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Chemical receptors in blood vessels (the carotid and aortic bodies located in the carotid artery and aorta, respectively) and in the brain stem constantly monitor various physical and chemical characteristics of the blood. Three characteristics important to the control of the breathing process are (1) the CO2 content, (2) the pH, and (3) the O2 content of the arterial blood. If any of these varies outside preset limits, the chemical control system signals the respiratory center to modify the breathing process to bring the errant level back into balance. If the chemical control system detects a rise in the blood level of CO2 and a decrease in the blood pH (blood becomes acidic), it signals the respiratory center to increase the rate and depth of respiration so that more CO2 can be eliminated from the lungs. If the CO2 level falls too low, which is usually accompanied by a rise in the blood pH level, the opposite occurs (that is, respiration is decreased to allow the CO2 level to rise back into the normal range).
The effects of variations in the blood O2 level are not as clear cut as the CO2 effects. If a slight decrease in the blood O2 level (hypoxia) occurs, the chemical control system signals the respiratory center to increase the rate and depth of breathing so that more O2 will be taken in. If, however, the blood O2 level drops below a critical level, the neurons of the respiratory center can become so depressed from the hypoxia (lack of oxygen) that they cannot send adequate nerve impulses to the respiratory muscles. This can cause breathing to decrease or stop completely.
A cough is a protective reflex stimulated by irritation or foreign matter in the trachea or bronchi. Moist coughs, also known as productive coughs, help an animal clear mucus and other matter from the lower respiratory passages. They are generally beneficial to the animal, and we usually do not try to eliminate them with medications. Dry coughs, also known as nonproductive coughs, are generally not beneficial and are often treated with cough-suppressant medications.
A sneeze is similar to a cough, but the irritation originates in the nasal passages. The burst of air is directed through the nose and mouth in an effort to eliminate the irritant or irritants.
A yawn is a slow, deep breath taken through a wide-open mouth. It may be stimulated by a slight decrease in the oxygen level of the blood, or it may result from boredom, drowsiness, or fatigue. Yawns can even occur in humans by the power of suggestion, such as seeing someone else yawn or even thinking about yawning. (Did you just yawn?)
A sigh is a slightly deeper-than-normal breath. It is not accompanied by a wide-open mouth like a yawn. A sigh breath may be a mild corrective action when the blood level of oxygen gets a little low or the carbon dioxide level gets a little high. It may also serve to expand the lungs more than the normal breathing pattern does. Anesthetized animals are often manually given deep sigh breaths periodically to keep their lungs well expanded. This is done to prevent the partial collapse of the lungs, which can occur in anesthetized animals as a result of respiratory system depression caused by general anesthetic drugs.
Hiccups are spasmodic contractions of the diaphragm accompanied by sudden closure of the glottis, causing the characteristic "hiccup" sound. Although hiccups can result from serious conditions such as nerve irritation, indigestion, and central nervous system damage, most of the time they are harmless and temporary and occur for unknown reasons.
The estrous cycle stages are proestrus, estrus, metestrus, and diestrus. Anestrus occurs in some animals between breeding seasons. Proestrus is the period of follicular development in the ovary. During this stage, follicles begin developing and growing. As they increase in size, the follicles' output of estrogen increases accordingly, causing many physical changes that prepare the rest of the reproductive tract for ovulation and breeding. These changes include thickening and development of the linings of the oviduct, uterus, and vagina. The epithelial lining of the vagina also begins cornifying, that is, forming a layer of tough keratin on its surface to help protect against the physical trauma of breeding that is about to come.
Estrus is the heat period, or the period of sexual receptivity in the female. It occurs when the estrogen level from the mature follicles reaches its peak. This high estrogen level causes physical and behavioral changes that signal the female's willingness to breed to the male.
Metestrus is the period after ovulation when the corpus luteum develops. The granulosa cells left in the now-empty follicle begin to multiply under continued stimulation from LH. They soon produce a solid structure, the corpus luteum (yellow body), which is about the same size as the former mature follicle. The hormone progesterone produced by the corpus luteum temporarily inhibits follicular development in the ovary, causes the lining of the uterus to get very thick in preparation for implantation of a fertilized ovum, and causes loss of the cornified epithelial lining that developed in the vagina during proestrus and estrus.
Diestrus is the active luteal stage when the corpus luteum has reached maximum size and exerts its maximum effect. If the animal is bred and becomes pregnant, the corpus luteum receives an endocrine signal from the developing embryo and is retained well into the pregnancy. If the animal is not pregnant, the corpus luteum degenerates at the end of diestrus. The animal either goes back into proestrus or the ovary shuts down and the animal goes into anestrus. Anestrus is a period of temporary ovarian inactivity seen in seasonally polyestrous, diestrous, and monoestrous animals. It is the period between breeding cycles when the ovary essentially shuts down temporarily.
The fertilized ovum is called a zygote. Immediately after fertilization, the nucleus of the spermatozoon is called the male pronucleus and the nucleus of the ovum is called the female pronucleus. Each carries the haploid chromosome number. The male and female pronuclei quickly join together to restore the diploid chromosome number and determine the unique genetic makeup of the offspring. As soon as the two pronuclei join to form a single nucleus, the zygote begins to divide rapidly by the normal process of mitosis. This rapid division is called cleavage. The single cell divides into 2 cells, which quickly divide into 4, then 8, then 16, and so on. Cleavage occurs so rapidly that the cells of the zygote do not have time to grow between divisions. The number of cells making up the zygote is increasing dramatically, but its overall size is still about the same as the original ovum, even after several days. While cleavage is taking place, the zygote slowly moves down the oviduct toward the uterus. Delicate, muscular contractions and the movements of cilia are gently propelling it along. After a few days the zygote is a solid mass of cells that looks like a tiny raspberry; this is known as the morula stage. The cells of the morula continue to divide and gradually form a hollow cavity in the center. By the time it reaches the uterus a few days later, it is formed into a hollow ball of cells with a bump on one side that eventually forms into the embryo. It is now called a blastocyst, and it is ready to implant itself in the lining of the uterus.