The trachea, also known as the windpipe, is a straight tube that extends from the larynx to the anterior part of the thoracic cavity. It is about 4-5 inches long and approximately 1 inch in diameter. The trachea is a crucial part of the respiratory system, providing the passage for air to enter and exit the lungs. The walls of the trachea are supported by C-shaped rings of cartilage, which prevent the trachea from collapsing and blocking the airway. Therefore, option D is the correct answer.

The partial pressure of oxygen in alveoli is more than that of the blood. This is due to the process of diffusion, where oxygen moves from an area of high concentration (alveoli) to an area of low concentration (blood). Similarly, the partial pressure of carbon dioxide in alveoli is less than that of the blood, as carbon dioxide moves from the blood to the alveoli through diffusion. Therefore, option C is the correct statement.

The pleura is a membrane that surrounds the lungs and lines the thoracic cavity. It is a double-layered membrane, consisting of an outer parietal layer and an inner visceral layer. The parietal pleura lines the inner surface of the thoracic cavity, while the visceral pleura covers the outer surface of the lungs. The two layers are separated by a thin layer of fluid called pleural fluid, which reduces friction and allows the lungs to expand and contract smoothly during breathing. Therefore, option C is the correct answer.

Lumen is not a part of the human respiratory system. It is a term used in anatomy to describe the inner space or cavity within a tubular structure. In contrast, the pharynx, alveoli, and trachea are all structures that play crucial roles in the respiratory system.
The pharynx is a muscular tube that connects the nasal cavity and mouth to the esophagus and trachea. It serves as a passageway for air and food.
The trachea, also known as the windpipe, is a tube that carries air from the pharynx to the bronchi of the lungs. It is made up of C-shaped rings of cartilage, which keep the trachea open.
The alveoli are small, air-filled sacs located at the end of the bronchioles in the lungs. They are the site of gas exchange between the air and the blood, allowing oxygen to enter the bloodstream and carbon dioxide to be removed.
In summary, the human respiratory system includes the pharynx, trachea, and alveoli, but not the lumen. The lumen belongs to the digestive system, where it plays a role in increasing the surface area for the absorption of food.

he trachea is a long tube-like structure that starts at the larynx and extends down to the mid-thoracic cavity, where it divides into two bronchi. The trachea divides at the fifth thoracic vertebra into left and right bronchi. This branching of the trachea is the first division of the respiratory system. Therefore, option C is the correct answer to this question. The other options (sixth, fourth, and second) are incorrect as the trachea does not branch at these thoracic vertebrae.

During inspiration, the thoracic cavity volume increases. This is due to the contraction of the diaphragm and the external intercostal muscles, which move the ribs upward and outward. In addition, the sternum moves towards the ventral and anterior direction. This movement of the sternum is known as the pump-handle movement and it increases the anterior-posterior dimension of the thoracic cavity, allowing more air to be drawn into the lungs.

Inspiration is an active process that requires muscular effort. During inspiration, the diaphragm and external intercostal muscles contract. This contraction causes the diaphragm to move downward and the ribcage to move upward and outward, which increases the volume of the thoracic cavity. As a result, the air pressure inside the lungs decreases, and air is drawn into the lungs through the airways. Therefore, option C is the correct answer.

The spleen is an organ that only receives oxygenated blood. It is a small, fist-shaped organ located in the left upper quadrant of the abdomen. The spleen plays an important role in the immune system by filtering out old or damaged red blood cells and producing white blood cells to fight infection. The oxygenated blood supply to the spleen comes from the splenic artery, which branches off from the celiac artery, a major blood vessel that supplies blood to the abdominal organs. Options A and B are incorrect because both the lungs and gills are involved in the exchange of gases between the respiratory surface and the bloodstream, and thus receive both oxygenated and deoxygenated blood. Option C is incorrect because the liver receives both oxygenated and deoxygenated blood from the hepatic artery and hepatic portal vein, respectively.

The alveoli are the tiny air sacs in the lungs where gas exchange occurs between the air in the lungs and the blood. The alveolar epithelium is the thin layer of cells that makes up the walls of the alveoli. This epithelium is composed of nonciliated squamous cells, which are very thin and flat, allowing for efficient gas exchange. These cells are also very delicate and are susceptible to damage from various factors such as pollution and smoking. Damage to the alveolar epithelium can lead to various lung diseases, such as emphysema.

The respiratory rhythm center is located in the medulla region of the brain. Specifically, it is located in the medullary reticular formation, which contains two groups of neurons known as the dorsal respiratory group (DRG) and the ventral respiratory group (VRG). The DRG is responsible for generating the basic respiratory rhythm, while the VRG remains inactive during normal respiration but can be activated during periods of increased respiratory demand or in response to certain stimuli. The respiratory rhythm center is responsible for regulating the rate and depth of breathing, and also receives input from various sensors in the body that detect changes in oxygen and carbon dioxide levels.