Question

Given alongside is a diagrammatic sketch of a part in human lungs.
i) Name the parts numbered $1 - 4$.
ii) What do the arrows $5$ and 6 indicate?

Answer 1

Gases are exchanged over a pressure/concentration gradient. Gases are exchanged between alveoli and blood, as well as between blood and tissues. Diffusion of gases through membranes is caused by a differential in partial pressures of $C{O_2}$ $\left( {pC{O_2}} \right)$ and ${O_2}$ $\left( {p{O_2}} \right)$ in alveolar tissues, blood, and other bodily tissues. At the end of the process, oxygen is transferred from the blood to the tissue fluid, while carbon dioxide is transferred from the tissue fluid to the blood.

Complete answer:
Part A: 1. Blood vessel(capillary)
2. Red blood cells
3. Low oxygen and high carbon dioxide levels in the blood.
4. Alveolus wall
Part B: Arrow $5$ indicates red blood cells carrying oxygen and arrow $6$ indicates red blood cells carrying carbon dioxide.

Additional information:
Gas is exchanged between the millions of alveoli in the lungs, as well as the capillaries that surround them. Inhaled oxygen travels from the alveoli to the capillaries, whereas carbon dioxide travels from the blood in the capillary to the air in the alveoli.
Process of respiration:
Pulmonary respiration (inhalation of ambient oxygen and exhalation of $C{O_2}$-rich alveolar air) $\to$ ${O_2}$ and $C{O_2}$ diffusion across the alveolar membrane $\to$ Gas transportation via blood $\to$ Oxygen and $C{O_2}$ diffusion between blood and alveoli $\to$ Cellular respiration (catabolism of food using ${O_2}$ to release energy and $C{O_2}$).

Note:
Gas transportation is a highly efficient method. Because haemoglobin has a high affinity for oxygen, oxygen is carried by red blood cells' haemoglobin. Each molecule of haemoglobin binds to four molecules of oxygen. The blood transports the oxygen that haemoglobin has gathered up to various tissues. Carbon dioxide is more soluble in water than oxygen, hence it is carried in dissolved form in human blood, though it is also carried by haemoglobin.