Bio Chapter 42

Ventricles (more)

-Compared to the atria, the ventricles have thicker walls and contract much more forcefully especially the left ventricle, which pumps blood throughout the body via the systemic circuit. -Although the left ventricle contracts with greater force than the right ventricle, it pumps the same volume of blood as the right ventricle during each contraction

Gills

-Outfoldings of the body surface that are suspended in the water -gills often have a total surface area much greater than that of the rest of the body's exterior

Ions

-Regulate pH of the blood -regulate the osmotic pressure of the blood -Plasma proteins also combat viruses and other foreign agents that invade the body (via immunoglobulins, or antibodies) -Plasma also contains fibrinogens, which are clotting factors that help plug leaks when blood vessels are injured -plasma also contains substances in transit from one part of the body to another, including nutrients, metabolic wastes, respiratory gases, and hormones. Plasma has a much higher protein concentration than interstitial fluid (otherwise the two fluids are similar)

Advantages & disadvantages of an open circulatory system

-The lower hydrostatic pressure of an open circulatory system make them less costly than closed systems in terms of energy expenditure -hydrostatic pressure generated by open circulatory systems can be used to extend body parts such as in spiders

Surfactant

-a mixture of phospholipids and proteins that are produced by the alveoli to prevent collapse under high surface tension

Capillary function

-at any given time only about 5-10% of the body's capillaries have blood flowing through them. However, each tissue has many capillaries so all tissues still have blood flowing to them -major organs are usually always filled with blood -the skin has varying levels of blood to regulate body temperature

Double circulation

-circulatory systems of amphibians, reptiles, and mammals -the pimps for the two circuits are combined into a single organ, the heart. Having both pumps within a single heart simplifies coordination of the pumping cycles. One pump, right side of the heart, delivers oxygen-poor blood to the capillary beds of the gas exchange tissues, where the movement of O2 into the blood and CO2 out of the blood. -after the oxygen-enriched blood leaves the gas exchange tissues, it enters the other pump, the left side of the heart. Contraction of the heart propels this blood to capillary beds in organs and tissues throughout the body.

Blood Pressure

-flows from areas of higher blood pressure to areas of lower blood pressure -the heart is the site of the highest blood pressure -the part of the force directed lengthwise in an artery causes the blood to flow away from the heart, the site of the highest pressure. The part of the force exerted sideways stretches the wall of the artery -by the time the blood enters the veins, this resistance has dissipated much of the pressure generated by the pumping heart

How an amphibian breathes

-muscles lower the floor of an amphibian's oral cavity, drawing in air through its nostrils. Next, with the nostrils and mouth closed, the floor of the oral cavity rises, forcing air down the trachea. During exhalation, air is forced back out by the elastic recoil of the lungs and compression of the muscular body wall

Blood flow in capillary beds

-one mechanism is constriction or dilation of the arterioles that supply capillary beds. -another mechanism is pre-capillary sphincters (rings of smooth muscle located at the entrance to capillary beds. Opening and closing these muscular rings regulate and redirect the passage of blood into particular sets of capillaries. -The signals regulating blood flow by these mechanisms include nerve impulses, hormones traveling throughout the blood stream, and chemicals produced locally. -ex. histamine released by cells at a wound site causes vasodilation. The result is increased blood flow and increased access of disease-fighting white blood cells to invading microorganisms

Platlets

-pinched-off cytoplasmic fragments of specialized bone marrow cells. -serve both structural and molecular functions in blood clotting

Benefits of Double Circulation

-provides a vigorous flow of blood to the brain, muscles, and other organs because the heart repressurizes the blood destined for these tissues after it passes through the capillary beds of the lungs or skin. -blood pressure is often much higher in the systemic circuit than in the gas exchange circuit. -in contrast, single circulation blood flows under reduced pressure directly from the gas exchange organs to other organs.

Benefits of closed circulatory systems

-relatively high blood pressure (enables the effective delivery of O2 and nutrients to the cells of larger and more active animals) -particularly well suited to regulating the distribution of blood to different organs

Countercurrent Exchange

-the efficiency of gas exchange is maximized by countercurrent exchange- the exchange of a substance or heat between two fluids flowing in opposite directions -countercurrent exchange mechanisms are remarkable efficient. In the fish gill, more than 80% of the O2 dissolved in the water is removed as the water passes over the respiratory surface.

