Exchange in Capillaries (A-level Biology)

Exchange in Capillaries

Tissue Fluid

  • Tissue fluid is a watery substance that bathes the cells of tissues. Tissue fluid is formed from blood plasma, the fluid that moves out of capillaries.
  • Tissue fluid is the fluid by which substances are exchanged between the blood and cells. It’s function is to supply tissues with essential solutes in exchange for waste products between blood and cells at the site of capillaries’ endothelial cells.
  • Made up of substances which are small enough to escape through gaps in the capillaries wall. This includes nutrients such as: oxygen, glucose, water, amino acids, fatty acids and ions.

Hydrostatic and Oncotic Pressure

  • Hydrostatic pressure is the residual pressure from the heart beating created when blood is forced through the capillaries.
  • Oncotic pressure is when there is movement of fluid out of the capillaries (due to hydrostatic pressure), the water potential of the capillaries becomes more negative (though this is usually stable).
  • Becoming negative causes water to move down the water potential gradient. This is from the tissue fluid to blood via osmosis.
  • Some fluid can be pushed back into the capillaries by osmotic pressure. This is due to both tissue fluid and blood having a negative water potential due to containing solutes.
  • Tissue Fluid is less negative (in water potential) than blood, therefore it is positive in comparison. This is because blood contains more solutes.

Below is a table outlining how high or low the pressures are at either end of the capillary:

Functions of Lymphatic System

  • Lymphatic system carries back remaining tissue fluid not pushed back into capillaries. The fluid is now known as lymphatic fluid. This process helps to prevent swelling by water retention.
  • Lymphatic system contains lymph fluid. This is similar to tissue fluid’s content, though contains less nutrients due to its main function being waste removal.
  • Lymph fluid travels through the lymphatic system and finally drains back into the blood at the subclavian vein.
  • Lymphatic system contains lymph nodes. These function to prevent bacteria and foreign materials entering the fluid.
  • Lymph nodes produce antibodies. They empty the antibodies into the blood to help destroy and invading pathogens, part of the immune system defences.
  • Lymph glands remove bacteria and other pathogens too.


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Tissue Fluid Formation and Return to the Circulatory System

Formed as a result of interplay between hydrostatic and oncotic pressure in the capillaries, the process of tissue fluid formation is outlined below:

  1. High hydrostatic (liquid) pressure exists at the arterial end of the capillary. The hydrostatic pressure inside the capillary is higher than the hydrostatic pressure in the tissue fluid.
  2. This difference in pressure forces water and other small molecules  out of the capillary, forming tissue fluid. Proteins and cells stay inside the capillary because they’re too large to leave.
  3. The hydrostatic pressure in the capillary reduces as water leaves the capillary.
  4. Water potential at the venule end of the capillary is lower than that of the tissue fluid. This is due to the loss of fluid from the capillary and an increasing concentration of proteins and cells that don’t leave the capillary.
  5. Some of the tissue fluid re-enters the capillary from the venule end via osmosis. The tissue fluid loses most of its oxygen and other nutrients to the cells but has gained carbon dioxide and waste materials.
  6. Excess tissue fluid is drained into the lymphatic system and then back into the circulatory system.
A-level Biology - Exchange in Capillaries
A-level Biology – Exchange in Capillaries


→What is an exchange in capillaries?

Exchange in capillaries refers to the transfer of oxygen, nutrients, and waste products between the blood and the tissues in the body. This occurs through the walls of the smallest blood vessels, called capillaries, which are located near the body’s cells.

→How does exchange in capillaries work?

Exchange in capillaries works through a process called diffusion, where substances move from an area of high concentration to an area of low concentration. In the case of exchange in capillaries, oxygen and nutrients diffuse from the blood into the tissues, and waste products diffuse from the tissues into the blood.

→What is the lymphatic system?

The lymphatic system is a network of vessels, tissues, and organs that helps to maintain fluid balance in the body and defend against infections. It consists of lymphatic vessels, lymph nodes, the spleen, thymus, and tonsils, among other structures.

Lymphatic vessels are similar to blood vessels, but they carry lymph, a clear fluid that contains immune cells and waste products, instead of blood. The lymphatic vessels collect lymph from the body’s tissues and return it to the bloodstream, helping to regulate fluid balance.

Lymph nodes are small, bean-shaped structures that filter lymph and trap bacteria, viruses, and other harmful substances. The lymph nodes contain immune cells that can recognize and destroy these invaders, helping to prevent infections from spreading.

The spleen, thymus, and tonsils are other parts of the lymphatic system that help to produce and store immune cells, which can help to fight off infections. Overall, the lymphatic system plays a crucial role in maintaining the body’s immune function and protecting against disease.

→What does the lymphatic system do?

