<< Chapter < Page Chapter >> Page >

Vessel length and diameter

The length of a vessel is directly proportional to its resistance: the longer the vessel, the greater the resistance and the lower the flow. As with blood volume, this makes intuitive sense, since the increased surface area of the vessel will impede the flow of blood. Likewise, if the vessel is shortened, the resistance will decrease and flow will increase.

The length of our blood vessels increases throughout childhood as we grow, of course, but is unchanging in adults under normal physiological circumstances. Further, the distribution of vessels is not the same in all tissues. Adipose tissue does not have an extensive vascular supply. One pound of adipose tissue contains approximately 200 miles of vessels, whereas skeletal muscle contains more than twice that. Overall, vessels decrease in length only during loss of mass or amputation. An individual weighing 150 pounds has approximately 60,000 miles of vessels in the body. Gaining about 10 pounds adds from 2000 to 4000 miles of vessels, depending upon the nature of the gained tissue. One of the great benefits of weight reduction is the reduced stress to the heart, which does not have to overcome the resistance of as many miles of vessels.

In contrast to length, the diameter of blood vessels changes throughout the body, according to the type of vessel, as we discussed earlier. The diameter of any given vessel may also change frequently throughout the day in response to neural and chemical signals that trigger vasodilation and vasoconstriction. The vascular tone    of the vessel is the contractile state of the smooth muscle and the primary determinant of diameter, and thus of resistance and flow. The effect of vessel diameter on resistance is inverse: Given the same volume of blood, an increased diameter means there is less blood contacting the vessel wall, thus lower friction and lower resistance, subsequently increasing flow. A decreased diameter means more of the blood contacts the vessel wall, and resistance increases, subsequently decreasing flow.

The influence of lumen diameter on resistance is dramatic: A slight increase or decrease in diameter causes a huge decrease or increase in resistance. This is because resistance is inversely proportional to the radius of the blood vessel (one-half of the vessel’s diameter) raised to the fourth power (R = 1/r 4 ). This means, for example, that if an artery or arteriole constricts to one-half of its original radius, the resistance to flow will increase 16 times. And if an artery or arteriole dilates to twice its initial radius, then resistance in the vessel will decrease to 1/16 of its original value and flow will increase 16 times.

The roles of vessel diameter and total area in blood flow and blood pressure

Recall that we classified arterioles as resistance vessels, because given their small lumen, they dramatically slow the flow of blood from arteries. In fact, arterioles are the site of greatest resistance in the entire vascular network. This may seem surprising, given that capillaries have a smaller size. How can this phenomenon be explained?

Questions & Answers

body, muscle and regional of structures in the body
Harmony Reply
when the heart beats, blood pumps through the body and making the body function
what is vagina
Hausa Reply
what is ventricular circulation
Maryam Reply
In a four-chambered heart, such as that in humans, there are two ventricles that operate in a double circulatory system: the right ventricle pumps blood into the pulmonary circulation to the lungs, and the left ventricle pumps blood into the systemic circulation through the aorta.
A 23-year-old basketball player mentally rehearses free throw shots while lying in bed. Which option best describesthe area of the brain that is involved in generating a motor image of this action in the absence of actual movement?
a 67-year-old man has a stroke. one week later, he experiences sudden and uncontrolled flailing, ballistic movements of his limbs. which part of the man’s brain is most likely to have been damaged by the stroke?
primary motor cortex.. (principal area of cerebrum)
what factors that affect the rate diffusion
Gift Reply
the mass of the solute the temperature of theenvironment the solvent density and the distance traveled
sorry what are the common meaning of haemostatis
Juma Reply
homeostasis is the steady internal physical and chemical conditions maintaining by living systems
what are the negatives feedback regulation of ADH
Nansi Reply
what is the the differences between DNA and RNA?
The major differences between the DNA and RNA are contain of double stranded and single stranded which the DNA contain duoble stranded and RNA contain single stranded.
what are the negatives feedback regulation of ADH before this we confused just verify long term of ADH firstly
How many genes consist of DNA?
omaryare muhyadiin when you talk about genes, is the material formed in a DNA genes have form like plasma have many genes round there
aldosterone, renin
DNA contains the sugar deoxyribone while RNA contains the sugar ribose the only difference between ribose and deoxyribone is that ribose has one more- OH group than deoxyribone which has -H attached to the second carbon in the ring DNA is a double stranded molecules RNA is a single strnded molecul
in human genes very in size from a few hundred DNA based to more than 2million bases
to know the different structures of the body To know how the body works To know more about our body parts
Deitdre Reply
hi Deitdre you tock about knowledge or you ask the question
do you need any explanation when reading this book?
janet Reply
Its Good
if have many ability just do it!!
we all waiting for it
compare and contrast the operation of homeostasis
Dinelle Reply
what is the difference between an ionic, polar covalent and nonpolar covalent bond?
In summary, the bond has different in electronegativity.
sorry help me to get the definition of hemostasis or the meaning
the definition of distal
Dinelle Reply
farthest away from the attachment point.
Distal, is the farthest possition from the origin or midle point
exercise physiologist how ?
Noor Reply
hi noorr. when you talk about physiologist its a person who study physiology but exercise physiologist is what an exercise doing by physiologist in physiologican
can I get the questions of human physiology that is present in HSC 2nd semester
Rafiullah Reply
in my lerning the question com like for example ni eassy qustion must understand the the part of the body and how it work or mechanisms of each part learned
how can I memorize
mukhtaar Reply
which part of the body produces blood
give me answer
Red blood cells are formed in the red bone marrow ofbones. Stem cells in the red bone marrow called hemocytoblasts give rise to all of the formed element
what is hemocytoblasts
hemocytoblasts are stem cells in red bone marrow which give rise the all of formed elements
Discuss clonal theory in physiology and its application in measles infection in a 6yr child? Can anyone help me
Isaac Reply
Capillary permeability
what do you want to know about it?
Hello, I want to search about the topic, information and pictures

Get the best Anatomy & Physiology course in your pocket!

Source:  OpenStax, Anatomy & Physiology. OpenStax CNX. Feb 04, 2016 Download for free at http://legacy.cnx.org/content/col11496/1.8
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Anatomy & Physiology' conversation and receive update notifications?