Cover
Preface
PART I: Introduction
1. CHAPTER 1: The Cell
  1. Introduction
  2. Components of the Human Cell
  3. Cytoplasmic Organelles
  4. Reference
  5. Further Reading
PART II: Nerve Muscle Physiology
1. CHAPTER 2: Nerve Physiology
  1. Introduction
  2. Physiology of Peripheral Nerve
  3. Transmission and Speed of nerve impulses
  4. Clinical Relevance
  5. References
  6. Further Reading
2. CHAPTER 3: Muscle Physiology
  1. Introduction
  2. Types of Muscle
  3. Anatomical Organisation of the Skeletal Muscle
  4. Contraction of the Muscle
  5. Neuromuscular Junction (NMJ)
  6. Smooth Muscle
  7. Clinical Relevance
  8. References
  9. Further Reading
PART III: Cardiovascular System
1. CHAPTER 4: Heart
  1. Introduction
  2. Gross Anatomy
  3. Functional Histology
  4. Cardiac Cycle
  5. Cardiac Indices
  6. Other Functions
  7. Clinical Relevance
  8. Reference
  9. Further Reading
2. CHAPTER 5: Circulation
  1. Introduction
  2. Classification
  3. Functional Histology
  4. Determinants of Circulation
  5. Rhythmic Contraction of the Heart
  6. Determinants of Arterial Blood Pressure
  7. Variations in Regional Blood Flow
  8. Coagulation and Fibrinolytic Systems
  9. Fluid Compartments of the Body
  10. Tissue Fluid Exchange
  11. Clinical Relevance
  12. Reference
  13. Further Reading
PART IV: Respiratory System
1. CHAPTER 6: Mechanics of Respiration
  1. Introduction
  2. Organisation of the Respiratory System
  3. Lung Volumes and Capacities
  4. Respiratory Muscles
  5. Mechanics of Pulmonary Ventilation
  6. Lung Compliance
  7. Clinical Relevance
  8. References
  9. Further Reading
2. CHAPTER 7: Gas Exchange and Transport
  1. Introduction
  2. Gas Diffusion
  3. Respiratory Indices
  4. Transport of Oxygen in Blood
  5. Transport of Carbon Dioxide in Blood
  6. Clinical Relevance
  7. References
  8. Further Reading
3. CHAPTER 8: Control of Breathing
  1. Introduction
  2. Peripheral and Central Chemoreceptors
  3. Neural Respiratory Centres
  4. Peripheral Receptors
  5. Clinical Relevance
  6. References
  7. Further Reading
PART V: Gastrointestinal System (GIT)
1. CHAPTER 9: GIT Movements
  1. Introduction
  2. Mastication
  3. Swallowing
  4. Movements of the Stomach
  5. Movements of the Small Intestine
  6. Movements of the Large Intestine
  7. Reference
  8. Further Reading
2. CHAPTER 10: GIT Secretions
  1. Introduction
  2. Saliva
  3. Gastric Secretion
  4. Pancreatic Secretion
  5. Bile
  6. Intestinal Juice
  7. References
  8. Further Reading
3. CHAPTER 11: GIT Digestion and Absorption
  1. Introduction
  2. Absorption in the Small Intestine
  3. Absorption in the Large Intestine
  4. Clinical Relevance
  5. Reference
  6. Further Reading
PART VI: Hepato Renal System
1. CHAPTER 12: Liver Physiology
  1. Introduction
  2. Functional Histology
  3. Functions of the Liver
  4. Clinical Relevance
  5. References
  6. Further Reading
2. CHAPTER 13: Renal Physiology
  1. Introduction
  2. Blood Supply
  3. Lymphatic Drainage and Innervation
  4. Functions of the Kidneys
  5. Functional Histology
  6. The Glomeruli
  7. The Glomerular Filtration Rate
  8. The Bowman’s Capsule
  9. The Plasma Filtrate
  10. The Renal Tubular System
  11. Selective Reabsorption
  12. Renal Clearance
  13. Regulation of Acid Base Balance
  14. Clinical Relevance
  15. Reference
  16. Further Reading
PART VII: Blood
1. CHAPTER 14: Blood Plasma and Cells
  1. Composition of Blood
  2. Production of Blood Cells
  3. Red Blood Cells
  4. Disorders of Red Blood Cells
  5. White Blood Cells
  6. Role of White Blood Cells in Inflammation
  7. Disorders of White Blood Cells
  8. Reference
  9. Further Reading
2. CHAPTER 15: Immune System
  1. Introduction
  2. The Innate Immune System
  3. Adaptive immunity
  4. Resolution of Immune Responses
  5. Immunopathology
  6. Further Reading
3. CHAPTER 16: Haemostasis
  1. Introduction
  2. Platelets
  3. Formation of Platelet Plug
  4. Activation of the Coagulation Cascade
  5. Fibrinolysis
  6. Coagulation Disorders
  7. Acquired Disorders
  8. Haemorrhage
  9. Further Reading
PART VIII: Endocrinology
1. CHAPTER 17: Endocrinology
  1. Introduction
  2. Pituitary Gland (Hypophysis)
  3. Thyroid Gland
  4. Regulation of Thyroid Hormones
  5. Hyperthyroidism
  6. Hypothyroidism
  7. Adrenal Gland
  8. Reference
  9. Further Reading
2. CHAPTER 18: Regulation of Blood Glucose
  1. Introduction
  2. Insulin
  3. Glucagon
  4. Diabetes Mellitus
  5. Reference
  6. Further Reading
3. CHAPTER 19: Regulation of Blood Calcium
  1. Introduction
  2. Disorders of Calcium
  3. Vitamin D
  4. Disorders of Vitamin D
  5. Parathormone
  6. Calcitonin
  7. Reference
  8. Further Reading
4. CHAPTER 20: Reproductive Hormones and Pregnancy
  1. Organisation of the Reproductive Tract
  2. Monthly Ovarian Cycle
  3. Physiologic Changes During Pregnancy
  4. Respiratory Changes During Pregnancy
  5. Cardiovascular Changes During Pregnancy
  6. Gastrointestinal Tract
  7. Genitourinary System
  8. Haematologic System
  9. Reference
  10. Further Reading
PART IX: Nervous System
1. CHAPTER 21: Central Nervous System
  1. Introduction
  2. Divisions of the Nervous System
  3. Clinical Relevance
  4. Physiology of Dental Pain
  5. Clinical Relevance
  6. References
  7. Further Reading
2. CHAPTER 22: The Autonomic Nervous System
  1. Introduction
  2. Anatomic Organisation of the ANS
  3. Antagonistic Functions of the Sympathetic and Parasympathetic Efferents
  4. Sympathetic and Parasympathetic Tone
  5. Clinical Relevance
  6. Reference
  7. Further Reading
3. CHAPTER 23: Special Senses
  1. Introduction
  2. Vision
  3. Hearing
  4. Equilibrium (Balance)
  5. Olfaction
  6. Reference
  7. Further Reading
Index
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List of Tables

