3.3.4.1: Mass transport in animals (Mass transport in humans)

Decided to do a separate post on this as its preeeety long...

Here is a summary:

• The aorta leaves the left ventricle and transports blood around the body
• The vena cava takes blood from the body into the right atrium
• The pulmonary artery takes blood from the right ventricle into the lungs
• The pulmonary vein taken blood from the lungs into the left ventricle
• The renal artery/vein taken blood out/in the kidneys, respectively

NOTE: The heart is also supplied with it’s own special set of blood vessels: the coronary arteries. These branch off the aorta and blockages of these lead to bad things such as myocardial infarctions because an area of the heart is deprived of blood and therefore deprived of oxygen and therefore cannot aerobically respire and so it dies. How cheery.

Structure of the human heart:

The human heart is formed from two atria (left and right) and two ventricles (left and right). Separating each atrium and ventricle is the left/right atrioventricular valve - these prevent backflow. Separating the left and right sides of the heart is the septum. We have separate (left and right) sides of the heart as it is essential to keep oxygenated and deoxygenated blood separate.

Okay so how does the heart actually make blood flow through it? Basically, it’s all to do with pressure changes:

• During diastole (relaxation), blood enters the atria
• When atrial pressure exceeds ventricular pressure, blood goes through the atrioventricular valve into the ventricle
• Atrial systole (contraction) occurs, pushing the remaining blood out of the atria into the ventricles. At this point, the semilunar valves are closed
• Atrioventricular valves close due to ventricular pressure exceeding atrial pressure
• Atria relax and a short delay occurs to allow the ventricles to fill with blood.
• Ventricular systole results in blood in the ventricle passes through the semilunar valves into the pulmonary artery/aorta

In the heart, valves prevent the backflow of blood:

• Atrioventricular valves: situated between the atrium and ventricle of each side and prevent backflow of blood from ventricle to atrium when ventricular pressure exceeds atrial pressure
• Semi-lunar valves: prevent backflow of blood from vessel into ventricles when the pressure in the vessel exceeds that of the ventricle. this arises when the elastic walls of the vessels recoil increasing the pressure inside them and the ventricular walls relax decreasing the pressure inside them

How valves work: they are made of a tough fibrous tissue and are cusp-shaped. When pressure is greater on the convex side they move apart to let blood through. When pressure is greater on the concave side they push together as blood collects within the bowls. Blood can now not enter.

Cardiac output: cardiac output is the volume of blood pumped by one ventricle of the heart in one minute…

cardiac output = heart rate x stroke volume

Heart rate: the rate at which the heart beats

Stroke volume: the volume of blood pumped out at each beat

 A tiny bit more now…we just need to know about the different types of blood vessels.

• Arteries carry blood away from the heart into arterioles
• tough fibrous outer layer to resist pressure changes
• thick muscle layer that can contract to control blood flow - pressure must be kept high
• thick elastic layer to help maintain blood pressure by stretch and recoil - maintains a high pressure
• thin smooth endothelium lining to reduce friction and allow fast diffusion
• lumen (the central cavity)
• Arterioles (smaller arteries) control blood flow from arteries to capillaries
• muscle layer thicker than arteries - the rest is the same as arteries apart from being a generally smaller vessel
• Capillaries are tiny vessels that link arterioles to veins
• mostly just the thin lining layer of smooth endothelium to reduce friction and allow fast diffusion meaning they are very thin. Spaces between the endothelial cells allow white blood cells to enter/exit
• narrow lumen (the central cavity)
• numerous
• highly branched
• Veins carry blood from capillaries to the heart
• tough fibrous outer layer to resist pressure changes
• thin muscle layer that can contract to control blood flow
• thin elastic layer to help maintain blood pressure by stretch and recoil
• thin smooth endothelium lining to reduce friction and allow fast diffusion
• large lumen (the central cavity)
• pocket valves!!! - ensure than when vein is squeezed (eg when skeletal muscles contract) blood flows toward the heart

Last little bit…tissue fluid:

Cells of multicellular organisms bathe in tissue fluid. It supplies the cells with oxygen/ions/amino acids/fatty acids/glucose etc (basically, nutrients) and in return receives waste products such as carbon dioxide. It is a means by which materials are exchanged. It is formed from blood plasma:

• Pumping of the heart creates a high hydrostatic pressure at the arteriole end of a capillary
• Blood pumped by the heart passes through the narrow capillaries
• The high hydrostatic pressure causes tissue fluid to move out of the blood plasma. This is opposed by the hydrostatic pressure of the tissue fluid on the outside of the capillary and the lower water potential of the blood due to plasma proteins etc that causes water to move back into the capillaries.
• The resulting pressure is only enough to push small molecules out of the capillary. This is known as ultrafiltration. Larger molecules eg blood cells and proteins remain in the blood as they are too large to cross the membrane.

Once exchanges are complete, tissue fluid must return to the blood:

• Loss of tissue fluid reduces the hydrostatic pressure in the capillaries (particularly in the venous end)
• The tissue fluid is forced back into the capillaries due to the higher hydrostatic pressure outside the capillaries
• The plasma has lost water but still contains proteins which further brings the tissue fluid back into the capillaries as it has a lower water potential than the tissue fluid so water enters the capillary by osmosis down a water potential gradient.
• The remainder of the tissue fluid is carried via the lymphatic system that drains its contents into the bloodstream via two ducts that join veins close to the heart.
• The contents of the lymphatic system are moved by the hydrostatic pressure of the tissue fluid that has left the capillary/contraction of body muscles that squeeze the lymph vessels. The lymph vessels have valves like veins to stop the backflow of this tissue fluid.