A & P chapter 20

Tunica interna (intima)

innermost layer made of simple squamous epithelium called the *endothelium* and a supporting basement membrane. Large arteries also possess a layer of elastic tissue called the *internal* *elastic membrane*.

Net filtration pressure step 3:

In the middle of the capillary bed CHP and BCOP are equal so that NFP = Zero. This means there is no net movement and materials are moving in and out equally.

Net filtration pressure step 4:

filtration is always greater than reabsorption causing the interstitial fluid to experience a net accumulation of water and materials. This excess fluid would increase pressure in the interstitial space and decrease vascular blood pressure if not corrected. Hence, the need for our lymphatic system (next chapter).

Aldosterone

secreted by the cortex of the adrenal gland; enhances the reabsorption of sodium ions (Na+) by the kidney which in turn increases water reabsorption by the kidneys and further reduces urine output. With the increase in water in the blood, blood volume increases and blood pressure increases.

Epinephrine and norepinephrine

secreted by the medulla of the adrenal gland; increases heart rate which in turn increases cardiac output. Increases in cardiac output causes increased blood pressure.

Capillaries

smallest, most abundant type of blood vessel; permits diffusion between the bloodstream and interstitial fluids in order to facilitate nutrient and waste exchange.

Angiology

study of blood vessels and the lymphatic system

Blood Vessels

the conducting tubes that distribute the blood; three primary types of blood vessels are in the human body. 1. Arteries 2. Capillaries 3. Veins

Net filtration pressure (NFP)

the difference between the forces that favor filtration versus those that oppose it (reabsorption). *NFP = CHP - BCOP*.

Fenestrated Capillaries

the endothelial lining of these capillaries possess many pores or "*fenestrations*" and intercellular clefts. Tight junctions are less numerous. Common in areas where filtration and absorption of dissolved particles is important organ functions. Found in the digestive tract, endocrine glands, kidneys, etc.

Excessive blood volume or low blood pressure triggers

the secretion of *Atrial natriuretic* *peptide (or ANP)* by the right atrium and *Brain natriuretic* *peptide (or BNP)* by the muscle cells of the ventricles. ANP and BNP both reduce sodium reabsorption so that the body experiences an increase in Na+ loss in urine. Water follows the Na+ so blood volume declines. ANP and BNP inhibit the release of ADH, aldosterone, epinephrine, and norepinephrine. All of these things together will lower blood pressure.

Structure and general characteristics of veins:

- Possesses *valves* which are formed from folds of the tunica interna function to prevent backflow of blood. If a valve fails, blood will pool abnormally in the vessel causing it to become grossly distended. If near the surface, it is visible through the skin and is called a *varicose vein*. Varicose veins of the anal canal are called *hemorrhoids*. - Serve as *blood reservoirs* (capacitance vessels) because the pressure is so low within these vessels compared to arteries or capillaries. In fact, 64% of blood total volume is in systemic veins at any given time.

Changes in blood pressure occur as blood flows through the cardiovascular system:

- The heart generates a pressure of around 100 mmHg as it pumps blood into the aorta which has a cross sectional area of roughly 4.5 cm2. - At each branching of the arterial system, the arterial pressure drops as blood is pushed into ever-increasing numbers of smaller and smaller branches. - At the start of peripheral capillaries, arterial pressure has fallen to approximately 35 mmHg, and by the time blood reaches the venules, it has fallen to approximately 18 mmHg. - By the time blood reaches the vena cavae, pressure is approximately 5 mmHg. - Because pressure is so low in veins, blood tends to pool in veins and the venous flow depends upon the *muscular pump* and *respiratory pump* to keep the blood moving toward the heart.

Arterial pressure is not constant; it rises during ventricular contraction and falls during ventricular relaxation as the elastic arterial walls stretch and recoil.

- The peak blood pressure measured during ventricular contraction is called the *systolic* *pressure*, and the minimum blood pressure at the end of the ventricular relaxation phase is called the *diastolic pressure*. In recording blood pressure, we separate systolic and diastolic pressure with a slash. Average adult blood pressure is 120/80 mmHg. - The difference between systolic and diastolic pressure is the *pulse* *pressure*. Systolic pressure - Diastolic pressure = pulse pressure. - The *mean arterial pressure* (or MAP) is calculated by adding one third of pulse pressure to the diastolic pressure. - Be sure you can identify the various *pulse points* of the body. See diagram. - In the laboratory, you will have an opportunity to measure your partner's blood pressure using a *sphygmomanometer* and *stethoscope*.