Chapter 02
1. Table 2.1 Classification of peripheral nerve fibres.
2. Table 2.2 Chemical composition of extracellular and intracellular fluid compartments.
Chapter 06
1. Table 6.1 Typical lung volume and capacity values for a resting 70 kg male.
2. Table 6.2 The main muscles used in expiration and inspiration.
Chapter 07
1. Table 7.1 The approximate composition of various gases found in the atmosphere.
2. Table 7.2 Normal physiological values for respiration in healthy individuals.
Chapter 10
1. Table 10.1 Characteristics of saliva.
Chapter 14
1. Table 14.1 Full blood counts.
2. Table 14.2 Characteristics and functions of white blood cells.
Chapter 15
1. Table 15.1 Immunoglobulin isoforms and functions.
Chapter 16
1. Table 16.1 Coagulation factors.
2. Table 16.2 Blood coagulation tests.
3. Table 16.3 Classification of haemorrhage.
Chapter 17
1. Table 17.1 Hormones secreted by the anterior pituitary.
2. Table 17.2 Features of hyper‐ and hypothyroidism.
Chapter 18
1. Table 18.1 Plasma glucose levels.
2. Table 18.2 Features of hypoglycaemia and hyperglycaemia.
Chapter 20
1. Table 20.1 Role of placental hormones in pregnancy.
Chapter 21
1. Table 21.1 Distribution and clinical testing of cranial nerves.
Chapter 22
1. Table 22.1 Actions of autonomic nervous system.

