B3.2 Transport Flashcards
What does a capillary do?
The function of capillaries is to exchange materials between the cells in tissues and blood travelling at low pressure (<10mmHg)
How small is a capillary and how is this optimised?
Arteries split into arterioles which in turn split into capillaries, decreasing arterial pressure as total vessel volume is increased.
The branching of arteries into capillaries therefore ensures blood is moving slowly and all cells are located near a blood supply.
After material exchange has occurred, capillaries will pool into venules which will in turn collate into larger veins.
What are the adaptations of the capillaries for maximum exchange of materials?
Capillaries have specialised structures in order to accomplish their task of material exchange:
They have a very small diameter (~ 5 µm wide) which allows passage of only a single red blood cell at a time (optimal exchange).
The capillary wall is made of a single layer of cells to minimise the diffusion distance for permeable materials.
They are surrounded by a basement membrane which is permeable to necessary materials.
They may contain pores to further aid in the transport of materials between tissue fluid and blood.
How does a capillaries place in the body affects it’s role?
Capillaries structure may vary depending on its location in the body and specific role:
The capillary wall may be continuous with endothelial cells held together by tight junctions to limit permeability of large molecules.
In tissues specialised for absorption (e.g. intestines, kidneys), the capillary wall may be fenestrated (contains pores).
How arteries vary to veins
Artery
Narrower lumen
Thicker wall
Circular
Corrugated inner surface
Fibres visible in wall
Vein
Wider lumen
Thinner wall
Circular or flattened
No corrugation
Few or no fibres visable
Summary:
Arteries have thick walls and narrow lumens because they transport blood at high pressure
Veins have thin walls with wide lumens and valves because they transport blood at low pressure
What is the function of the arteries?
The function of arteries is to convey blood at high pressure from the heart ventricles to the tissues of the body and lungs
They have a narrow lumen (relative to wall thickness) to maintain a high blood pressure (~ 80 – 120 mmHg).
They have a thick wall containing an outer layer of collagen to prevent the artery from rupturing under the high pressure.
The arterial wall also contains an inner layer of muscle and elastic fibres to help maintain pulse flow (it can contract and stretch).
What is the flow of blood like in the arteries?
Blood is expelled from the heart upon ventricular contraction and flows through the arteries in repeated surges called pulses
This blood flows at a high pressure and the muscle and elastic fibres assist in maintaining this pressure between pumps
The muscle fibres help to form a rigid arterial wall that is capable of withstanding the high blood pressure without rupturing
Muscle fibres can also contract to narrow the lumen, which increases the pressure between pumps and helps to maintain blood pressure throughout the cardiac cycle
The elastic fibres allow the arterial wall to stretch and expand upon the flow of a pulse through the lumen
The pressure exerted on the arterial wall is returned to the blood when the artery returns to its normal size (elastic recoil)
The elastic recoil helps to push the blood forward through the artery as well as maintain arterial pressure between pump cycles
How do you measure your pulse rate?
Your pulse rate is usually measured at rest and can be determined manually by feeling the radial artery or the carotid artery, which is on the neck, beside the trachea.
You should place your index and middle finger on the artery and count the number of pulses that occur in 15 seconds, and then multiply this value by four to get the number of pulses that occur in one minute.
Pulse rate can also be measured using a stethoscope, which is placed on the sternum.
What is the flow of blood like in the veins
Blood is at very low pressure in the veins which can make it difficult for the blood to move against the downward force of gravity
The veins contain numerous one-way valves in order to maintain the circulation of blood by preventing backflow
Veins typically pass between skeletal muscle groups, which facilitate venous blood flow via periodic contractions
When the skeletal muscles contract, they squeeze the vein and cause the blood to flow from the site of compression
Veins typically run parallel to arteries, and a similar effect can be caused by the rhythmic arterial bulge created by a pulse
What are the causes of occlusion of the coronary arteries?
Buildup of plaque, a fatty substance that is mostly made of cholesterol.
Over time, this hardens into a deposit called atherosclerotic plaque, which narrows the diameter of the coronary artery and reduces blood flow to the heart.
Sometimes parts of the atherosclerotic plaque can rupture, exposing and damaging the tissue underneath and resulting in the formation of a blood clot, or thrombus, which can further restrict blood flow.
