Sentence:  At first the patient felt stronger , and the <WSD>hemoglobin</WSD> rose to 13.8 gm. , but on Oct. 20 , 1958 , he complained of `` caving in '' in his knees . 
Meaning of the word marked with <WSD> tags:  a hemoprotein composed of globin and heme that gives red blood cells their characteristic color; function primarily to transport oxygen from the lungs to the body tissues