Blood Vessels Ch. 19 Module 3: Section 19.06-19.10 Dynamic Study Module Flashcards

kidneys

Ex.

The major long-term mechanism of blood pressure control is provided by the kidneys.

Unlike short-term controls of blood pressure that alter peripheral resistance and cardiac output, long-term controls alter blood volume. Renal mechanisms mediate long-term regulation by the kidneys.

140/90

Ex.

Hypertension is defined physiologically as a condition of sustained arterial pressure above 140 / 90.

Chronically elevated blood pressure is called hypertension and is characterized by a sustained increase in either systolic pressure (above 140 mm Hg) or diastolic pressure (above 90 mm Hg). The American Heart Association considers individuals to have prehypertension if their blood pressure values are elevated but not yet in the hypertension range. These individuals are at higher risk for developing full-blown hypertension and are often advised to change their lifestyles to reduce their risk of developing full-blown hypertension.

Chronic hypertension is a common and dangerous disease. An estimated 30% of people over age 50 are hypertensive. Although this “silent killer” is usually asymptomatic for the first 10 to 20 years, it slowly but surely strains the heart and damages the arteries. Prolonged hypertension is the major cause of heart failure, vascular disease, renal failure, and stroke. The higher the pressure, the greater the risk for these serious problems. Because the heart is forced to pump against greater resistance, it must work harder, and over time the myocardium enlarges. When finally strained beyond its capacity, the heart weakens and its walls become flabby. Hypertension also ravages the blood vessels, accelerating the progress of atherosclerosis. As the vessels become increasingly blocked, blood flow to the tissues becomes inadequate and vascular complications appear in the brain, heart, kidneys, and retinas of the eyes.

Hormonal control of blood flow

Ex.

Angiotensin II provides hormonal control of blood flow.

Hormones also help regulate blood pressure, both in the short term via changes in peripheral resistance and in the long term via changes in blood volume. Paracrines (local chemicals), on the other hand, primarily serve to match blood flow to the metabolic need of a particular tissue. In rare instances, massive release of paracrines can affect blood pressure. Here are the short-term effects of selected hormones.

• Adrenal medulla hormones. During periods of stress, the adrenal gland releases epinephrine and norepinephrine (NE) to the blood. Both hormones enhance the sympathetic response by increasing cardiac output and promoting generalized vasoconstriction.

• Angiotensin II. When blood pressure or blood volume are low, the kidneys release renin. Renin acts as an enzyme, ultimately generating angiotensin II (an″je-o-ten′sin), which stimulates intense vasoconstriction, promoting a rapid rise in systemic blood pressure. It also stimulates release of aldosterone and ADH, which act in long-term regulation of blood pressure by enhancing blood volume.

• Atrial natriuretic peptide (ANP). The atria of the heart produce the hormone atrial natriuretic peptide (ANP), which leads to a reduction in blood volume and blood pressure. As noted in Chapter 16, ANP antagonizes aldosterone and prods the kidneys to excrete more sodium and water from the body, reducing blood volume. It also causes generalized vasodilation.

• Antidiuretic hormone (ADH). Produced by the hypothalamus, antidiuretic hormone (ADH, also called vasopressin) stimulates the kidneys to conserve water. It is not usually important in short-term blood pressure regulation. However, when blood pressure falls to dangerously low levels (as during severe hemorrhage), much more ADH is released and helps restore arterial pressure by causing intense vasoconstriction.

The difference between systolic and diastolic pressure

Ex.

The difference between the systolic and diastolic pressures is called the pulse pressure. It is felt as a throbbing pulsation in an artery (a pulse) during systole as ventricular contraction forces blood into the elastic arteries and expands them.

Increased stroke volume and faster blood ejection from the heart (a result of increased contractility) raise pulse pressure temporarily. Atherosclerosis chronically increases pulse pressure because the elastic arteries become less stretchy.

Nervous system control of blood flow

Ex.

Sympathetic impulses provide nervous system control of blood flow.

Blood flow through body tissues, or tissue perfusion, is involved in (1) delivering oxygen and nutrients to tissue cells, and removing wastes, (2) exchanging gases in the lungs, (3) absorbing nutrients from the digestive tract, and (4) forming urine in the kidneys. The rate of blood flow to each tissue and organ is almost exactly the right amount to provide for proper function—no more, no less. This is achieved by intrinsic controls (autoregulation) acting automatically on the smooth muscle of arterioles that feed any given tissue. We will examine these intrinsic mechanisms in the next section.

