PBT 1

1. What are the basic mechanics and physiology of breathing (anatomy of thorax and lungs)?

Breathing The process of taking air into and expelling it from the lungs The two lungs are the primary organs of the respiratory system. Other components of the respiratory system conduct air to the lungs, such as the trachea (windpipe) which branches into smaller structures called bronchi. The process of breathing (respiration) is divided into two distinct phases, inspiration (inhalation) and expiration (exhalation). During inspiration, the diaphragm contracts and pulls downward while the muscles between the ribs contract and pull upward. This increases the size of the thoracic cavity and decreases the pressure inside. As a result, air rushes in and fills the lungs. During expiration, the diaphragm relaxes, and the volume of the thoracic cavity decreases, while the pressure within it increases. As a result, the lungs contract and air is forced out.

Movement in the two cavities A simplified image of the human body divides the torso into two cavities, the thoracic and abdominal. These cavities share some properties, and have important distinctions as well. Both contain vital organs: the thoracic contains the heart and lungs; the abdominal contains the stomach, liver, gall bladder, spleen, pancreas, small and large intestines, kidneys, bladder, among others. Both cavities are bounded posteriorly by the spine. Both open at one end to the external environment the thoracic at the top, and the abdominal at the bottom. Both share an important structure, the diaphragm the roof of the abdominal cavity and the floor of the thoracic.

Change in the abdominal cavity: shape, not volume Although both the abdominal and thoracic cavities change shape, there is an important structural difference in how they do so. The abdominal cavity changes shape like a flexible fluid-filled structure such as a water balloon. The same principle applies when the abdominal cavity is compressed by the movements of breathing: a squeeze in one region produces a bulge in another. In the context of breathing, the abdominal cavity changes shape, but not volume.

Change in the thoracic cavity: shape and volume It behaves like a flexible gas-filled container, similar to an accordion bellows. When you squeeze an accordion, you create a reduction in the volume of the bellows and air is forced out, and when you pull the bellows open, its volume increases and the air is pulled in. This is because the accordion is compressible and expandable.

Volume and pressure Volume changes in the thoracic cavity result in movement of air. Volume and pressure are inversely related when volume increases, pressure decreases, and when volume decreases, pressure increases. Since air always flows towards areas of lower pressure, increasing the volume inside the thoracic cavity will decrease pressure and cause air to flow into it. This is an inhale.

Pressure/volume shift and shape change During an inhale, the thoracic cavity expands its volume. This pushes downward on the abdominal cavity, which changes shape as a result of the pressure from above. During relaxed, quiet breathing (such as while sleeping) an exhale is a passive reversal of this process. The thoracic cavity and lung tissue which have been stretched open during the inhale spring back to their initial volume, pushing the air out and returning the abdominal cavity to its previous shape. This is referred to as a passive recoil. Its important to note that any reduction in the elasticity of these tissues will result in a reduction of the bodys ability to exhale passively leading to an increase of muscular breathing effort and a host of respiratory problems. In breathing that involves active exhaling (such as blowing out candles, speaking, singing, as well as various Yoga exercises), the musculature surrounding the two cavities contracts in such a way that the abdominal cavity is pushed upward into the thoracic, or the thoracic is pushed downward into the abdominal, or any combination of the two.

Breathing occurs in a context Gravity, posture, activity, habit, intention can affect breathing Shape, depth, rhythm and volume of our breath is a reflection of our habits, training, intentions, body position and state of mind Since our metabolism changes with activity, so much our breathing patterns Breathing and spinal movement are intimately connected: flexion of the spine is the shape change that reduces thoracic volume (exhale) and spinal extension is the shape change that increases thoracic volume (inhale)

Diaphragm Is the principal muscle that causes three dimensional shape change in the thoracic and abdominal cavities Divides the torso into the thoracic and abdominal cavities it is the floor of the thoracic cavity and the roof of the abdominal cavity The uppermost part reaches the space between the third and fourth ribs, and its lowest fibres attach to the front of the third lumbar vertebra (nipple to navel) Has an asymmetrical double-domed shape, with the right dome rising higher than the left. This is because the liver pushes up from below the right dome, and the heart pushes down from above the left dome.

