Chapter 21 Cardiovascular System: Blood Vessels and Circulation

Carry blood away from heart Types: Elastic or conducting arteries-Named Arteries Largest diameters, pressure high More elastic tissue than muscle. Prevents decrease in pressure but also dampens pressure to ensure smaller vessels are not damaged Muscular (medium/small arteries)-Named/Unnamed Arteries Thick tunica media allows vessels to regulate blood supply by constricting or dilating. Arterioles Transport blood from small arteries to capillaries. Smallest arteries where the three tunics can be differentiated. Vasoconstriction and dilation.

Arteries

Blood flow can increase 7 to 8 times as a result of vasodilation of metarterioles and precapillary sphincters in response to increased rate of metabolism. Vasomotion: periodic contraction and relaxation of precapillary sphincters. Long-term local control: capillaries become more dense in a region that regularly has an increased metabolic rate.

Autoregulation of Blood Flow

Baroreceptors in the carotid sinus and aortic arch monitor blood pressure. 2. The glossopharyngeal and vagus nerves conduct action potentials to the cardioregulatory and vasomotor centers in the medulla oblongata. 3. Increased parasympathetic stimulation of the heart decreases the heart rate. 4.Increased sympathetic stimulation of the heart increases the heart rate and stroke volume. 5. Increased sympathetic stimulation of blood vessels increases vasoconstriction.

Baroreceptor Reflex Control of Blood Pressure

Determined by: Flow-volume of blood flowing through any given vessel, or through entire system, per minute=Cardiac Output Resistance-anything that prevents flow or creates friction Viscosity, vessel length, vessel diameter Pressure-measure of force exerted by blood against the wall. Blood moves through vessels because of blood pressure.

Blood Circulation

When Flow Increases Pressure is high Resistance is low When Resistance Increases Flow will decrease Pressure is low When Pressure Increases Flow will increase Resistance is low

Blood Circulation Summary

Rate of flow through a tube (vessel) is expressed as the volume that passes a specific point per unit of time. Flow = (P1 − P2/R). P1 and P2 are pressures in the vessel at points one and two Flow is directly proportional to pressure differences. Increased pressure=increased flow and vice versa R is the resistance to flow. Resistance in vessels that needs to be overcome in order to push blood through system and create flow Flow decreases when resistance increases and vice versa. Resistance is proportional to blood vessel diameter so constriction of a blood vessel increases resistance and thus decreases flow.

Blood Flow

Measured directly using cannula into blood vessel, or indirectly using auscultatory method. Auscultatory method: Sphygmomanometer, blood pressure cuff, and stethoscope BP cuff inflated until brachial artery is collapsed=no sound Pressure in BP cuff lowered until below systolic pressure (blood flow during ventricular systole). Stethoscope picks up turbulence in blood flow called Korotkoff sounds Pressure during first sound: systolic. BP cuff pressure lowered more Korotkoff sounds change tone/loudness When pressure drops below normal flow (laminar blood flow), normal flow restored, and sounds disappear: diastolic. Normal BP: 120 (systolic)/80 (diastolic)

Blood Pressure

Most abundant Blood from arterioles flows into capillaries Capillary wall consists of an endothelium (simple squamous epithelium), basement membrane, and a delicate layer of loose CT Substances move across capillaries by diffusion. Lipid-soluble and small water-soluble molecules through plasma membrane. Larger water-soluble molecules pass through fenestrae or gaps between endothelial cells. Red Blood Cells flow through one at a time

Capillaries

Blood flows from arterioles through metarterioles, then through capillary network. Flow through thoroughfare channel fairly consistent while flow through arterial capillaries is intermittent. Smooth muscle in arterioles, metarterioles, precapillary sphincters regulates blood flow.

Capillary Network

1. Chemoreceptors in the carotid and aortic bodies monitor blood O2, CO2, and pH. 2. Chemoreceptors in the medulla oblongata monitor blood CO2 and pH. 3. Decreased blood O2, increased CO2, and decreased pH decrease parasympathetic stimulation of the heart, which increases the heart rate. 4. Decreased blood O2, increased CO2, and decreased pH increase sympathetic stimulation of the heart, which increases the heart rate and stroke volume. 5. Decreased blood O2, increased CO2, and decreased pH increase sympathetic stimulation of blood vessels, which increases vasoconstriction.

