Chapter 13

Vessels carry blood to peripheral tissues, and the nature of the exchange that occurs between the blood and interstitial fluids.• form a closed tube that carries blood away

from the heart, to the cells, and back again. Vessels consist of: arteries,

arterioles, capillaries, venules, and veins

Blood coming from the heart to the capillaries travel through the arteries and arterioles.

Arteries are strong, elastic vessels adapted for carrying high-

pressure blood. Arteries become smaller as they

divide and give rise to arterioles.

The wall of an artery and vein consist of an endothelium(tunica

interna) tunica media (smooth muscle), and tunica externa (connective tissue).

Arteries are capable of vasoconstriction as directed by the

sympathetic impulses; when impulses are inhibited, vasodilation results.

An artery has a thicker wall and small lumen than a vein.

Comparison of a Artery and Vein

Only blood vessels whose wall permit exchange between the blood and surrounding interstitial fluid.

Capillaries are the smallest vessels, consisting only of a layer of endothelium through which substances are exchanged with tissue cells.

Areas with a great deal of metabolic activity (leg muscles, for example) have higher densities of capillaries.

Capillaries do not function as individual unites but as part of a interconnected network called a capillary bed.

Blood entering capillaries contains high concentrations of oxygen and nutrients that diffuse out of the capillary wall and into the tissues.

Capillary Bed

Hydrostatic pressure drives the passage of fluids and very small molecules out of the capillary at this point.

At the venule end, osmosis, due to the osmotic pressure of the blood, causes much of the tissue fluid to return to the bloodstream.

Venules leading from capillaries merge to form veins that return blood to the heart.

Veins have relatively thin walls because they do not need to withstand much pressure.

Veins have the same three layers as arteries have and have a flap-like valve inside to prevent backflow of blood.

Pressure and resistance both affect blood flow to tissues, but they have opposing effects.

Blood flow and pressure are directly related: when pressure increases, flow increases.

Blood flow and resistance are inversely related: when resistance increases, flow decreases.

When a pressure difference exists, a liquid will flow from an area of higher pressure toward an area of lower pressure.

Flow rate is directly proportional to this: the greater the difference in pressure, the faster the flow.

Largest pressure difference is found between the base of the aorta and entrance to the right atrium.

Resistance is any force that opposes movement. In the cardiovascular system, it opposes the movement of blood.

The circulatory pressure much be great enough to over the total peripheral resistance.

Greatest pressure difference occurs in the arterial network

Blood pressure is the force of blood against the inner walls of blood vessels anywhere in the cardiovascular system, although the term "blood pressure" usually refers to arterial pressure.

Arterial blood pressure rises and falls following a pattern established by the cardiac cycle.

During ventricular contraction, arterial pressure is at its highest (systolic pressure).

When ventricles are relaxing, arterial pressure is at its lowest (diastolic pressure).

Blood pressure is measured by using a sphygmomanometer.

An inflatable cuff is placed around the arm, when inflated the cuff squeezes the brachial artery. A stethoscope is placed over the artery.

A tube connects the cuff to a pressure gauge that measure the pressure inside the cuff.

Air is slowly let out of the cuff.

When the pressure in the cuff falls below systolic pressure, blood can enter the artery again.

First, blood enters at peak systolic blood pressure and then continues to fall below diastolic blood pressure, where blood flow become continuous

Read as “120/80”

Systolic Pressure

Diastolic Pressure

Cardiovascular regulation is to ensure that blood flow changes occur at the appropriate time, right area, and with drastically altering blood pressure and blood flow to organs.

Controlled by Autoregulation and Neural and Endocrine mechanisms.

Changes in tissue conditions act directly on precapillary sphincters to alter peripheral resistance, producing changes in the pattern of blood flow.

Factors that promote the dialation of precapillary sphincters are called vasodilators.

Factors that stimulate the constriction of precapillary sphincters are called vasoconstrictors.

Cardiovascular centers detect changes in tissue demand by monitoring arterial blood, especially blood pressure, pH, and gas concentration.

Baroreceptor reflexes- monitor changes in blood pressure

Chemoreceptor reflexes- respond to changes in chemical composition.

The endocrine system provides short-term regulation of cardiac output and peripheral resistance with epinephrine and norepinephrine from the adrenal medullae.

Hormones involved in long-term regulation of blood pressure and volume are antidiuretic hormone (ADH) and angiotensinII, erythropoietin(EPO) and atrial naturetic peptide(ANP)

ADH and angiotensinII promote peripheral vasoconstriction.

ADH and aldosterone promote water and electrolyte retention and stimulate thirst.

EPO stimulates red blood cell production ANP encourages sodium loss, fluid loss ,

reduces blood pressure, inhibits thirst, and lowers peripheral resistance.

Shock is an acute circulatory crisis marked by low blood pressure and inadequate blood flow.

Causes of shock: Fall in cardiac output after fluid loss, damage to the heart, external pressure on the heart, or extensive peripheral vasodialation

When blood volume declines by more than 35%, homeostatic mechanisms become unable to cope with the situation

Low blood pressure and low venous return lead to decreased cardiac output and myocardial damage, reducing cardiac output.

Carotid sinus baroreceptors trigger a massive activation of sympathetic vasoconstrictors

Which reduces blood flow to peripheral tissues in order to maintain adequate blood flow to the brain.

Must receive immediate treatment to help eliminate fatal consequences.