Exchange

-two opposing forces control the movement of fluid between the capillaries and the surrounding tissues 1. blood pressure tends to drive fluid out of the capillaries, 2. the presence of blood proteins tends to pull fluid back. (many blood proteins are too large to pass readily through the endothelium and they remain in the capillaries. these dissolved proteins are responsible for much of the blood's osmotic pressure) -on average, blood pressure is greater than the opposing forces, leading to a net loss of fluid from the capillaries. The net loss is generally greatest at the arterial end of these vessels, where blood pressure is the highest

Regulation of Blood Pressure

-vasoconstriction and vasodilation are often coupled to changes in cardiac output that also affect blood pressure. This coordination of regulatory mechanisms maintains adequate blood flow as the body's demands on the circulatory system change. -ex. arterioles in working muscle dilate and the increased blood flow into the capillaries causes the blood pressure to drop (and therefore blood flow). Cardiac output increases and the blood pressure remains constant.

Leukocytes

-white blood cells -their function is to fight infections. -some are phagocytic, engulfing and digesting microorganisms as well as debris from the body's own dead cells. Other leukocytes, called lymphocytes develop into B cells and T cells that mount immune responses against foreign substances.

Control of Heart Rhythm

1. Impulses from the SA node first spread rapidly through the walls of the atria, causing both atria to contract 2. During atrial contraction, the impulses originating at the SA node reach other auto-rhythmic cells located in the wall between the left and right atria. These cells form a relay point called the atrioventricular (AV) node

Mammalian Circulation

1. The right ventricle contracts and pumps blood to the lungs via 2. The pulmonary arteries. Ad blood flows through 3. capillary beds in the left and right lungs, it loads O2 and unloads CO2. Oxygen-rich blood returns from the lungs via the pulmonary veins to 4. the left atrium of the heart. Next, the oxygen rich blood flows into the 5. hearts left ventricle, which pumps the oxygen-rich blood out to body tissues through the systemic circuit. Blood leaves the left ventricle via the 6. aorta. Which conveys blood to arteries leading throughout the body. The first branches leading from the aorta are the coronary arteries, which supply blood to the heart muscle itself. Branches lead to 7. capillary beds in the head and arms (forelimbs). The aorta then descends into the abdomen, supplying oxygen-rich blood to arteries leading to 8. capillary beds in the abdominal organs and legs (hind limbs). Within capillaries, there is a net diffusion of O2 from the blood to the tissues and of CO2 (produced by cellular respiration) into the blood. Capillaries rejoin, forming venules, which convey blood into veins. Oxygen-poor blood from the head, neck, and forelimbs is channeled into a large vein, 9. the superior vena cava. another large vein, the inferior vena cava, drains blood from the trunk and hind limbs. The two venae cavae empty their blood into t 11. the right atrium, from which the oxygen-poor blood flows into the right ventricle

Adaptations that permit effective exchange

1. one adaptation is a body plan that places many or all cells in direct contact with the environment. Each cell can thus exchange materials directly with the surrounding medium. This type of body plan is only found in certain invertebrates, including cnidarians and flatworms. 2. The second found in all other animals, is a circulatory system. Such systems move fluid between each cell's immediate surroundings and the body tissues where exchange with the environment occurs.

pH of human blood

7.4

Cl0sed Circulatory systems

A circulatory fluid called blood is confined to vessels and is distinct from the interstitial fluid. One or more hearts pump blood into large vessels that branch into smaller ones that infiltrate the organs. Chemical exchange occurs between the blood and the interstitial fluid, as well as between the interstitial fluid and body cells.

Fluid return by the Lymphatic System

Adult humans lose 4-8 L of fluid from capillaries to the surrounding tissues each day. There is also some leakage of blood proteins. -lost fluid and proteins are returned to the blood via the lymphatic system, which includes a network of tiny vessels intermingled among capillaries of the cardiovascular system, as well as larger vessels into which smaller vessels empty.

