Aging, changes in the blood and lymph that occur with
The BLOOD undergoes a number of normal changes across the span of the lifetime. Blood cells have life spans ranging from a few hours to decades. The blood continually renews itself, producing millions of erythrocytes and thousands of leukocytes every hour. Blood cell production accelerates to meet unique health needs, such as PREGNANCY or INFECTION.
The LIVER is the first organ in the developing fetus to produce blood cells, primarily erythrocytes, with supplemental production from the SPLEEN and the THYMUS. At about five gestational months the BONE MARROW has developed enough to begin taking over blood cell production and by birth is the primary structure for HEMATOPOIESIS. Through childhood (until about age 16), nearly all the bone marrow is red bone marrow that actively produces blood cells. As the body matures yellow bone marrow, a fibrous structure of connective tissue and fat, gradually replaces the red bone marrow. By adulthood only about 60 percent of the bone marrow is red. This level remains fairly constant until around age 70, when some red bone marrow, primarily in the long bones, again transitions to yellow marrow.
The bone marrow slows ERYTHROCYTE (red blood cell) production in advanced age, putting fewer erythrocytes into the blood’s circulation. The reduced erythrocyte volume correspondingly decreases the amount of available HEMOGLOBIN in the blood, which diminishes the amount of oxygen the blood can carry to the cells with each heartbeat. This reduction commonly results in decreased AEROBIC CAPACITY, showing lessened ENDURANCE and longer recovery times with strenuous physical activity, and may cause ANEMIA. The spleen’s efficiency at removing old and defective erythrocytes from the circulation declines, an accommodation that is somewhat a double-edged sword. While this slowed hemolytic action allows more erythrocytes to remain in the blood to improve the blood’s capacity to carry oxygen, a greater number of those erythrocytes are less effective in this role.
The LYMPH structures and functions also change with age. The lymphatic system becomes active at about age two months when the child’s IMMUNE SYSTEM begins to replace the protection from the mother’s IMMUNITY. Most of the lymphocytes the lymph tissues produce swarm to the THYMUS, where they will come to maturity. The thymus contains nearly the lifetime complement of immature T-cell lymphocytes (called thymocytes in immaturity) by about age 16, at which point the thymus reaches its peak size and level of function. As the T-cells reach maturity they leave the thymus and migrate to other lymph structures throughout the body where they reside until the immune system needs them. As T-cell maturation winds down the activity of the thymus decreases and the thymus begins to shrink, diminishing by early adulthood to a few clusters of lymph tissue. After adulthood, the body can make only limited additional T-cells. Health conditions that affect Tcells, such as HIV/AIDS, that destroys them can deplete the body’s supply of these vital protective cells, depriving the body of its front line immune defense.
With age lymphocyte production also decreases, resulting in fewer circulating lymphocytes and a corresponding reduced resistance to infection. Later in life the spleen diminishes in size, ultimately retreating to about half its size in early adulthood. Its functional capacity decreases as well, resulting in the spleen becoming less efficient at filtering aged erythrocytes from circulation. The spleen also becomes less effective in fighting infection, reducing the body’s resistance.
Other changes in the body that occur with advancing age affect the ability of the LYMPH VESSELS to collect fluid from the tissues and transport it back to the bloodstream. Diminished MUSCLE tone and reduced movement slow the flow of fluids into and through the lymph vessels. Other health conditions such as congestive HEART FAILURE and kidney disease affect the body’s ability to move fluids through the blood vessels, creating a backlog. By about age 70, however, the body begins to decrease the total amount of water its tissues retain. This results in less water in the blood and a lower blood volume, somewhat lowering the BLOOD PRESSURE though increasing the risk for blood clots (thrombosis).
See also AGING, CARDIOVASCULAR CHANGES THAT OCCUR WITH; AGING, PULMONARY CHANGES THAT OCCUR WITH; CANCER RISK FACTORS; SENESCENCE.
