The spinal cord is a long tubular bundle of nervous tissue extending from the medulla oblongata to the level of the first/second lumbar vertebra (L1-L2), where it tapers into the conus medullaris. Below this, nerves form the cauda equina. It transmits impulses between the brain and body and coordinates reflexes.
Explanation:
The thymus gland is a primary lymphoid organ where T lymphocytes mature and become self-tolerant. Bone marrow is where B cells mature; spleen and lymph nodes are secondary.
Explanation:
Pulmonary surfactant is a complex mixture of lipids (primarily dipalmitoylphosphatidylcholine) and proteins secreted by Type II alveolar cells (septal cells) in the lungs. Its primary function is to reduce the surface tension of the fluid lining the alveoli. Without surfactant, the alveoli would collapse at the end of expiration (atelectasis) due to high surface tension, making it extremely difficult to re-inflate the lungs during inspiration.
Explanation:
Veins, especially in limbs, have one-way valves that prevent the backflow of blood as it returns to the heart against gravity. Arteries have thick walls to withstand pressure and lack valves. Capillaries have no valves.
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