First, let’s step back and look at the big picture. What do you think would happen if all of the arterioles in your body dilated at once? Because there is only a finite amount of blood, blood pressure would fall. Critical tissues, such as the brain, would be deprived of the oxygen and nutrients they need and would stop functioning. Extrinsic controls keep this from happening by acting on arteriolar smooth muscle to maintain blood pressure.

The extrinsic controls act via the nerves (sympathetic nervous system) and hormones of the nervous and endocrine systems, the two major control systems of the body. They reduce blood flow to regions that need it the least, maintaining a constant MAP and allowing intrinsic mechanisms to direct blood flow to where it is most needed.

A source of resistance related to the distance blood has to travel through blood vessels to reach a destination

Ex.

Total blood vessel length is a source of resistance related to the distance blood has to travel through blood vessels to reach a destination.

There are three important sources of resistance: blood viscosity, vessel length, and vessel diameter.

Total Blood Vessel Length

The relationship between total blood vessel length and resistance is straightforward: the longer the vessel, the greater the resistance. For example, an infant’s blood vessels lengthen as he or she grows to adulthood, and so both peripheral resistance and blood pressure increase.

Blood viscosity

The internal resistance to flow that exists in all fluids is viscosity (vis-kos’ĭ-te) and is related to the thickness or “stickiness” of a fluid.

Blood Vessel Diameter

The relationship between blood vessel diameter and resistance is also straightforward: the smaller the diameter, the greater the resistance. (That’s why a wider straw is easier to drink from.)

Blood pressure is defined as the force per unit area exerted on a vessel wall by the contained blood.

Lowest level of aortic pressure

Ex.

Diastolic pressure is the lowest level of aortic pressure.

During diastole, the aortic valve closes, preventing blood from flowing back into the heart. The walls of the aorta (and other elastic arteries) recoil, maintaining sufficient pressure to keep the blood flowing forward into the smaller vessels. During this time, aortic pressure drops to its lowest level (approximately 70 to 80 mm Hg in healthy adults), called the diastolic pressure (di-as-tah’lik). You can picture the elastic arteries as pressure reservoirs that operate as auxiliary pumps to keep blood circulating throughout the period of diastole, when the heart is relaxing. Essentially, the volume and energy of blood stored in the elastic arteries during systole are given back during diastole.

Peak of aortic pressure

Ex.

The volume of blood flowing through a vessel, an organ, or the entire circulation in a given period

Ex.

Blood flow is the volume of blood flowing through a vessel, an organ, or the entire circulation in a given period (ml/min).

If we consider the entire vascular system, blood flow is equivalent to cardiac output (CO), and under resting conditions, it is relatively constant. At any given moment, however, blood flow through individual body organs may vary widely according to their immediate needs.

There are three important sources of resistance: blood viscosity, vessel length, and vessel diameter.

1. Total Blood Vessel Length

The relationship between total blood vessel length and resistance is straightforward: the longer the vessel, the greater the resistance. For example, an infant’s blood vessels lengthen as he or she grows to adulthood, and so both peripheral resistance and blood pressure increase.

2. Blood viscosity

The internal resistance to flow that exists in all fluids is viscosity (vis-kos’ĭ-te) and is related to the thickness or “stickiness” of a fluid.

3. Blood Vessel Diameter

The relationship between blood vessel diameter and resistance is also straightforward: the smaller the diameter, the greater the resistance. (That’s why a wider straw is easier to drink from.)

Blood pressure is the force per unit area exerted on a blood vessel wall by the contained blood.

Opposition to flow (a measure of the amount of friction blood encounters as it passes through the vessels)

Ex.

Pressure that propels blood to the tissues

Ex.

Metabolic control of blood flow

Ex.

Nitric oxide contributes to metabolic control of blood flow.

When blood flow is too low to meet a tissue’s metabolic needs, oxygen levels decline and metabolic products (which act as paracrines) accumulate. These changes serve as stimuli that lead to automatic increases in tissue blood flow.

The metabolic factors that regulate blood flow are low oxygen levels, and increases in H⁺ (from CO₂ and lactic acid), K⁺, adenosine, and prostaglandins. The relative importance of these factors is not clear. Many of them act directly to relax vascular smooth muscle, but some may act by causing vascular endothelial cells to release nitric oxide.

