Veterinary Anatomy and Physiology Flashcards

A 4-year-old dog presents with persistent regurgitation. Fluoroscopic contrast study reveals a focal dilation of the esophagus at the level of the heart base with normal peristalsis cranial and caudal to the lesion. Which embryological remnant is MOST likely responsible?

Answer: Persistent right aortic arch (PRAA) compressing the esophagus

A persistent right aortic arch (PRAA) is the most common vascular ring anomaly in dogs. Normally the left fourth aortic arch persists; when the right persists instead, it forms a vascular ring with the ligamentum arteriosum and pulmonary trunk that encircles and constricts the esophagus at the heart base. This produces a focal segmental dilation cranial to the constriction, with normal esophageal function elsewhere — the classic fluoroscopic pattern described.

During a rectal examination of a mature stallion, the examiner palpates an accessory sex gland with a firm, lobulated texture located dorsolateral to the bladder neck, medial to the ductus deferens ampullae. Histologically, this gland would MOST likely demonstrate which characteristic secretory epithelium?

Answer: Tall columnar epithelium with prominent secretory blebs consistent with apocrine secretion

The structure described — firm, lobulated, dorsolateral to the bladder neck, medial to the ampullae — is the seminal vesicle (vesicular gland) of the stallion. Equine vesicular glands are compound tubuloacinar glands lined by tall columnar epithelium that displays prominent apocrine secretory blebs (cytoplasmic projections released into the lumen). This apocrine morphology is a distinguishing histological feature compared to the prostatic glandular epithelium, which is more typically simple cuboidal to low columnar with merocrine secretion.

In ruminants, the reticulo-omasal orifice (ROO) plays a critical gating role in digesta flow. Which neurological mechanism is PRIMARILY responsible for the reflex relaxation of the ROO that allows passage of liquid digesta from the reticulum to the omasum?

Answer: Vagal efferent (parasympathetic) non-adrenergic, non-cholinergic (NANC) inhibitory neurons releasing VIP

Relaxation of the reticulo-omasal orifice is mediated by vagal efferent non-adrenergic, non-cholinergic (NANC) inhibitory neurons whose primary neurotransmitter is vasoactive intestinal peptide (VIP), along with nitric oxide. This vagal NANC pathway is coordinated with the biphasic reticulo-ruminal contraction cycle — the ROO relaxes during the second phase of the reticulo-ruminal contraction, allowing fluid digesta to pass. The sympathetic system generally inhibits GI motility but is not the primary gating reflex here; motilin is more relevant to the migrating myoelectric complex in monogastrics.

A feline patient undergoing adrenalectomy for a unilateral pheochromocytoma suddenly develops severe bradycardia and hypotension immediately after ligation of the adrenal vein during surgery. Which anatomical variant MOST likely explains why venous ligation in this location affected cardiac output so acutely in the cat compared to other species?

Answer: In cats, the right adrenal gland is embedded within the hepatic parenchyma, and its vein drains directly into the caudal vena cava very close to the hepatic veins

In cats (and to a lesser degree dogs), the RIGHT adrenal gland is intimately associated with the wall of the caudal vena cava — it can be partially embedded in it — and its vein drains directly into the CVC immediately adjacent to the right hepatic veins. This makes right adrenalectomy in cats particularly hazardous: ligation or manipulation in this region can acutely compromise venous return to the heart or trigger vagal reflexes via stretch of the CVC wall, causing rapid bradycardia and hemodynamic collapse. This is a well-recognized intraoperative risk in feline adrenal surgery not as pronounced in dogs or humans.

Avian species lack a diaphragm, yet achieve effective ventilation through a unique air sac system. In a bird at rest, which air sacs receive FRESH (oxygen-rich, CO2-poor) air during INSPIRATION?

Answer: Caudal thoracic and abdominal air sacs

Avian respiratory flow is unidirectional through the parabronchi across two full breathing cycles. During INSPIRATION, fresh air bypasses the lungs almost entirely and flows directly into the CAUDAL air sacs (caudal thoracic and abdominal). Simultaneously, air that was stored in the caudal sacs during the previous cycle moves through the parabronchi for gas exchange. During expiration, the now-spent air moves into the cranial air sacs and is expelled. This counter-current/cross-current system means that only the caudal sacs receive fresh inspired air — cranial sacs receive parabronchial effluent.

A 7-year-old intact male Greyhound presents with hindlimb ataxia and is found to have lumbosacral instability. MRI reveals compression at L7-S1. The clinician notes that the cauda equina at this level contains nerve roots responsible for innervating the external urethral sphincter. Which specific nerve and its spinal cord origin correctly describes the primary somatic motor supply to the external urethral sphincter in the dog?

Answer: Pudendal nerve, originating from spinal cord segments S1–S3

The external urethral sphincter is a striated (skeletal) muscle under voluntary control, and its somatic motor innervation in the dog is provided by the pudendal nerve, which arises from sacral spinal cord segments S1–S3 (occasionally including S2–S3 depending on the source). The pudendal nerve exits via the lesser sciatic foramen and supplies both the external urethral sphincter and the external anal sphincter. The pelvic nerve (parasympathetic) and hypogastric nerve (sympathetic) supply smooth muscle of the bladder wall and internal sphincter, respectively. The genitofemoral nerve is sensory/motor to the inguinal region and has no role in urethral sphincter control.