Changes in Blood Pressure During the Cardiac Cycle

Arteriole blood pressure is highest when the heart contracts during ventricular systole. The pressure at this time is called systolic pressure

Natural selection & Double circulation

As ectotherms, birds and mammals use about ten times as much energy as equal-sized ectotherms. Their circulatory systems therefore need to deliver about ten times as much fuel and O2 to their tissues and remove about ten times as much CO2 and other wastes. -This large traffic of substances is made possible by the separate and independently powered systemic and pulmonary circuits and by large hearts that pump the necessary volume of blood. -A POWERFUL FOUR-CHAMBERED HEART AROSE INDEPENDENTLY IN THE DISTINCT ANCESTORS OF BIRDS AND MAMMALS AND THUS REFLECTS CONVERGENT EVOLUTION

Blood flow velocity

Blood slows as it moves from arteries to arterioles and to the much narrower capillaries. -The reason is that the number of capillaries is enormous. -each artery conveys blood to so many capillaries that the total cross-sectional area is much greater in capillary beds than in the arteries or any other part of the circulatory system

Circulatory system

By transporting fluid throughout the body, the circulatory system functionally connects the aqueous environment of the body cells to the organs that exchange gases, absorb nutrients, and dispose of wastes.

Bohr shift

CO2 reacts with water to form carbonic acid, which lowers pH of the surroundings. Low pH, in turn, decreases the affinity of hemoglobin for O2. Where CO2 production is higher, hemoglobin releases more O2, which can then be used to support more cellular respiration

Organization of Vertebrate Circulatory Systems

Cardiovascular system- blood circulates to and from the heart through an extensive network of vessels.

Platlets

Cell fragments that are involved in the clotting process

Valves

Four valves in the heart prevent back flow and keep blood moving in the correct direction. -they are made of flaps of connective tissue (the valves open when pushes from one side and closed when pushed from the other)

Gastrovascular cavities

GVC function in the distribution of substances throughout the body, as well as in digestion. an opening at one end connects the cavity to the surrounding water. Fluid bathes both the inner and outer tissues layers, facilitating exchange of gasses and cellular waste. Only the cells lining the cavity have direct access to nutrients released by digestion. Nutrients can diffuse only short distance to reach the cells of the outer tissue layer.

Blood pressure and gravity

Gravity has a significant effect on blood pressure.

How a Mammal breathes

Negative pressure breathing- pulling, rather than pushing, air into their lungs. Using muscle contraction to actively expand the thoracic cavity, mammals lower air pressure in their lungs below that of the air outside their body. Because gas flows from a region of higher pressure to a region of lower pressure, air rushes through the nostrils and mouth and down the breathing tubes to the alveoli. During exhalation, the muscles controlling the thoracic cavity relax and the volume of the cavity is reduced. -INHALATION IS ALWAYS ACTIVE AND REQUIRES WORK, WHEREAS EXHALATION IS USUALLY PASSIVE

Capillary beds

Networks of capillaries that infiltrate tissues, pass within a few cell diameters of every cell in the body -across the thin walls of capillaries, chemicals, including dissolved gases, are exchanged by diffusion between the blood and the interstitial fluid around the tissue cells.

Respiratory Media

O2 is plentiful in air making up about 21% of the Earth's atmosphere by volume. -Air is much less dense and viscous than water (easier to move and force through small passageways) -as a result, breathing air is relatively easy and need not be particularly efficient -Gas exchange in water is much more demanding -the amount of O2 dissolved in a given volume of water varies but is always much less than dissolved in the air -Water's lower O2 content, greater density, and greater viscosity mean that aquatic animals such as fishes and lobsters must expend considerable energy to carry out gas exchange.

Cardiac Cycle

One complete sequence of pumping and filling is referred to as the cardiac cycle.

Regulating heart rate

Physiological cues alter heart tempo by regulating the pacemaker function of the SA node. two portions of the nervous system, the sympathetic and parasympathetic divisions are largely responsible for this regulation. 1. -sympathetic speeds up -parasympathetic slows down 2. -hormones (such as epinephrine) also affect the pacemaker 3. -Body temperature is the third type of input that affects the pacemaker.

Erythrocytes

Red blood cells -the most numerous blood cells -O2 transport (structure is related to function)

Sinoatrial (SA) Node

Sets the rate and timing at which all cardiac muscle cells contract (in contrast, some arthropods have pacemakers located in the nervous system, outside the heart) -group of auto-rhythmic cells located in the wall of the right atrium, near where the superior vena cava enters the heart -SA node produces electrical impulses much like those produced by nerve cells. because cardiac muscle cells are electrically coupled through gap junctions, impulses from the SA node spread rapidly within heart tissue.

Endothelium

The blood vessels contain a central lumen lined with an endothelium -A single layer of flattened epithelial cells -the smooth surface of the endothelium minimizes resistance to the flow of blood.

