48 & 49 Gas Exchange and the Circulatory System in Animals.

48 & 49Gas Exchange and the

Circulatory System in Animals

48 Physical Processes of Respiratory Gas Exchange

• The respiratory gases are oxygen (O2) and carbon dioxide (CO2).

• Cells require O2 from the environment to produce ATP by cellular respiration.

• Cellular respiration produces CO2 as an end product, which must be lost to the environment to prevent toxic effects.

• Diffusion is the only means to exchange these gases.

48 Gas Exchange in Human Lungs

• The air pathway in humans consists of the following components:

An oral or nasal cavity, followed by the pharynx (an area for both food and air).

The larynx (voice box), which leads to the trachea.

The trachea branches into two bronchi (both of these have cartilage support).

The bronchi branch repeatedly into bronchioles, which terminate in the alveoli.

Figure 48.10 The Human Respiratory System (Part 1)


The alveoli are thin-walled air sacs and are the sites of gas exchange.


Capillary blood vessels closely surround the alveoli, resulting in a diffusion path of less than 2 mm, which is less than the diameter of a red blood cell.


• Cells lining the airways produce a sticky mucus that captures dirt and microbes.

• This mucus is cleared by cilia beating upward toward the trachea and pharynx, where it is swallowed.

• This phenomenon has been called the mucus escalator, and it can be immobilized by smoking.

• In cystic fibrosis, a faulty chloride channel leads to mucus that is thick and difficult to clear, resulting in blockage and infection.


• A surfactant is a chemical substance that reduces the surface tension of a liquid.

• The aqueous lining of the lung has surface tension that must be overcome to permit inflation.

• Cells in the alveoli produce surfactant molecules when they are stretched.

• Premature babies may develop respiratory stress syndrome if they are born before cells in the alveoli are producing surfactant.


• The human lungs are suspended in the thoracic cavity in separate, closed pleural cavities.

• The thoracic cavity is bounded by the shoulder girdle, rib cage, and the diaphragm muscle.

• With inhalation, the diaphragm muscle contracts downward to create suction, and air flows into the lung.

• When the diaphragm relaxes, it pushes upward, and exhalation occurs.

• In the rib cage, intercostal muscles lift the ribs up and down to increase thoracic cavity volume.

Figure 48.11 Into the Lungs and Out Again

48 Blood Transport of Respiratory Gases

• As O2 diffuses from the alveoli into the blood, it is swept away and delivered to the cells and tissues of the body.

• Most O2 is carried by the oxygen-binding pigment hemoglobin in red blood cells.

• Hemoglobin is a protein consisting of four polypeptide subunits, each with a heme (iron-containing) group.

• Each heme group can reversibly bind a molecule of O2.


• Carbon monoxide (CO) binds to hemoglobin with a much higher affinity than does O2.

• CO is a deadly poison, as it destroys the ability of hemoglobin to transport O2.

• Myoglobin in muscle cells is an oxygen-binding molecule that can take up one molecule of O2.

• It has a higher affinity for O2 than hemoglobin does and provides an oxygen reserve for high metabolic demand or when blood flow is interrupted.


• CO2 is highly soluble, moving easily through cell membranes into the blood, where the partial pressure of CO2 is lower.

• Most CO2 is transported as bicarbonate ion (HCO3–).

48 Circulatory Systems: Pumps, Vessels, and Blood

• A circulatory system is composed of a pump (heart), fluid (blood), and conduits (blood vessels).

• This is also called a cardiovascular system.

48 The Human Heart: Two Pumps in One

• The left and right sides of the human heart may be thought of as separate pumps.

• The left pump delivers blood to the systemic circuit.

• The right pump delivers blood to the pulmonary circuit.

• Atrioventricular valves between the atria and ventricles prevent backflow into the atria when the ventricles contract.

• The pulmonary valve and aortic valve prevent backflow into the ventricles.

48Figure 49.3 The Human Heart and Circulation (Part 2)


• The right atrium receives blood from the superior and inferior vena cavas.

• From the right atrium, blood goes to the right ventricle.

• The right ventricle sends blood through the pulmonary artery to the lung.

• Pulmonary veins return oxygenated blood to the left atrium.

• From the left atrium, blood goes to the left ventricle.

• The left ventricle sends blood through the aorta to the body and the capillary beds.

