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Category: Medical

Topic: Cellular Physiology

Level: Paramedic

Next Unit: Types of Tissue

6 minute read

Cells are similar to multicellular “social” organisms because each cell has within it components, each with specific jobs that work together to help the cell perform certain functions. Cells communicate electrochemically and coordinate specific bodily functions via hormones produced by the endocrine system, bioactive substances by the exocrine system, and chemo- and neurotransmitters that act on specific receptor sites.


The Endocrine System

The endocrine system is made up of glands that produce the hormones regulating metabolism, growth and development, tissue function, sexual function, reproduction, sleep, and mood—in short, all internal functions. They secrete their products directly into the blood, as opposed to using ducts or pores (as used by exocrine glands).

Glands in the Endocrine System include:

  • hypothalamus (makes hormones that participate in the stimulation and inhibition of other glands via positive and negative feedback loops),
  • pituitary (makes numerous hormones under the direction of the hypothalamus),
  • thyroid (makes thyroid hormone),
  • parathyroid (makes parathyroid hormone to raise calcium as part of a process involving bone),
  • adrenal (makes catecholamines and other metabolic hormones),
  • pineal (makes melatonin, which participates in pigment deposition and sleep cycle health),
  • gonads (ovaries and testes making testosterone and estrogen, and produces sperm and ova), and the
  • pancreas (secretion of insulin via "islet"--or beta cells and secretion of glucagon via alpha cells).

The Exocrine System

The exocrine system is the system that secretes substances out of the body through ducts.

Exocrine system glands include:

  • salivary glands (salivary duct--e.g., parotid duct),
  •  bile-producing glands of the liver (hepatic and bile ducts),
  • prostate gland (prostatic duct and urethra),
  • gastric and intestinal glands (mucus, secreted into the lumen),
  • apocrine (sweat, via openings onto the skin),
  • sebaceous glands (sebum, via openings onto the skin),
  • lacrimal gland (tears, via lacrimal ducts),
  • mammary glands (milk, via mammary ducts),
  • ceruminous (ear wax into the external ear canal),
  • pancreas (digestive enzymes, via pancreatic ducts), and
  • others.

NOTE: The pancreas is both an endocrine (insulin, glucagon) and an exocrine (digestive enzymes) gland. 



  • CHEMORECEPTORS: respond to chemical stimuli (high CO2 or low O2),
  • BARORECEPTORS:  respond to changes in pressure,
  • ADRENERGIC RECEPTORS: respond to catecholamines (epinephrine and norepinephrine), and
  • other receptors with more specific triggers and jobs, such as interactions between neurotransmitters and their respective neuroreceptors.