Human Anatomy and Physiology: Key Systems, Fluids, and Processes

Posted on Mar 3, 2026 in Biology

Nervous System

Nervous system: The nervous system is the most complex system of the human body, containing a vast network of nerve cells. It is the major controlling, coordinating, regulatory and communicating system of the body. The nervous system is the body’s command centre and the centre of all mental activities including thought, learning, and memory. It is also responsible for maintaining homeostasis.

Organization of the Nervous System

  • Central Nervous System: Brain, Spinal cord
  • Peripheral Nervous System:
    • Autonomic nervous system: Sympathetic, Parasympathetic
    • Somatic nervous system

Cells of the Nervous System

There are basically two types of cells in the nervous system:

  1. Neurons (nerve cells)
  2. Neuroglia (glial cells)

Neurons

Neurons are the basic structural and functional units of the nervous system. They generate, carry and transmit nerve impulses. They are also known as nerve cells.

Structure of a Neuron

A neuron is basically composed of three parts:

  1. Cell body (soma)
  2. Dendrites
  3. Axon

Cell Body

The cell body, or soma, is the central region containing the nucleus and is the site of major metabolic activity of the cell. It is approximately 4–100 micrometers in diameter. The cytoplasm of a neuron is called neuroplasm. The ribosomes of neurons are called Nissl granules. Dendrites and axons are extensions of the cell body.

Axon

An axon is a thin, long cylindrical extension that arises from the cell body. Axons carry and transmit nerve impulses from one neuron to another. Most axons are covered by a fatty substance called the myelin sheath, which is formed by Schwann cells (in the peripheral nervous system). The regions of axon where myelin is absent are known as the Nodes of Ranvier. The site from which the axon extends from the cell body is the axon hillock. The terminal endings are called axon terminals.

Dendrites

Dendrites are extensions of the cell body that receive stimuli or nerve impulses from other neurons and conduct them to the cell body.

Brain

The brain: The brain is one of the largest organs in the body and coordinates most body activities. It is the control centre of the body. The adult human brain weighs on average about 1.4–1.5 kg (males average ≈ 1370 g; females ≈ 1200 g). It is made up of about 100 billion neurons and is one of the most complex known biological structures.

Ventricles of the Brain

The ventricles are cavities (hollow spaces) filled with cerebrospinal fluid (CSF). There are four interconnected ventricles in the brain. Each ventricle is lined by ependymal cells that form the choroid plexus, which produces CSF.

Four Ventricles

  • Right lateral ventricle
  • Left lateral ventricle
  • Third ventricle
  • Fourth ventricle

Lateral Ventricles

There are two lateral ventricles, one in each cerebral hemisphere. They are roughly C-shaped, lined by ependymal cells and filled with CSF. Both lateral ventricles are separated by the septum pellucidum (septum lucidum).

Third Ventricle

The third ventricle is located below the lateral ventricles. It is a narrow funnel-shaped cavity filled with CSF and is connected to the fourth ventricle by the cerebral aqueduct.

Fourth Ventricle

The fourth ventricle is a diamond-shaped cavity located below the third ventricle; it communicates with the third ventricle via the cerebral aqueduct.

Cerebrospinal Fluid (CSF)

CSF is a clear, colourless, transparent fluid found in the ventricles, subarachnoid space and the central canal of the spinal cord. It is mainly formed by the choroid plexus of the lateral ventricles.

Choroid Plexus

The choroid plexus is a network of blood vessels lined by ependymal cells that produces CSF. The CSF volume is approximately 100–150 ml. It is secreted at a rate of about 0.5 ml/min (≈ 500–700 ml/day). Its pH is slightly alkaline, around 7.3.

Composition of CSF

CSF is composed primarily of water (~99%) and about 1% solids. Inorganic ions include sodium, potassium, calcium, magnesium, bicarbonates, and chlorides. Organic components include proteins, glucose, uric acid and creatinine.

Functions of CSF

CSF supports and protects the central nervous system and acts as a shock absorber. It also helps provide nutrients to the brain and spinal cord.

Major Parts of the Brain

The brain can be divided into four major parts: cerebrum, cerebellum, brain stem and diencephalon.

Cerebrum

The cerebrum is the largest portion of the brain and is divided into two cerebral hemispheres.

Digestive System / Gastrointestinal Tract (GIT)

Digestive system: The major objective of eating food is to obtain nutrients. Digestion is the conversion of complex food into simple and absorbable forms. The digestive system breaks down food into nutrients.