Diaphragm origin and insertion Origin: lower edges of the diaphragms circumference originate from three distinct regions: the bottom of the sternum, the base of the ribcage, and the front of the lower spine. These three regions form a continuous rim of attachment for the diaphragm, and the only bony components of this rim are the back of the xiphoid process and the front surfaces of the first three lumbar vertebrae. The majority of the diaphragm (over 90%) originates on flexible tissue: the costal cartilage of ribs 6 through 10 and the arcuate ligaments which bridge the span from the 10th ribs cartilage to the floating 11th and 12th ribs, and from there to the spine. Insertion: all the muscular fibres of the diaphragm rise upward in the body from their origins. They eventually arrive at the flattened, horizontal top of the muscle, the central tendon, into which they insert. In essence, the diaphragm inserts onto itself its own central tendon, which is fibrous non-contractile tissue.

The diaphragms relations: organic connections The central tendon of the diaphragm is a point of anchorage for the connective tissue that surrounds the thoracic and abdominal organs. Pleura which surround the lungs Pericardium which surrounds the heart Peritoneum which surrounds the abdominal organs Every organ has a membrane that tightly enwraps it, called the visceral membrane. Outside of the visceral membrane is another layer that anchors the organ to the body. This outer membrane is the parietal membrane. It is the parietal membranes that attach the organs to the diaphragm and the inner surfaces of the thoracic and abdominal cavities.

The diaphragms actions: basics The muscular fibres of the diaphragm are oriented primarily along the vertical axis of the body, and this is the direction of its muscular action. Recall that the horizontal central tendon is non-contractile, and can move only in response to the contraction of the muscular fibres, which insert onto it. Like any other muscle, the contracting fibres of the diaphragm pull its insertion and origin (the central tendon and the base of the ribcage) towards each other.

The diaphragms action: origin/insertion stable/mobile The muscular action of the diaphragm is usually associated with a bulging movement in the upper abdomen (belly breath), but this is only the case if the diaphragms origin (the base of the ribcage) is stable, and its insertion (the central tendon) is mobile. If the central tendon is stabilized, and the ribs are free to move, a diaphragmatic contraction will cause an expansion of the ribcage (chest breath), which many people must be caused by the action of muscles other than the diaphragm. This mistaken idea can create a false dichotomy between diaphragmatic and non-diaphragmatic breathing. The unfortunate result of this error is that many people receiving breath training who exhibit chest movement (rather than belly movement) are told that they are not using their diaphragm, which is false. Except in cases of paralysis, the diaphragm is always used for breathing. The issue is whether it is being used efficiently or not. If it were possible to release all of the diaphragms stabilizing muscles, and allow its origin and insertion to freely move towards each other, both the chest and abdomen would move simultaneously. This rarely occurs, as the need to stabilize the bodys mass in gravity will cause many of the respiratory stabilizing muscles (which are also postural muscles) to remain active through all phases of breathing.

The accessory muscles Accessory breathing muscles include the abdominal group, intercostal group, sternocleidomastoids, scalenes, pectoralis minor, serratus anterior, and a host of other muscles that stabilize them It is important to note that we are influencing breathing, not air. Just because a particular region of the chest is moving more than another does not mean that there is more air going into the lung just beneath that movement. A look at the structure of the bronchial tree will reveal the pathway of lung tissue ventilation. This is not altered by the pattern of abdominal/thoracic shape-change. Its understandable that we make this error, because we dont have direct sensory awareness of lung tissue, but we do have direct feedback from the breathing musculature. Thus, it is easy to confused one with the other.

