23.8 Breathing Rate and Homeostasis PDF

Page 939 23.8 Breathing Rate and Homeostasis The respiratory center stimulates breathing to occur at a rate of 12 to 15 times per minute and a tidal volume depth of 500 mL at rest (see section 23.5c). The respiratory center adjusts the rate and depth of breathing to maintain homeostasis in response...

Page 939 23.8 Breathing Rate and Homeostasis The respiratory center stimulates breathing to occur at a rate of 12 to 15 times per minute and a tidal volume depth of 500 mL at rest (see section 23.5c). The respiratory center adjusts the rate and depth of breathing to maintain homeostasis in response to various stimuli, including changes in blood Pco2, levels of blood H+, and changes in blood Po2. Recall from section 23.5c that blood Pco2 level is the most important stimulus. Changes in breathing rate generally help to maintain homeostatic levels of respiratory gases and pH in the blood. However, changes in breathing rate may also result in homeostatic imbalances. Here we cover the changes that occur with hyperventilation, hypoventilation, and exercise, including how homeostasis may be affected. 23.8a Effects of Hyperventilation and Hypoventilation on Cardiovascular Function LEARNING OBJECTIVES 48. Explain how hyperventilation and hypoventilation influence the chemical composition of blood. 49. Describe how breathing rate and depth affect venous return of blood and lymph. Hyperventilation is a breathing rate or depth that is increased above the body’s demand. Hyperventilation may be caused by anxiety, panic, or ascending to a high altitude (as the individual breathes faster to compensate for the lower oxygen levels). Hyperventilation may also be performed voluntarily when you consciously inspire and expire excessively at an accelerated rate. Po2 levels increase and Pco2 levels decrease in the alveoli. One significant result is that additional carbon dioxide leaves the blood to enter the alveoli due to a steeper Pco2 gradient. Consequently, arterial blood Pco2 decreases below normal levels, a condition called hypocapnia (hī′pō-kap′nē-ă; kapnos = smoke or vapor). Low arterial blood Pco2 causes vasoconstriction of systemic blood vessels. The blood vessels within the brain are especially vulnerable to these changes. Ironically, one result of hyperventilation is decreased oxygen delivery to the brain due to this generalized vasoconstriction. Low blood Pco2 may also result in a decrease in blood [H+] (an increase in blood pH) because, as CO2 decreases, this equation is driven to the left ( ) and H+ levels decrease. If the body’s buffering capacity is exceeded, this may result in respiratory alkalosis, a condition discussed in more detail in section 25.6b. Symptoms that accompany hyperventilation may include feeling faint or dizzy, numbness, tingling of the mouth and fingertips, muscular cramps, and tetany. If hyperventilation is prolonged, it can cause disorientation, loss of consciousness, coma, and possibly death. If the cause is panic, breathing rate typically returns to a normal level once the individual loses consciousness. A person who is hyperventilating is sometimes directed to breathe into and out of a paper bag, because this action is thought to slow the loss of CO2. Hypoventilation is breathing that is either too slow (called bradypnea, brad-ip-nē′ă) or too shallow (called hypopnea, hī-pop'nē-ă) to adequately meet the metabolic needs of the body. Causes of hypoventilation are varied and include airway obstruction, pneumonia, brainstem injury, obesity (which restricts lung expansion), and any other condition that interferes with pulmonary ventilation or pulmonary gas exchange. Oxygen levels decrease and carbon dioxide levels increase in the alveoli. This results in smaller partial pressure gradients between the alveoli and blood for both O2 and CO2. Thus, the diffusion of the respiratory gases is altered during pulmonary gas exchange as follows: Lower amounts of oxygen diffuse from the alveoli into the blood, and blood Po2 decreases. This is a condition called hypoxemia (hī-pok′sē′mē-ă), which may lead to low oxygen in the tissue called hypoxia (hī-pok′sē-ă). Lower amounts of carbon dioxide diffuse from the blood into the alveoli. This causes blood Pco2 to increase, a condition called hypercapnia (hī′pĕr-kap′nē-ă). Low