Ventricles

The chambers responsible for pumping blood out of the heart

Electrocardiogram (EKG)

The currents are recorded by electrodes placed on the skin. The resulting graph of current against time has a characteristic shape that represents the stages in the cardiac cycle

Capillaries

The small vessels of the arterioles convey blood to the capillaries -microscopic vessels with very thin, porous walls.

Atria (more)

The two atria have relatively thin walls and serve as collection chambers fro blood returning to the heart from the lungs or other body tissues. -Much of the blood that enters the atria flows into the ventricles while all heart chambers are relaxed. The remainder is transferred by contraction of the atria before the ventricles begin to contract.

Cardiac Output

The volume of blood each ventricle pumps per minute -Two factors determine cardiac output: 1.the rate of contraction (heart rate-number of beats per minute) 2. stroke volume (the amount of blood pumped by a ventricle in a single contraction)

Arteries & Veins

The wall of arteries and veins have a more complex organization than those of capillaries. -both arteries and veins have two layers of tissue surrounding the endothelium. -the outer layer is formed by connective tissue that contains elastic fibers, which allow the vessel to stretch and recoil and collagen, which provides strength -the layer next to the endothelium contains smooth muscle and more elastic fibers.

Veins

Venules converge into veins -the vessels that carry blood back to the heart -arteries and veins are distinguished by the direction in which they carry blood, not by the O2 content or other characteristics of blood the contain.

Plasma

Vertebrate blood is a connective tissue consisting of cells suspended in a liquid matrix called plasma. cellular elements (cells and cell fragments) occupy about 45% of the volume of blood. The remainder is plasma. -dissolved in the plasma are ions and proteins that, together with the blood cells, function in osmotic regulation, transport, and defense. -among the many solutes in plasma are inorganic salts in the form of dissolved ions, sometimes referred to as blood electrolytes. The dissolved ions are an essential component of the blood b/c they buffer the blood at a pH of 7.4

Circulatory systems link exchange surfaces with cells throughout the body

When there is a difference in concentration, diffusion can result in net movement. But such movement is very slow for distances of more than a few millimeters. The time it takes for a substance to diffuse from one place to another is proportional to the square of the distance. -Given that net movement by diffusion is rapid only over very small distances: natural selection has resulted in two basic adaptations that permit effective exchange for all of an animal's cells.

Circulatory system three basic components

a circulatory fluid, a set of interconnecting vessels, and a muscular pump, the heart

Tracheal System

a network of air tubes that branch throughout the body. The largest tubes, called tracheae, open to the outside. The finest branches extend close to the surface of nearly every cell, where gas is exchanged by diffusion across the moist epithelium that lines the tips of the tracheal branches -because the tracheal system brings air within a very short distance of virtually every body cell in an insect, it can transport O2 and CO2 without the participation of the animal's open circulatory system

Diaphragm

a sheet of skeletal muscle that forms the bottom wall of the cavity. -Contracting one set of rib muscles expands the rib cage, the front wall of the thoracic cavity by pulling the ribs upward and the sternum outward

PO2 in Alveoli

always lower than in the air

Sickle-cell disease

an abnormal form of hemoglobin polymerizes into aggregates. Because of the concentration of hemoglobin in erythrocytes is so high, these aggregates are large enough to distort the erythrocyte into an elongated, curved shape that resembles a sickle. This abnormality results from an alteration in the amino acid sequence of hemoglobin at a single position. -significantly impairs the function of the circulatory system. Sickled cells often lodge in arterioles and capillaries, preventing O2 and nutrient delivery and removal of CO2 and wastes.

Respiratory Pigments

animals transport most of their O2 bound to proteins called respiratory pigments. These pigments circulate with the blood or hemolymph and are often contained within specialized cells. -greatly increase the amount of O2 that can be carried in the circulatory fluid. -the respiratory pigment of almost all vertebrates and many invertebrates is hemoglobin. It is contained in erythrocytes and has four subunits, each with a cofactor called a heme group that has an iron atom at its center. Each iron atom binds one molecule of O2 so a hemoglobin molecule can carry four molecules of O2 -WHEN O2 BINDS TO ONE SUBUNIT, THE OTHERS CHANGE SHAPE SLIGHTLY, INCREASING AFFINITY FOR O2, WHEN FOUR O2 MOLECULES ARE BOUND AND ONE SUBUNIT UNLOADS ITS P2, THE OTHER THEREE SUBUNITS MORE READILY UNLOAD O2 AS AN ASSOCIATED SHAPE CHANGE LOWERS THEIR AFFINITY FOR O2

