).
    • Biological Vector: The pathogen undergoes a part of its life cycle inside the vector’s body before being transmitted to the host (e.g., the female Anopheles mosquito transmitting the Plasmodium parasite for malaria). The work of Sir Ronald Ross in 1897 in Secunderabad, India, demonstrated the mosquito’s role in malaria transmission, earning him the Nobel Prize in 1902.

HELMINTHIC DISEASES

These diseases are caused by parasitic worms (helminths), which can be flatworms (platyhelminths) or roundworms (nematodes).

• Ascariasis: Caused by the roundworm Ascaris lumbricoides. Infection occurs by ingesting contaminated food or water containing the worm’s eggs.
• Taeniasis: Caused by tapeworms (flatworms), such as Taenia solium (pork tapeworm). Infection typically occurs from consuming undercooked, infected pork.
• Lymphatic Filariasis (Elephantiasis): Caused by filarial worms transmitted by mosquitoes. The worms lodge in the lymphatic system, leading to chronic swelling and thickening of the skin, most commonly in the legs, resembling an elephant’s limbs.
• Hookworm Infection: Caused by roundworms (Ancylostoma duodenale or Necator americanus). The larvae penetrate the skin, usually from walking barefoot on contaminated soil.

PROTOZOAN DISEASES

These are caused by single-celled eukaryotic organisms called protozoa.

• Examples: Amoebiasis (caused by Entamoeba histolytica), Giardiasis (caused by Giardia lamblia), and Malaria.
• Malaria: A life-threatening disease caused by Plasmodium parasites. The most common and dangerous species are P. falciparum and P. vivax.
  • Vector: Transmitted to humans through the bites of infected female Anopheles mosquitoes.
  • Transmission Cycle:
    1. An infected mosquito injects Plasmodium sporozoites into a human’s bloodstream.
    2. The parasites travel to the liver, where they mature and multiply (asexual reproduction).
    3. They then re-enter the bloodstream and infect red blood cells (RBCs), where they multiply further.
    4. The infected RBCs burst, releasing more parasites to infect other RBCs. This cyclical bursting of RBCs causes the characteristic fever and chills of malaria.
    5. Some parasites develop into male and female gametocytes.
    6. When another mosquito bites the infected person, it ingests these gametocytes.
    7. Inside the mosquito’s gut, the gametocytes fuse (sexual reproduction) and develop into new sporozoites, which migrate to the mosquito’s salivary glands, ready to infect another person.

FUNGAL DISEASES

These diseases, also known as mycoses, are caused by fungi.

• Ringworm (Dermatophytosis): A common skin infection caused by various fungi, not a worm. It causes a characteristic itchy, red, circular rash.
• Athlete’s Foot (Tinea pedis): A fungal infection that typically begins between the toes.
• Candidiasis: An infection caused by yeast of the Candida genus, most commonly Candida albicans. It can affect the mouth (thrush), vagina, or skin.

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Prelims Pointers

• Excretory System

  • Primary function: Waste removal and osmoregulation (regulation of water and ion balance).
  • Composition of urine: 95% water, 2.5% urea, 2.5% other wastes (uric acid, creatinine, ions).
  • Kidney stones are most commonly formed from calcium oxalate.
  • The functional unit of the kidney is the nephron.

• Circulatory System

  • Components: Heart, blood, and blood vessels.
  • The heart has four chambers: right atrium, left atrium, right ventricle, left ventricle.
  • Arteries carry blood away from the heart; they are thick-walled and deep-seated.
  • Veins carry blood towards the heart; they are thin-walled, superficial, and may have valves.
  • Pulmonary artery is the only artery carrying deoxygenated blood.
  • Pulmonary veins are the only veins carrying oxygenated blood.
  • Humans have a double circulation system (pulmonary and systemic circulation).

• Glands and Hormones

  • Exocrine glands have ducts (e.g., liver, salivary glands).
  • Endocrine glands are ductless and secrete hormones directly into the blood.
  • Hypothalamus produces releasing and inhibiting hormones that control the pituitary gland.
  • Anterior Pituitary Hormones: TSH, Prolactin, ACTH, FSH, GH, LH.
  • Posterior Pituitary Hormones: ADH (Vasopressin) and Oxytocin.
  • Thyroid Gland: Secretes Thyroxine, which regulates metabolism.
  • Parathyroid Gland: Secretes Parathyroid Hormone, which regulates calcium and phosphate levels.
  • Adrenal Medulla: Secretes Adrenaline and Noradrenaline (emergency hormones).
  • Adrenal Cortex: Secretes Mineralocorticoids (regulate water/mineral balance) and Glucocorticoids (carbohydrate metabolism).
  • Pancreas: Secretes Insulin (lowers blood glucose) and Glucagon (raises blood glucose).