Normal breathing to tidal volume and breathing rate The volume of air entering the lungs with each inspiration and expiration cycle is called tidal volume The minute-ventilation of the lungs is tidal volume per minute Changes in minute-volume always reflect changes in metabolism in a healthy individual High minute-volume reflects increased activity such as running, low minute-volume reflects a low level of activity such as rest In a healthy individual, breathing rate usually follows minute-volume Rapid breathing accompanies a high minute-volume, while slower breathing goes with a lower minute-volume Weight is an indicator of how many cells that person has to oxygenate on a moment-to-moment basis, and minute-volume is a measure of what the body is doing to provide that oxygen Inexplicably rapid or slow breathing, or high or low minute-volume, can indicate trouble and can also cause it it tells us the body is compensating for something unusual

Ventilation Movement of air between the environment and the lungs via inhalation and exhalation Minute ventilation = tidal volume * respiratory rate (the total volume of gas entering the lungs per minute) Alveolar ventilation = (tidal volume dead space) * respiratory rate (the volume of gas per unit time that reaches the alveoli, the respiratory portion of the lungs where gas exchange occurs) Dead space ventilation dead space * respiratory rate (is the volume of gas per unit time that does not reach these respiratory portions but instead remains in the airways (trachea, bronchi, etc.) Dead space volume of air which is inhaled that does not take part in the gas exchange, not all the air in each breath is available for the exchange of oxygen and carbon dioxide It remains in the conducting airways It reaches alveoli that are not perfused or poorly perfused Notion of taking deep, slow breaths to maximize oxygen intake and carbon dioxide elimination at all times is a recipe for metabolic mayhem Our breathing rate/volume is eliminating carbon dioxide from our system faster than it is being produced by our metabolism Oxygen in excess is toxic to our body The entire process of respiration is driven by CO2 from the impulse that brings air into the body, to the chemical balancing act that delivers oxygen to our tissue A rise in blood CO2 eventually signals our brains respiratory centre to send an electrical impulse through the phrenic nerve to contract your diaphragm It is also the presence of CO2 in the blood which allows the hemoglobin to transport the oxygen from the blood into all body tissues When weve blown off too much CO2, the bloods acid-base balance is thrown into excessive alkalinity. When this happens, the hemoglobin holds too tightly onto the oxygen molecules and doesnt release them into the bodys tissues. So, even if we could maximize CO2 loss and O2 gain, this effect could only go as far as the bloodstream where the oxygen will remain undelivered, bound to the hemoglobin. From this perspective, hyperventilation is a paradoxical state in which theres too much oxygen in the bodys bloodstream but not enough in its tissues. Its interesting to note that hyperventilation refers to the chemical condition of blood, not to a particular pattern of rapid or shallow breathing. It is just as possible to hyperventilate while breathing slowly and deeply as it is while breathing rapidly and shallowly. The only requirement is that the minute-volume exceeds the bodys ability to replace the CO2 thats being blown off.

2. What is the role of the cardiovascular system in gas transport (anatomy of the heart and blood vessels at basic level; pulmonary and systemic circulation)?

The human heart pumps blood through the arteries, which connect to smaller arterioles and then even smaller capillaries. It is here that nutrients, electrolytes, dissolved gases, and waste products are exchanged between the blood and surrounding tissues. The capillaries are thin-walled vessels interconnected with the smallest arteries and smallest veins. Approximately 7,000L of blood is pumped by the heart every day. In an average persons life, the heart will contract about 2.5 billion times. Blood flow throughout the body begins its return to the heart when the capillaries return blood to the venules and then to the larger veins. The cardiovascular system consists of a closed circuit: the heart, arteries, arterioles, capillaries, venules, and veins. The venules and veins are part of the pulmonary circuit because they send deoxygenated blood to the lungs to receive oxygen and unload carbon dioxide. The arteries and arterioles are part of the systemic circuit because they send oxygenated blood and nutrients to the body cells while removing wastes. Pulmonary circulation movement of blood from the heart, to the lungs, and back to the heart again. Oxygenated blood leaves the heart, goes to the lungs, and then re-enters the heart. Oxygenated blood leaves the right ventricle through the pulmonary artery. Systemic circulation is the part of the cardiovascular system which carries oxygenated blood away from the heart to the body, and returns deoxygenated blood back to the heart.

