L3 - tissue fluid

• Blood contains:

  -cells (leucocytes, erythrocytes)

  -cell fragments (platelets)

  -plasma (liquid containing dissolved $O_2,\ CO_2,\$salts, glucose, amino acids, hormones and plasma proteins such as albumin and globulins)
• Role of blood:

  -transport oxygen to the respiring cells and transport carbon dioxide away

  -transport food molecules from the small intestine

  -transport nitrogenous waste products to the excretory organs

  -transport hormones

  -transport platelets to damaged areas

  -transport cells and antibodies involved in the immune response
• Key points:

  -blood is the main transport fluid in the body, but this is only part of the story

  -tissue fluid is the other main player in the transfer of substances from the blood to the cells

  -a third liquid called lymph is also part of the circulatory system
• Capillaries have a smooth endothelial surface to reduce resistance to blood flow
• Capillaries have a single layer of endothelium to ensure a short diffusion distance
• Capillaries have a small diameter (7nm) to slow the flow rate of blood and to ensure a short diffusion distance
• Capillaries have gaps between endothelial cells (fenestrated) to allow plasma to pass out of capillaries
• Tissue fluid is a watery fluid containing glucose, amino acids, fatty acids, salts, $O_2$and $CO_2$

  -tissue fluid is the medium by which substances are transferred between cells and blood. it is formed from blood plasma
• Key points for tissue formation:

  -hydrostatic pressure

  -oncotic pressure

  -(large) plasma proteins
• Hydrostatic pressure in blood vessels is the pressure of the blood against the walls of the vessel (pushing force)
• Oncotic pressure is the tendency of water to move into the blood via osmosis as a result of the plasma proteins (pulling force)
• Large plasma proteins (particularly albumin) give the capillaries a low water potential compared to the surrounding tissue fluid. As a result, water moves into the capillaries via osmosis
• Formation of tissue fluid:

  -as blood flows through capillaries, some plasma passes into the tissues because of high (hydrostatic) pressure (4.6kPa)

  -tissue fluid is very similar to plasma but without large plasma proteins or red blood cells

  -this fluid bathes every cell in the body supplying them with glucose, amino acids, fatty acids, salts, and oxygen by diffusion and facilitated diffusion

  -oncotic pressure moves some water back into the blood by osmosis due to the presence of plasma proteins (lower than hydrostatic pressure) (-3.3kPa)
• Returning of fluid to the blood:

  -at the venous end, hydrostatic pressure is lower as fluid has moved out (2.3kPa)

  -oncotic pressure is greater than the hydrostatic pressure so fluid moves into the capillary by osmosis
• Filtration pressure = hydrostatic pressure - oncotic pressure
• Arterial end filtration pressure:

  -high hydrostatic of 4.6kPa

  -4.6 minus 3.3 is 1.3kPa

  -therefore net flow of fluid out of the capillary to form tissue fluid
• Venous end filtration pressure:

  -low hydrostatic pressure of 2.3kPa

  -2.3 minus 3.3 is negative 1.0kPa

  -therefore net flow of fluid back into capillary
• There are more solutes in the blood than in the tissue fluid so the blood has a lower water potential than the tissue fluid. So this effect would cause water to move in to the capillary from the tissue fluid by osmosis (down the water potential gradient). At the venous end the blood has a much lower hydrostatic pressure. So the overall ‘effective blood pressure’ causes fluid to move back into the capillary there
• First step of capillary exchange: blood arrives at the arterioles end under pressure from the surge of blood when the heart contracts
• Second step of capillary exchange: this hydrostatic pressure is higher than the oncotic pressure, so fluid is forced out of the capillaries to surround the cells
• Third step of capillary exchange: this fluid is called tissue fluid. Diffusion of substances (oxygen, glucose, fatty acids, salts) occurs from the blood to the cells via the tissue fluid
• Fourth step of capillary exchange: as the blood moves through the capillaries towards the venous system, the hydrostatic pressure drops as fluid has moved out and the pulse surge is lost
• Fifth step of capillary exchange: large plasma proteins (like albumin) do not pass through into the tissue fluid and remain in the capillaries. Creating a low water potential in the capillaries at the venule end
• Sixth step of capillary exchange: this creates a strong oncotic pressure (osmotic pressure), which is higher than the hydrostatic pressure, so water moves into the capillaries via osmosis (with carbon dioxide and nitrogenous waste)
• Seventh step of capillary exchange: by the time the blood returns to the veins, 90% of the fluid is back in the blood containing waste products like carbon dioxide. The other 10% becomes lymph
• Hydrostatic pressure decreases as you move from the arteriole end to the venule end of the capillary. Oncotic pressure is created due to the large proteins that remain in the capillaries. This created a water potential gradients which increases towards the venous end. Once the hydrostatic pressure reduces, water (an waste product) moves back into the capillary via osmosis
• Lymph:

  -not all fluid returns to the blood capillary

  -10% enters a separate system of microscopic tubes called lymph capillaries

  -these are part of the lymph system

  -lymph vessels have valves that let fluid enter but not leave them

  -the lymph capillaries join to form lymph vessels
• Lymph:

  -most of the lymph re enters the blood stream at the subclavian veins under the collar bones

  -lymph is a milky looking fluid

  -digested fat is absorbed into the lymph through the lacteals in the villi of the small intestine
• Lymph:

  -at intervals along the lymph vessels are structures called lymph nodes

  -these are part of the immune system

  -lymphocytes are produces in the lymph nodes and found in lymph
• Lymphocytes:

  -white blood cells that engulf and destroy bacteria

  -have been filtered out of the lymph fluid by the lymph nodes

  -lymph nodes swell up at times of infection