BI0005 - Lecture 11 - Blood vessels
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What is an artery?
Arteries (from Greek ἀρτηρία, meaning "windpipe, artery") are blood vessels that carry blood away from the heart.
What is a vein?
In the circulatory system, veins (from the Latin vena) are blood vessels that carry blood toward the heart.
What is an arteriole?
An arteriole is a small diameter blood vessel in the microcirculation that extends and branches out from an artery and leads to capillaries.
What is a venule?
- A venule is a very small blood vessel in the microcirculation that allows blood to return from the capillary beds to the larger blood vessels calledveins.
- Venules range from 7 to 50μm in diameter.
What is a capillary?
Capillaries are the smallest of a body's blood vessels and are parts of its microcirculation. Their endothelial linings are only one cell layer thick. These microvessels, measuring around 5 to 10 micrometre in diameter, connect arterioles and venules, and they help to enable the exchange of water, oxygen, carbon dioxide, and many other nutrients and waste chemical substances between blood and the tissues surrounding them.
What are the three main layers of blood vessels?
- Tunica intima
- Tunica media
- Tunica adventitia
What is the tunica intima?
- The tunica intima (or just intima) is the innermost tunica (layer) of an artery or vein.
- It is made up of one layer of endothelial cells and is supported by an internal elastic lamina.
- The endothelial cells are in direct contact with the blood flow.
What is the tunica media?
- The tunica media (or just media) is the middle tunica (layer) of an artery or vein.
- It lies between the tunica intima on the inside and thetunica externa on the outside.
- Tunica media is made up of smooth muscle cells and elastic tissue.
What is the tunica adventitia?
- The tunica externa, also known as the tunica adventitia (or just adventitia), is the outermost tunica (layer) of a blood vessel, surrounding the tunica media.
- It is mainly composed of collagen and, in arteries, is supported by external elastic lamina. The collagen serves to anchor the blood vessel to nearby organs, giving it stability.
Why do veins have valve?
Valves in the veins maintain a unidirectional flow of blood in these vessels.
How does blood pressure and flow change in the different blood vessels?
Blood pressure is highest in the aorta and other arteries, and flows at its fastest rate (49 cm/s) in the aorta.
Blood flow is much slower in the capillaries (0.1 cm/s) because the total cross-sectional area of the capillaries is much greater than in any other part of the circulatory system. This flow is essential to allow gas and nutrient exchange to take place by diffusion.
Blood flow speeds up as it enters the venules and veins, as they have a smaller total cross-sectional area than the capillaries.
What is a dicrotic notch?
The graph of aortic pressure throughout the cardiac cycle displays a small dip (the "incisure" or "dicrotic notch") which coincides with the aortic valve closure. The dip in the graph is immediately followed by a brief rise (the "dicrotic wave") then gradual decline.
How can you measure arterial blood pressure?
1. A Sphygomomanometer, an inflatable cuff attached to a pressure gauge, measures blood pressure in an artery. The cuff is inflated until the pressure closes the artery, so that no blood flows past the cuff. When this occurs, the pressure exerted by the cuff exceeds the pressure in the artery.
2. The cuff is allowed to deflate gradually. When the pressure exerted by the cuff falls just below that in the artery, blood pulses into the forearm, generating sounds that can be heard with the stethoscope. The pressure measured at this point is the systolic pressure.
3. The cuff is allowed to deflate further, just until the blood flows freely through the artery and the sounds below the cuff disappear. The pressure at this point is diastolic pressure.
What effects venous blood pressure?
Gravity is a consideration for blood flow in veins, especially those in the legs. Although blood pressure in veins is relatively low, several mechanisms assist the return of venous blood to the heart.
First, rhythmic contractions of smooth muscles in the walls of venules and veins aid in the movement of the blood.
Second, and more important, the contraction of skeletal muscles during exercise squeezes blood through the veins towards the heart
What can Atherosclerosis cause?
Atherosclerosis can create a 'bruit' (the unusual sound that blood makes when it rushes past an obstruction) over an artery, leading to angina, hypertension, eneurysm, death.
First, there is a creation of a fatty streak - excess LDL-cholesterol accumulates in acellular layer between endothelium and connective tissue
Second, progression to fibrous plaque - smooth cells divide and thus thicken arterial walls.
Third, formation of calcified plaque - 'Hardening of Arteries'.
What factors affect smooth muscle tone?
Intrinsic factors: Stretch, temperature, locally released chemical factors
Extrinsic: Autonomic nervous system and circulating hormones.
How does blood pressure change?
Blood pressure fluctuates on a longer time scale (than change of blood pressure within cardiac cycle) in response to signals that change the state of smooth muscles in arteriole walls.
For example, physical or emotional stress cantrigger nervous and hormonal responses that cause smooth muscles in the arteriole walls to contract, a process called vasoconstriction. When that happens the arterioles narrow, thereby increasing blood pressure upstream in the arteries.
When smooth muscles relax, the arterioles undergo vasodilation, an increase in diameter that causes blood pressure in the arteries to fall.
What is hypertension ?
Hypertension (HTN) or high blood pressure, sometimes called arterial hypertension, is a chronic medical condition in which the blood pressure in the arteries is elevated.
- Hypertension is classified as either primary (essential) hypertension or secondary hypertension; about 90–95% of cases are categorized as "primary hypertension" which means high blood pressure with no obvious underlying medical cause.
- The remaining 5–10% of cases (secondary hypertension) are caused by other conditions that affect the kidneys, arteries, heart or endocrine system.
Hypertension puts strain on the heart, leading to hypertensive heart disease and coronary artery disease if not treated. Hypertension is also a major risk factor for stroke,aneurysms of the arteries (e.g. aortic aneurysm), peripheral arterial disease and is a cause of chronic kidney disease.
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