List of Illustrations

Chapter 01
1. Figure 1.1 Components of a cell.
2. Figure 1.2 Fluid mosaic model of plasma membrane.
3. Figure 1.3 A human osteoblast cell stained for actin cytoskeletal filaments (green), plasma cell membrane (orange) and nucleus (blue).
Chapter 02
1. Figure 2.1 (a) Unipolar sensory neuron and (b) multipolar motor neuron.
2. Figure 2.2 Typical multipolar motor neuron found at the neuromuscular junction. The arrows show the direction of impulses in an efferent motor neuron.
3. Figure 2.3 Examples of unipolar sensory receptors for touch (a, b), pressure (c), and pain or temperature (d).
4. Figure 2.4 (a) Changes in ion flow through voltage‐gated channels during the depolarising and repolarising phases of an action potential. Leak channels and sodium–potassium pumps are not shown. (b) Action potential or impulse. The action potential arises at the trigger zone (here, at the junction of the axon hillock and the initial segment) of the neuronal cell body, which propagates along the axon to the axon terminals. The green and yellow regions of the neuron indicate parts that typically have voltage‐gated Na⁺ and K⁺ channels (axolemma and axon terminals).
5. Figure 2.5 Propagation of action potential in a neuron. Dotted lines indicate ionic current flow along the direction of the arrow. The insets show local current circuits. (a) In continuous conduction, along an unmyelinated axon, ionic currents flow across each adjacent segment of the membrane. (b) In saltatory conduction, along a myelinated axon, the action potential (nerve impulse) at the first node of Ranvier generates ionic currents in the cytosol and interstitial fluid that open voltage‐gated Na⁺ channels in the axolemma at the second node, and so on at each subsequent node. Thus action potential jumps from node to node.
Chapter 03
1. Figure 3.1 Structure of different muscle types: skeletal, cardiac, and smooth muscle.
2. Figure 3.2 Organisation and structure of the skeletal muscle. Tendons attach the skeletal muscle to the bone. The muscle organ is divided into numerous bundles called fascicles. Each fascicle consists of a number of muscle fibres (or cells), which in turn contain many myofibrils, made up of thinner myofilaments. Each component of the skeletal muscle is covered by its own outer layer of connective tissue: the epimysium, perimysium, and endomysium respectively.
3. Figure 3.3 Microscopic organisation of skeletal muscle. Muscles arise from the fusion of myoblasts. The sarcolemma or muscle cell membrane has numerous folds which form a system of transverse tubules or T‐tubules, through which the rapid transmission of muscle action potentials takes place.
4. Figure 3.4 Ultrastructure of the muscle fibre, showing light (I) and dark (A) banding patterns, giving the muscle a striated appearance. The bands contain contractile proteins. The I‐bands contain the thin actin myofilament and the A‐bands contain the thick myofilament.
5. Figure 3.5 Structure of contractile proteins (a) myosin, thick myofilament with the two binding sites, one for actin and the other for ATPase (b) actin, thin myofilament with two binding sites for myosin. At rest the myosin‐binding sites are blocked by the regulatory protein tropomyosin which forms a complex with troponin to prevent cross‐bridge formation and thus muscle contraction.
6. Figure 3.6 Sliding filament model of muscle contraction. (a) Relaxed muscle with no overlap between actin and myosin. Muscle fibre at rest and not contracted. (b) Partially contracted muscle, with some degree of overlap between thick and thin myofilaments. Length of muscle cell is slightly shorter than at rest. (c) Maximally contracted muscle, with actin filaments from both sides fully overlapping myosin. The length of muscle shortened considerably, owing to inward movement of Z‐discs being pulled inwards by the actin filaments to which they are attached. The widths of I'band and H‐zone have decreased.
7. Figure 3.7 The Sliding filament mechanism. Describes the contraction cycle of the skeletal muscle.
8. Figure 3.8 Neuromuscular junction. (a) Axon collaterals of motor neurons synapsing with muscle sarcolemma; (b) the arrival of a nerve impulse causes neurotransmitter ACh release into synaptic cleft; (c) binding of ACh to its specific receptor giving rise to muscle action potential. Also shows the breakdown of ACh to prevent further muscle depolarisation which would lead to sustained muscle contraction.
9. Figure 3.9 Summary of events in the excitation‐contraction mechanism of skeletal muscle fibre.
10. Figure 3.10 Motor units in smooth muscle. A series of axon‐like swelling, called varicosities or boutons, from autonomic neurons form motor units throughout the smooth muscle.
Chapter 04
1. Figure 4.1 Layers of the heart wall.
2. Figure 4.2 Cardiac valves.
3. Figure 4.3 Systemic and pulmonary circulation.
4. Figure 4.4 Histology of the cardiac muscle.
5. Figure 4.5 Conduction system of the heart.
6. Figure 4.6 Pacemaker potentials (green) and action potentials (black) in autorhythmic fibres of SA node.
7. Figure 4.7 Action potentials in ventricular contractile fibre (resting membrane potential –90 mV).
8. Figure 4.8 Events in a cardiac cycle.
Chapter 05
1. Figure 5.1 Comparative structure of blood vessels.
2. Figure 5.2 Role of baroreceptors in the regulation of blood pressure.