The heart The human heart is a muscular organ containing four chambers that is situated just to the left of the midline of the thoracic cavity. It is approximately the size of a mans closed fist. The upper two chambers (atria) are divided by a wall-like structure called the interatrial septum. The lower two chambers (ventricles) are divided by a similar structure called the interventricular septum. Between each atrium and ventricle, valves allow blood to flow in one direction, preventing backflow. Blood that is low in oxygen flows into the right atrium from the veins known as the superior vena cava and inferior vena cava. The superior vena cava carried blood from the heart, neck, chest, and arms. The inferior vena cava carries blood from the remained of the trunk and the legs. Blood in the right atrium then flows through the right atrioventricular (tricuspid) valve into the right ventricle. From here it begins the pulmonary circuit, with deoxygenated blood flowing into the right and left pulmonary arteries and their smaller branches. The blood becomes oxygenated (and carbon dioxide released) while moving through the lungs capillary beds.

Structures of the heart The heart lies inside the thoracic cavity, resting on the diaphragm. It is hollow and cone-shaped, varying in size. The heart is within the mediastinum in between the lungs. Its posterior border is near the vertebral column, and its anterior border is near the sternum. An average adult has a heart that is about 14cm long by 9cm wide. The base of the heart is actually the upper portion, where it is attached to several large blood vessels. This portion lies beneath the second rib. The distal end of the heart extends downward, to the left, ending in a blunt point called the apex, which is even with the fifth intercostal space. The three layers comprising the wall of the heart are the outer pericardium, middle myocardium, and inner endocardium. The pericardium consists of connective tissue and some deep adipose tissue, and it protects the heart by reducing friction. The thick myocardium is mostly made of cardiac muscle tissue that is organized in planes and richly supplied by blood capillaries, lymph capillaries, and nerve fibres. It pumps blood out of the chambers of the heart. The endocardium is made up of epithelium and connective tissue with many elastic and collagenous fibres. It also contains blood vessels and specialized cardiac muscle fibres known as Purkinje fibres. The inside of the heart is divided into four hollow chambers, with two on the left and two on the right. The upper chambers are called atria and receive blood returning to the heart. They have auricles, which are small projections that extend anteriorly. The lower chambers are called ventricles and receive blood from the atria, which they pump out into the arteries. The left atria and ventricle are separated from the right atria and ventricle by a solid wall-like structure (septum). This keeps blood from one side of the heart from mixing with blood from the other side (except in a developing fetus). The atrioventricular valve (AV valve), which consists of the mitral valve on the left and the tricuspid valve on the right, ensures one-way blood flow between the atria and ventricles. The right atrium receives flood from two large veins called the superior vena cava and the inferior vena cava as well as a smaller vein (the coronary sinus), which drains blood into the right atrium from the hearts myocardium. The tricuspid valve has projections (cusps) and lies between the right atrium and ventricle. This valve allows blood o move from the right atrium into the right ventricle while preventing backflow. The cusps of the tricuspid valve are attached to strong fibres called chordae tendineae, which originate from small papillary muscles that project inward from the ventricle walls. These muscles contract as the ventricle contracts. When the tricuspid valve closes, they pull on the chordae tendineae to prevent the cusps from swinging back into the atrium. The right ventricles muscular wall is thinner than that of the left ventricle, as it only pumps blood to the lungs with a low resistance to blood flow. The left ventricle is thicker because it must force blood to all body part, with a much higher resistance to blood flow. As the right ventricle contracts, its blood increases in pressure to passively close the tricuspid valve. Therefore, this blood can only exit through the pulmonary trunk, which divides into the left and right pulmonary arteries that supply the lungs. At the trunks base, there is a pulmonary valve with three cusps that allow blood to leave the right ventricle while preventing backflow into the ventricular chamber. Four pulmonary veins (two from each of the lungs) supply the left atrium with blood. Blood passes from the left atrium into the left ventricle through the mitral valve (bicuspid valve), preventing blood from flowing back into the left atrium from the ventricle. Like the tricuspid valve, the papillary muscles and chordae tendineae prevent the mitral valves cusps from swinging back into the left atrium when the ventricle contracts. The mitral valve closes passively, directing blood through the large artery known as the aorta. At the base of the aorta is the aortic valve, with three cusps. The valve opens to allow blood to leave the left ventricle during contraction. When the ventricle relaxes, the valve closes to prevent blood from backing up into the ventricle. The mitral and tricuspid valves are known as atrioventricular valves because they lie between the atria and ventricles. The pulmonary and aortic valves have half-moon shapes and are therefore referred to as semilunar valves. The right atrium receives low-oxygen blood through the vena cava and coronary sinus. As the right atrium contracts, the blood asses through the tricuspid valve into the right ventricle. As the right ventricle contracts, the tricuspid valve closes. Blood moves through the pulmonary valves into the pulmonary trunk and pulmonary arteries. It then enters the capillaries of the alveoli of the lungs, where gas exchanges occur. This freshly oxygenated blood then returns to the heart through the pulmonary veins, into the left atrium. The left atrium contracts, moving blood through the mitral valve into the left ventricle. When the left ventricle contracts, the mitral valve closes. Blood moves through the aortic valve into the aorta and its branches. The first two aortic branches are called the right and left coronary arteries. They supply blood to the heart tissues, with openings lying just beyond the aortic valve. The body tissues require continual beating of the heart because they need freshly oxygenated blood to survive. Coronary artery branches supply many capillaries in the myocardium. These arteries have smaller branches with connection called anastomoses between vessels providing alternate blood pathways (collateral circulation). These pathways may supply oxygen and nutrients to the myocardium when blockage of a coronary artery occurs. Branches of the cardiac veins drain blood from the myocardial capillaries, joining an enlarged vein, the coronary sinus, which empties into the right atrium.