Nitric oxide (NO) is a powerful vasodilator, but is quickly destroyed so its potent vasodilator effects are very brief. Even so, NO is the major player in controlling local vasodilation, often overriding sympathetic vasoconstriction when tissues need more blood flow.

Myogenic responses keep tissue perfusion fairly constant despite most variations in systemic pressure by responding directly to passive stretch (caused by increased intravascular pressure) with increased tone, or by responding to reduced stretch with vasodilation, increasing blood flow into the tissue.

The extrinsic controls act via the nerves (sympathetic nervous system) and hormones of the nervous and endocrine systems, the two major control systems of the body. They reduce blood flow to regions that need it the least, maintaining a constant MAP and allowing intrinsic mechanisms to direct blood flow to where it is most needed.

A source of resistance related to the thickness, or "stickiness," of the blood

Ex.

Myogenic control of blood flow

Ex.

Stretch of vascular smooth muscle provides myogenic control of blood flow.

Organs regulate their own blood flows by varying the resistance of their arterioles. These intrinsic control mechanisms may be classed as metabolic (chemical) or myogenic (physical). Generally, both metabolic and myogenic factors determine the final autoregulatory response of a tissue. For example, reactive hyperemia (hi”per-e’me-ah) refers to the dramatically increased blood flow into a tissue that occurs after the blood supply to the area has been temporarily blocked. It results both from the myogenic response and from the metabolic wastes that accumulated during occlusion. The figure below summarizes the various intrinsic (local) and extrinsic controls of arteriolar diameter.

The force per unit area exerted on a vessel wall by the contained blood

Ex.

Blood pressure (BP), the force per unit area exerted on a vessel wall by the contained blood, is expressed in millimeters of mercury (mm Hg). For example, a blood pressure of 120 mm Hg is equal to the pressure exerted by a column of mercury 120 mm high.

Unless stated otherwise, the term blood pressure means systemic arterial blood pressure in the largest arteries near the heart. The pressure gradient—the differences in blood pressure within the vascular system—provides the driving force that keeps blood moving, always from an area of higher pressure to an area of lower pressure, through the body.

Blood flow is the volume of blood flowing through a vessel, an organ, or the entire circulation in a given period.

Opposition to blood flow is resistance and arises from three sources:

1. Total Blood Vessel Length

The relationship between total blood vessel length and resistance is straightforward: the longer the vessel, the greater the resistance. For example, an infant’s blood vessels lengthen as he or she grows to adulthood, and so both peripheral resistance and blood pressure increase.

2. Blood viscosity

The internal resistance to flow that exists in all fluids is viscosity (vis-kos’ĭ-te) and is related to the thickness or “stickiness” of a fluid.

3. Blood Vessel Diameter

The relationship between blood vessel diameter and resistance is also straightforward: the smaller the diameter, the greater the resistance. (That’s why a wider straw is easier to drink from.)

Declining impulses from baroreceptors stimulate cardioacceleratory center and stimulate vasomotor center.

vascular shock

Ex.

Loss of vasomotor tone that results in a huge drop in peripheral resistance is known as vascular shock.

In vascular shock, blood volume is normal, but circulation is poor as a result of extreme vasodilation. A huge drop in peripheral resistance follows, as revealed by rapidly falling blood pressure.

A common cause of vascular shock is loss of vasomotor tone due to anaphylaxis (anaphylactic shock), a systemic allergic reaction in which the massive release of histamine triggers bodywide vasodilation. Two other common causes are failure of autonomic nervous system regulation (neurogenic shock), and septicemia (septic shock), a severe systemic bacterial infection (bacterial toxins are notorious vasodilators).

Hypertension results when blood pressure is chronically elevated.

In atherosclerosis, small patchy thickenings called atheromas form that make the walls of our arteries thicker and stiffer, resulting in hypertension.

Vasoconstriction is a decrease in the diameter (lumen size) of blood vessels due to contraction of the smooth muscle in the tunica media.

Varicose veins are veins that are tortuous and dilated because of incompetent (leaky) valves.

Vasomotor fibers stimulate vasoconstriction, causing an increase in total peripheral resistance (TPR).

Ex.

The step in the homeostatic response to low blood pressure indicated by “E” is vasomotor fibers stimulate vasoconstriction, causing an increase in total peripheral resistance .

Blood pressure rises.

changes in blood pressure

Ex.

Most neural controls of blood pressure involve input from baroreceptors, which are sensitive to changes in blood pressure.

