Tissue level of organization

Tissue Types

  • Epithelial Tissue: covers body surfaces, lines hollow organs, body cavities and ducts

  • Connective Tissue: protects & supports the body & its organs, store energy reserves as fat

  • Muscle Tissue: generates physical force needed to make body structures move

  • Nervous Tissue: detects changes in internal & external environment

Source of Tissue

  • Primary germ layers:

  1. Ectoderm

  2. Mesoderm

  3. Endoderm

Epithelial Tissue

  • General Features

    • Cells arranged in continuous sheets either in single or multiple layers

    • Closely packed, held tightly together by many cell junctions

    • Avascular

    • Substances exchanged between tissues by diffusion

    • High rate of cell division due to stress, wear & tear

  • Functions

    • Covering/lining

    • Barrier

    • Protection

    • Absorption

    • Combines with nervous tissue to form specialised organs for senses

    • Excretion (myoepithelia)

    • Secretion

    • Filtration

  • Classification

    • Two types:
      1. Covering & Lining
      2. Glandular

    • Covering & Lining Epithelia classified :
      1. cell layer arrangement
      2. shape of the cells

Glandular Epithelium

  • Endocrine glands: secrete hormones into interstitial fluid which diffuses directly into the blood stream

  • Exocrine Glands: secrete products into ducts that empty onto the surface or covering and lining epithelium

  • Mixed glands: contain both endocrine and exocrine tissue

EXOCRINE GLANDS

  • Two criteria for categorizing multicellular glands:

    • Whether the ducts are branched or unbranched...

    • Simple gland the duct does not branch.

    • Compound gland the duct branches.

    • The shape of the secretory portion of the gland

    • Tubular glands have tubular secretory parts.

    • Acinar glands have rounded secretory parts.

    • Tubuloacinar glands have features of both.

FUNCTIONAL CLASSIFICATION

Type of secretion

  • Mucous

  • Serous

  • Sebaceous

  • Mixed

Secretion mechanism

  • Merocrine: Secrete by exocytosis

  • Apocrine: Secretion is pinched off cell

  • Holocrine: Secretory cell matures, ruptures and is secreted product

Epithelial Membranes

  • Mucous membranes line “interior” body surfaces open to the outside:

    • Digestive tract

    • Respiratory tract

    • Reproductive tract

  • Serous membranes line some closed internal surfaces:

    • Parietal layer next to body wall

    • Serous fluid between layers

    • Visceral layer next to organ

Covering and lining epithelium

  • Endothelium is a specialized simple squamous epithelium that lines the entire circulatory system from the heart to the smallest capillary – it is extremely important in reducing turbulence of flow of blood.

  • Mesothelium is found in serous membranes such as the pericardium, pleura, and peritoneum.

    • Unlike other epithelial tissue, both are derived from embryonic mesoderm (the middle layer of the 3 primary germ layers of the embryo).

Connective tissue

  • Embryonic Connective Tissue

  • Mature Connective Tissue

  • Cartilage

  • Bone Tissue

  • Blood Tissue

  • Lymph

Functions

  • binding

  • supporting

  • protecting

  • forming blood

  • storing fats

  • filling space

  • transport

Ground Substance

Function:

  • Transport of nutrients and waste by-products between suspended cells and circulatory system

Composition

  • H2O + glycoproteins and proteoglycans

  • Glycosaminoglycans (GAGS)

    • Hyaluronic Acid

    • Chondroitin Sulphate

    • Adhesion proteins e.g. fibronectin

Fibres

Collagen: Composed of collagen

  • Thickest CT fibres Abundant in tendons & eyeball

Elastic: Composed of elastin.

  • Form branching networks. Stretch readily.

  • Skin, ligaments & arterial walls

Reticular Fibres: Consists of collagen arranged in fine bundles and a coating of glycoprotein.

  • Abundant in reticular CT. Spleen, lymph nodes

Cells of Connective Tissues

  • Chondrocytes make the various cartilaginous C.T.

