"The anorectal angle (ARA) is measured between the longitudinal axis of anal canal and the posterior rectal line, parallel to the longitudinal axis of the rectum (Fig. 1). It can be difficult to measure because the posterior wall of the rectum is often not clearly delineated and the angle becomes highly subjective. At rest, its average value is 95-96° (physiologic range, 65-100°) without noticeable differences between men and women. ARA is an indirect indicator of the puborectal muscle activity. During muscle contraction, ARA becomes more acute, while during relaxing phase it becomes obtuse.
The second important parameter for evaluation is the shift of the anorectal junction (ARJ) during straining. ARJ is the uppermost point of the anal canal. ...
... While the patient is asked to strain (Fig. 2C), the ARA increases with partial to complete loss of puborectal impression and the pelvic floor descends. The degree of caudal migration of ARJ is considered normal when less than 3.5 cm relative to the resting position.
During evacuation (Fig. 2D), wide opening of the anal canal and funneling of the anorectum are seen with near complete loss of puborectal sling impression. The ARA increases with the relaxation of anal sphincter and puborectalis muscle."
Journal of Neurogastroenterology and Motility. 2011 Oct; 17(4): 416-420.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3228983/
How to Interpret a Functional or Motility Test - Defecography
"Hemorrhoids are one of the most common reasons that patients seek consultation from a colon and rectal surgeon. ... It is thought that clinical disease develops as a result of dilation and distension of the veins along with weakening of the supporting connective tissue.
Hemorrhoids are highly vascular submucosal cushions that generally lie along the anal canal in three columns—the left lateral, right anterior, and right posterior positions. These vascular cushions are made up of elastic connective tissue and smooth muscle, but because some do not contain muscular walls, these cushions may be considered sinusoids instead of arteries or veins. Clinically evident bleeding arises from the perisinusoidal arterioles and are therefore arterial in nature. Hemorrhoids play a significant physiologic role in protecting the anal sphincter muscles and augment closure of the anal canal during moments of increased abdominal pressure (e.g., coughing, sneezing) to prevent incontinence and contribute 15 to 20% of the resting anal canal pressure. Increases in abdominal pressure increase the pressure in the inferior vena cava that cause these vascular cushions to engorge and prevent leakage. This tissue is also thought to help differentiate stool, liquid, and gas in the anal canal.
The dentate line differentiates external and internal hemorrhoids. External hemorrhoids are located below the dentate line and drain via the inferior rectal veins into the pudendal vessels and then into the internal iliac vein. These vessels are covered by anoderm that is comprised of modified squamous epithelium. As a result, these tissues contain pain fibers and affect how patients present and are treated. Internal hemorrhoids lie above the dentate line and are covered by columnar cells that have visceral innervations. These drain via the middle rectal veins into the internal iliac vessels. Internal hemorrhoids are classified further into the degree of prolapse. First-degree hemorrhoids protrude into the anal canal, but do not prolapse out of the canal. Second-degree hemorrhoids prolapse outside of the canal, but reduce spontaneously. Third-degree hemorrhoids prolapse out of the canal and require manual reduction; fourth-degree hemorrhoids are irreducible."
"Patients frequently complain of bleeding with or without defecation, a swelling, mild discomfort or irritation. Other symptoms may include soilage or mucous discharge, pruritis, difficulties with hygiene, and a sense of incomplete evacuation. Internal hemorrhoids are otherwise painless unless they are thrombosed, prolapsed with edema, or strangulated. External hemorrhoids result in pain when a thrombosis occurs and bleeding if ulceration occurs from pressure necrosis. Skin tags may form from prior acutely edematous or thrombosed external hemorrhoids."
"Pain is the primary complaint with acutely thrombosed hemorrhoids and is most often external in nature. A painful swelling suddenly appears and on examination a bluish-colored lump is noted. Patients may report straining, lifting, or prolonged sitting prior to the thrombosis, but many recall no antecedent event. Treatment is aimed at controlling the pain, but because the pain is due to edema and pressure, topical agents tend not to be helpful. The pain and edema have been shown to peak at 48 hours and subside after 4 days."
Clinics in Colon and Rectal Surgery. Mar 2011; 24(1): 5-13.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3140328/
Hemorrhoids
"The pathogenesis of symptomatic hemorrhoids is not completely understood but likely involves weakening of the anchoring connective tissue, which can then cause prolapse of internal hemorrhoids into the anal canal and protrusion of external hemorrhoids below the anal sphincter. Swelling and engorgement of the hemorrhoidal plexi occur due to factors that increase intra-abdominal pressure, such as straining, constipation, pregnancy, and prolonged sitting."
Gastroenterology & Hepatology (N Y). 2014 May; 10(5): 294-301.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4076876/
Common Anorectal Disorders