Diffusion occurs continuously across capillary walls, but different substances use different routes and mechanisms for movement into and out of the bloodstream:

- Water, ions, and small organic molecules diffuse between adjacent endothelial cells or through the pores of the fenestrated capillaries. - Some ions, including Na+, K+, Ca+, or Cl-, can diffuse across endothelial cells by pass through their plasma membrane channels. - Large water-soluble compounds are unable to enter and leave the bloodstream except at fenestrated capillaries. - Lipids and lipid-soluble materials can cross capillary walls by diffusion through the endothelial plasma membrane. - Plasma proteins are normally unable to cross the endothelial lining anywhere except in sinusoidal capillaries like those of the liver.

Capillary Beds

- are also called "microcirculation". - possess several important structures: - *True capillaries* where exchange occurs (about 10 to 100 per capillary bed). - *Precapillary sphincters* circular muscles which open and close to allow blood to flow into the true capillary vessels or to bypass them. - *Metarteriole*—thoroughfare channels (a vascular shunt) these are the vessels that allow the blood to bypass the true capillaries and be redirected to tissues that need an increased blood supply. - *Arteriovenous* anastomosis may bypass the capillary bed and lead directly to the venous system. These serves as collateral circuits.

veins walls characteristics

- tend to have a flattened, collapsed, or irregularly shaped lumen with relatively thin walls. - the tunica interna of ________ is often smooth and the internal elastic membrane is absent. - have a thin tunica media composed of smooth muscle and collagen fibers. - lack an external elastic membrane. - The tunica externa of _______ possesses collagen and smooth muscle and the nervi vasorum and vasa vasorum are present in the tunica externa and the tunica media.

Arteries walls characteristics

- tend to have a round lumen and relatively thick wall. - The tunica interna of ___________ is rippled and possesses an internal elastic membrane. - have a thick tunica media dominated by smooth muscle and elastic fibers. - possess an external elastic membrane within its tunica media. - have a tunica externa made of collagen and elastin fibers with a nervi vasorum and vasa vasorum present.

Blood velocity

- the rate of blood flow through a blood vessel. - is inversely proportional to the cross-sectional surface area of the vessels; the more surface area - the more friction - the slower blood flows. - Even though individual capillaries are small, there are so many of them they collectively have a very high surface area. - Capillaries have the highest cross-sectional area and blood traveling through them is therefore very slow. - Arteries and veins are larger with high individual surface area but they are far less numerous than capillaries so collectively they generate less cross-sectional surface area so blood moves through them faster.

Filtration and reabsorption are determined by the interplay of two important pressures:

1. Capillary Hydrostatic Pressure 2. Blood Colloid Osmotic Pressure

Blood volume

As __________ increases (as with pregnancy) blood pressure increases. As __________ decreases (as with hemorrhaging) blood pressure decreases. Hypervolemia Hypovolemia

Cardiac output

As cardiac output increases (as during exercise) blood pressure increases. As cardiac output decreases (as with rest) blood pressure decreases.

Length versus diameter vessels

Differences in diameter have much more significant effects on resistance than do differences in length. If two vessels are equal in diameter but one is twice as long as the other, the longer vessel offers twice as much resistance to blood flow. But for two vessels of equal length, one twice the diameter of the other, the narrower one offers 16 times as much resistance to blood flow. For this reason, the brain typical initiates changes in blood vessel diameter (by *vasoconstriction* and *vasodilation*) to regulate blood pressure. However, abnormal narrowing of a vessel due to a plaque buildup (as with *atherosclerosis*) leads to hypertension and increased risk of heart attack (*myocardial* *infarction*) and stroke (*cerebral* *vascular accident*).

neural regulation

Neurotransmitter binding [Ca2+] in sarcoplasm; either graded (local) potential or action potential; Response depends on neurotransmitter released and type of receptor molecules

Net filtration pressure step 1:

On the arterial end of the capillary bed, CHP is greater than BCOP so that NFP is positive. The positive number indicates a net movement of materials out of the bloodstream (such as water, nutrients, and oxygen).

