Overview
The aorta leaves the left ventricle and carries oxygenated blood to the whole body; the caudal vena cava returns blood from the abdomen, pelvis and pelvic limbs to the right atrium. Both run close to the vertebral column but part company at the level of the liver and the heart.
Aorta
- Aortic arch: curves dorsally and caudally from the base of the heart. As in ruminants, it gives a single branch, the brachiocephalic trunk, which gives off both subclavian arteries and the bicarotid trunk.
- Thoracic aorta: runs in the mediastinum, ventral to the thoracic vertebrae and slightly left, and gives the intercostal arteries. It enters the abdomen through the aortic hiatus of the diaphragm.
- Abdominal aorta: ventral to the lumbar vertebrae; it gives the coeliac artery (stomach, liver, spleen), the cranial mesenteric artery (small intestine, caecum and ascending colon), the renal arteries, the caudal mesenteric and the gonadal arteries. It ends at the pelvic inlet by dividing into the external and internal iliac arteries.
- Uterine artery: in the mare it arises from the external iliac artery and runs in the broad ligament.
Caudal vena cava
It forms at the pelvic inlet from the union of the common iliac veins and runs along the abdominal roof to the right of the aorta. Cranially it is embedded in the dorsal border of the liver, where it receives the hepatic veins, and passes through the diaphragm at the caval foramen to the right atrium.
Clinical relevance
- Verminous arteritis: Strongylus vulgaris larvae migrate in the wall of the cranial mesenteric artery and its branches, causing arteritis and thrombi. Emboli lead to intestinal infarction and thromboembolic colic. It is less common today thanks to deworming.
- Uterine artery rupture: in older mares, at or shortly after foaling, it causes haemorrhage into the broad ligament or the abdomen, with colic, pallor and sometimes sudden death.
- Aortoiliac thrombosis: a thrombus at the aortic termination causes pelvic limb lameness or weakness that appears with exercise; the arterial pulse is reduced and transrectal ultrasound confirms the thrombus.
- Jugular vein: in the jugular groove, it is the usual site for blood sampling and intravenous injection. In the cranial half of the neck the omohyoideus muscle separates the jugular from the common carotid artery; caudally there is no such separation and the risk of intracarotid injection, which causes immediate seizures, increases. Indwelling catheters can cause thrombophlebitis.