I. Gas Exchange Respiratory Surfaces Respiratory Surface: Common characteristics of respiratory surfaces: a) Moist: allows for the RAPID diffusion of dissolved gasses across its surface. Whereas the respiratory surfaces of aquatic organisms are external structures, those of land animals must be must be internalized to prevent from drying out. b) Thin-walled: facilitates the RAPID diffusion of respiratory gasses across its surface. c) Large Surface Area: maximizes contact with the respiratory medium (air or water) to promote the RAPID diffusion of gasses. d) Vascularized: necessary in complex multicellular organisms in which the majority of cells are not in direct contact with the external environment. Figure 1: Respiratory Surfaces Across Species
II. Respiratory Adaptations Insect Arthropods Tracheal System: composed of branched air tubes continuous with openings in the body wall called Spiracles. The finest branches extend to the surface of nearly every cell, where gas exchange is accomplished by diffusion. Mammalian Gas Exchange Lungs: because the respiratory surface of a lung is not in direct contact with all other parts of the body, the gap must be bridged by the circulatory system, which transports gases between the lungs & the rest of the body. Figure 2: Human Respiratory System Nasal Cavity: a) Mucus serves to moisten incoming air, facilitating its diffusion across the respiratory surface. It also traps debris suspended in air. Capillary beds just below the mucous membranes serve to warm incoming air to maintain a constant body temperature. Pharynx: a) At the top of the trachea lies a cartilaginous structure called the Larynx. Humans & other mammals use the larynx as a voice box as air is exhaled, it passes over vocal cords, causing them to vibrate & produce sound.
Respiratory Tree: a) Trachea: tube reinforced w/cartilaginous rings to keep passageway open at all times. b) Bronchi: formed as trachea branches off as it enters the lungs into 2 smaller tubes. Within the lung, the bronchi branch 23 times with each branch becoming progressively smaller & thinner. c) Bronchioles: describes the smallest, thinnest branches of the respiratory tree. d) Alveoli: grape-like clusters of air sacs found at the end of the bronchioles. In addition to being 1 cell thick, each alveolus is surrounded by a capillary bed derived from the pulmonary arterioles. Figure 3: Alveoli Lungs: a) Each lung is divided into lobes & enclosed in a double membrane called the Pleura. Each of the pleural membranes is separated by a thin fluid-filled space. b) The inner surface of the pleura adheres snugly to the lung surface, while the other is attached to the wall of the ribcage. The membranes can transfer the motion of the ribcage & diaphragm to the lungs while breathing. Diaphragm: III. Mechanics of Breathing Figure 4: Breathing Mechanics
IV. Gas Transport Oxygen Transport Respiratory Pigment: a) Hemoglobin consists of 4 subunits, each with a region that has a Fe atom at its center. Since it is the Fe that binds the O2, Hb can carry 4 molecules of O2. Hemoglobin that is bound to O2 is called Oxyhemoglobin. Carbon Dioxide Transport Hb also plays a role in CO2 transport. The methods by which CO2 is transported in the blood include: a) Carboxyhemoglobin: 23% of the CO2 in transport. CO2 does not compete with O2 for the same binding site on Hb. b) Dissolved CO2: 7% of the CO2 in transport. Reacts with water in blood plasma to form carbonic acid (H2CO3). c) Bicarbonate Ion: 70% of CO2 in transport. Reacts with water w/in the cytoplasm of RBC s to form carbonic acid (H2CO3). Carbonic acid the breaks down to form H + & HCO3 -, where the H + binds to Hb to prevent the blood from becoming too acidic. The bicarbonate ions (HCO3 - ) diffuse into the blood plasma, serving to buffer the blood (prevents ph from becoming too low, a condition that may denature blood proteins). Upon reaching the lungs, the bicarbonate ion is converted back into CO2 where it enters the alveoli & is expelled from the body. Figure 7: Carbon Dioxide Transport
V. Control of Breathing Breathing Center Breathing rate is controlled by Breathing Control Centers located in the medulla & the pons of the brain. The breathing control center in the medulla sends impulses to the diaphragm & rib muscles, stimulating them to contract & causing inhalation. The rate of breathing is controlled by sensors in blood vessels that monitor changes in blood ph that directly reflects CO 2 concentrations. When the medulla registers a slight drop in ph (increase in CO 2 ), it increases the depth & rate of breathing, & the excess CO 2 is expelled with exhaled air. Normally, a rise in CO 2 concentration is a good indicator in a fall in O 2 concentration, for CO 2 is produced by the same process (cellular respiration) that consumes O 2. Figure 8: Control of Breathing