What are the consequences of occlusion of the coronary arteries
If blood flow is severely or completely impeded, the part of the heart supplied by that vessel will not receive blood, and therefore the cardiac muscle cells in that part of the heart do not obtain the oxygen and nutrients.
That part of the heart can experience tissue damage or death, leading to a myocardial infarction (heart attack).
The lack of oxygen and nutrients to the heart tissue also results in a shortness of breath, fatigue and pain called angina.
Release and reuptake of tissue fluid in capillaries
Tissue fluid (interstitial fluid), is formed from the blood plasma that is pushed through the capillary walls and into the surrounding tissues.
Tissue fluid contains:
Water
small solutes such as ions
Hormones
Nutrients
Tissue fluid bathes cells and allows the exchange of substances between the blood and the cells.
Plasma proteins, platelets and red blood cells are large and will remain in the capillary, although oxygen will diffuse out of red blood cells into the plasma, and then into the tissue fluid.
The amount of fluid pushed out of the capillary wall is influenced by the hydrostatic pressure exerted by the blood on the capillary wall.
Hydrostatic pressure is high at the arteriole end of the capillary, so fluid is pushed out of the capillary wall into the surrounding tissue.
Hydrostatic pressure is lower at the venule end of the capillary, so fluid is drawn back into the capillaries.
A small amount of fluid will not return to the capillaries and will drain into the lymph ducts, forming lymph.
Exchange of substances between tissue fluid and cells in tissues
This can occur through diffusion or active transport.
Nutrients, hormones and oxygen will move from the tissue fluid into cells.
This means that the concentrations of these substances are lower in the tissue fluid than in the blood plasma at the arterial end of the capillaries.
This causes these materials to move from the blood plasma into the tissue fluid and then into the cells.
Excess water and waste products, including carbon dioxide, will move from the cells into the tissue fluid.
This means the concentration of these substances will be higher in the tissue fluid than the blood plasma at the arterial end of the capillaries.
This causes materials to move from the cells into the tissue fluid and then into the blood plasma.
Drainage of excess tissue fluid into lymph ducts
Around 90% of tissue fluid will drain back into capillaries and the remaining 10% will move into lymphatic capillaries, which are found in the interstitial space. The walls of lymphatic capillaries are thin and permeable (made of a single layer of endothelial cells which contain small gaps between cells).
Inside the lymphatic system, the fluid is called lymph. Lymph is a colourless fluid containing excess tissue fluid, white blood cells, proteins and other substances. Lymph functions to help transport immune cells and remove foreign particles and toxins from the body.
Lymphatic capillaries drain into lymphatic ducts, which transport lymph to lymph nodes. Pressure is low in the lymphatic vessels, which rely on the contraction of surrounding skeletal muscle to squeeze the vessels to move lymph along. Like veins, lymphatic vessels contain valves to ensure that lymph flows in one direction.
Lymph nodes act as a filter, trapping or destroying anything harmful that the body does not need or could cause harm, such as foreign particles and toxins. Dendritic cells (a type of lymphocyte) sample the lymph for pathogens or parts of pathogens.
Lymph can then be returned to the circulatory system. Once in the blood, the waste products and destroyed bacteria can be removed from the blood by the liver or kidneys.
Single and double circulatory systems
Bony fish have a single circulatory system. This means that to complete one circulation around the body, blood has to pass through the heart once.
In contrast, mammals have a double circulatory system. This means that to complete one circulation around the body, blood has to pass through the heart twice.
This double circulation is composed of:
- the pulmonary circulation, where blood is pumped from the heart to the lungs, and then returned to the heart
- the systemic circulation, where blood is pumped from the heart to the rest of the organism and back to the heart.
Advantages of a double circulatory system
The mammalian heart is divided into a right side and a left side by the septum. Deoxygenated blood returns to the right-hand side of the heart and is then pumped to the lungs. In the lungs this blood is oxygenated, and returns to the left-hand side of the heart where it is pumped to the rest of the body.
The physical separation of oxygenated and deoxygenated blood helps to maintain high concentration gradients, allowing more efficient removal of carbon dioxide and a higher rate of oxygen delivery per unit of blood flow.
In this way, the double circulatory system can meet the high metabolic demands of mammals.
