Summary
Summary BLOOD SYSTEM WEEK 2
- Institution
- Chamberlain College Of Nursng
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BIOS255 – Anatomy & Physiology IIIProf Haneen Salhieh, M.S.
Week 3 - The Cardiovascular System: Blood Vessels
We previously discussed the different structures of the heart and learned about the flow of blood to and
from the lungs. This was our introduction to a few of the major vessels including the pulmonary artery and
veins. Let’s look at the three main types of vessels: arteries, veins, and capillaries.
Arteries are vessels that transport blood away from the heart towards the capillaries. These
vessels are typically oxygen-rich.
Veins are vessels that drain blood from the capillaries and transport it back to the heart. These
vessels are typically oxygen-poor.
Capillaries are microscopic, thin-walled vessels that allow for the exchange of substances
between the blood and tissues. These vessels connect the smallest arteries to the smallest veins.
ONLY Exception:
Pulmonary Artery: Deoxygenated blood to lungs
Pulmonary Veins: Oxygenated blood to heart
Umbilical Artery: Deoxygenated blood
Umbilical Veins: Oxygenated blood
Tunics (Layers) of Arterial and Venous Walls
Regardless of the direction of blood flow or function, the walls of arteries and veins consist of three layers
called tunics. Capillaries do not contain these three layers.
The tunica interna (tunica intima) is the most inner layer of the vessel. This layer directly touches blood.
It consists of a simple squamous epithelium called the endothelium, with a thin layer of areolar connective
tissue underneath. The tunica interna acts as a permeable membrane to specific materials that enter or
leave the bloodstream. The endothelium secretes chemicals that will stimulate changes in the vessel’s
,BIOS255 – Anatomy & Physiology III
Prof Haneen Salhieh, M.S.
diameter. When this layer is damaged and the underlying connective tissue is exposed, platelets can
adhere to it and form a blood clot to help reduce blood loss.
The tunica media is the middle layer that consists of smooth muscle, elastic tissue, and collagen fibers. It
usually is the thickest layer. The smooth muscle here contracts to regulate the diameter of the blood
vessel.
The tunica externa (tunica adventitia) is the outermost layer that consists of connective tissue that
anchors the vessel to nearby tissues and provides a passageway to lymphatics, small nerves, and small
blood vessels that supply the tissues of larger vessels.
Because blood vessel walls contain different types of tissues, they will require oxygenation, nutrition, and
waste removal. The inner half of vessel walls is believed to be nourished by diffusion from blood in the
lumen. A network of small blood vessels called the vasa vasorum supplies the outer half of the vessel
walls.
Arteries
Recall that arteries are vessels that carry blood away from the heart. These vessels are built to withstand
the surge of pressure as blood is ejected into them with each heartbeat. For this reason, they are
sometimes called resistance vessels. Arteries have a high blood pressure that fluctuates and pulsates as
blood flows through the vessel. As we trace blood flow in arteries, we find that it splits off into smaller and
smaller branches. Let’s examine the different artery types.
Elastic arteries, aka conducting or large arteries, are the biggest (2.5-1.0 cm in diameter). The
tunica media consists of 40-70 layers of elastic tissue with alternating layers of smooth muscle
and collagen fibers. These vessels stretch to accommodate blood being ejected from the left
ventricle and the elastic fibers help them ‘rebound’ to their original diameter. Examples include
the aorta, pulmonary trunk, common iliac arteries, and subclavian arteries.
Distributing arteries, aka muscular or medium arteries, are smaller branches that carry blood to
specific organs or body regions (1.0 cm – 0.3 mm). Their names are specific to the organs they
distribute blood to such as the femoral, renal, splenic, and brachial arteries. They typically have
up to 40 layers of smooth muscle in their tunica media, which is proportionally thicker than in
elastic arteries. The extra muscle allows these vessels to narrow (vasodilation) and widen
(vasoconstriction) to regulate the distribution of blood to specific body regions or organs.