Net filtration pressure step 2:

On the venous end of the capillary bed, CHP has dropped while BCOP has remained relatively constant. The net effect is that BCOP is now greater than CHP and NFP is negative. The negative number indicates a net movement of materials into the bloodstream (such as water, wastes, and carbon dioxide).

Coronary circuit blood flow

Rest: 250 mL/min light exercise: 350 mL/min Heavy exercise: 750 mL/min

brain blood flow

Rest: 750 ml/min light exercise: 750 ml/min Heavy exercise:750 ml/min

cardiovascular control centers

The _____________ are primarily located in the hypothalamus and require the stimulation of baroreceptors and chemoreceptors. The medulla oblongata can coordinate respiratory activities to enhance control.

Circulatory Shock

There are several recognized forms of shock which can occur with the failure to maintain blood pressure or blood flow: 1. Hypovolemic shock 2. Cardiogenic shock 3. Distributive shock 4. Obstructive shock

factors affecting resistance and hence blood pressure:

Vessel length Vessel diameter

Capillaries connect

________ arteries to veins and are the site of nutrient and waste exchange. A capillary is a microscopic channel that supplies blood to the tissues themselves, a process called perfusion.

Large veins

________ include the superior and inferior venae cavae and their tributaries. All three vessel wall layers are present in all _____________. The slender tunica media is surrounded by a thick tunica externa composed of a mixture of collagen and elastin fibers. Some __________ are flattened with extremely thin walls. These are called *venous sinuses* and are found in the heart (coronary sinus) and the brain (dural sinuses).

Vessel diameter

_________ Friction also occurs between layers of fluid moving at different speeds. The layer of blood closest to the vessel wall is slowed down by friction with the endothelial surface. The adjacent layer of blood is slowed down by friction with the more superficial layer. As blood proceeds toward the capillaries, the diameter of arteries decreases markedly; as blood returns toward the heart, the diameter of veins increases. This gradually diminishes as the distance from the wall increases. In a small-diameter vessel, all the blood is slowed to some degree and therefore resistance is high. In a large diameter vessel the central region is unaffected by events at the periphery, so the resistance is relatively low.

Vessel length

_________ Friction occurs between the moving blood and the walls of the vessel. The longer the________, the greater the surface area in contact with the blood, and therefore the greater the resistance. The most dramatic changes in blood ___________ occur between birth and adulthood. Once the reach adulthood, ____________ is relatively constant.

Blood flow (F)

_________ also called *tissue* *perfusion*, is defined as the volume of blood flowing through a vessel, an organ, or the entire circulation. - Systemic tissue perfusion occurs along the following path: left ventricle of the heart aorta, elastic and muscular arteries, arterioles, capillaries, venules, small veins, medium-sized veins, large veins, and finally superior and inferior vena cava to the right atrium of the heart. - The cardiovascular centers make extensive adjustments to cardiac output and blood distribution during changes in physical activity.

Venules

_________ drains capillary beds into veins. These are extremely porous and are often more similar in structure and function to capillaries than to veins. Many _________ possess only endothelium and pericytes while others may have scanty tunica media and a thin tunica externa.

blood pressure (BP)

_________ is defined as the amount of force per unit area exerted on the wall of a blood vessel by the blood contained within it. Pressure is measured in mmHg. - In general terms, *blood flow (F)* is directly proportional to the blood pressure (increased pressure results in increased flow), and inversely proportional to peripheral resistance (increased resistance results in decreased flow).

Anti-diuretic hormone (ADH)

_________ is secreted by the pituitary gland.__________enhances water reabsorption in the kidney so that urine output is reduced and blood volume rises. Increased blood volume increases blood pressure.

Cardiogenic shock

_________ results from the inability of the heart to maintain cardiac output. Most often, it results from a myocardial infarction (heart attack), but it may also be caused by arrhythmias, valve disorders, cardiomyopathies, cardiac failure, or simply insufficient flow of blood through the cardiac vessels. Treatment involves repairing the damage to the heart or its vessels to resolve the underlying cause, rather than treating _____________directly.

