01 / Location & anatomical relationships
At the end of the digestive tract, below the pelvis.
02 / Structure & organization
The anal canal ends at the external opening. Internal smooth-muscle and external skeletal-muscle sphincters provide different forms of control. The lining changes along the canal, as do aspects of sensory supply and drainage.
03 / Function
Rings of muscle help control when stool passes out.
04 / Mechanism
Inner and outer rings of muscle help keep the opening closed between bowel movements. Nerve signals coordinate relaxation when stool passes. Some of this control is automatic and some is learned voluntary control.
Working together: digestive system
Teeth break food into pieces, the stomach mixes it, and the small intestine absorbs nutrients. Your blood then carries these nutrients around your body.
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Connect structure with function
The internal sphincter contributes involuntary resting tone, while external sphincter activity permits voluntary control. Pelvic-floor coordination is also important during defecation.
Check your understanding
Locate the anus in relation to its neighbors. Explain how the structural features described above support its function, then distinguish it from a nearby structure with a different role.
Keep learning
Further reading: NIDDK: Your Digestive System & How It Works.
Continue with OpenStax Anatomy and Physiology for system-level study. This atlas is educational and has not undergone independent medical peer review.