  • Adipocytes store triglycerides.

  • Osteocytes make bone.

  • White blood cells provide immunity

Loose Connective Tissue

  • Areolar Connective Tissue: most widely distributed in the body.

  • Contains several types of cells and all fibre types.

  • Used to attach skin and underlying tissues, and as a packing between glands, muscles, and nerves.

  • Adipose tissue is located in the subcutaneous layer deep to the skin and around organs and joints.

    • It reduces heat loss and serves as padding and as an energy source

  • Reticular connective tissue is a network of interlacing reticular fibers and cells.

    • It forms a scaffolding used by cells of lymphoid tissues such as the spleen and lymph nodes.

Mature Connective Tissues

  • Dense Connective Tissues

    • Dense Irregular Connective Tissue consists predominantly of
      fibroblasts and collagen fibers randomly arranged.

      • It provides strength when forces are pulling from many different directions.

    • Dense regular Connective Tissue comprise tendons, ligaments, and other strong attachments where the need for strength along one axis is mandatory (a muscle pulling on a bone).

    • Elastic Connective Tissue consists predominantly of fibroblasts and freely branching elastic fibers.

      • It allows stretching of certain tissues like the elastic arteries (the
        aorta).

BONE TISSUE

1. Functions of Bone Tissue

  • Support: Provides structural support for the body.

  • Protection: Shields vital organs (e.g., cranium, vertebrae, rib cage).

  • Movement: Serves as a mechanical lever system for body movement.

  • Mineral Homeostasis: Maintains calcium (Ca++) balance.

  • Haematopoiesis: Produces blood cells.

  • Energy Reserve: Stores triglycerides for energy.

2. Composition of Bone Tissue

  • Dynamic Nature: Continuously repaired and remodeled.

  • Cell Types:

    • Osteoblasts: Bone-forming cells that can divide.

    • Osteocytes: Mature bone cells.

    • Osteoclasts: Multinucleated cells formed from macrophages.

  • Matrix:

    • Inorganic: 67% of the matrix; primarily composed of calcium (Ca), phosphorus (P), and carbon (C).

    • Organic: 33%; mainly collagen fibers.

    • Water (H2O): Present in the matrix.

3. Classification of Bone Tissue

  • By Morphology:

    • Compact Bone: Dense and forms the outer layer.

    • Spongy Bone: Lighter and found within bone interiors.

  • By Shape:

    • Long Bones: e.g., femur, humerus.

    • Short Bones: e.g., carpals, tarsals.

    • Flat Bones: e.g., skull bones, ribs.

    • Irregular Bones: e.g., vertebrae, certain facial bones.

  • By Site of Formation:

    • Intramembranous: Directly from mesenchyme.

    • Endochondral: Pre-existing cartilage model.

4. Anatomy & Histology of Bone

  • Compact Bone: Dense bone tissue with osteons.

  • Spongy Bone: Contains trabeculae, spaces filled with red or yellow bone marrow.

  • Bone Formation & Growth: Involves processes like ossification and lengthening.

  • Bone Remodeling: The ongoing replacement of old bone tissue with new.

  • Horns: Structures found on some animals that differ from antlers, composed of bone covered by a keratin sheath.

MUSCLE TISSUE

1. General Characteristics

  • Function: Generates force for movement.

  • Types:

    • Skeletal Muscle: Voluntary, striated, multi-nucleated.

    • Smooth Muscle: Involuntary, non-striated, single nucleus.

    • Cardiac Muscle: Involuntary, striated, connected by intercalated discs.

2. Skeletal Muscle Anatomy

  • Myocyte Structure:

    • Multi-nucleated with numerous myofibrils.

    • Myofibrils: Composed of sarcomeres, which are the contractile units.

    • Sarcomere: Composed of actin (thin filament) and myosin (thick filament).

  • Muscle Fibers:

    • Slow and Fast Fibers: Differentiated by myosin ATPase activity and metabolic properties.

  • Connective Tissue:

    • Fascia: Connective tissue linking muscles, bones, and organs.