Stages of Digestion

  • Ingestion — receiving food
  • Propulsion — movement of food
  • Digestion — mechanical and chemical breakdown
  • Absorption — absorption of nutrients
  • Excretion — removal of undigested material

Components of the Digestive System

The digestive system consists mainly of the gastrointestinal tract and accessory digestive glands. The major organs of the GIT include:

  1. Mouth
  2. Pharynx
  3. Oesophagus
  4. Stomach
  5. Small intestine
  6. Large intestine
  7. Rectum
  8. Anus

Mouth

The mouth, or oral (buccal) cavity, is the first part of the gastrointestinal tract. It is surrounded by the upper and lower lips and contains the cheeks, gums, teeth, salivary glands and tongue. The space between the gums and cheeks is the vestibule. The roof of the oral cavity is the palate, which has two parts: the hard palate and the soft palate.

Teeth

Teeth perform mechanical digestion by breaking down food into smaller pieces. There are typically 32 teeth in the adult mouth: incisors (8), canines (4), premolars (8), and molars (12).

Tongue

The tongue lies on the floor of the mouth and is attached to the hyoid bone. It mixes saliva with food to form a bolus and contains taste receptors for sweet, salty, sour and bitter tastes.

Pharynx

The pharynx is a muscular tube-like structure located between the mouth and the oesophagus. It is a common pathway for the digestive and respiratory tracts and is also known as the throat. The pharynx passes the bolus into the oesophagus. The epiglottis ensures that food is directed from the pharynx into the oesophagus. The pharynx is subdivided into three parts: nasopharynx, oropharynx and laryngopharynx.

Oesophagus

The oesophagus (food pipe) is a hollow tube that carries food (bolus) from the pharynx to the stomach. It lies behind the trachea and in front of the vertebral column. No digestion occurs in the oesophagus because digestive enzymes are absent; the oesophagus opens into the stomach.

Stomach

The stomach is a pouch-like organ located on the left side of the abdominal cavity.

ATP (Adenosine Triphosphate)

ATP: Adenosine triphosphate (ATP) is a multifunctional nucleotide coenzyme and the primary chemical energy currency of all cells. It is produced by phosphorylation and cellular respiration and is used by enzymes and structural proteins in many cellular processes. One molecule of ATP contains three phosphate groups. Energy from ATP is released by hydrolysis of high-energy phosphoanhydride bonds:

  • ATP + H2O → ADP + Pi
  • ADP + H2O → AMP + Pi
  • AMP + PPi (pyrophosphate)

Biological Significance / Functions of ATP

ATP is the primary source of energy within all living organisms. It is used in muscle contraction, active transport across cell membranes, impulse formation, and various metabolic processes. ATP is also the precursor of cyclic AMP (cAMP).

Mechanisms of ATP Formation

There are three main mechanisms by which organisms form ATP:

  1. Substrate-level phosphorylation — direct transfer of a phosphate group from a substrate to ADP to form ATP (occurs during glycolysis and the citric acid cycle). Example: 1,3-bisphosphoglycerate + ADP → 3-phosphoglycerate + ATP.
  2. Oxidative phosphorylation — occurs in mitochondria and uses energy derived from the electron transport chain to phosphorylate ADP to ATP; the major mechanism for ATP synthesis in aerobic respiration.
  3. Photophosphorylation — occurs in chlorophyll-containing plant cells or in certain photosynthetic bacteria; light energy is used to generate ATP.

Role of ATP

ATP stores and releases energy in its phosphate bonds. When ATP is broken down into ADP and Pi, energy is released to power various cellular processes. ATP provides energy for the synthesis of macromolecules such as proteins, lipids and nucleic acids, and is involved in active transport processes across cell membranes.

Metabolism and Basal Metabolic Rate (BMR)

Metabolism refers to the complex set of biochemical reactions that occur within living organisms to sustain life. These reactions enable the organism to acquire, transform, store and utilize energy and nutrients essential for growth, reproduction, maintenance and response to the environment. Metabolism can be divided into two types:

  1. Catabolism
  2. Anabolism

Catabolism

Catabolism is the breakdown of complex molecules into simpler molecules.

Anabolism

Anabolism is the synthesis of complex molecules from simpler ones.

Basal Metabolic Rate (BMR)

BMR stands for basal metabolic rate. It refers to the amount of energy needed by an individual in an awake state at rest to maintain vital body functions (e.g., respiration, circulation, nerve functions, kidney function, ion transport across membranes and other cellular activities) in the post-absorptive phase.