3. Figure 5.3 Restoration of blood pressure in hypovolaemic shock.
Chapter 06
1. Figure 6.1 The airway passages, with their generation number, in relation to other structures in the thorax.
2. Figure 6.2 A water spirometer is used to produce a spirogram, which gives four lung volumes and four capacities.
3. Figure 6.3 Measurements of FEV₁ and FVC in different disease states.
4. Figure 6.4 Respiratory muscles used for inhalation (inspiration) and exhalation (expiration).
5. Figure 6.5 Volume, pleural pressure, alveolar pressure, and airflow changes during inspiration and expiration in a healthy subject at rest.
6. Figure 6.6 Steps required for breathing to take place in a healthy subject at rest.
Chapter 07
1. Figure 7.1 Partial pressures of O₂ and CO₂ across air passages and blood (in kPa).
2. Figure 7.2 Diffusion of oxygen molecules across the alveolar‐capillary membrane.
3. Figure 7.3 Changes in the _A/ ratio, ventilation (_A), and perfusion () taken across a healthy upright lung.
4. Figure 7.4 (a) O₂‐Hb dissociation curve. The solid line is the standard oxygen dissociation curve and the dashed line shows four factors which shift the curve to the right. These are low pH, high PCO₂, high temperature, and high 2,3‐DPG; (b) the Hb molecule showing the four haem subunits, with attached O₂ molecules.
5. Figure 7.5 A red blood cell showing the transport mechanisms for CO₂.
Chapter 08
1. Figure 8.1 (a) Anatomical location of the carotid and aortic bodies. (b) Histology of the carotid body.
2. Figure 8.2 The relationship between PaO₂ and _A, while PaCO₂ remains constant and also when PaCO₂ is raised.
3. Figure 8.3 Anatomical location, histology, and biochemistry of the central chemoreceptors.
4. Figure 8.4 Neural respiratory pathways.
Chapter 09
1. Figure 9.1 The gastrointestinal tract extends from the mouth to the anus.
2. Figure 9.2 Layers of the gastrointestinal tract showing enteric nervous system.
3. Figure 9.3 Diagrammatic representation of oral (a), pharyngeal (b), and oesophageal (c) phases of swallowing.
4. Figure 9.4 Structure of the stomach.
5. Figure 9.5 External anatomy of the small intestine (Anterior view).
6. Figure 9.6 Anatomy of the large intestine.
Chapter 10
1. Figure 10.1 Major salivary glands include parotid, submandibular, and sublingual glands (a); microscopic structure of the submandibular gland containing serous and mucous acini (b).
2. Figure 10.2 Control of salivary secretions by the autonomic nervous system.
3. Figure 10.3 Sectional view of the stomach mucosa depicting component cell‐types (a); histological structure of the fundic mucosa (b).
4. Figure 10.4 Relationship of the duodenum with pancreas, liver, and gall bladder (a); ducts carrying bile and pancreatic juice into the duodenum (b).
Chapter 11
1. Figure 11.1 Absorption of digested nutrients in the small intestine.
2. Figure 11.2 Structure of the large intestine.
Chapter 12
1. Figure 12.1 Dual blood supply of the liver.
2. Figure 12.2 Internal structure of the liver showing hepatocytes and sinusoids.
3. Figure 12.3 Anatomy of the liver depicting transverse section of a lobule.
4. Figure 12.4 Breakdown of haemoglobin and bilirubin metabolism.
Chapter 13
1. Figure 13.1 Urinary system in a female. Urine formed in the kidneys is transported to the ureter to be stored in the urinary bladder before being excreted through the urethra.
2. Figure 13.2 Internal structure of the kidneys showing the outer (light) cortex and inner (dark) medulla and associated features.
3. Figure 13.3 Renal vascular system showing a highly branched network of blood vessels. The smallest branches of the renal artery are the afferent arteriole and the glomerular capillaries, which supply each kidney with oxygenated blood. The smallest branches of the renal vein, which carry away deoxygenated blood to the heart, are the efferent arteriole and peritubular capillaries.
4. Figure 13.4 Structure of a nephron, which forms the functional unit of the kidneys.
5. Figure 13.5 Relationship of the nephron structure to three basic functions: glomerular filtration, tubular reabsorption, and tubular secretion.
Chapter 14
1. Figure 14.1 Origin of blood cells.
2. Figure 14.2 Regulation of erythropoiesis.
Chapter 15
1. Figure 15.1 Cells of the innate immune system: key functions and potential pathology.
2. Figure 15.2 Major histocompatibility complex and antigen presentation.
3. Figure 15.3 The complement cascade.
4. Figure 15.4 Major classes of adaptive immune cells.
5. Figure 15.5 Antibody anatomy: the Fc region is highly variable to recognise a (almost) limitless range of antigenic shapes (epitopes). A process called ‘somatic hyper‐mutation’, where genes for antigen binding sites are rearranged, gives rise to millions of possible antigen binding sites. The Fc region is constant; it is recognised by Fc receptors on phagocytes, NK cells, mast cells, basophils and eosinophils, and promotes phagocytosis and degranulation.
Chapter 16
1. Figure 16.1 The coagulation cascade.
Chapter 17
1. Figure 17.1 Pituitary gland: Location, blood supply and histology.
2. Figure 17.2 Control and actions of antidiuretic hormone secretion.
3. Figure 17.3 Thyroid gland: Location, blood supply and histology.
4. Figure 17.4 Adrenal glands: Location, blood supply and histology.
Chapter 18
1. Figure 18.1 Position, blood supply, and histology of the pancreas.
Chapter 19
1. Figure 19.1 Steps in the activation of vitamin D.