Chemoreceptor Reflex Control

Vascular compliance: Tendency for blood vessel volume to increase as blood pressure increases. More easily the vessel wall stretches, the greater its compliance and vice versa Venous system has a large compliance (24 times greater than that of arteries)

Compliance

Local control: in most tissues, blood flow is proportional to metabolic needs of tissues. Nervous System: responsible for routing blood flow and maintaining blood pressure. Hormonal Control: sympathetic action potentials stimulate epinephrine and norepinephrine.

Control of Blood Flow in Tissues

Total cross-sectional area for all vessels of a given type considered. Cross sectional area multiplied by the number of that type of blood vessels in the body Only one aorta with a cross-sectional area of 5 cm2. That is the total cross-sectional area for the aorta Individual capillary very small cross-sectional area on its own ~10 billion capillaries in the body Total cross sectional area of capillaries is ~2500 cm2. So the total cross-sectional area in the capillaries much greater than the aorta. Velocity of blood flow in particular blood vessel type inversely proportional to total cross-sectional area Greatest velocity of blood flow in the body exists in the aorta where the total cross-sectional area is smallest Capillaries have the lowest velocity of blood flow even though total cross-sectional area is large As veins grow in diameter, total cross-sectional area decreases, velocity of flow increases Stream that flows rapidly through a narrow gorge but flows slowly through a broad plane cross sectional area of vessels will dictate flow

Cross-sectional Area

Capillary exchange: the movement of substances into and out of capillaries. Most important means of exchange: diffusion. Lipid soluble cross capillary walls diffusing through plasma membrane. for example, O2, CO2, steroid hormones, fatty acids. Water soluble diffuse through intercellular spaces or through fenestrations of capillaries. for example, glucose, amino acids. Blood pressure, capillary permeability, and osmosis affect movement of fluid from capillaries. Fluid moves out of capillaries at arterial end and most but not all returns to capillaries at venous end. That which remains in tissues is picked up by the lymphatic system then returned to venous circulation.

Forces acting across capillary walls

Net filtration pressure (NFP)- force responsible for moving fluid across capillary walls. Two forces affect pressure. Hydrostatic pressure: physical pressure of blood flowing through the vessels or of fluid in interstitial spaces. Osmotic pressure: force required to prevent water from moving by osmosis across selectively permeable membrane Large proteins do not freely pass through the capillary walls and the difference in protein concentrations between the blood and interstitial fluid is responsible for osmosis.

Forces acting across capillary walls

Local factors regulate metarterioles and precapillary sphincters. Increased metabolism of a tissue results in vasodilator substances (for example, carbon dioxide, lactate, potassium) that cause vasodilation and relaxation of sphincters; blood flow increases to serve the working tissue. Lack of nutrients can also result in increased blood flow to a tissue.

Functional Characteristics of the Capillary Bed

Portal system: vascular system that begins and ends at a capillary bed with no pumping mechanism in between. Hepatic portal- liver; renal portal- kidney; hypothalamohypophyseal portal between hypothalamus and pituitary. Blood entering the hepatic portal vein is rich with nutrients collected from the intestines, but may also contain toxic substances. Both nutrients and toxic substances will be regulated by the liver. Nutrients either taken up and stored or modified chemically and used by other parts of the body. Biotransformation: Toxic substances can be broken down by hepatocytes or can be made water soluble. To be transported in blood and excreted by the kidneys.

Hepatic Portal System

Important in minute-to-minute regulation of local circulation. Provides a means by which blood can be shunted from one large area of the peripheral circulatory system to another area by increasing resistance. Sympathetic division most important. Innervates all vessels except capillaries, precapillary sphincters, and most metarterioles. Vasomotor center in lower pons and upper medulla oblongata. Excitatory part is tonically active. Causes vasomotor tone. Norepinephrine. Inhibitory part can cause vasodilation by decreasing sympathetic output.

Nervous Control of Blood Flow in the Tissues

Sympathetic stimulation of adrenal medulla causes output of norepinephrine and epinephrine into circulation. Causes vasoconstriction in vessels (α-adrenergic receptors) except in skeletal muscle where vasodilation takes place (ß-adrenergic receptors).