Typical BMR values (approximate): males 35–38 Cal/sq-m/hr; females 32–35 Cal/sq-m/hr.

Determination of BMR

When determining BMR, ensure the person is:

  • Awake
  • At rest
  • In the post-absorptive phase
  • At neutral ambient temperature
  • Mental rest

There are two methods for determining BMR: the direct method and the indirect method.

Factors Affecting BMR

Gender, age, physical activity, hormones and stress are among the factors that affect BMR.

Stomach Acid Production

Hydrochloric acid (HCl) is produced by parietal cells of the stomach. HCl secretion is an active process that takes place in the canaliculi of parietal cells. Carbon dioxide produced by parietal cell metabolism combines with H2O to form carbonic acid (H2CO3) in the presence of the enzyme carbonic anhydrase, which is present in high concentrations in parietal cells. Carbonic acid dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3−). Hydrogen ions are actively pumped into the canaliculus via the H+/K+ ATPase (proton pump); the energy for this process is derived from cellular metabolism. Chloride ions (Cl−) derived from plasma NaCl move into the canaliculus and combine with H+ to form HCl in the stomach lumen.

Respiratory System

Respiration is the process of exchange of gases between body tissues and the external environment. Cells continuously use oxygen for metabolic reactions that release energy from nutrient molecules (ATP production). Carbon dioxide produced during these reactions must be excreted. The supply of O2 and excretion of CO2 occur via the respiratory system.

Stages of Respiration

  1. Breathing (ventilation)
  2. Internal respiration
  3. Cellular respiration

Definitions

Breathing: exchange of gases between the environment and the lungs.

Internal respiration: exchange of gases between the blood and the lungs (pulmonary gas exchange).

Cellular respiration: metabolic processes within cells where oxygen is used and carbon dioxide is produced.

Types of Respiration

  • Aerobic respiration: occurs in the presence of oxygen; produces CO2 and water and yields more energy.
  • Anaerobic respiration: occurs in the absence of oxygen; may or may not produce CO2 and yields less energy.

Lungs

The lungs are the principal organs of respiration. They are paired, spongy, air-filled organs located on either side of the chest. Each lung is covered by a double membrane called the pleura: the outer membrane (parietal pleura) and the inner membrane (visceral pleura). Pleural fluid is present between these membranes. The left lung is slightly smaller than the right lung. The right lung has three lobes while the left lung has two lobes.

Surfaces and Lobes

Surfaces of the lungs include the apex (narrow superior portion), the base (broad inferior portion), the costal surface (against the ribs) and the medial surface (adjacent to the mediastinum).

Lobes

  • Right lung: superior, middle and inferior lobes
  • Left lung: superior and inferior lobes

Lung Volumes

Lung volumes (respiratory volumes) refer to the volume of gas in the lungs at a given time. Major lung volumes include:

  • Tidal volume: amount of air inhaled or exhaled during a normal breath (~500 ml)
  • Inspiratory reserve volume: maximum additional air inhaled after a normal inspiration (~2500–3000 ml)
  • Expiratory reserve volume: maximum additional air exhaled after a normal expiration (~1000–1200 ml)
  • Residual volume: air remaining in lungs after forceful exhalation (~1200–1500 ml)

Lung Capacities

Lung capacities are combinations of different lung volumes. Major capacities include total lung capacity, vital capacity, expiratory capacity, residual capacity and inspiratory capacity.

Total Lung Capacity

Total lung capacity is the total volume of air a person can hold after a forced inhalation (sum of all volumes). Approximate values: males ≈ 6000 ml; females ≈ 4500 ml.

Vital Capacity

Vital capacity is the amount of air a person can move in and out of the lungs; it is the sum of tidal volume, inspiratory reserve volume and expiratory reserve volume.

Inspiratory Capacity

Inspiratory capacity is the amount of air that can be inhaled after a normal tidal inhalation.

Nephrons (Kidney Functional Units)

Types of Nephrons

There are two basic types of nephrons:

  • Cortical nephrons: about 85% of nephrons; have a short loop of Henle
  • Juxtamedullary nephrons: about 15% of nephrons; have a long loop of Henle

Physiology of Urine Formation

Urine formation is a blood-cleansing function. Normally about 1300 ml of blood enters the kidney per minute (note: cardiac output supplies kidneys); the kidney excretes unwanted substances from the blood as urine. Normal urine output is about 1–1.5 litres per day.