2. Figure 19.2 Location, blood supply, and histology of parathyroid glands.
Chapter 20
1. Figure 20.1 Female reproductive system (posterior view).
2. Figure 20.2 Hormonal interactions in the female sexual cycle.
3. Figure 20.3 A pyogenic granuloma in the upper right anterior region in a 23‐year‐old pregnant lady.
Chapter 21
1. Figure 21.1 (a) Major divisions of the nervous system: central nervous system, CNS (beige) and peripheral nervous system, PNS (purple). (b) The CNS is divided into the brain and spinal cord. The PNS is subdivided into the afferent or sensory and efferent or motor divisions. The afferent division (blue) conveys information from the periphery via somatic, autonomic, and enteric neurons and receptors, sensory and visceral stimuli. The efferent division (brown) sends information from the CNS to the periphery, via somatic, autonomic, and enteric divisions to the effector organs, like muscles and glands.
2. Figure 21.2 Overview of major brain components showing four major divisions: cerebrum, diencephalon, brainstem, and cerebellum.
3. Figure 21.3 Surface anatomy of the brain. Anterior or superior view of the brain. A longitudinal fissure divides the brain into the left and right hemispheres. The inset shows that the surface of the brain is thrown into numerous folds or convolutions called gyri (singular gyrus) and sulci (singular sulcus).
4. Figure 21.4 The four lobes of the cerebrum: frontal lobe, temporal lobe, parietal lobe and occipital lobe. The ‘insula’ is an area of the brain involved in taste sensation, which lies deep inside the brain. It is not visible externally and has therefore been projected to the surface of the brain.
5. Figure 21.5 The afferent sensory pathway from mechanoreceptors in the right hand (used for writing or handling dental surgical tools) to the primary sensory cortex in the left brain (steps 1–5). The efferent pathway from primary motor cortex in left brain to the skeletal muscle in the right hand on contralateral side (steps 6–8).
6. Figure 21.6 Inferior aspect of the brain showing 12 pairs of cranial nerves (CN) in yellow.
7. Figure 21.7 Section through the spinal cord (a) showing white and grey columns (matter), the posterior (dorsal) and the anterior (ventral) horns, the dorsal root through which sensory nerves enter the spinal cord, and the ventral root through which the motor nerve exits the spinal cord (b) substantia gelatinosa, nucleus dorsalis (Clarke’s column), and nucleus proprius.
8. Figure 21.8 The spinal cord and spinal nerves. A total of 31 pairs of spinal nerves, originate from different levels of the spinal cord: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal. Spinal nerves are mixed nerves formed by the fusion of the dorsal (sensory) and a ventral (motor) roots.
9. Figure 21.9 Pain pathways in the head and face.
10. Figure 21.10 Modulation of pain via the descending pathway (in green); +/− indicate nociceptive and inhibition of nociceptive responses at the spinal cord. Modulation occurs through the interaction between ascending (excitatory) pain pathway with the descending analgesic pathway at the level of midbrain, medulla and spinal cord.
Chapter 22
1. Figure 22.1 Structure of the sympathetic division of the autonomic nervous system. Solid lines represent preganglionic axons; dashed lines represent postganglionic axons. The preganglionic nerves exit from the thoracolumbar regions of the spinal cord to synapse with postganglionic fibres within autonomic ganglia. The ganglia on each side of the spinal cord, form chains, which run parallel to the cord (only left chain ganglia shown).
2. Figure 22.2 Structure of the parasympathetic division of the autonomic nervous system. Solid lines represent preganglionic axons; dashed lines represent postganglionic axons. The preganglionic fibres arise from the craniosacral regions of the spinal cord. The cranial preganglionic fibres enter discrete ganglia to synapse with the postganglionic fibres. The innervation of the organs is shown on one side only. Note the rich innervation of the orofacial region by the facial nerve (VII) and glossopharyngeal nerves (IX).
3. Figure 22.3 Neurotransmitter release in somatic (voluntary) and autonomic (involuntary) motor neurons. Preganglionic nerves (solid lines) and postganglionic nerve (dashed lines); (a) somatic motor neurons release acetylcholine (shown only for purposes of comparison with sympathetic and parasympathetic (autonomic) neurons; (b) sympathetic preganglionic nerve fibres release acetylcholine and postganglionic fibres release norepinephrine; parasympathetic pre‐ and postganglionic fibres release only acetylcholine. The cells of the adrenal medulla are modified postganglionic neurons (Chromaffin cells) because they lack axons that would normally project to target tissues.
Chapter 23
1. Figure 23.1 Anatomy of the eyeball.
2. Figure 23.2 Microscopic structure of the retina.
3. Figure 23.3 Anatomy of the ear.
4. Figure 23.4 Structure of the internal ear.
5. Figure 23.5 (a) Location of olfactory epithelium in nasal cavity. (b) Anatomy of olfactory receptor cells, consisting of first‐order neurons whose axons extend through the cribriform plate and terminate in the olfactory bulb.
6. Figure 23.6 The relationship of gustatory receptor cells in taste buds to tongue papillae.
7. Figure 23.7 The gustatory pathway.