Nervous Control of Blood Flow in the Tissues

1. Vasodilation of precapillary sphincters.Precapillary sphincters relax as the tissue concentration of nutrients, such as O2, glucose, amino acids, and fatty acids, decreases. The sphincters also relax as the concentration of vasodilator substances, such as CO2, lactic acid, adenosine, adenosine monophosphate, adenosine diphosphate, nitric oxide, and K+, increase, and as the pH decreases. 2. Constriction of precapillary sphincters.Precapillary sphincters contract as the tissue concentration of nutrients, such as O2 , glucose, amino acids, and fatty acids, increases. The sphincters also contract as the tissue concentration of metabolic by-products, such as CO2 , lactic acid, adenosine, adenosine monophosphate, adenosine diphosphate, nitric oxide, and K+, decrease, and as the pH increases.

Precapillary sphincters and Local Control of Blood Flow Through Capillary Beds

Baroreceptor reflexes: change peripheral resistance, heart rate, and stroke volume in response to changes in blood pressure.-SHORT TERM BP stretches arterial walls, mechanically gated Na+ channels open, AP sent to midbrain, results in dilation of arterioles to reduce resistance and/or reduce HR to lower cardiac output Adrenal medullary mechanism: activated by substantial increase in sympathetic stimulation of the heart and blood vessels, such as sudden large decrease in blood pressure, substantial increase in exercise, or stressful conditions. SHORT TERM Chemoreceptor reflexes: sensory receptors sensitive to oxygen, carbon dioxide, and pH levels of blood. SHORT TERM

Regulation of Blood Pressure

1. Kidneys detect decreased blood pressure, resulting in increased renin secretion. 2. Renin converts angiotensinogen, a protein secreted from the liver, to angiotensin I. 3. Angiotensin-converting enzyme in the lungs converts angiotensin I to angiotensin II. Angiotensin II is a potent vasoconstrictor, resulting in increased blood pressure. 4. Angiotensin II stimulates the adrenal cortex to secrete aldosterone. 5. Aldosterone acts on the kidneys to increase Na+ reabsorption. As a result, urine volume decreases and blood volume increases, causing blood pressure to rise.

Renin-Angiotensin-Aldosterone Mechanism

Exercise Heart beats with greater force Increased pressure in the aorta. Blood flow through aorta increases Capillaries to skeletal muscle increase in diameter Decrease in vascular resistance Increase in blood flow.

Resistance, Pressure and Flow Example

Tunica intima. Endothelium. Basement membrane. Lamina propria (C.T. layer). Internal elastic membrane. Fenestrated layer of elastic fibers. Tunica media: smooth muscle cells arranged circularly around the blood vessel. Vasoconstriction: smooth muscles contract, decrease in blood flow. Vasodilation: smooth muscles relax, increase in blood flow. Tunica externa (adventitia): connective tissue, varies from dense regular near the vessel to loose that merges with the surrounding C.T. All vessels except small vessels

Structural Features of Blood Vessels

Continuous. No gaps between endothelial cells. No fenestrae. Less permeable to large molecules than other capillary types. (muscle, nervous tissue). Fenestrated. Endothelial cells have numerous fenestrae-areas where cytoplasm is absent and plasma membrane is made of a thin, porous diaphragm. Highly permeable. (intestinal villi, ciliary process of eye, choroid plexus, glomeruli of kidney). Sinusoids. Large diameter sinusoidal capillaries. Sparse basement membrane. Large molecules/cells can move across. (liver, bone marrow). Venous sinuses are similar in structure to sinusoids but even larger. (spleen).

Types of Capillaries

Valves found in all veins greater than 2 mm in diameter. Folds in intima form two flaps that overlap. More valves in veins of lower extremities than in veins of upper extremities.

Valves

Carry blood back to the heart Types Venules drain capillary network. Endothelial cells and basement membrane with a few smooth muscle cells. As diameter of venules increases, amount of smooth muscle increases. Small veins. Receive blood from venules. Smooth muscle cells form a continuous layer. Addition of tunica adventitia made of collagenous connective tissue. Medium veins. Go-between between small veins and large veins. Large veins. Tunica intima is thin: endothelial cells, relatively thin layer of C.T and a few scattered elastic fibers. Tunica media has circularly arranged smooth muscle cells. Adventitia is predominant layer.

Veins

Viscosity: measure of resistance of liquid to flow. Viscosity increases, pressure required to flow needs to increase. Viscosity influenced largely by hematocrit Dehydration and/or uncontrolled production of RBCs can lead to increased viscosity which increases the workload on the heart.

Viscosity