    • Endomysium, Perimysium, Epimysium: Surround individual muscle fibers, fascicles, and the entire muscle, respectively.

3. Cardiac Muscle Anatomy

  • Characteristics:

    • Cross-striations and intercalated discs for cell communication.

    • Autorhythmic: Can contract without external stimulation.

    • Adaptations: Larger mitochondria for aerobic ATP production; slow calcium (Ca++) channels.

  • Physiology:

    • Action Potentials: Initiate contraction and spread through gap junctions.

    • Modified Myofibers: Purkinje fibers conduct impulses to myocardium.

4. Smooth Muscle Anatomy

  • Types:

    • Multiunit: Individual fibers controlled by one nerve terminal.

    • Visceral/Unitary: Few myocytes innervated, connected by gap junctions, often autorhythmic.

  • Microscopic Anatomy:

    • No Sarcomeres: Myofilaments are arranged differently.

    • Contraction Mechanism: Calmodulin binds Ca++, activating myosin light chain kinase for contraction.

CONNECTIVE TISSUE

1. General Functions

  • Support and Protection: Provides structural integrity and protects organs.

  • Energy Storage: Adipose tissue stores fat.

  • Transport: Blood transports nutrients and waste.

  • Repair: Connective tissues aid in tissue repair.

2. Types of Connective Tissue

  • Embryonic Connective Tissue: Includes mesenchyme.

  • Mature Connective Tissue:

    • Loose Connective Tissue:

      • Areolar: Most widespread; attaches skin to underlying tissues.

      • Adipose: Stores energy, insulates, and pads organs.

      • Reticular: Supports lymphoid organs.

    • Dense Connective Tissue:

      • Dense Regular: Parallel fibers found in tendons and ligaments.

      • Dense Irregular: Randomly arranged fibers; found in dermis and joint capsules.

      • Elastic: Contains elastic fibers for stretching; found in arteries.

3. Ground Substance

  • Composition: Water, glycoproteins, proteoglycans, and glycosaminoglycans (e.g., hyaluronic acid, chondroitin sulfate).

  • Function: Facilitates nutrient and waste transport between cells and circulatory system.

4. Connective Tissue Fibers

  • Collagen: Thick fibers; abundant in tendons and ligaments.

  • Elastic: Thin, stretchable fibers found in skin and arteries.

  • Reticular: Thin collagen fibers; forms scaffolding in lymphoid tissues.

5. Connective Tissue Cells

  • Chondrocytes: Produce cartilage.

  • Adipocytes: Store triglycerides.

  • Osteocytes: Maintain bone tissue.

  • White Blood Cells: Provide immunity.

NEUROGLIA & NERVOUS TISSUE

1. Peripheral Nervous System (PNS) Neuroglia

  • Schwann Cells: Form myelin sheath around PNS axons, aiding regeneration.

  • Satellite Cells: Support neurons in ganglia.

2. Myelination

  • Oligodendrocytes: Form myelin sheath in the central nervous system (CNS).

3. Neural Circuits

  • Simple Circuit: Involves sensory, interneurons, and motor neurons.

  • Electrical Signals:

    • Action Potentials: Long-distance communication.

    • Graded Potentials: Short-distance communication.

4. Muscle Proteins

  • Contractile Proteins: Myosin and actin.

  • Regulatory Proteins: Troponin and tropomyosin.

  • Structural Proteins: Titin, myomesin, nebulin, dystrophin.

Skeletal Muscle and the Sliding Filament Mechanism of Contraction

  • M Line: Myosin cross-bridges pull on thin filaments.

  • I Band: Thin filaments slide inward.

  • Z Discs: Move toward each other.

  • Sarcomeres: Shorten, causing muscle fibers and the muscle itself to contract.

  • Note: Thick and thin filaments do not change in length.

Thick Filaments (Myosin):

  • Composed of myosin, resembling two golf clubs twisted together.

  • Myosin heads (cross-bridges) extend toward thin filaments and are held in place by M line proteins.

