Chapter 21. Chapter 21: Circulation and Respiration

With the increasing size of multicellular organisms, most of an organism’s cells are not in direct contact with the external environment, so essential nutrients and wastes cannot diffuse into and out of cells adequately. A dedicated delivery and removal system (a circulatory system) is necessary.

Open circulatory systems have no clear distinction between the circulating fluid and the interstitial fluid that bathes cells and tissues. In closed circulatory systems, the circulating fluid and interstitial fluid are kept separate.

Single-circuit cardiovascular systems are found in fishes. Blood passes once through the heart to the gills, then through the body and back to the heart. Gill capillaries slow down the blood flow, so the body receives blood at a relatively low pressure, which decreases the rate of oxygen supply to the cells. Double circulation is found in amphibians, reptiles, birds, and mammals. Blood flows from the heart to the lungs and back to the heart, then from the heart to the body and back to the heart. Blood leaves the heart at a relatively high pressure, especially in birds and mammals, which increases the rate of food and oxygen supply to cells and the rapid removal of wastes—a system better suited to high energy demand.

The blood picks up oxygen and gives up carbon dioxide in the pulmonary circuit.

The sinoatrial node is the heart’s natural pacemaker; if it malfunctions, an artificial pacemaker may be required.

The pressure of blood in the veins is only about one-tenth of the pressure in the arteries. Friction in the capillaries slows the blood so that when it passes into the veins, the blood pressure is much diminished.

(a) Red blood cells transport oxygen from the lungs to the rest of the body, contain few organelles, and are packed full of hemoglobin (the oxygen carrier). (b) White blood cells are part of the immune system and fight disease. (c) Platelets are cellular fragments filled with enzymes and chemicals that help patch damaged blood vessels.

With high blood pressure, the heart must work harder, potentially weakening it, and arteries are less able to expand and accommodate the increased blood flow during exertion. More cholesterol sticks to the more rigid artery walls, narrowing the diameter and further reducing the efficiency of the circulatory system and heart. These problems increase the risk of heart attack and stroke.

To reduce the risk of heart disease: (a) avoid all types of tobacco, (b) exercise, (c) achieve and maintain a healthy body weight, and (d) limit alcohol consumption.

Lymph travels through the body in lymphatic vessels that are squeezed by adjacent muscles as they contract, pushing the fluid onward toward the shoulders, where it enters the bloodstream.

In animals with gills, gas exchange occurs as water rushes across the gills, passing between lamellae (disks of elaborately branched capillaries). Dissolved oxygen passes from the inhaled water to the blood by direct diffusion through the capillaries, and dissolved carbon dioxide passes by direct diffusion from the blood to the exhaled water. In animals with lungs, air moves down a trachea into the lungs, where oxygen diffuses into capillaries and thus into the bloodstream, and carbon dioxide diffuses from the blood into air, which is exhaled.

Hemoglobin releases oxygen when it encounters a low partial pressure of oxygen in the tissues, such as in actively contracting muscle tissue.

By orienting vessels so that the blood flows in the opposite direction to the water, gills extract significantly more oxygen from the water than if blood and water flowed in the same direction.

Amphibians make up for reduced lung capacity by conducting some gas exchange through their skin.

Birds have highly efficient gas exchange systems, extracting more oxygen from the air than do other terrestrial vertebrates. Air moves in one direction through the lungs, and during both inhalation and exhalation, fresh air continues flowing through the lungs.

During exhalation, the diaphragm and intercostal muscles relax, the volume of the chest cavity decreases, and the lungs are compressed, forcing air out of the lungs.

At lower air pressure, the amount of oxygen in the air is lower, and it is harder to push air into the lungs and to drive the oxygen into the blood.

Humans living under the low-oxygen conditions of higher elevations are likely to have hemoglobin that is more sticky, with a higher affinity for oxygen, enabling the hemoglobin to become saturated with oxygen even though the air is relatively “oxygen-poor.”