Topic: Endocrine Glands and Hormones

Introduction

• Endocrine glands are duct­less glands, mean­ing they lack ducts.
• Their secre­tions are called hor­mones.

Classical vs. Modern Definition of Hormones

Key Points about the Modern Definition

• Hor­mones are non-nutri­ent chem­i­cals.
• They act as inter­cel­lu­lar mes­sen­gers, facil­i­tat­ing com­mu­ni­ca­tion between cells.
• They are pro­duced in trace amounts, high­light­ing their poten­cy.
• The mod­ern def­i­n­i­tion expands the scope of hor­mones beyond those secret­ed by orga­nized endocrine glands. Many oth­er tis­sues also pro­duce sig­nal­ing mol­e­cules that fit the mod­ern def­i­n­i­tion of a hor­mone.

Endocrine Systems in Different Organisms

• Inver­te­brates: Pos­sess sim­ple endocrine sys­tems with rel­a­tive­ly few hor­mones.
• Ver­te­brates: Exhib­it com­plex endocrine sys­tems with a large num­ber of diverse chem­i­cal mes­sen­gers act­ing as hor­mones, pro­vid­ing intri­cate coor­di­na­tion of phys­i­o­log­i­cal process­es.