Proteins of Muscle - Myosin:

  • Myosin Head:

    • ATP-binding sites.

    • Actin-binding sites.

    • Hinged structure allows swiveling, which contributes to muscle contraction.

Thin Filaments:

  • Composed of actin, troponin, and tropomyosin.

  • Actin: Two coiled chains.

  • Tropomyosin: Wraps around actin chains, covering the myosin-binding sites in a relaxed muscle.

  • Troponin: Positioned at the ends of tropomyosin.

Muscle Structure:

  • Sarcomere: The segment from one Z line to the next.

  • Z Lines: Anchor the thin filaments.

Neural Circuit for Muscle Contraction

  • Components: Sensory neuron, interneuron, motor neuron.

  • Communication: Electrochemical signals.

Electrical Signals in Neurons:

  • Action Potentials:

    • Communicate over both short and long distances in the body.

    • Involves rapid changes in membrane potential.

  • Graded Potentials:

    • Short-distance communication only.

Resting Membrane Potential:

  • Factors contributing to its maintenance.

Ion Channels in Neurons: Facilitate signal transmission.

Action Potential Phases:

  • Depolarization: Initial phase of the action potential.

  • Repolarization: Return to resting state.

  • All-or-Nothing Law: Action potentials only occur if the membrane potential reaches a certain threshold.

Refractory Periods:

  • Absolute Refractory Period: No new action potential can be triggered during this time.

  • Direction of Travel: Action potentials move one-way along the axon due to the downstream channels opening.

Brain Protection and Functions

  • Protection:

    • Skull: 7 mm thick.

    • Cerebrospinal Fluid: Cushions and protects.

    • Cranial Meninges: Three connective tissue layers:

      • Dura Mater.

      • Arachnoid Mater.

      • Pia Mater.

  • Blood-Brain Barrier: Restricts the passage of substances to maintain a stable environment.

Brain Functions:

  • Receive and process signals.

  • Control voluntary and involuntary muscle movement.

  • Produce and regulate hormones.

  • Facilitate cognitive processing.

Major Brain Components:

  • Brain Stem.

  • Cerebellum.

  • Diencephalon.

  • Cerebrum.

Hypothalamus:

  • Integrates autonomic nervous system and pituitary gland activities.

  • Regulates emotional and behavioral patterns, circadian rhythms, body temperature, and feeding/drinking behavior.

  • Produces oxytocin and antidiuretic hormone.

  • Maintains wakefulness and sleep patterns.

Pituitary Gland:

  • Composed of two parts:

    • Anterior Pituitary (Adenohypophysis).

    • Posterior Pituitary (Neurohypophysis).

CNS Pathologies

  • Prion Diseases:

    • Scrapie.

    • Bovine Spongiform Encephalopathy (BSE).

  • Brain Conditions:

    • Head Trauma: Causes edema.

    • Stroke.

    • Neurodegenerative Diseases:

      • Alzheimer's, Parkinson's, motor neuron disease, Huntington's (progressive neuron death).

    • Mental Illnesses:

      • Depression, schizophrenia, bipolar disorder, PTSD.

    • Infectious Diseases: Bacterial or viral.

    • Genetic Disorders:

      • Tay-Sachs, Fragile X syndrome, Down syndrome.

OLFACTION

Olfactory Epithelium

  • Location: Found in the superior nasal cavity, covering the cribriform plate and superior nasal concha.

  • Function: Contains olfactory receptors that detect airborne molecules.

Key Features

  1. Turbinate Bones:

    • Divide the nasal cavity into multiple passageways, increasing surface area.

    • Anterior Portion: Covered by a vascularized mucous membrane that acts as a heat exchanger.

    • Posterior Portion: Covered by olfactory epithelium (OE).

    • Comparison: Humans have ~5 cm² of OE with ~5 million olfactory cells; dogs have ~150 cm² and ~250 million cells.

  2. Olfactory Receptors:

    • Bipolar neurons with cilia.