Hypovolemic shock

____________ in adults is typically caused by hemorrhage, although in children it may be caused by fluid losses related to severe vomiting or diarrhea. Other causes for __________ include extensive burns, exposure to some toxins, and excessive urine loss related to diabetes insipidus or ketoacidosis. Typically, patients present with a rapid, almost tachycardic heart rate; a weak pulse often described as "thread;" cool, clammy skin, particularly in the extremities, due to restricted peripheral blood flow; rapid, shallow breathing; hypothermia; thirst; and dry mouth. Treatments generally involve providing intravenous fluids to restore the patient to normal function and various drugs such as dopamine, epinephrine, and norepinephrine to raise blood pressure.

Filtration

____________ refers to the movement of materials OUT of the bloodstream while *reabsorption* refers to the movement of materials INTO the bloodstream.

Distributive shock (vascular shock)

______________ occurs when arterioles lose their normal muscular tone and dilate dramatically. A common cause is *sepsis* (or septicemia), also called "blood poisoning," which is a widespread bacterial infection that results in an organismal-level inflammatory response known as septic shock. Neurogenic shock is a form of vascular shock that occurs with cranial or spinal injuries that damage the cardiovascular centers in the medulla oblongata or the nervous fibers originating from this region. Anaphylactic shock is a severe allergic response that causes the widespread release of histamines, triggering vasodilation throughout the body.

Obstructive shock

__________________, as the name would suggest, occurs when a significant portion of the vascular system is blocked. It is not always recognized as a distinct condition and may be grouped with cardiogenic shock. The most common cause is a pulmonary embolism, a clot that lodges in the pulmonary vessels and interrupts blood flow. Other causes include stenosis of the aortic valve; cardiac tamponade, in which excess fluid in the pericardial cavity interferes with the ability of the heart to fully relax and fill with blood (resulting in decreased preload); and a pneumothorax, in which an excessive amount of air is present in the thoracic cavity, outside of the lungs, which interferes with venous return, pulmonary function, and delivery of oxygen to the tissues.

Compliance

__________is the ability of any vessel to expand to accommodate increased content. A metal pipe, for example, is not compliant, whereas a balloon is. Veins are more compliant than arteries and can expand to hold more blood resulting in less pressure. When vascular disease causes stiffening of arteries (as with *arteriosclerosis*), _________ is reduced and resistance to blood flow is increased. The result is higher pressure within the vessel and reduced blood flow.

Renin Angiotensin II

a drop in blood pressure or blood volume is detected by *baroreceptors* of the kidney. Renin is released and activates angiotensin I (already in the bloodstream) and is converted to the activated angiotensin II, a potent vasoconstrictor. This increases peripheral resistance and blood pressure rises. Plus, angiotensin stimulates the release of other hormones.

Lumen

a hollow passageway through which blood flows

Nitric oxide

a potent vasodilator which reduces peripheral resistance which in turn, reduces blood pressure.

Capillary Hydrostatic Pressure (CHP)

a push out of the bloodstream generated by fluids within the vessel.

Blood Colloid Osmotic Pressure (BCOP)

a suction into the bloodstream generated by large immovable plasma proteins.

types of veins

a. Venules b. Medium-sized veins c. Large veins

Hypervolemia

abnormally high levels of fluid and blood within the body.

Hypovolemia

abnormally low levels of fluid and blood within the body

Elastic arteries

also known as conducting vessels, found near the heart because they can withstand the highest blood pressures (aorta and its immediate branches); diameter = 2.5 cm to 1 cm; possesses large amounts of elastin fibers in the tunica media forming a "holey sheet" (like Swiss cheese); has substantial smooth muscle in the tunica media, however they have little ability to vasoconstrict.

Muscular Arteries

also known as distribution vessels; delivers blood to specific body organs (renal artery, mesenteric artery, gastric artery, etc.); diameter = 1 cm to .3 mm; the tunica media has relatively more smooth muscle and less elastin than elastic arteries; very active in vasoconstriction and are less distensible.

Arterioles

also known as resistance vessels; regulate blood flow into capillaries; diameter = .3 mm to 10 micrometers; the tunica media is almost entirely smooth muscle w/ little to no elastin fibers; tunica externa is poorly defined.

Arteries

always carry blood away from the heart to the capillaries, typically red on models because they generally carry oxygenated blood.

