Human Sensory Receptors and Endocrine System Functions

Chemoreceptors

Detect chemicals (taste, smell, CO₂, O₂ levels).

Thermoreceptors

Detect temperature changes (heat or cold).

Mechanoreceptors

Detect touch, pressure, vibration, and stretch.

Photoreceptors

Detect light (found in retina—rods and cones).

Pain (Nociceptors)

Detect tissue damage or potentially harmful stimuli.

Neuronal Adaptation

The gradual decrease in sensitivity to a constant stimulus.

Free Nerve Endings

Detect pain, temperature, and crude touch.

Meissner's Corpuscles

Detect light touch and texture (most concentrated in fingertips and lips).

Pacinian Corpuscles

Detect deep pressure and vibration.

Focusing from Near → Far Objects

The ciliary muscles relax, the lens flattens, and light is focused on the retina.

Fovea (Fovea Centralis)

A small central pit in the retina with the highest density of cone cells; provides sharp central vision.

Rods

Detect dim light (black, white, gray); responsible for night and peripheral vision.

Cones

Detect color (red, green, blue) and function best in bright light.

Tympanic Membrane (Eardrum)

Converts sound waves in the air into mechanical vibrations.

Malleus, Incus, and Stapes (Ossicles)

Transmit and amplify vibrations from the eardrum to the inner ear (cochlea).

Cochlea

Converts sound vibrations into electrical impulses that travel through the auditory nerve to the brain.

Semicircular Canals

Detect rotational (dynamic) movements of the head for balance.

Stretch Receptors

Detect stretching of muscles and tendons, helping maintain posture and muscle tone.

Static Equilibrium

Sensing body position relative to gravity (controlled by the vestibule—utricle and saccule).

Dynamic Equilibrium

Detects rotation or motion (controlled by semicircular canals).

Taste Receptors

Located in taste buds on the tongue (mostly papillae).

Hormone

A chemical messenger secreted by endocrine glands into the bloodstream.

Job of Hormones

Regulate metabolism, growth, reproduction, and homeostasis.

Hormone Release Mechanism

Secreted by endocrine cells directly into the blood in response to signals (neural, hormonal, or humoral).

Exocrine Gland

Releases substances through ducts to body surfaces or cavities (e.g., sweat, saliva).

Endocrine Glands

Ductless glands that release hormones directly into the blood (e.g., thyroid, adrenal).

Three Benefits of Hormones

Long-lasting effects, widespread influence, require only small amounts for major changes.

Receptors for Lipid-Soluble Hormones

Inside the cell (cytoplasm or nucleus).

Receptors for Water-Soluble Hormones

On the cell membrane surface.

Negative Feedback

A system that reverses a change to maintain homeostasis.

Hypothalamus

Links the nervous and endocrine systems; controls the pituitary gland.

Oxytocin

Stimulates uterine contractions and milk ejection during breastfeeding.

Antidiuretic Hormone (ADH)

Reduces water loss by increasing kidney water reabsorption.

Thyroxine (T₄)

Released by the thyroid to increase metabolic rate when metabolism is too low.

Parathyroid Hormone (PTH)

Released by the parathyroid glands to increase blood calcium when calcium is too low.

Cortisol

Released by the adrenal cortex; increases blood glucose and suppresses inflammation.

Epinephrine (adrenaline)

Released by the adrenal medulla for 'fight-or-flight' response.

Norepinephrine

Released by the adrenal medulla for 'fight-or-flight' response.

Insulin

Lowers blood glucose by helping cells take in sugar.

Glucagon

Raises blood glucose by stimulating glycogen breakdown in the liver.

Erythrocyte (Red Blood Cell)

Transports oxygen and carbon dioxide via hemoglobin.

Blood as an Organ

Made up of several tissue types working together (cells + plasma).

Blood as a Suspension

Blood cells are suspended in plasma and can settle out if left undisturbed.

Erythropoietin (EPO)

Hormone from kidneys that stimulates red blood cell production.

Neutrophils

Most common WBC; phagocytize bacteria ('first responders').

Eosinophils

Attack parasites and reduce allergic reactions.

Basophils

Release histamine and heparin during inflammation/allergies.

Monocytes

Become macrophages; engulf pathogens and dead cells.

Lymphocytes

Include B and T cells; part of immune response (antibody production and defense).

Platelets

Cell fragments that help blood clot (hemostasis).

Hematopoietic Stem Cells

Found in bone marrow; give rise to all blood cells.

Thrombus

A blood clot that forms in an unbroken vessel.

Embolus

A traveling clot that can block vessels elsewhere.

Agglutination

Clumping of red blood cells when antibodies bind to foreign antigens.

Antibody

Protein made by B cells that targets specific antigens.

Antigen

A molecule (often on cell surfaces) that triggers an immune response.

Type B Antigen

B antigen present on Type B RBCs.

Type B Antibody

Anti-A antibody present in Type B blood.

Universal Recipient

Type AB (has A and B antigens, no antibodies).

Universal Donor

Type O (no A or B antigens on RBCs).

Rh- Mother and Rh+ Fetus Problem

Mother's immune system may form anti-Rh antibodies, which can attack the red blood cells of an Rh+ baby during later pregnancies (hemolytic disease of the newborn).