    • Humans have 10–100 million olfactory receptors.

  3. Other Cells:

    • Supporting Cells: Provide structural and metabolic support.

    • Basal Cells: Stem cells that regenerate olfactory receptors.

Olfactory Pathway

  • Nerve impulses travel through:

    • Olfactory nerves → Olfactory bulbs → Olfactory tract → Primary olfactory area in the temporal lobe.

GUSTATION (Taste)

Primary Tastes

  • Five Primary Tastes: Sour, sweet, bitter, salty, and umami.

    • Umami: Stimulated by monosodium glutamate (MSG).

  • Other flavors, like chocolate or coffee, result from combinations of the five tastes plus olfactory and tactile sensations.

Taste Perception

  • Influenced by:

    • Context (e.g., prior experience, hunger, or satiety).

    • Sensory inputs such as visual cues, texture, temperature, and irritants (e.g., chili peppers).

ANATOMY OF THE EYE

Key Structures

  1. Optic Disc:

    • Entry/exit point for the optic nerve and retinal vessels.

    • Lacks photoreceptors, creating a "blind spot" that the brain compensates for.

  2. Retina:

    • Contains photoreceptor cells:

      • Rods: ~120 million per eye; highly sensitive to low light; produce low-resolution black-and-white images.

      • Cones: Function in bright light; provide high-resolution color vision (red, green, and blue types).

  3. Lens:

    • Avascular, transparent structure located posterior to the pupil.

    • Fine-tunes the focusing of light on the retina.

  4. Eye Cavities and Chambers:

    • Anterior Cavity: Divided into anterior and posterior chambers, both filled with aqueous humor.

    • Posterior Cavity: Contains the vitreous body, which holds the retina flush against the choroid for clear image reception.

Fluid Dynamics

  • Aqueous Humor:

    • Produced by the ciliary body.

    • Flows through the posterior chamber → Pupil → Anterior chamber → Reabsorbed into the scleral venous sinus.

VISUAL TRANSDUCTION

Process

  • Photopigments in rods and cones convert light into neural signals.

  • Photopigments:

    • Contain opsin (glycoprotein) and retinal (vitamin A derivative).

  • Adaptation to light/dark:

    • Light adaptation: Quick (seconds).

    • Dark adaptation: Slower (up to 40 minutes).

The Visual Pathway

  1. Graded potentials generated by photoreceptors are processed by retinal neurons (e.g., horizontal and bipolar cells).

  2. Axons of retinal ganglion cells form the optic nerve, exiting the retina at the vitreous surface.

  3. At the optic chiasm, some axons cross to the opposite side while others remain uncrossed.

  4. Axons continue as optic tracts, with most terminating in the thalamus.

    • From the thalamus, neurons project to the primary visual cortex in the occipital lobe.

The Auditory Pathway

  • Nerve Impulses: Travel via CN VIII to the medulla, pons, midbrain, and thalamus, then to the primary auditory cortex in the temporal lobe.

  • Sound Localization: Timing differences in nerve impulses from both ears at the superior olivary nuclei in the pons help determine the source of sound.

Equilibrium

  • Controlled by the Vestibular Apparatus:

    • Components:

      • Saccule and utricle of the vestibule.

      • Three semicircular canals.

  • Types of Equilibrium:

    • Static Equilibrium: Balance relative to gravity.

    • Dynamic Equilibrium: Balance during sudden movements.

The Inner Ear: Vestibular Apparatus

  • Semicircular Canals:

    • Located above the vestibule; each ends in an ampulla (involved in dynamic equilibrium).

  • Vestibule:

    • The central part of the bony labyrinth.

    • Contains the membranous labyrinth with the utricle and saccule (involved in static equilibrium).

Static Equilibrium

  • Sensory Hairs: Located within the macula of the utricle and saccule.

  • Otolithic Membrane:

    • Contains dense calcium carbonate crystals called otoliths.

    • Responds to gravity; changes in head position move the otoliths, opening transduction channels in hair cells.