Formation of Urine

Urine formation mainly involves three steps:

  1. Glomerular filtration
  2. Tubular reabsorption
  3. Tubular secretion

Glomerular Filtration

Glomerular filtration is the process by which plasma is filtered from glomerular capillaries into Bowman’s capsule via the filtration membrane. It is the first step of urine formation. Plasma proteins are generally not filtered; the filtered fluid is known as glomerular filtrate.

Glomerular Filtration Rate (GFR)

GFR is defined as the total volume of filtrate formed in all the nephrons of both kidneys per unit time. Normal GFR is approximately 125 ml/min (≈ 180 L/day).

Factors Affecting GFR

Renal blood flow, glomerular capillary pressure, colloid osmotic pressure and hydrostatic pressure in Bowman’s capsule affect GFR.

Tubular Reabsorption

About 180 L of filtrate is formed per day but only about 1–1.5 L of urine is excreted; approximately 99% of filtrate is reabsorbed into the blood. Tubular reabsorption is the process by which water and necessary substances are reabsorbed from the renal tubule back into the bloodstream. Reabsorbed substances move into the interstitial fluid of the renal medulla and then into tubular capillaries. Tubular reabsorption is selective: renal tubular cells reabsorb substances necessary for the body while allowing unwanted substances to remain in the filtrate for excretion.

Tubular Secretion

Tubular secretion is the process by which substances are transported from the blood into renal tubules. Unwanted substances not filtered at the glomerulus may be secreted into the renal tubule later in this process.

Renin–Angiotensin–Aldosterone System (RAAS)

RAAS: The renin–angiotensin–aldosterone system is a hormonal system involved in the regulation of arterial blood pressure and plasma sodium concentration. Renin is an enzyme/hormone secreted by the juxtaglomerular apparatus in the kidney. Angiotensinogen is a plasma protein released by the liver.

Pathway (simplified):

Angiotensinogen (liver) —(renin)→ Angiotensin I —(angiotensin-converting enzyme, ACE)→ Angiotensin II → vasoconstriction (increased blood pressure), increased sympathetic activity, and stimulation of aldosterone release from the adrenal cortex → increased Na+ and water reabsorption → increased blood volume and blood pressure.

Endocrine System

Endocrine system: The human endocrine system consists of a group of ductless glands (endocrine glands) that regulate body processes by secreting chemical messengers called hormones. Endocrine glands secrete hormones directly into the bloodstream. The nervous and endocrine systems together control and coordinate body functions. The study of the endocrine system is called endocrinology.

Endocrine Glands

Endocrine glands secrete hormones into the extracellular spaces and these diffuse into blood capillaries to be carried throughout the body by the circulatory system. Major endocrine glands include:

  1. Pituitary gland
  2. Pineal gland
  3. Thyroid gland
  4. Parathyroid glands
  5. Thymus gland
  6. Adrenal glands
  7. Pancreatic islets
  8. Ovaries (females)
  9. Testes (males)

Hormones

Hormones are chemical messengers primarily produced by endocrine glands. Hormones regulate a wide range of physiological functions and are responsible for various cellular activities, especially growth and metabolism.

Types of Hormones

Hormones can be broadly classified into two types:

  • Steroidal hormones
  • Non-steroidal hormones

Steroidal Hormones

Steroidal hormones are generally synthesized from cholesterol-based lipids. They are lipid-soluble (water-insoluble) and can cross cell membranes to act on intracellular receptors.

Adrenal Gland

There are two adrenal glands, one atop each kidney. Each is approximately 4 cm long, 3 cm thick and around 4 g in weight. Each adrenal gland has two main parts: the adrenal cortex and the adrenal medulla.

Adrenal Cortex

The adrenal cortex is the outer part of the gland and develops from mesodermal (renal-related) tissue. It produces three groups of steroid hormones collectively known as corticosteroids (corticoids) and is subdivided into zones:

  • Zona glomerulosa — mineralocorticoids (e.g., aldosterone)
  • Zona fasciculata — glucocorticoids (e.g., cortisol)
  • Zona reticularis — gonadocorticoids (sex hormones, e.g., androgens)

Mineralocorticoids (Aldosterone)

Aldosterone is the major mineralocorticoid. It helps maintain water and electrolyte balance by stimulating sodium reabsorption and potassium excretion. Aldosterone release is stimulated by increased blood potassium and activation of RAAS.

Glucocorticoids

Cortisol, corticosterone and cortisone are major glucocorticoids. They are essential for life, regulate metabolism and responses to stress. Release of cortisol is triggered by stress. Functions include gluconeogenesis, lipolysis, sodium and water retention, anti-inflammatory action, suppression of immune response, and delay of wound healing.