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Names: Ali, Kamran, editor. | Prabhakar, Elizabeth, editor.
Title: Essential physiology for dental students / edited by Kamran Ali, Elizabeth Prabhakar.
Description: Hoboken, NJ : John Wiley & Sons, 2019. | Includes bibliographical references and index. |
Identifiers: LCCN 2018022260 (print) | LCCN 2018023041 (ebook) | ISBN 9781119271611 (Adobe PDF) | ISBN 9781119271758 (ePub) | ISBN 9781119271710 (pbk.)
Subjects: | MESH: Physiological Phenomena
Classification: LCC RK76 (ebook) | LCC RK76 (print) | NLM QT 104 | DDC 617.60071/1–dc23
LC record available at https://lccn.loc.gov/2018022260

Cover Design: Wiley
Cover Image: © S.Y. Melina Kam

CHAPTER 1
The Cell: Structure and Function

Vehid Salih and Kamran Ali

Introduction

A cell is the fundamental structural, functional, and biological unit of all known living organisms (except viruses). Individual cells range from 1 to 100 μm and are visible only under a microscope as the human eye is unable to see anything smaller than 100 μm. Living organisms are described as unicellular (microorganisms) or multicellular (e.g. plants and animals). Unicellular organisms are also classified as prokaryotes and multicellular organisms as eukaryotes. Prokaryotes lack a nucleus and cytoplasmic organelles and are represented by bacteria. Eukaryotes have a nucleus as well as cytoplasmic organelles and include microorganisms such as fungi, protozoa, algae as well as animals and humans. Nevertheless, both types possess a cell membrane and contain deoxyribonucleic acid (DNA). Viruses are neither prokaryotes nor eukaryotes as they lack characteristics of living organisms, apart from the ability to replicate. They are best regarded as obligate parasites as they can only replicate in living cells.

An adult human body comprises approximately 75–100 trillion cells, and more than 200 varieties of specialised cells have already been identified. The cells in the human body join to form tissues. Four basic human tissues include epithelium, connective tissue, nervous tissue, and muscle. Different tissues are grouped to form organs which in turn join to form various systems of the human body. The function of the human body is maintained by thousands of control systems at the level of cells, tissues, organs as well as systems allowing the body to maintain a constant internal environment, or homeostasis. Nevertheless, all physiological processes as well as disease mechanisms can be described at, and ascribed to, the cellular level. Cells are diverse and vary tremendously in their morphology and function. Figure 1.1 shows the main features of a typical human cell.

Image described by caption and surrounding text. — **Figure 1.1** Components of a cell.

*Source:* Tortora and Derrickson (2013).

Components of the Human Cell

Cell Membrane

The cell membrane (plasmalemma) forms the outer boundary of the cell. The selective permeability of the cell membrane allows the cell to interact with its environment in a controlled way. The cell membrane is composed of a fluid combination of lipids (phospholipids and cholesterol), proteins and a small amount of carbohydrates. The basic structure of the cell membrane is formed by a phospholipid bilayer (Figure 1.2). The hydrophilic head region of the phospholipids faces the exterior (extracellular fluid or interstitial or tissue fluid) or aqueous interior (intracellular fluid or cytoplasmic face) of the cell, while the hydrophobic tails remain isolated. A variety of proteins attach to the surface of the phospholipid bilayer, while others traverse it partly or completely. The membrane proteins perform a variety of roles including: channel proteins, which facilitate passive transport across the cell membrane; protein pumps for active transport (Chapter 2); and receptor proteins for hormones and other endogenous as well as exogenous chemicals. Carbohydrates are either found in combination with proteins (glycoproteins) or lipids (glycolipids) and function as recognition markers, allowing the immune system to differentiate ‘self’ from foreign cells.

Illustration of a human cell structure with arrow pointing to the fluid mosaic model of plasma membrane with components labeled phospholipids, cholesterol, peripheral protein, pore, cytosol, etc. — **Figure 1.2** Fluid mosaic model of plasma membrane.

*Source:* Tortora and Derrickson (2013).