Vasoconstriction

as the smooth muscles in arterioles walls contract, the arterioles narrow. Narrowing of the arterioles increases blood pressure upstream in the arteries

How a bird breathes

birds use eight or nine air sacs situated on either side of the lungs. The air sacs do not function directly in gas exchange but act as bellows that keep air flowing through the lungs. Instead of alveoli, which are dead ends, the sites of gas exchange in bird lungs are tine channels called parabronchi. Passage of air through the entire system-lungs and air sacs- requires two cycles of inhalation and exhalation

Breathing control (more)

breathing control is effective only if ventilation is matched to blood flow through alveolar capillaries. DURING EXERCISE, INCREASED BREATHING RATE, WHICH ENHANCES O2 UPTAKE AND CO2 REMOVAL WITH AN INCREASE IN CARDIAC OUTPUT

Venules

capillaries converge into these vessels.

Portal Veins

carry blood between pairs of capillary beds -ex. hepatic portal vein carries blood from the capillary beds in the digestive system to capillary beds in the liver

Arteries

carry blood from the heart to organs throughout the body

Hear Murmur

caused by blood squirting backwards through a defective valve

Respiratory surfaces

cells that carry out gas exchange have a plasma membrane that must be in contact with an aqueous solution (respiratory surfaces are therefore always moist) -The movement of O2 and CO2 across respiratory surfaces takes place by diffusion -respiratory organs that are extensively folded or branched, thereby enlarging the available surface area for gas exchange (gills, tracheae, and lungs)

Bronchioles

fine tubes

Systemic Circuit

following the exchange of O2 and CO2, as well as nutrients and waste products, the now oxygen-poor blood returns to the heart

Atrioventricular (AV) node

here the impulses are delayed for about .1 seconds before spreading to the heart apex. This delay allows the atria to empty completely before the ventricles contract. Then the signals from the AV node are conducted to the heart apex and throughout the ventricular walls by specialized structures called bundle branches and Purkinje fibers

Open Circulatory Systems

in open circulatory systems: the circulatory fluid, called hemolymph, is also the interstitial fluid that bathes body cells. Arthropods, such as grasshoppers and mollusks have open circulatory systems.

Pulmocutaneous circuit

includes capillaries in both the lungs and the skin, as in many amphibians

positive pressure breathing (amphibians)

inflating the lungs with forced airflow.

Atrioventricular (AV) valve

lies between each atrium and ventricle -AV valves are anchored by strong fibers that prevent them from turning inside out -pressure generated by the powerful contraction of the ventricles closes the AV valves, keeping blood from flowing back into the atria

Lungs

localized respiratory organs -representing an infolding of the body surface, they are typically subdivided into numerous pockets. -The entire system of air ducts is covered by cilia and a thin film of mucus. The mucus traps dust, pollen, and other particulate contaminants and the beating cilia move the mucus upward to the pharynx where it can be swallowed

Semilunar Valves

located at the two exits of the heart: where the aorta leaves the left ventricle and where the pulmonary artery leaves the right ventricle. -these valves are pushed open by the the pressure generated during contraction of the ventricles. When the ventricles relax, blood pressure built ip in the aorta and pulmonary artery closes the semilunar valves and prevents significant backflow

Lymphatic system

lost fluid and proteins return to the blood via the lymphatic system, which includes a network of tiny vessels intermingled among capillaries of the cardiovascular system, as well as larger vessels into which smaller vessels empty

Nitric Oxide (NO)

major inducer of vasodilation and endothelin, a peptide, as the most potent inducer of vasoconstriction.

Ventilation

movement of the respiratory medium over the respiratory surface -maintains the partial pressure gradients of O2 and CO2 across the gill that are necessary for gas exchange

Heart rate

number of beats per minute

Negative Pressure Breathing (mammals)

pulling, rather that pushing, air into the lungs.

Pulse

rhythmic bulging of the artery walls with each heartbeat

Lymph nodes

small, lymph-filtering organs that play an important role in the body's defense. Inside each lymph node is a honeycomb of connective tissue with spaces filled by white blood cells, which function in defense

Capillaries

smallest blood vessels, having a diameter only slightly greater than that of a red blood cell. -thin walls, which consist of just an endothelium and a surrounding extracellular layer called the basal lamina -the exchange of substances between the blood and interstitial fluid occurs only in capillaries because there are blood vessel walls thin enough to permit this exchange.

