Question

Which of the following statements is true regarding capillaries? A. In a capillary bed, nutrients and oxygen diffuse into cells. B. In a capillary bed, plasma is often forced into spaces between cells, and is ultimately picked up by a lymphatic capillary to be returned to the general circulation. C. In a capillary bed, urea and carbon dioxide diffuse into the blood. D. All of the above are true of capillary beds.

Short answer

D. All of the above are true of capillary beds.

Step-by-step solution

Identifying the role of capillaries in nutrient and gas exchange

Determine if statement A is correct by identifying that capillaries are the sites of nutrient and oxygen exchange where these substances diffuse from the blood into surrounding cells.

Understanding the fluid exchange in capillary beds

Consider statement B which refers to the process of filtration and reabsorption in capillary beds, where plasma can leave the capillaries and enter the interstitial space, and is often collected by the lymphatic system.

Recognizing waste product diffusion

Evaluate statement C which indicates the role of capillaries in waste removal, where substances like urea and carbon dioxide are transferred from cells into the blood.

Combining validated statements

After validating that statements A, B, and C describe true functions of capillary beds, conclude whether statement D is true.

Key concepts

Gas Exchange in Capillaries

The process of gas exchange in capillaries is a critical function in the human body's respiratory system. Here, oxygen and carbon dioxide are exchanged between the blood and the surrounding tissues. This process is driven by a gradient of partial pressures, which means that oxygen, which is higher in partial pressure in the blood, diffuses into the body's tissues where the pressure is lower. Conversely, carbon dioxide, which is higher in the tissues, diffuses into the capillary blood to be carried away and ultimately exhaled. This diffusion is facilitated by the thin walls of the capillaries, which are only one cell layer thick, allowing for efficient gas transfer.

This concept is related to statement A, which correctly states that in capillary beds, nutrients and oxygen diffuse into cells, supplying them with the essentials for cellular metabolism. The presence of a large surface area and the slow blood flow through capillaries enhances this crucial exchange, sustaining cellular function and overall health.

Capillary Fluid Exchange

The capillary fluid exchange refers to the movement of water and dissolved substances between capillaries and the interstitial space—the area surrounding the cells. This process is governed by the balance between hydrostatic pressure, which pushes fluid out of capillaries, and oncotic pressure, caused by plasma proteins, which pulls fluid in. This balance results in either filtration, where fluid moves out of capillaries and into the surrounding tissues, or reabsorption, where fluid moves from the tissues back into the bloodstream.

Statement B in the exercise outlines a scenario where plasma is forced into the spaces between cells—this is an example of filtration. The lymphatic system plays a role in collecting this excess fluid, ensuring that it does not accumulate in the tissues, but is instead returned to the circulation, maintaining fluid homeostasis.

Waste Diffusion in Capillaries

The waste diffusion in capillaries is a detoxifying function whereby metabolic waste products such as urea, carbon dioxide, and other byproducts generated by cells are removed efficiently. Similar to oxygen, these waste substances follow a concentration gradient. They move from areas of higher concentration within the cells to lower concentrations in the blood within the capillaries. This essential process prevents the accumulation of toxic substances which could impair cellular function.

According to statement C, urea and carbon dioxide diffuse into the blood within capillary beds. This is a vital function of the capillaries—to collect and transport these waste products to organs like the lungs and kidneys, where they can be excreted from the body. It is an integral part of the body's regulatory mechanisms, keeping the internal environment stable and safe for all cellular processes.