Conduction system Strands and clumps of specialized cardiac muscle contain only a few myofibrils and are located throughout the heart. These areas initiate and distribute impulses through the myocardium, comprising the cardiac conduction system that coordinates the cardiac cycle. The sinoatrial node (SA node) is a small mass of specialized tissue just beneath the epicardium, in the right atrium. It is located near the opening of the superior vena cava, with fibres continuous with those of the atrial syncytium. The SA nodes cells can reach threshold on their own, initiating impulses through the myocardium, stimulating contraction of cardiac muscle fibres. Its rhythmic activity occurs 70-80 times per minute in a normal adult. Since it generates the hearts rhythmic contractions, it is often referred to as the pacemaker. The path of a cardiac impulse travels from the SA node into the atrial syncytium, and the atria begin to contract almost simultaneously. The impulse passes along junctional fibres of the conduction system to a mass of specialized tissue called the atrioventricular node (AV node), located in the inferior interatrial septum, beneath the endocardium. The AV node provides the only normal conduction pathway between the atrial and ventricular syncytia. Impulses are slightly delayed due to the small diameter of the junctional fibres. The atria, therefore, have more time to contract and empty all of their blood into the ventricles before ventricular contraction occurs. When the cardiac impulse reaches the distal AV node, it passes into a large AV bundle (bundle of His), entering the upper part of the interventricular septum. Nearly halfway down the septum, these branches spread into enlarged Purkinje fibres, extending into the papillary muscles. They continue to the hearts apex, curving around the ventricles and passing over their lateral walls. The Purkinje fibres have numerous small branches that become continuous with cardiac muscle fibres and irregular whorls. Purkinje fibre stimulation cause the ventricular walls to contract in a twisting motion, to force blood into the aorta and pulmonary trunk. An electrocardiogram (EKG) is used to record electrical changes in the myocardium during the cardiac cycle. The most important ions that influence heart action are potassium and calcium. Excess extracellular potassium ions (hyperkalemia) decrease contraction rates and forces, while deficient extracellular potassium ions (hypokalemia) may cause a potentially life threatening abnormal heart rhythm (arrhythmia). Excess extracellular calcium ions (hypercalcemia) can cause the heart to contract for an abnormally long time, while low extracellular calcium ions (hypocalcemia) depress heart action.

