The walls of alveoli are surrounded by _______.

Keep practicing — related questions

• Alveoli are surrounded by blood vessels.
• Tracheids have _____________.
• Xylem vessels do not contain ________________.
• The respiratory passage is lined by ________ epithelium.
• As compared to arteries, veins have ________.
• The lungs are covered by two thin membranes called _______.
• Which of the following has very thin walls and are supplied with blood capillaries?
• The elements of xylem are ______________.