Veins

always return blood from capillaries to the heart; typically blue on models because they generally carry deoxygenated blood.

types of capillaries

continuous, fenestrated, sinusoids

types of arteries

elastic, muscular, arterioles

Capillary exchange

is a dynamic process that includes diffusion by filtration and reabsorption mechanisms. Recall that diffusion is the net movement of ions or molecules from an area where their concentration is higher to an area where their concentration is lower.

Sinusoidal Capillaries

least common of the three; leaky capillaries because tight junctions are rare and *intercellular clefts* are abundant. Commonly found in areas where cells must leave circulation and move to the tissues such as bone marrow, liver, and lymph organs. *Kupffer cells* are specialized phagocytic cells found in the sinusoidal capillaries of the liver. They act as macrophages that extend into the lumen of the capillary to capture prey.

Baroreceptors

located in the *carotid sinuses*, aorta, and right atrium; detect changes in blood pressure. When the baroreceptors detect an increase in blood pressure, they send a signal to the hypothalamus which in turn, triggers vasodilation, a decrease in heart rate, and a reduction in cardiac output. When they detect a decrease in blood pressure, they send a signal to the hypothalamus which in turn, triggers vasoconstriction, an increase in heart rate, and an increase in cardiac output.

Chemoreceptors

located in the carotid sinuses and aortic arch; detect changes in blood composition. When the chemoreceptors detect an increase in carbon dioxide or a decrease in oxygen concentration or pH, they stimulate mechanisms such as vasoconstriction or increased cardiac output and blood pressure.

Tunica media

middle layer which functions in *vasodilation* and *vasoconstriction*; composed mostly of smooth muscle, collagen, and elastin fibers. Large arteries also possess an external elastic membrane.

Continuous Capillaries

most abundant; these capillaries possess an uninterrupted layer of squamous cells. The cells are connected by *tight junctions* and have few *intercellular clefts*. These are common in skin, muscle, and brain tissues.

Tunica externa (adventitia)

outermost layer made of connective tissue which serves to anchor, protect, and prevent over-stretching.

Medium-sized veins

range from 2 - 9 mm in diameter; the tunica media is thin and contains smooth muscle cells and collagen fibers. The thickest layer is the tunica externa, which contains smooth muscle cells and longitudinal bundles of elastic and collagen fibers. Valves, particularly within veins of the limbs, are common.

Peripheral resistance

resistance is the amount of opposition (or friction) blood encounters as it flows through the vessels. As resistance increases blood pressure increases. As resistance decreases blood pressure decreases.

kidney blood flow

rest: 1100 mL/min light exercise: 900 mL/min heavy exercise: 600 mL/min

Skeletal muscles blood flow

rest: 1200 mL/min light exercise: 4500 mL/min heavy exercise: 12500 mL/min

Abdominal organs blood flow

rest: 1400 mL/min light exercise: 1100 mL/min heavy exercise: 600 mL/min

skin blood flow

rest: 500 mL/min light exercise: 1500 mL/min heavy exercise: 1900 mL/min

TOTAL body blood flow

rest: 5800 mL/min light exercise: 9500 mL/min heavy exercise: 17,500 mL/min

Other tissues blood flow

rest: 600 mL/min light exercise: 400 mL/min heavy exercise: 400 mL/min

Structure and general characteristics of capillaries

the smallest of all blood vessels with diameters less than 10 micrometers; composed of a single layer of squamous epithelium and a sparse *basal lamina*. May possess *pericytes* associate with the epithelium to help stabilize the layer.

Viscosity of blood

thick fluids are more sluggish than thin fluids. Liquids with low _________ (like water) flow at low pressures. Thick, syrupy fluids, such as molasses, flow under higher pressures. Whole blood has a ___________ 4 -5 times that of water due to the presence of plasma proteins and the formed elements. Increases in ______________, as when someone is significantly dehydrated, will cause an increase in blood pressure. A decrease in _________ will decrease blood pressure.

Structure and general characteristics of arteries

thick tunica media compared to tunica externa; lack valves, and may possess large amounts of elastic tissue.

Arterial System

transport blood away from the heart, generally red, generally transport oxygenated blood.

Venous System

transport blood towards the heart, generally blue, and generally transport deoxygenated blood.

Structure of Vessel Walls

tunica intima, tunica media, tunica externa