Functions of the heart The heart chambers are coordinated so that their actions are effective. The atria contract (atrial systole) as the ventricles relax (ventricular diastole). Likewise, ventricles contract (ventricular systole) as atria relax (atrial diastole). Then a brief period of relaxation of both atria and ventricles occurs. This complete series of events makes up a heartbeat, also called a cardiac cycle. One cardiac cycle causes pressure in the heart chambers to rise and fall and valve to open and close. Early during diastole, pressure in the ventricles is low, causing the AV valves to open and the ventricles to fill with blood. Nearly 70% of returning blood enters the ventricles before contraction. As the atria contract, the remaining 30% is pushed into the ventricles. As the ventricles contract, ventricular pressure rises. When ventricular pressure exceeds atrial pressure, the AV valves close and papillary muscles contract, preventing the cusps of the AV valves from bulging into the atria excessively. During ventricular contraction, the AV valves are closed, and atrial pressure is low. Blood flows into the atria while the ventricles are contracting, so that the atria are prepared for the next cardiac cycle. As ventricular pressure exceeds pulmonary trunk and aorta pressure, the pulmonary and aortic valves open. Blood is ejected from the ventricles into these arteries, and ventricular pressure drops. When ventricular pressure is lower than in the aorta and pulmonary trunk, the semilunar valves close. When ventricular pressure is lower than atrial pressure, the AV valves open, and the ventricles begin to refill. A heartbeat makes a characteristic double thumping sound when heard through a stethoscope. This is due to the vibrations of the heart tissues related to the valve closing. The first thumping sound occurs during ventricular contraction when the AV valves close. The second sound occurs during ventricular relaxation when the pulmonary and aortic valves close. Cardiac muscle fibres are similar in function to skeletal muscle fibres, but are connected in branched networks. If any part of the network is stimulated, impulses are sent throughout the heart, and it contracts as a single unit. A functional syncytium is a mass of merging cell that functions as a unit. There are two of these structures in the heart one in the atrial walls and another in the ventricular walls. A small area of the right atrial floor is the only part of the hearts muscle fibres that is not separated by the hearts fibrous skeleton. Here, cardiac conduction system fibres connect the atrial syncytium and the ventricular syncytium. Newly oxygenated blood flows into the left and right pulmonary veins, returning to the left atrium. Blood then flows through the left atrioventricular (bicuspid or mitral) valve into the left ventricle, passing through the aortic semilunar valve into the systemic circuit (via the ascending aorta). The systemic circuit moves blood to the body tissues, supplying their required oxygen. Should blood flow backward at this point due to a valve malfunction, a heart murmur will result. To summarize, the right side of the heart pumps oxygen-poor blood to the lungs, and the left side pumps oxygen-rich blood toward the body tissues. The contraction of the heart is called systole, and its relaxation is called diastole. The systolic blood pressure is the first number in a blood pressure reading, measuring the strength of contraction. The diastolic blood pressure is the second number in a blood pressure reading, measuring the strength of relaxation. The right ventricle does not need to pump blood with as much force as the left ventricle. This is so because the right ventricle supplies blood to the nearby lungs and the pulmonary vessels are wide and relatively short. This means that the walls of the right ventricle are thinner and less muscular than those of the left ventricle, which must pump blood to the entire body.