• Health and Disease

  • WHO definition of health: A state of complete physical, mental, and social well-being.
  • Congenital diseases are present from birth (e.g., Down syndrome).
  • Acquired diseases develop after birth.
  • Communicable diseases are caused by pathogens (e.g., Malaria, COVID-19).
  • Non-communicable diseases are not caused by pathogens (e.g., cardiovascular disease, cancer).
  • Minamata disease is caused by mercury poisoning.
  • Silicosis is an occupational lung disease caused by silica dust.

• Disease Transmission and Causative Agents

  • Vector: An organism that transmits a pathogen but does not cause the disease itself.
  • Mechanical Vector: Carries pathogen on its body surface (e.g., housefly).
  • Biological Vector: Pathogen’s life cycle occurs within the vector (e.g., Anopheles mosquito for malaria).
  • Ascariasis: Caused by a roundworm (Ascaris).
  • Taeniasis: Caused by a flatworm/tapeworm (Taenia solium).
  • Lymphatic Filariasis: Also known as Elephantiasis.
  • Malaria: Protozoan disease caused by Plasmodium.
  • Vector for Malaria: Female Anopheles mosquito.
  • Ringworm: A fungal disease, not caused by a worm.

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Mains Insights

General Studies Paper I (Society)

• Social Determinants of Health: The WHO definition of health includes “social well-being.” This links directly to societal issues. The prevalence of both communicable (e.g., Tuberculosis in crowded slums) and non-communicable diseases (e.g., lifestyle diseases in urban populations) is deeply influenced by social factors like poverty, education, sanitation, housing, and cultural practices. Mains questions can explore the relationship between social inequality and health outcomes in India.
• Urbanization and Health: Rapid and unplanned urbanization creates conditions conducive to the spread of diseases. Water-borne diseases like cholera and vector-borne diseases like dengue are rampant due to poor sanitation and water management. Simultaneously, sedentary lifestyles and changing dietary patterns in urban areas contribute to the epidemic of NCDs like diabetes and hypertension.

General Studies Paper II (Governance, Social Justice)

• Public Health Infrastructure: The classification of diseases and their modes of transmission are fundamental to designing effective public health policies. The government’s response to COVID-19 highlighted both the strengths and weaknesses of India’s health infrastructure. Mains questions often revolve around the need for a robust, decentralized public health system, focusing on prevention, as seen in schemes like the National Health Mission (NHM) and Ayushman Bharat.
• Disease Burden and Policy Interventions: India faces a “dual burden” of disease—a high incidence of communicable diseases alongside a rapidly growing epidemic of non-communicable diseases (NCDs). This poses a significant challenge for the healthcare system. Analysis of government programs like the National Vector Borne Disease Control Programme (NVBDCP), the National Programme for Prevention and Control of Cancer, Diabetes, Cardiovascular Diseases and Stroke (NPCDCS), and the goal to eliminate Malaria by 2030 and TB by 2025 can be part of an answer.
• Global Health Governance: The spread of infectious diseases like COVID-19 underscores the importance of international cooperation and the role of organizations like the WHO. Issues like vaccine equity, sharing of pathogen data, and coordinated responses to pandemics are critical aspects of global governance.

General Studies Paper III (Economy, Environment, S&T)

• Economic Impact of Diseases: A high disease burden impacts economic productivity through loss of workdays, reduced efficiency, and high healthcare expenditure. NCDs, being chronic, impose a long-term economic burden on families and the national economy. Questions can analyze how investing in health (“Health as a Capital Good”) can lead to demographic dividends.
• Environment and Health Linkages: Environmental factors are a major cause of disease. Minamata disease (mercury poisoning) is a classic example of industrial pollution’s devastating health impact. Air pollution is a leading cause of respiratory diseases in India. Climate change is altering the geographic range of vectors like mosquitoes, potentially increasing the incidence of diseases like malaria and dengue in new regions. This links to the concept of “One Health,” which recognizes the interconnectedness of human, animal, and environmental health.
• Science & Technology in Healthcare: Understanding the biology of pathogens (e.g., the life cycle of Plasmodium) is crucial for developing diagnostics, drugs, and vaccines. S&T plays a vital role in disease surveillance (e.g., using GIS mapping for vector control), developing new treatment modalities, and promoting telemedicine to bridge the urban-rural healthcare divide.

General Studies Paper IV (Ethics)

• Equity in Healthcare Access: The disparity in health outcomes between different socio-economic groups raises ethical questions about justice and equity. Is access to healthcare a fundamental right? The challenges in providing affordable care for chronic NCDs or expensive treatments for diseases like cancer pose significant ethical dilemmas for policymakers.
• Ethics in Public Health Interventions: Public health measures, such as mandatory vaccinations or lockdowns during a pandemic, often involve a trade-off between individual liberty and collective well-being. These situations require a careful ethical analysis based on principles of proportionality, necessity, and transparency.
• Occupational and Environmental Health Ethics: The prevalence of occupational diseases like silicosis raises ethical questions about corporate responsibility and the duty of the state to protect its workers. Similarly, environmental pollution leading to diseases reflects an ethical failure to balance economic development with public health and environmental protection (inter-generational equity).