Cardiovascular center activity is modified by inputs from (1) baroreceptors (pressure-sensitive mechanoreceptors that respond to changes in arterial pressure and stretch), (2) chemoreceptors (receptors that respond to changes in blood levels of carbon dioxide, H⁺, and oxygen), and (3) higher brain centers.

Antidiuretic hormone

Ex.

Antidiuretic hormone acts on both the kidneys and blood vessels to raise blood pressure.

Produced by the hypothalamus, antidiuretic hormone (ADH, also called vasopressin) stimulates the kidneys to conserve water. It is not usually important in short-term blood pressure regulation. However, when blood pressure falls to dangerously low levels (as during severe hemorrhage), much more ADH is released and helps restore arterial pressure by causing intense vasoconstriction.

Impulses from baroreceptors stimulate cardioinhibitory center and inhibit vasomotor center.

Decline in cardiac output and peripheral resistance return blood pressure to homeostatic range.

Baroreceptors are inhibited.

Baroreceptors are stimulated.

Pumping action of the heart

Ex.

The major force generating blood flow is the pumping action of the heart.

Any fluid driven by a pump through a circuit of closed channels operates under pressure, and the nearer the fluid is to the pump, the greater the pressure exerted on the fluid. Blood flow in blood vessels is no exception, and blood flows through the blood vessels along a pressure gradient, always moving from higher- to lower-pressure areas. Fundamentally, the pumping action of the heart generates blood flow. Pressure results when flow is opposed by resistance.

There are three important sources of resistance: blood viscosity, vessel length, and vessel diameter.

Total Blood Vessel Length

The relationship between total blood vessel length and resistance is straightforward: the longer the vessel, the greater the resistance. For example, an infant’s blood vessels lengthen as he or she grows to adulthood, and so both peripheral resistance and blood pressure increase.

Blood viscosity

The internal resistance to flow that exists in all fluids is viscosity (vis-kos’ĭ-te) and is related to the thickness or “stickiness” of a fluid.

Blood Vessel Diameter

The relationship between blood vessel diameter and resistance is also straightforward: the smaller the diameter, the greater the resistance. (That’s why a wider straw is easier to drink from.)

circulatory shock

Ex.

Any condition in which blood vessels are inadequately filled and blood cannot circulate normally is called circulatory shock.

In circulatory shock blood flow is inadequate to meet tissue needs. If circulatory shock persists, cells die and organ damage follows.

Hypertension results when blood pressure is chronically elevated.

In atherosclerosis, small patchy thickenings called atheromas form that make the walls of our arteries thicker and stiffer, resulting in hypertension.

Arteriosclerosis is a hardening of the arteries.

Varicose veins are veins that are tortuous and dilated because of incompetent (leaky) valves.

multiple heart attacks

Ex.

Cardiogenic shock is most likely to result from multiple heart attacks (myocardial infarctions).

Cardiogenic shock, or pump failure, occurs when the heart is so inefficient that it cannot sustain adequate circulation. Its usual cause is myocardial damage, as might follow numerous myocardial infarctions (heart attacks).

A systemic allergic reaction or a severe bacterial infection could both cause vascular shock, which is a condition in which blood volume is normal, but circulation is poor as a result of extreme vasodilation.

Large-scale blood loss would result in hypovolemic shock, which results from massive blood or fluid loss, as might follow acute hemorrhage, severe vomiting or diarrhea, or extensive burns.

Increase in sympathetic impulses to heart causes increase in heart rate, contractility, and cardiac output.

increasing cardiac output

Ex.

Blood flow would be increased by increasing cardiac output.

Blood flow is the volume of blood flowing through a vessel, an organ, or the entire circulation in a given period (ml/min). If we consider the entire vascular system, blood flow is equivalent to cardiac output (CO), and under resting conditions, it is relatively constant. At any given moment, however, blood flow through individual body organs may vary widely according to their immediate needs. To summarize:

• Blood flow (F) is directly proportional to the difference in blood pressure (ΔP) between two points in the circulation, that is, the blood pressure, or hydrostatic pressure, gradient. Consequently, when ΔP increases, blood flow increases, and when ΔP decreases, blood flow declines.

• Blood flow is inversely proportional to the total peripheral resistance (TPR) in the systemic circulation; if TPR increases, blood flow decreases.

These relationships can be expressed by the formula: F = ΔP / TPR.

Decreasing blood vessel diameter or increasing blood vessel length would increase total peripheral resistance and decrease blood flow.

Decreasing blood pressure would decrease ΔP and decrease blood flow.