Gonadocorticoids (Sex Hormones)

Adrenal cortical sex hormones are mainly androgens (male sex hormones) and, to a lesser extent, estrogens. The adrenal contribution to circulating sex hormones is generally small compared with secretion from the testes and ovaries.

Adrenal Medulla

The adrenal medulla is the inner part of the gland and develops from nervous tissue. It secretes catecholamines: adrenaline (epinephrine) and noradrenaline (norepinephrine).

Adrenaline and Noradrenaline

Adrenaline and noradrenaline are released into the bloodstream from the adrenal medulla during activation of the sympathetic nervous system. They produce similar effects such as increasing heart rate and increasing blood pressure, and they augment sympathetic activity.

Female Reproductive System

The female reproductive system is a complex network of organs and structures that function together to facilitate reproduction. Its primary roles are to produce eggs (ova), provide a suitable environment for fertilization, support embryo development during pregnancy and enable childbirth.

Classification

The female reproductive system can be classified into:

  • External genitalia (vulva)
  • Internal genitalia

External Genitalia (Vulva)

The external genital organs collectively called the vulva include:

  1. Labia majora
  2. Labia minora
  3. Clitoris
  4. Perineum
  5. Vestibular glands (Bartholin’s glands)

Labia Majora

The labia majora are two large folds forming the boundary of the vulva. They are composed of skin, fibrous tissue and fat, and contain numerous sebaceous and sweat glands. They are also known as the “outer lips” or “greater lips” and help protect the internal genital structures.

Internal Genitalia

Internal genital organs include the vagina, cervix, uterus, fallopian tubes (uterine tubes) and ovaries.

Male Reproductive System

The male reproductive system is a complex network of organs and structures responsible for producing, storing and delivering sperm cells to fertilize an egg. The system is subdivided into external and internal genital organs.

External Genital Organs

  • Penis
  • Scrotum
  • Testes

Internal Genital Organs

  • Epididymis
  • Vas deferens
  • Accessory glands and ducts (seminal vesicles, prostate gland, Cowper’s glands)
  • Ejaculatory ducts

Functions of the Male Reproductive System

Testes produce sperm and testosterone. The epididymis stores and matures sperm. The penis deposits sperm into the female reproductive tract and also serves in urine excretion. The scrotum maintains a lower temperature for optimal spermatogenesis.

Oogenesis

Oogenesis is the process of formation of female gametes (ova). Oogenesis begins during embryonic development in the ovary (in ovarian follicles). Primordial germ cells differentiate into oogonia, which then enter meiosis.

Timeline and Numbers

Oogenesis begins around 6–7 weeks of fetal development. At about 20 weeks of gestation there are approximately 6–7 million oogonia; at birth this number decreases to about 2 million follicles. At puberty the number of primary follicles is around 60,000–80,000.

Stages of Oogenesis

  1. Oogonium formation
  2. Primary oocyte formation
  3. Completion of meiosis I
  4. Secondary oocyte and meiosis II
  5. Fertilization and completion of meiosis II

Oogonium Formation

During fetal development, primordial germ cells migrate to the ovaries and differentiate into oogonia, which are diploid cells.

Primary Oocyte Formation

Oogonia enter meiosis and become primary oocytes. They begin meiosis I but are arrested at prophase I until puberty.

Completion of Meiosis I

At puberty, during each menstrual cycle, a primary oocyte resumes meiosis and completes the first meiotic division, producing a secondary oocyte and a first polar body (which typically degenerates).

Secondary Oocyte and Meiosis II

The secondary oocyte begins the second meiotic division but is arrested at metaphase II. It will complete meiosis II only if fertilization occurs.

Fertilization and Completion of Meiosis II

If fertilization occurs, the secondary oocyte completes meiosis II, yielding a mature ovum and a second polar body.

Chromosomes

Chromosomes are thread-like structures composed of DNA and proteins that are found in the nucleus of eukaryotic cells. They carry genetic information necessary for growth, development and functioning of living organisms.

Structure

Chromosomes are primarily composed of deoxyribonucleic acid (DNA) and associated histone proteins that help organize and compact DNA into a manageable structure.

Number

Except for reproductive cells (sperm and ovum), human somatic cells have 46 chromosomes arranged in 23 pairs: 22 pairs of autosomes and 1 pair of sex chromosomes.

Types of Chromosomes

  • Autosomes — chromosomes not involved in determining sex (22 pairs in humans)
  • Sex chromosomes — determine the sex of an individual (females: XX; males: XY)