Residual Volume

the air that remains after a forced exhalation

Breathing

the alternating inhalation and exhalation of air

Stroke Volume

the amount of blood pumped by a ventricle in a single contraction

Single circulation

the blood passes through the heart once in each complete circuit through the body -in single circulation, blood that leaves the heart passes through two capillary beds before returning to the heart. When blood flows through a capillary bed, blood pressure drops substantially. The drop in blood pressure in the gills limits the rate of blood flow in the rest of the animal's body. As the animal swims, the contraction and relaxation of its muscles help accelerate the relatively sluggish pace of circulation

Systolic pressure

the blood pressure when the heart contracts

Atria

the chambers that receive blood entering the heart.

Systole

the contraction phase of the cardiac cycle

Diastolic Blood Pressure

the elastic walls of the arteries snap back during diastole and there is a lower but substantial blood pressure as the ventricles are relaxed -because the arteries remain pressurized throughout the cardiac cycle, blood continuously flows into arterioles and capillaries

lymph

the fluid lost by capillaries is called lymph; its composition is about the same as that of interstitial fluid -the joining of the lymphatic and cardiovascular systems enables lipids to be transferred from the small intestine to the blood. -the movement of lymph from peripheral tissues to the heart relies on much the same mechanisms that assist blood flow in veins. Lymph vessels like veins have valves that prevent the back flow of fluid. rhythmic contraction of the vessel walls help draw fluid into the small lymphatic vessels. Skeletal muscle plays a role in moving lymph

Hemoglobin

the iron-containing protein that transports O2

pH of blood

the medulla uses the pH of the surrounding tissue fluid as an indicator of blood CO2 concentration. The reason pH can be used in this way is that blood CO2 is the main determinant of the pH of cerebrospinal fluid, the fluid surrounding the spinal chord and brain. -the blood O2 level usually has little effect on the breathing control centers.

Control of Breathing in Humans

the neurons mainly responsible for regulating breathing are in the medulla oblongata, near the base of the brain. Neural circuits in the medulla form a pair of breathing control centers that establish the breathing rhythm. When you breathe deeply, a negative-feedback mechanism prevents the lungs from over expanding. -the medulla uses the pH of the surrounding tissue fluid as an indicator of blood CO2 concentration. The reason pH can be used this way is that blood CO2 is the main determinant of the pH of cerebrospinal fluid

Partial pressure

the pressure exerted by a particular gas in a mixture of gases -gas always undergoes net diffusion from a region of higher partial pressure to a region of lower partial pressure

Rate of Diffusion in respiratory surfaces

the rate of diffusion is proportional to the surface area across which it occurs an inversely proportional to the square of the distance through which molecules must move -gas exchange is fast when the area for diffusion is short (respiratory surfaces tend to be large and thin)

Diastole

the relaxation phase of the cardiac cycle

Pulmonary Circut

the right side of the heart, delivers oxygen-poor blood to the gas exchange tissues, where there is a net movement of Os into the blood and of CO2 out of the blood.

Larynx

the upper part of the respiratory tract

Gas Exchange

the uptake of molecular O2 from the environment and the discharge of CO2 to the environment

Tidal Volume

the volume of air inhaled and exhaled with each breath

Arteries (more)

the walls of arteries are thick and strong, accommodating blood pumped at high pressure by the heart. They are elastic. When the heart relaxes between contractions, the arterial walls recoil, helping maintain blood pressure and flow to capillaries -signals from the nervous system and hormones circulating in the blood act on the smooth muscle in arteries and arterioles dilating or constricting these vessels and thus controlling blood flow to different parts of the body

Vital capacity

tidal volume during maximal inhalation and exhalation

Bronchi

trachea branch into two bronchi, one leading to each lung. Within the lung, the bronchi branch repeatedly into finer and finer tubes called bronchioles

Veins (more)

veins convey blood back to the heart at a lower pressure and therefore do not require thick walls. -a vein is only about a third as thick as an artery -unlike arteries, veins contain valves, which maintain a unidirectional flow of blood despite the low blood pressure in these vessels

Vasodilation

when the smooth muscles relax, an increase in diameter that causes blood pressure in the arteries to fall

Aveoli

where gas exchange in mammals occurs -air sacs clustered at the tops of the tiniest bronchioles -lacking cilia or significant air currents to remove particles from their surface, alveoli are highly susceptible to contamination -white blood cells patrol the alveoli, engulfing foreign particles

Trachea

windpipe

Arterioles

within organs arteries branch into arterioles