blood oxygen levels may result in insufficient oxygen delivery to systemic cells, with a subsequent decrease in aerobic cellular respiration (see section 3.4). High blood Pco2 may result in a decrease in pH, because as CO2 increases, this equation is driven to the right ( ) and H+ levels increase. If the body’s buffering capacity is exceeded, this shift may lead to respiratory acidosis, described in more detail in section 25.6b. Symptoms resulting from insufficient blood Po2, increased blood Pco2, or both include lethargy, sleepiness, headache, polycythemia (low oxygen triggers release of erythropoietin), and cyanotic tissues in which skin color appears blue as a consequence of low oxygen saturation of hemoglobin. If hypoventilation is prolonged, it may lead to convulsions, unconsciousness, and possibly death. Hypoventilation, or even the cessation of breathing, can be performed voluntarily. However, you cannot hold your breath long enough to die. The accumulation of CO2 in the blood stimulates chemoreceptors to relay nerve signals to the respiratory center to initiate inspiration before or after loss of consciousness—but always before the brain suffers damage from lack of oxygen. WHAT DID YOU LEARN? 39 How does blood Pco2 change if an individual is hyperventilating? What happens to oxygen delivery to the brain and why? 23.8b Breathing and Exercise LEARNING OBJECTIVE 50. Explain the changes in breathing that accompany exercise. A number of observations have been made regarding breathing and exercise, although not all aspects are fully understood. INTEGRATE CONCEPT CONNECTION Breathing rate also influences venous return of blood and lymph, as was described, respectively, for the cardiovascular system (see section 20.5a) and lymphatic system (see section 21.1b). The contraction and relaxation of the skeletal muscles of breathing cause rhythmic pressure changes during breathing, and this cycle is referred to as the respiratory pump. The action of the respiratory pump increases during hyperventilation, increasing venous return of blood and lymph, whereas during hypoventilation, decreased action of the respiratory pump decreases venous return of blood and lymph. Page 940 Page 941 INTEGRATE CONCEPT OVERVIEW Figure 23.35 The Movement of Oxygen and Carbon Dioxide. Oxygen moves from the atmosphere to systemic cells, and carbon dioxide moves from systemic cells to the atmosphere, through the processes of respiration. These movements are occurring simultaneously. Watch Video: Gas Exchange During Respiration Page 942 While participating in vigorous exercise, a person’s breathing depth increases while the breathing rate remains the same. This type of breathing, which is deeper, but not faster, is referred to as hyperpnea (hī′pĕr-nē-ă). Hyperpnea can be differentiated from hyperventilation in that hyperventilation is a condition where both breathing depth and rate are increased beyond the body’s demands. Both oxygen consumption and carbon dioxide production increase in response to elevated rates of cellular respiration during exercise. However, blood Po2 and Pco2 levels remain relatively the same. This occurs because deeper breathing, increased cardiac output, and increased blood flow are able to deliver the additional oxygen needed and eliminate the greater amount of carbon dioxide produced (i.e., supply increases to meet demand). Thus, blood Po2 and Pco2 levels are not thought to be the stimuli that cause breathing to change because they remain relatively constant during exercise. Although questions remain as to the exact cause, stimulation of the respiratory center generally is thought to occur from one or more of the following causes: Sensory signals relayed from proprioceptors in muscles, tendons, and joints in response to movement Motor output originating in the cerebral cortex that initiates muscular movement during exercise, simultaneously relaying signals to the respiratory center The conscious anticipation of participating in exercise WHAT DO YOU THINK? 8 Does exercise increase or decrease venous return of blood and lymph? Will the change in blood return affect heart rate? Why or why not? WHAT DID YOU LEARN? 40 How do blood Po2 and Pco2 change during exercise? Explain. 41 What are three possible reasons that breathing changes during exercise?