Ture capillaries do not have smooth muscle.

Ventilation in birds is accomplished by expansion and contraction of the ( ). A: air sacs B: trachea C: lungs D: diaphragm E: air capillaries

Which of the following structures can not be found in the epidermis？ A: Duct of sweat glands B: Capillaries C: Nerve endings D: Melanocytes

Which of the blood vessels conduct blood away from the ventricles of the heart and lead to the capillaries? A: arteries and veins B: arterioles and venules C: arteries and venules D: arteries and arterioles

Moisture in food can be divided into "bound water" and "free water" according to different properties. Which of the following is not bound water? A: The first layer of water molecules adsorbed to glucose molecules B: Water molecules with unpaired electrons entering the outer empty orbits of sodium ions C: Water molecules adsorbed in capillaries in food tissue D: Water molecules bound to the negative ion of glutamic acid on the protein peptide chain