Blood vessels and circulation The blood vessels of the human body carry blood to every type of tissue and organ. Vessels decrease in size as they move away from the heart (arteries and arterioles), ending in the capillaries, and then increase in size as they move toward the heart (venules and veins). The largest artery in the body is the aorta, with the largest veins being the vena cava. There are five general classes of blood vessels in the cardiovascular system: arteries, arterioles, capillaries, venules, and veins. Arteries are elastic vessels that are very strong, able to carry blood away from the heart under high pressure. The subdivide into thinner tubes that give rise to branches, finer arterioles. An arterys wall consist of three distinct layers. The innermost tunica interna is made up of a layer of simple squamous epithelium known as endothelium. It rests on a connective tissue membrane with many elastic, collagenous fibres. The endothelium helps prevent blood clotting and may also help in regulating blood flow. It releases nitric oxide to relax the smooth muscle of the vessel. Vein walls are similar but not identical to artery walls. The middle tunica media makes up most of an arterial wall, including smoth muscle fibres and a thick elastic connective tissue layer. The outer tunica externa (tunica adventitia) is thinner, mostly made up of connective tissue with irregular fibres it is attached to the surrounding tissues. Smooth artery and arteriole muscles are innervated by the sympathetic nervous system. Vasomotor fibres receive impulses to contract and reduce blood vessel diameter (vasoconstriction). When inhibited, the muscle fibres relax and the vessels diameter increases (vasodilation). Changes in artery and arteriole diameters greatly affect blood flow and pressure. Larger arterioles also have three layers in their walls, which get thinner as arterioles lead to capillaries. Very small arteriole walls only have an endothelial lining and some smooth muscle fibres, with a small amount of surrounding connective tissue. The smallest-diameter blood vessels are capillaries, which connect the smallest arterioles to the smallest venules. The walls of capillaries are also composed of endothelium and form the semipermeable layer through which substances in blood are exchanged with substances in tissue fluids surrounding cells of the body. Capillary walls have thin slits where endothelial cells overlap. These slits have various sizes, affecting permeability. Capillaries of muscles have smaller openings than those of the glands, kidneys, and small intestine. Tissues with higher metabolic rates (such as muscles) have many more capillaries than those with slower metabolic rates (such as cartilage). Some capillaries pass directly from arterioles to venules while others have highly branched networks. Precapillary sphincters control blood distribution through capillaries. Based on the demand of cells, these sphincters constrict or relax so that blood can follow specific pathways to meet tissue cellular requirements. Gases, metabolic by-products, and nutrients are exchanged between capillaries and the tissue fluid surrounding body cells. Capillary walls allow the diffusion of blood with high levels of oxygen and nutrients. They also allow high levels of carbon dioxide and other wastes to move from the tissues into the capillaries. Plasma proteins usually cannot move through the capillary walls due to their large size, so they remain in the blood. Blood pressure generated when capillary walls contract provides force for filtration via hydrostatic pressure. Blood pressure is strongest when blood leaves the heart and weaker as the distance from the heart increases because of friction (peripheral resistance) between the blood and the vessel walls. Therefore, blood pressure is highest in the arteries, less so in the arterioles, and lowest in the capillaries. Filtration occurs mostly at the arteriolar ends of the capillaries because the pressure is higher than at the venular ends. Plasma proteins trapped in capillaries create an osmotic pressure that pulls water into the capillaries (colloid osmotic pressure). Capillary blood pressure favours filtration while plasma colloid osmotic pressure faours reabsorption. At the venular ends of capillaries, blood pressure has decreased due to resistance so that reabsorption can occur. More fluid usually leaves capillaries than returns to them. Lymphatic capillaries have closed ends and collect excess fluid to return it via lymphatic vessels to the venous circulation. Unusual events may cause excess fluid to enter spaces between tissue cells, often in response to chemicals such as histamine. If enough fluid leaks out, lymphatic vessels can be overwhelmed, and affected tissues swell and become painful.