  • Nerve Impulses: Local potentials generated from hair cell stimulation sum up to form an action potential (AP).

Dynamic Equilibrium

  • Sensory Hairs: Found in the ampulla of the semicircular canals.

  • Crista: A small elevation in each ampulla containing hair cells and supporting cells.

  • Cupula: Gelatinous material covering the hair cells in the crista.

  • Endolymph Movement:

    • With head movement, the endolymph lags behind the cupula, creating inertial forces that bend the hair bundles and generate nerve impulses.

The Urinary System

  • Components:

    • 2 Kidneys

    • 2 Ureters

    • Bladder

    • Urethra

    • Associated nerves and blood vessels

Function of the Kidneys

Kidneys help maintain homeostasis of body fluids by:

  • Regulating blood ionic composition

  • Maintaining pH balance

  • Controlling blood volume

  • Regulating blood pressure

  • Maintaining blood osmolarity

  • Producing hormones

  • Regulating blood glucose levels

  • Excreting wastes and foreign substances

Renal Anatomy

  • External Structures:

    • Renal capsule: Protective covering of the kidney.

    • Renal hilum: Entry/exit point for blood vessels and ureter.

  • Internal Structures:

    • Renal cortex: Outer layer of the kidney.

    • Renal medulla: Inner region containing renal pyramids.

    • Renal columns: Tissue between the pyramids.

    • Renal pyramid: Cone-shaped structures in the medulla.

    • Renal papilla: Tip of the pyramid where urine drains.

  • Nephron: The functional unit of the kidney.

  • Collecting duct: Collects urine from multiple nephrons.

Path of Urine Drainage

  1. Minor calyx

  2. Major calyx

  3. Renal pelvis

  4. Ureter

  5. Urinary bladder

Cardiovascular System

Propagation of Action Potentials Through the Heart

  • Autorhythmic Cells:

    • Conduct action potentials throughout the heart.

    • Impulse is delayed at the atrioventricular (AV) node, then carried to the ventricles.

    • Ensures atria depolarize and contract before the ventricles.

Electrocardiogram (ECG)

  • A recording of the electrical activity of the heart, not its contractions.

  • Electrical activity is a good indicator of heart pumping efficiency.

Key Features

  1. Initiates contractions by spreading impulses.

  2. Indicates heart rate (HR) and rhythm.

Sources: Sjaastad, Hove & Sand (2003); Tortora & Derrickson (2005); Campbell et al. (1999).

Regulation of Heart Rate (HR)

  • SA Node: Pacemaker of the heart.

  • HR Increases:

    • Stimulated by sympathetic nerves and hormones (e.g., epinephrine).

    • Accelerates SA node membrane potential to threshold.

  • HR Decreases:

    • Mediated by the parasympathetic nervous system.

Cardiac Output (CO)

  • Definition: Volume of blood pumped by each ventricle per minute.

    • Formula: CO = Stroke Volume (SV) × Heart Rate (HR).

    • Stroke Volume: Blood pumped per beat = End Diastolic Volume (EDV) – End Systolic Volume (ESV).

Effect of Exercise

  • Rest vs. Exercise in Horses:

    • Resting HR: 40 bpm → Exercise HR: 220 bpm.

    • Resting CO: 30 L/min → Exercise CO: 220 L/min.

  • Stroke Volume increases during strenuous exercise.

  • Fitter animals exhibit:

    • Lower HR.

    • Higher SV.

Blood Pressure (BP)

Arterial Function

  • Systole: Heart contracts, increasing arterial pressure; arteries expand.

  • Diastole: Heart relaxes; arteries recoil, maintaining blood flow.

  • Mean Arterial Pressure (MAP): Function of CO and total peripheral resistance (TPR).

Regulation of Blood Pressure

  1. Neural and Hormonal Regulation:

    • Sympathetic activity → Arteriole constriction → Increased resistance → Higher BP.

  2. Baroreceptors:

    • Located in the carotid sinus and aorta.

