Hemorrhoids: This is the most common cause of painless, bright red bleeding during defecation. Hemorrhoids are swollen veins in the lowest part of the rectum or anus, and they can bleed when strained during bowel movements.

Anal Fissure: Although typically painful, some anal fissures may present with painless bleeding, especially if the fissure has become chronic and less painful over time.
Diverticulosis: This condition involves the formation of small, bulging pouches (diverticula) in the digestive tract. When these pouches bleed, it can lead to bright red blood in the stool.
Rectal Prolapse: A condition where the rectum loses its normal attachments inside the body, allowing it to protrude out through the anus, which can cause bleeding.

Colorectal Cancer: Although less common than other causes, colorectal cancer can present with painless rectal bleeding. It is crucial to rule out this potentially life-threatening condition.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can cause rectal bleeding, among other symptoms. Early diagnosis and treatment are vital to manage the disease and prevent complications.
Angiodysplasia: A condition characterized by the presence of abnormal blood vessels in the colon, which can cause bleeding. It's more common in older adults and can be a significant cause of gastrointestinal bleeding.

Rectal Varices: Enlarged veins in the rectum, similar to varicose veins in the legs, which can bleed.
Solitary Rectal Ulcer Syndrome: A condition that can cause rectal bleeding due to ulceration of the rectal mucosa, often associated with chronic constipation or straining during bowel movements.
Vascular Malformations: Rare congenital conditions that can cause abnormal connections between blood vessels, potentially leading to bleeding.