• Blood returns to the right atrium via veins.

48Figure 49.3 The Human Heart and Circulation (Part 1)


• The left ventricle is more muscular because the resistance of the systemic circuit is much greater than that of the pulmonary circuit.

• In the cardiac cycle, ventricle contraction is called systole and ventricle relaxation is called diastole.

• At the end of diastole, the atria contract.

• The sounds of the cardiac cycle (the “lub-dub”) are caused by the closure of heart valves.

• Defective valves produce heart murmurs, whooshing sounds following the “lub.”

• The cardiac cycle can also be felt in artery pulsation, the surge of blood during systole.


• Cardiac muscle cells are in electrical contact with one another through gap junctions.

• This permits coordinated contraction for effective blood pumping.

• The primary pacemaker of the heart is the sinoatrial node located at the juncture of the superior vena cava and the right atrium.

• The atrioventricular node is stimulated by depolarization of the atria; with a slight delay it generates action potentials that are conducted to the ventricles via a bundle of fibers called the bundle of His.

48Figure 49.7 The Heartbeat

48 The Vascular System:Arteries, Capillaries, and Veins

• Large artery walls are elastic to withstand high pressures and to squeeze blood along their lumens by elastic rebound.

• Smooth muscle cells in arteries and arterioles contract and relax, varying the vessel diameters.

• As the diameter changes, resistance to flow also changes, allowing blood to be distributed to different tissues.

48 Figure 49.10 Anatomy of Blood Vessels (Part 1)

48 Figure 49.10 Anatomy of Blood Vessels (Part 2)

Capillary beds lie between arterioles and venules and exchange materials between blood and tissue fluid through

their thin walls.

Blood flows slowly here, facilitating this exchange.


• Blood tends to accumulate in veins. Veins are called capacitance vessels because of their high capacity to store blood.

• Pressure in veins is very low, and blood movement back to the heart relies on gravity, vessel squeezing by skeletal muscles, breathing, and limited smooth muscle contraction.

• Contraction of skeletal muscles pushes blood toward the heart because one-way valves in veins prevent backflow.

• If veins become stretched, the valves no longer do their job and varicose veins develop.

48Figure 49.13 One-Way Flow


• Heart attack or stroke is often the end result of atherosclerosis (hardening of arteries).

• When the smooth internal lining of arteries becomes damaged, deposits called plaque form at damaged sites.

• Swelling and lipid/cholesterol deposition invite fibrous connective tissue and calcium deposits.

• Blood platelets stick in the plaque and form blood clots (a thrombus), further blocking the artery.


• If the coronary arteries are affected, blood supply to the heart decreases.

• A thrombus here (coronary thrombosis) can block an artery, causing a heart attack (myocardial infarction, or MI).

• If part of the thrombus breaks away (an embolism), lodges in the brain, and blocks blood flow, stroke may occur.

• The best approach to reducing heart disease is prevention.

• Risk factors include high-fat/high-cholesterol diets, smoking, a sedentary life style, and obesity.

48 Blood: A Fluid Tissue

• Blood is connective tissue: it consists of living cells within an extracellular matrix.

• The fluid matrix is called plasma.

• The cellular components of blood are the red blood cells (erythrocytes), the white blood cells (leukocytes), and the platelets (cell fragments).

• Bone marrow makes about 2 million red blood cells per second.

• Each red blood cell lives about 120 days and then breaks down.


• Blood clotting involves many chemical steps in a cascade that activates circulating substances in the blood, many of which come from the liver.

• Cell damage and platelet activation lead to conversion of an inactive enzyme in the blood, prothrombin, to its active form, thrombin.

• Thrombin causes a plasma protein, fibrinogen, to polymerize, forming fibrin threads.

• These threads form a meshwork to seal the damaged vessel and provide a base for scar tissue.

48 Figure 49.16 Blood Clotting (Part 1)


• Plasma contains gases, ions, nutrients, proteins, hormones, and other chemicals.

• Predominant ions are Na+ and Cl–, giving blood a salty taste.

• Nutrient molecules in plasma include glucose, amino acids, lipids, lactic acid, and cholesterol.

• Circulating proteins include albumin, antibodies, hormones, and carrier molecules.

• Plasma is similar in composition to tissue fluid but has a higher concentration of proteins.

48 & 49 Gas Exchange and the Circulatory System in Animals.

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