Most of the blood vessels in the small-tailed Han sheep maintain a high degree of stability once they are fully developed. The changes caused by aging are slow and almost imperceptible. Only in the case of disease or trauma can the inherent regeneration ability of blood vessels be aroused. However, the blood vessels of the endometrium are different from the general ones. They replace stability with a high degree of variability, which is more obvious on sheep B-ultrasound. There are two types of this variability: one is the increase and decrease of blood vessels that occur with the regularly repeated estrus cycle; the other is the irregular increase and decrease of blood vessels due to pregnancy. The adaptability of blood vessels to this necessary procedure determines the different results of normal physiology and clinical pathology: and the factors that produce vascular variability also control the cyclical changes of endometrial parenchyma tissue through blood vessels. Therefore, blood vessels should be regarded as a factor affecting endometrial activity, and blood vessels play a leading role in uterine physiology. It is also important to observe the changes of blood vessels through sheep B-ultrasound.

One or several branches of the uterine artery and ovarian artery penetrate the outer edge of the uterine wall at an oblique angle and reach the middle layer of the uterine myometrium. The arched arterial crown is formed at this branch. Radial arteries emanate from the crown of the arcuate artery, and they penetrate the endometrium in a perpendicular course to the uterine cavity. Just after passing through the junction of the myometrium and the endometrium, each radial artery is divided into the basilar artery and the endometrial spiral artery, which is also more obvious when observed on sheep B-ultrasound. The basilar artery supplies the basal layer of the endometrium, which is not affected by hormonal stimulation; the spiral artery supplies the functional layer of the endometrium and is extremely sensitive to the influence of hormones. The endometrial spiral artery bears most of the functional activities of the endometrium. During pregnancy, the uterine blood vessels become thicker and more branched, especially the middle uterine artery and the uterine branch of the vaginal artery, which is also obvious when observed on sheep B-ultrasound. As the blood vessels become thicker, the wrinkles of the arterial intima increase and thicken, and the connection with the myometrium is loose, so when the blood flows, it changes from the original clear pulsation to intermittent and inconspicuous tremor, which is called the pregnancy pulse.

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