The functions of lymphatic system includes the following:

Maintaining fluid balance: The lymphatic system helps to regulate the amount of fluid in the body’s tissues by collecting excess fluid and returning it to the bloodstream. Without the lymphatic system, the body would swell with excess fluid.

Fighting infections: The lymphatic system plays a vital role in the body’s immune response by producing and storing immune cells, such as lymphocytes, and filtering out harmful bacteria and viruses from the lymphatic fluid. The lymph nodes act as checkpoints, detecting and trapping foreign particles, and activating immune cells to destroy them.

Absorbing fats: The lymphatic system also absorbs dietary fats and fat-soluble vitamins from the intestines and transports them to the bloodstream.

Removing waste: The lymphatic system collects waste products and toxins from the body’s tissues and transports them to the lymph nodes to be eliminated.

→What is a tissue fluid?

Tissue fluid, also known as interstitial fluid, is a clear, colorless fluid that surrounds the cells and tissues of the body. It is derived from blood plasma and contains water, ions, nutrients, and waste products. Tissue fluid plays an important role in providing nutrients to cells and removing waste products from them.

Tissue fluid formation happens when blood plasma is filtered through the walls of capillaries, the smallest blood vessels in the body. This filtration process is driven by the pressure of the blood, which forces water and other small molecules out of the capillaries and into the spaces between the cells. The tissue fluid then bathes the cells, delivering oxygen and nutrients and collecting waste products.

After the tissue fluid has delivered its nutrients and collected waste products, it is collected by the lymphatic system and returned to the bloodstream. This helps to maintain the balance of fluid in the body and ensures that the cells receive the nutrients they need to function properly.

→How is tissue fluid formed in A-level Biology?

Tissue fluid is formed by the process of ultrafiltration, which occurs in the capillaries, the smallest blood vessels in the body.

The capillaries have walls that are made up of a single layer of endothelial cells, which are thin enough to allow small molecules, such as water, oxygen, and nutrients, to pass through. Blood is under pressure as it moves through the capillaries, and this pressure forces fluid out of the capillaries and into the spaces between the cells, known as the interstitial spaces.

The fluid that enters the interstitial spaces is called tissue fluid. Tissue fluid contains water, oxygen, glucose, amino acids, and other nutrients that are needed by the cells in the tissues. The tissue fluid also collects waste products, such as carbon dioxide and urea, from the cells.

→Why is exchange in capillaries important?

Exchange in capillaries is important because it allows for the proper functioning of the body’s cells. Oxygen is needed for cellular respiration, while nutrients provide energy and building blocks for the cells. Waste products, such as carbon dioxide, need to be removed to prevent toxicity.

→How does the structure of capillaries facilitate exchange?

The structure of capillaries is designed to facilitate exchange. Capillaries have a very thin wall, allowing for easy diffusion of substances. They also have a high surface area-to-volume ratio, which increases the amount of exchange that can occur.

→What are the different types of exchange in capillaries?

There are two main types of exchange in capillaries: passive and active. Passive exchange occurs through diffusion and does not require energy. Active exchange, on the other hand, requires energy and can move substances against a concentration gradient.

→Can exchange in capillaries be affected by certain conditions?

Yes, exchange in capillaries can be affected by certain conditions, such as cardiovascular disease, high blood pressure, and diabetes. These conditions can damage the capillaries, making exchange less efficient, and potentially leading to other health problems.

→What is hydrostatic and oncotic pressure?

Hydrostatic pressure and oncotic pressure are two types of pressure that affect the movement of fluids across cell membranes and through blood vessels.

Hydrostatic pressure is the pressure exerted by a fluid on the walls of a container. In the context of the circulatory system, hydrostatic pressure is the pressure exerted by blood on the walls of blood vessels. Blood pressure is a type of hydrostatic pressure. The hydrostatic pressure in the capillaries is responsible for forcing water and solutes out of the capillaries and into the interstitial spaces, where they form tissue fluid.

Oncotic pressure, also known as colloid osmotic pressure, is the pressure exerted by the proteins in the blood plasma. Proteins in the blood, such as albumin, create an osmotic pressure that draws water back into the capillaries. This is because the proteins are too large to pass through the capillary walls and into the interstitial spaces. As a result, water is drawn back into the capillaries by the oncotic pressure, which helps to prevent excess fluid accumulation in the tissues.

Hydrostatic and oncotic pressure are important in maintaining fluid balance in the body. If the hydrostatic pressure is too high, fluid can accumulate in the tissues, leading to edema. If the oncotic pressure is too low, fluid can be drawn out of the blood vessels and into the interstitial spaces, also leading to edema. The balance between these pressures is essential for maintaining normal fluid balance in the body.

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