Resistance arteries, or small arteries, are variable in location and number and do not have
individual names (300-10 μm). The smallest class of these vessels is known as arterioles. These
vessels deliver blood to capillaries where gas and nutrient exchange occurs. Arterioles control
how much blood an organ or tissue receives.
In some areas, there are short vessels called metarterioles that directly connect arterioles to
venules, bypassing capillary es.
Clinical Correlation: Aneurysm
An aneurysm is a weak point in the wall of a vessel, most commonly arteries. This forms a thin walled,
bulging sac that pulsates each time the heart beats, and can put pressure on nerves, airway passages,
and adjacent tissues. The primary concern is the risk of aneurysm rupturing, which can lead to severe
internal bleeding and other complications depending on the location. Aneurysms can develop in different
parts of the body, most commonly in the abdominal aorta, renal arteries, and within the Circle of Willis at
the base of the brain.
Aneurysms can develop for several reasons:
Genetic factors: Some individuals may have a genetic predisposition to developing aneurysms
that can result in a congenital weakness of the vessel wall.
, BIOS255 – Anatomy & Physiology III
Prof Haneen Salhieh, M.S.
Arteriosclerosis: This is a general term that refers to the thickening and hardening of the arterial
walls that can result from a buildup of fatty deposits, genetic predisposition, aging, and long-term
hypertension.
Hypertension: Elevated blood pressure can strain arterial walls, making them more susceptible
to aneurysm formation.
Trauma: Injuries to blood vessels can lead to the formation of aneurysms.
When treating aneurysms, there are conservative measures and interventional procedures. Conservative
measures include monitoring small, stable aneurysms and lifestyle changes of exercise routines and
diets. Interventional procedures include surgical repair of high-risk aneurysms to replace/reinforce the
weakened section with a graft, as well as endovascular repair where a catheter is used to place a stent or
graft to support the weakened blood vessel wall.
Arterial Sense Organs
Certain arteries have sense organs within their walls that monitor the composition of blood and blood
pressure. These sensory receptors relay information to the brainstem that will send output to regulate
blood vessel diameters, heart rate, and respiration. There are 3 types of arterial sense organs:
Carotid sinuses are sensory receptors that monitor blood pressure (baroreceptors). Per the
name, these receptors are found in the walls of the internal carotid arteries. The carotid sinuses
respond to changes in blood pressure. Baroreceptors detect stretch
Carotid bodies are chemoreceptors located in the common carotid arteries. These sensory
organs monitor changes in blood composition and send signals to the brainstem to adjust
breathing patterns to regulate blood pH and O2 and CO2 levels.
Aortic bodies are chemoreceptors located in the aortic arch. These function similarly to carotid
bodies, monitoring the chemical composition of blood gases and blood pH. These sense organs
transmit signals to the brainstem by the vagus nerves.
Carotid Sinus: internal carotids / blood pressure and baroreceptors
Carotid Bodies: common carotids/ PH O2 and CO2 breathing patterns
Aortic Bodies: aortic arch / blood gases and blood pH / vagus nerve
Capillaries
As blood travels closer to the tissues it will oxygenate, the vessels diameter will become smaller until they
are barely wider than single red blood cells. The smallest and most numerous vessels in the body that will
facilitate the exchange of nutrients, gases, and wastes between blood and tissues are capillaries. These
vessels connect the smallest arteries to the smallest veins. There are three main types:
Continuous capillaries are the most common type of capillaries. These are found in most
tissues such as skin, muscles, and the CNS. The endothelial cells of these vessels are held
together by tight junctions, creating a continuous and uninterrupted lining. There are small gaps
that allow for exchange of substances such as gases via simple diffusion.
Fenestrated capillaries are found in tissues with high rates of filtration and absorption such as
the kidneys and small intestines. These vessels are similar to continuous capillaries but contain
fenestrations, or pores, that allow for an increased exchange of fluids and solutes.
Sinusoidal capillaries are vessels characterized by irregularly shaped endothelial cells with
large gaps between them. These large openings allow the passage of larger molecules such as
proteins and formed elements. Sinusoidal capillaries are found in organs with a high demand for
blood supply and where exchange of larger molecules is essential, such as the liver, spleen, and
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