3. How is gas transported by the blood how does gas get across the blood gas barrier, ventilation/perfusion relationship, how are gases moved to the peripheral tissues.

Alveoli Walls are extremely thin They have a large surface area in relation to volume They are fluid lined enabling gases to dissolve They are surrounded by numerous capillaries Alveoli are tiny balloon-like structures that inflate with each inhalation. The membranes that surround these tiny sacs are only one cell thick, and they are coated with a special fluid to enable inflation and dissolve gases. This fluid contains a substance that reduces the surface tension, which could otherwise cause the alveoli to collapse. The sacs have tiny blood vessels in direct contact with them, and these blood vessels have walls that are only one cell thick. The alveoli are inflated when the diaphragm contracts and expands the chest cavity. This causes the pressure in the alveoli to drop below atmospheric pressure and air to rush in to inflate them. Gas exchange of oxygen and carbon dioxide takes places in the alveoli. Oxygen from the inhaled air diffuses through the walls of the alveoli and adjacent capillaries into the red blood cells. Oxygen diffuses from the air in the alveoli into the blood and carbon dioxide diffuses from the blood into the air in the alveoli. Oxygen atom would first have to travel through the alveolar epithelial cell, then it would have to pass through any materials connecting the alveolar epithelium to the capillary. Finally it would have to pass through the capillary wall which is only one, simple squamous cell thick. When blood first arrives are the pulmonary capillary at its arteriole end, the partial pressures of carbon dioxide and oxygen are 45 mmHg and 40 mmHg. The partial pressure of carbon dioxide and oxygen in the alveoli are 40 mmHg and 104 mmHg (not the same at the atmospheric partial pressures due to residual capacity and residual volume, which is air that doesnt normally leave the respiratory tract; alveolar air is a mixture of atmospheric air and functional residual capacity air). Thus, carbon dioxide leaves the blood along its pressure gradient, while oxygen enters the blood along its pressure gradient. Once through the alveolar and capillary walls, the oxygen combines with hemoglobin to form oxyhemoglobin and is transported within the bloodstream. Carbon dioxide enters the red blood cells as waste product. In the RBC, it reacts with water to form carbonic acid (CA). CA dissociates to bicarbonate ions and hydrogen ions, where they diffuse into plasma and hydrogen ions are buffered by hemoglobin. Approximately 5% of total body CO2 is carried as carboxyhemoglobin on proteins and approximately 90% is carried as bicarbonate ions in the plasma. Ventilation/perfusion ratio of V/Q ratio is a measurement used to assess the efficiency and adequacy of the matching of ventilation (the air that reaches the alveoli) and perfusion (the blood that reaches the alveoli) Ventilation Gravity and lungs weight act on ventilation by increasing pleural pressure at the base (making it less negative) and thus reducing the alveolar volume. The lowest part of the lung in relation to gravity is called the dependent region. At the dependent region smaller volumes mean the alveoli are more compliant (more distensible) and so capable of wider oxygen exchanges with the external environment. The actual values in the lung vary depending on the position within the lung. If taken as a whole, the typical value is approximately 0.8. Because the lung is centered vertically around the heart, part of the lung is superior to the heart and part is inferior, which has a resulting impact on the V/Q ratio Apex of lung higher Base of lung lower In a subject standing in orthostatic position (upright) the apex of the lung shows higher V/Q ratio, while at the base of the lung, the ratio is lower but nearer to the optimal value for reaching adequate blood oxygen concentrations. The main reason for lower V/Q ratio at the base is that both ventilation and perfusion increase when going from the apex to the base, but Q does it more strongly thus lowering the V/Q ratio. A lower V/Q ratio impairs pulmonary gas exchange and is a cause of low arterial partial pressure of oxygen (PAO2) and excretion of carbon dioxide is also impaired. Chronic bronchitis and acute pulmonary edem A higher V/Q ratio decreases PACO2 and increases PAO2, typically associated with pulmonary embolism Ventilation is wasted, as it fails to oxygenate any blood