Overview of the Respiratory Control System Breathing is one of those things in life that you almost never think about until something goes wrong with it. However, those with pulmonary disease become intensely aware of breathing, as do people who overexert themselves, especially at high altitude. The feeling of dyspnea that they experience (see p. 701 ) is one of the most unpleasant sensations in life…
Although diffusion is at the very heart of the gas exchange, as we discussed in Chapter 30 , two other parameters are also extremely important. Ventilation and perfusion—both of which require energy—are critical because they set up the partial-pressure gradients along which O 2 and CO 2 diffuse. Ventilation is the convective movement of air that exchanges gases between the atmosphere and the alveoli. In Chapter…
The complex anatomy of the pulmonary tree, the mechanics of the respiratory system, and the sophisticated carriage mechanisms for O 2 and CO 2 combine to serve two essential purposes: the ready diffusion of O 2 from the alveoli to the pulmonary-capillary blood, and the movement of CO 2 in the opposite direction. In this chapter, we consider principles that govern these diffusive events and factors…
Carriage of O 2 The amount of O 2 dissolved in blood is far too small to meet the metabolic demands of the body Blood carries oxygen in two forms. More than 98% of the O 2 normally binds to hemoglobin within the erythrocytes, also known as red blood cells or RBCs (see pp. 434–435 ). A tiny fraction physically dissolves in the aqueous phases of…
Acid-base physiology is really the study of the proton, or hydrogen ion (H + ). Although they are present in exceedingly low concentrations in most intracellular and extracellular fluids, protons nevertheless have a major impact on biochemical reactions and on a variety of physiological processes that are critical for the homeostasis of the entire body and individual cells. Not surprisingly, the body has evolved sophisticated systems…
The field of pulmonary mechanics —the physics of the lungs, airways, and chest wall—deals with how the body moves air in and out of the lungs, producing a change in lung volume (V L ). When we examine these mechanical properties while no air is flowing, we are studying static properties. The situation becomes more complicated under dynamic conditions, when the lungs are changing volume and…
Comparative Physiology of Respiration External respiration is the exchange of O 2 and CO 2 between the atmosphere and the mitochondria For millennia, people have regarded breathing as being synonymous with life. Life begins and ends with breathing. The Bible states that God “breathed into [Adam's] nostrils the breath of life” and then later used part of Adam's ventilatory apparatus—a rib—to give life to Eve. In…
In the preceding chapters, we examined cardiovascular regulation at several different levels. Powerful systemic mechanisms operate over both the short term and the long term to control mean arterial pressure and cardiac output. Operating independently of these are local mechanisms of control that regulate blood flow at the microcirculatory level. In addition, individual organs have their own unique tools for managing specific circulatory requirements. In this…
In the preceding chapters, we considered blood flow to peripheral capillary beds as if the “periphery” were a single entity. In this chapter, we break that entity down into some of its component parts. Because each organ in the body has its own unique set of requirements, special circulations within each organ have evolved with their own particular features and regulatory mechanisms. Especially for times of…
When faced with a patient who appears seriously ill, clinicians focus their immediate attention on the patient's vital signs: temperature, respiratory rate, pulse, and blood pressure. These parameters are aptly named vital because they reflect the most fundamental aspects of health and even survival; a significant abnormality in any of these components indicates that emergent care is required. In this chapter, we focus on blood pressure,…
The Cardiac Cycle The sequence of mechanical and electrical events that repeats with every heartbeat is called the cardiac cycle. The duration of the cardiac cycle is the reciprocal of the heart rate: For example, for a heart rate of 75 beats/min, the cardiac cycle lasts 0.8 s or 800 ms. The closing and opening of the cardiac valves define four phases of the cardiac cycle The cardiac…
Different cardiac cells serve different and very specialized functions, but all are electrically active. The heart's electrical signal normally originates in a group of cells high in the right atrium that depolarize spontaneously; it then spreads throughout the heart from cell to cell ( Fig. 21-1 ). As this action potential propagates through the heart—sometimes carried by cells that form specialized conducting pathways and sometimes by…
The microcirculation serves both nutritional and non-nutritional roles The primary function of the cardiovascular system is to maintain a suitable environment for the tissues. The microcirculation is the “business end” of the system. The capillary is the principal site for exchange of gases, water, nutrients, and waste products. In most tissues, capillary flow exclusively serves these nutritional needs. In a few tissues, however, a large portion…
Arterial Distribution and Venous Collection Systems Hemodynamics is the study of the physical laws of blood circulation. It therefore addresses the properties of both the “content” (i.e., blood) and the “container” (i.e., blood vessels). In Chapter 18 , we discussed the properties of the blood. This chapter is primarily concerned with the properties of the blood vessels. The circulation is not a system of rigid tubes.…
Blood is a complex fluid consisting of plasma—extracellular fluid rich in proteins—and of formed elements—red blood cells (RBCs), white blood cells (WBCs), and platelets. Total blood volume is ~70 mL/kg body weight in the adult woman and ~80 mL/kg body weight in the adult man (see Table 5-1 ). Blood Composition Whole blood is a suspension of cellular elements in plasma If you spin down a sample of…
Elements of the Cardiovascular System The circulation is an evolutionary consequence of body size Isolated single cells and small organisms do not have a circulatory system. They can meet their metabolic needs by the simple processes of diffusion and convection of solutes from the external to the internal milieu ( Fig. 17-1 A ). The requirement for a circulatory system is an evolutionary consequence of the…
Elements of Neural Circuits Neural circuits process sensory information, generate motor output, and create spontaneous activity A neuron never works alone. Even in the most primitive nervous systems, all neurons participate in synaptically interconnected networks called circuits. In some hydrozoans (small jellyfish), the major neurons lack specialization and are multifunctional. They serve simultaneously as photodetectors, pattern generators for swimming rhythms, and motor neurons. Groups of these…
Sensory receptors convert environmental energy into neural signals Sensation is a cognitive process that requires the full powers of the central nervous system (CNS). Sensation begins with the sensory receptors that actually interface with the world, and these receptors use energy from the environment to trigger electrochemical signals that can be transmitted to the brain—a process called sensory transduction. An understanding of transduction processes is crucial…
When we are awake, we are constantly aware of sensory input from our external environment, and we consciously plan how to react to it. When we are asleep, the nervous system has a variety of mechanisms to dissociate cortical function from sensory input and somatic motor output. Among these mechanisms are closing the eyes, blocking the transmission of sensory impulses to the cortex as they pass…
After meticulous study of spinal reflexes, Charles Sherrington N10-2 deduced that neurons somehow communicate information, one to the next, by a mechanism that is fundamentally different from the way that they conduct signals along their axons. Sherrington had merged his physiological conclusions with the anatomical observations ( Fig. 13-1 ) of his contemporary, the preeminent neuroanatomist Santiago Ramón y Cajal. N10-1 Ramón y Cajal had proposed…