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Chapter 23: 12-15

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Circulation: Blood and Blood Vessels

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Blood has two kinds of elements: plasma, and cells. Plasma is a bit more than half the blood by volume, and itself is over 90% water. The rest of plasma contains nutrients like proteins, glucose, and minerals that come from the digestive system, carbon dioxide in transport as a waste from cell respiration, and hormones being transported from their origin in gland tissues to target cells. Red blood cells which transport O2 and CO2 gases, white blood cells which provide immunity, and platelet proteins that govern clotting make up the non-plasma blood components.

Blood Components

Plasma Water Solvent for all the other components
Salts Sodium, potassium, calcium, magnesium, chloride, bicarbonate Osmotic balance, buffer pH, and regulation of membrane permeabilities
Proteins Albumen, Fibrinogen, Immunoglobulins Osmotic balance, pH buffers, clotting, immunity
Substances Transported by Blood Glucose, fatty acids, vitamins; wastes; respiratory gases, hormones Nutrients, gases, respirationgases
Cells Erythrocytes Red cells Transport of oxygen and carbon dioxide
Leukocytes White cells Immunity
Platelets Platelets Blood Clotting

Blood Cells

Blood Cells

Electron scanning microscope comparison of a red blood cell on left, platelet in the middle, and white blood cell (leukocyte) on right.

The flat "plate-shaped" red blood cells carry gases to and from tissue cells. When the red cells pass through the small vessels of the alveoli in the lungs, the oxygen concentration in the surrounding tissues is higher than the concentration of oxygen in the blood, so oxygen moves into the blood vessels, where it locks onto the hemoglobin and displaces the carbon dioxide there. The displaced CO2 is flushed through the alveoli walls and out of the body, back into the atmosphere. As the blood moves through the capillaries, the lower oxygen concentration in the body tissues pulls the oxygen out, and allows CO2 to move in for transport back to the heart and then to the lungs as waste.

The gas exchange follows the laws of concentrations and diffusion. The CO2 concentration in the cells, coming as waste from cellular respiration, is higher than that in the blood fluid, so the hemoglobin (which has lost is oxygen) picks up the CO2. But it isn't quite that simple. The CO2 reacts with the water in the plasma, and forms H2CO2 (carbonic acid), which breaks down into H+ ions and HCO3- ions. The hemoglobin then attaches to the H+ ions to prevent their presence from changing blood pH to acidic levels. The carbon dioxide diffuses out of the blood when it reaches the alveoli, and the decreasing concentration of CO2 causes the HCO3- to release CO2 and reform H2O.

We will take up white blood cells as we study the immune system.

The lymphatic system

The circulation system includes not only the blood vessel system and heart, but also the pulmonary blood system (in birds and mammals), and the exchange between interstitial fluids and the blood system. Because vertebrates have closed blood systems, the body cells themselves are bathed in an interstitial fluid. Some of the fluid which leaves the blood system for the body cells is not collected again by capillaries, but by vessels which are part of the lymphatic system. These vessels also collect many of the waste products from cellular metabolism for removal from the body.