Nucleus

The nucleus is a double membrane‐bound structure and measures approximately 3–14 μm in most cells. It stores the DNA and associated proteins (= chromatin) in the form of chromosomes. The nuclear membrane (nucleolemma) isolates the DNA from the cytoplasm and is continuous with the endoplasmic reticulum (Figure 1.1). The nucleolemma contains pores to allow passage of messenger RNA (mRNA) units of nucleic acid. The gel‐like portion of the nucleus is known as nucleoplasm. Within the nucleus, proteins, DNA, and ribonucleic acid (RNA) are concentrated around specific chromosomal regions to form the nucleolus (plural: nucleoli). The nucleolus itself is not bound by a membrane and is responsible for synthesis of ribosomes.

The functions of the cell are coded in the genes. The genetic code or nucleotide sequence of DNA in the genes is used to direct protein synthesis, a process known as gene expression. First, the DNA is used as a template to link nucleotides, forming a strand of an mRNA molecule. This process is referred to as transcription and is facilitated by the enzyme RNA polymerase. The mRNA is then transferred across the nucleolemma into the cytoplasm and is used as a template by ribosomes to synthesise proteins, a process referred to as translation. Further processing of the proteins such as addition of phosphate (phosphorylation) or carbohydrates (glycosylation) takes place through a process known as post‐translation modification.

The nucleus is present in all cells of the body, excluding red blood cells and platelets. Liver cells (hepatocytes) can have one or two nuclei, while the osteoclasts and skeletal muscle cells are multinucleated.

Cytoplasm

The cytoplasm refers to the contents of the cell bounded by the cell membrane on the outer aspect and the nucleus in the inner part of the cell (Figure 1.1). The liquid portion of the cytoplasm is termed cytosol, and contains mainly water (70–85%), proteins (10–20%), lipids (2%), carbohydrates (1%), and electrolytes. Two distinct components of the cytoplasm include:

Organelles, which are membrane‐bound structures with a specific metabolic function. The membrane around different organelles serves to isolate the chemical reactions in the cytoplasm from each other.
Inclusions, which are non‐membrane‐bound particulate matter in the cytoplasm and do not have any specific metabolic function.

Cytoplasmic Organelles

Ribosomes
Ribosomes are small spherical structures (15 nm in diameter) which may exist as free‐floating particles or can be found in clusters lining the outer membrane of the rough endoplasmic reticulum. Ribosomes are the site of protein synthesis and translate the message obtained from the nucleus via transcription and effectively function as the working template or conveyor belt for assembling proteins.
Endoplasmic Reticulum
ER consists of a complex and multi‐folded system of parallel membranes and tubules which is contiguous with the nuclear membrane and Golgi apparatus.

The rough endoplasmic reticulum (rER) is studded with ribosomes. rER works with the ribosomes to synthesise proteins which are collected in spaces (cisternae) within the rER. A limited amount of packaging of proteins also takes place in the rER (pancreas).

The smooth endoplasmic reticulum (sER) contributes to lipid metabolism, including synthesis of steroid hormones, the lipid portion of lipoprotein in the liver, and lipid absorption and resynthesis of triglycerides in the intestinal mucosa. sER in skeletal and cardiac muscle, known as the sarcoplasmic reticulum, sequesters calcium for the cytosol. Finally, ER inactivates harmful by‐products of metabolism and drugs (liver).
Golgi Apparatus
The Golgi apparatus (GA) like the ER, is also a stacked series of folded membranous sacs. Its main function is to process and package both proteins and non‐proteins. The GA is the central delivery system from the cell and ‘packages’ cellular products into secretory vesicles which bud off from main Golgi membranes to enable them to migrate to and merge with the plasma membrane, releasing their contents outside the cell by a process known as exocytosis. The GA additionally produces lysosomes and digestion‐related organelles.
Lysosomes
Lysosomes are small membranous compartments (25–50 nm in diameter) which emanate from the GA. Lysosomes contain several degradative enzymes and assist in the degradation of toxic and waste cell products, including cell debris as well as microbial organisms (e.g. bacteria). Although found in all cells (except erythrocytes), lysosomes are particularly prominent in macrophages and neutrophils. Lysosomes also play a key role in programmed cell death, or apoptosis.
Peroxisomes
Peroxisomes are membrane‐bound structures, somewhat larger than lysosomes (0.3–1.5 μm in diameter), which arise from ER. They contain specific enzymes (oxidases and catalases) which help the breakdown of long‐chain lipids and are involved in the detoxification of certain chemicals (e.g. converting hydrogen peroxide to water). Apart from macrophages, peroxisomes are prominent in liver cells where they are involved in the degradation of alcohol.
Secretory Vesicles
These have a predominantly storage and transport role in cells and originate from the GA. These typically are transported by non‐constitutive secretion, which means that the secretions are released continuously regardless of any external factors. This is the standard mechanism and ensures proteins and other important molecules are continuously maintained at the plasma membrane. Non‐constitutive transport is very much regulated and requires specific signalling pathways to get the proteins delivered to the cell membrane.
Mitochondria
Mitochondria (singular: mitochondrion) are 0.75–3 μm in diameter and produce energy required for cell functions, effectively functioning as the powerhouse of the cell. They consist of a double‐layered membrane. Their inner membrane is highly folded into cristae, which markedly increase its surface area. Mitochondria have their own DNA, which allows independent replication of mitochondria and synthesis of enzymes and proteins involved in cellular respiration. They are studded with enzyme‐rich proteins responsible for production of energy molecules known as adenosine triphosphate (ATP). Mitochondria use oxygen (cellular respiration) to liberate energy stored in the sugars through a process known as oxidative phosphorylation. The energy is stored in the ATP molecules and is used for various cell functions such as protein synthesis and transport. Mitochondria are most numerous in cells with high energy requirements such as the cells in muscle and liver. In addition, mitochondria also store intracellular calcium.
Cytoskeletal Components
The cells are supported by a cytoskeletal network of extensive microtubules and microfilaments and intermediate filaments, which act as a scaffold for cell integrity, provide cell shape as well as being an additional means of transport of substances throughout the cell.
Microtubules
Microtubules are hollow cylindrical structures (25 nm in diameter) and are predominantly made up of α‐tubulin and β‐tubulin proteins. They have a role in the cellular transport of organelles such as mitochondria and vesicles. They also contribute to the axoneme, a cytoskeletal core in cellular appendages like cilia (respiratory tract, fallopian tubes) and flagella (sperms). Finally, microtubules also contribute to centrioles which help in cell division.
Intermediate Filaments
Intermediate filaments are made of fibrous proteins (8–10 nm in diameter) and contribute to the structural integrity and shape of the cell. Examples include: cytokeratin (epithelia, hair, nails); vimentin (connective tissue, blood cells); desmin and synemin (muscle, Z‐line proteins); and neurofilaments (neurons).
Microfilaments
Microfilaments are narrow (7 nm) filamentous structures made of the globular protein actin – a very common cell filament (Figure 1.3). They are located near to the inner surface of the cell membrane and help the cell to maintain its shape, and are involved in contraction (muscle) and movement (white blood cells).