    • Detect arterial wall stretch (BP changes).

    • Relay information to the medulla, leading to:

      • Increased sympathetic activity.

      • Decreased parasympathetic activity.

      • Elevated HR, SV, and peripheral resistance.

  3. Response to BP Decrease:

    • Reduced stretch in arterial walls detected by baroreceptors.

    • Leads to:

      • Increased HR and SV.

      • Vasoconstriction of arterioles and veins.

      • BP restoration.

Species-Specific Adaptations

Birds

  • High BP minimizes circulatory effects from rapid accelerations during flight.

Giraffes

  • Complex BP regulation between carotid and aortic baroreceptors compensates for large height differences between the heart and brain.

Respiratory System

Partial Pressures of O₂ and CO₂

  • Partial Pressure: Drives gas exchange in the lungs and tissues.

  • Sources: Sjaastad, Hove & Sand, Physiology of Domestic Animals.

Transport of Oxygen (O₂)

  • In Blood:

    1. Dissolved: ~1.5% at rest.

    2. Bound to Hemoglobin (Hb):

      • 1 Hb molecule can bind up to 4 O₂ molecules.

      • Binding is weak (iron is not oxidized).

  • Factors Affecting Hb-O₂ Binding:

    • pO₂: Higher pO₂ → Increased binding.

    • pH: Lower pH → Decreased binding (Bohr effect).

    • Temperature: Higher temperature → Decreased binding.

    • Type of Hb: Fetal Hb has higher affinity than adult Hb.

    • Carbon Monoxide (CO): Reduces Hb's ability to carry O₂.

Transport of Carbon Dioxide (CO₂)

  • In Blood:

    1. Dissolved in Plasma: ~7% (CO₂ is 20x more soluble than O₂).

    2. Bound to Proteins: ~23% (mainly Hb, binds to amino groups).

    3. As Bicarbonate (HCO₃⁻): ~70%

      • Enzyme: Carbonic anhydrase.

      • Reaction:
        CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺

Sources: Sjaastad, Hove & Sand, Physiology of Domestic Animals.

Regulation of Respiration (Ventilation)

  • Mechanism:

    • Controlled by the respiratory center in the medulla.

    • Inspiration: Stimulated by nerve impulses.

    • Expiration: Passive (elastic recoil of lungs and thoracic cage); active during exercise.

  • Chemoreceptors:

    • Central (Medulla): Sensitive to arterial CO₂ levels.

    • Peripheral: Found in carotid bodies and aortic arch.

      • Detect CO₂, O₂, and H⁺ in arterial blood.

  • Stretch Receptors: Prevent overinflation of lungs.

Sources: Trotora & Derrickson, Principles of Anatomy & Physiology.

Respiration in Birds

Gas Exchange

  • More efficient than mammals:

    • Unidirectional airflow through lungs during both inspiration and expiration.

    • Easier to saturate Hb at lower atmospheric pO₂, enabling high-altitude flight.

Avian Respiratory Anatomy

  • Lungs: Small, rigid, and constant in volume (site of gas exchange).

  • Air Sacs:

    • Nine thin-walled sacs (10x the volume of lungs).

    • No direct gas exchange.

    • Cranial and caudal sacs facilitate airflow.

  • No Diaphragm: Ventilation driven by respiratory muscle contractions.

Ventilation Cycle

  • Inspiration:

    • Air enters caudal sacs (from atmosphere).

    • Air enters cranial sacs (from lungs).

  • Expiration:

    • Air moves into lungs (from caudal sacs).

    • Air exits into atmosphere (from cranial sacs).

  • Complete Cycle: Takes two respiratory cycles to move a single breath through the system.

Other Roles of Air Sacs in Birds

  • Provide structural support: Wrap around organs, muscles, and skeleton.

  • Functions:

    1. Vocalization.

    2. Thermoregulation.

    3. Protection from trauma.

    4. Visual displays.

    5. Energy efficiency: Reduce muscle effort during gliding and soaring.