Image described by caption. — **Figure 1.3** A human osteoblast cell stained for actin cytoskeletal filaments (green), plasma cell membrane (orange) and nucleus (blue).

Figure courtesy Dr. Vehid Salih

Inclusions

Inclusions represent particulate regions in the cytoplasm which are not membrane‐bound. These include stored food (e.g. glycogen particles, fat droplets); pigments (melanin, lipofuscin, haemosiderin). Pathological inclusions include bacteria, viruses, and exogenous particulate matter, such as carbon or asbestos in the lungs (not seen in Figure 1.1).

Cell Regeneration and Repair

Cells are said to be in a homeostatic state when they can perform their function normally. However, cells may get damaged from a variety of causes, such as lack of oxygen, physical or chemical agents, nutritional deficiencies, infection, genetic defects, and immunological reactions. Human cells vary in their ability to withstand injury as well as in their ability to regenerate. For example, the cells in the skin and lining mucosae continue to be replaced throughout life while brain and heart cells have limited regenerative ability. Irreversible cell injury results in cell death, which can translate into organ dysfunction, disease, and even death. The dental tissues possess limited capacity for regeneration and repair. The tooth enamel does not contain living cells and cannot regenerate. However, enamel can be remineralised in the early stages of tooth decay prior to cavitation. The dentin‐pulp complex and the periodontium, on the other hand, contain living cells (including stem cells) and can undergo regeneration and/or repair to a limited extent, depending on the severity of the injury.

Stem Cells

Stem cells are undifferentiated cells of a multicellular organism which can divide indefinitely to give rise to more cells of the same type (totipotent), and from which certain other types of cell arise by differentiation (multipotent, pluripotent). Stem cells have the capacity therefore to self‐renew and to differentiate into different cell types or tissues during embryonic development and typically throughout adulthood, too, although their differentiation potential declines with age. They are classified into two groups based on their origin: embryonic stem cells, which are pluripotent and can develop into any type of cell, and adult stem cells, which are deemed multipotent and these can develop into many cell types.

Embryonic stem cells offer immense promise but their clinical use and feasibility are limited, owing to ethical concerns. Adult stem cells may be harvested from different kind of tissues, like bone marrow, umbilical cord, amniotic fluid, brain tissue, liver, pancreas, cornea, various periodontal tissues, and even adipose tissue. Stem cells are currently used in a variety of research applications and for treating diseases of importance (e.g. dementia) as well as debilitating genetic conditions. The role of future stem cell therapy suggests that an adult’s own (autologous) cells could be derived, differentiated, and utilised for cell therapy interventions.

Reference

Tortora, G.J. and Derrickson, B. (2013). Principles of Anatomy and Physiology. Hoboken, NJ: Wiley.

Essential Physiology for Dental Students

List of Contributors

Preface

About the Companion Website

PART I
Introduction