Three Principal Categories of Blood Vessels

1. Arteries
2. veins
3. capillaries

Aside from their general location adn direction of blood flow, these three categories of vessels also differ in the histological structure of their walls.

Define Vein

the afferent vessels that carry blood back to the heart.

Define Arteries

the efferent vessels of the cardiovascular system, that is, vessels that carry blood away from the heart.

Define Capillaries

microscopic thin-walled vessels that connect the smallest arteries to the smallest veins

The wall of veins and arteries are composed of three layers called:

tunics

1. tunica interna
2. tunica media
3. tunica externa

Tunica interna

- lines the inside of the blood vessel and is exposed to blood

- it consists of endothelium overlying a basement of membrane and a sparse layer of loose connective tissue, it is continous with the endocardium of the heart.

Endothelium

-acts as a permeable barrier to materials entering or leaving the bloodstream; it secretes chemicals that stimulate dilation or constriction of the vessel; and it normally repels blood cells and platelets so that they flow freely without sticking to the vessel wall.

-when damaged platelets may from a blood clot.

-when tissue around blood vessel is inflamed, endothelial cells produce cell-adhesion molecules that induce leukocytes to sdhere to the surface.

Tunica Media

-the middle layer of vessel walls and usually the thickest.

- consist of smooth muscle, collagen, and in some cases, elastic tissue.

-smooth muscle and elastic tissue amounts vary from one vessel to another

- strengthens the vessels and prevents blood pressure from ruptutring them. Also produces vasomotion, changes in the diameter of blood vessels.

Tunica Externa

-the outermost layer of blood vessel.

- consist of loose connective tissue that often merges with that of neighboring blood vessels, nerves, or other organs.

-anchors vessel and provides passage for small nerves, lymphatic vessels, and smaller blood vessels that supply tissue tissue of the larger vessel.

Vasa Vasorum

nourish at least the outer half of the wall of a larger vessel. Tissue of the inner half of the wall are thought to be nourished by diffusion from blood in the lumen.

Arteries are some times called:

Resistance vessels of the cardiovascular system because they have a relatively strong, resilient tissue structure. More muscular than veins and can retain their round shape even when empty.

Three Artery classes by size:

1. Conducting arteries (Biggest)

2. Distribtuing arteries (medium)

3. Resistance arteries (smallest)

Conducting arteries

(examples)

-aorta, common cartid and subclavian arteries, pulmonary trunk, and common iliac

Conducting Arteries

- expand during ventrixular systole to recieve blood, and recoil during diastole

- their expansion takes some pressure off the blood so smaller arteries are subjected to less systolic stress

- their recoil between heart beats prevents BP from dropping too low while heart is relaxing.

Distributing Arteries

Simplest and most common route of blood flow

Portal system

-blood flows through two consecutive capillary networks before returning to the heart.

Where does the portal system occur?

occurs in the kidneys; connecting the hepothalamus and the anterior pituitary and connecting the intestine to the liver

Define Anastomosis and 3 types

A point where two blood vessels merge.

1. arteriovenous anastomosis

2. venous anastomoses

3. Arterial anastomoses

Arteriovenous Anastomosis (shunt)

-blood flows from an artery directly into a vein and bypasses the capillaries.

-occurs in the fingers, palms, toes, and ears, where they reduce heat loss in cold weather by allowing warm blood to bypass these exposed surfaces.

Venous Anastomoses

-Most common anastomoses

-one vein empties directly into another, providing alternative routes of drainage from an organ.

Arterial anastomoses

- two arteries merge, providing collateral routes of blood supply to a tissue.

- common around joints where movement may temporarily compress an artery and obstruct and obstruct one pathway.

Contrast veins and arteries

Contrast:

- V: have a greater capacity for blood containment than A.( at rest, V: 64% and A: 13% of blood)

-Larger A: BP averages 90 to 100 mm Hg where in V: it averages 10 mm Hg

-In V: the blood flow is steady, where as in A: it is pulsating with the heart beat

-V: collaspes when empty= relatively flattened shape in histological sections

A: maintain round shape when empty= appear relatively circular in tissue sections

-blood flow in A: splitting off into smaller branches. Smaller V: merge to form larger and larger as they approach the heart.

Compare veins and arteries

-Both are types of vessels

-Both are vascular

Types of Capillaries

1.     Continuous capillaries

2.     Fenestrated capillaries

3.     Sinusoids capillaries

Continuous Capillaries

Occurs in most tissue, such as skeletal muscles

Fenestrated capillaries

Important in organs that engage in rapid absorption or filtration- the kidneys, endocrine glands, small intestine, and choroid plexuses of the brain

Sinusoids capillaries

Irregular blood-filled spaces in the liver, bone marrow, spleen, and some other organs.

Hypotension

-consequence of blood loss, dehydration, anemia, and is normal in people approaching death.

Hypertension

-can weaken the small arteries and cause aneurysms and promotes the  development of atherosclerosis.

Peripheral resistance

-opposition to flow that the blood encounter in vessels away from the heart

What are the machine movements through the capillary walls?

1.     Diffusion

2.     Transcytosis

3.     Filtration and reabsorption

Diffusion

-most important mechanism of exchange

-Glucose and oxygen (more concentrated in the systematic blood than in tissue fluid) diffuse out of the blood

-Carbon dioxide and other wastes (being more concentrated in tissue fluid) diffuse into the blood.

Transcytosis

-Process in which endothelial cells pick up material on one side of the plasma membrane by pinocytosis or receptor-mediated endocytosis, transport the vesicles across the cell, and discharge the material on the other side by exocytosis

-Accounts for a small fraction of solute exchange across the capillary wall

-fatty acids, albumin, and some hormones such as insulin move across the endothelium by this mechanism

Filtration and Reabsorption

-Fluid filters out of the arterial end of a capillary and osmotically reenters it at the venous end.

-This fluid delivers material to the cells and removes their metabolic wastes

-Capillary can give off fluid at one point and reabsorb at another because of the shift in balance between hydrostatic and osmotic forces.

Hydrostatic pressure

-The physical force exerted by a liquid against a surface such as capillary wall

            -Ex: blood pressure

-Typical value is -3 mm Hg---The negative indicates that this is a slight suction, which helps draw fluid out of capillaries. The positive hydrostatic pressure with the capillary and the negative interstitial pressure work in the same direction, creating a total outward force of 33 mm Hg.

Colloid osmotic pressure (COP)

-The portion of the osmotic pressure due to protein.

-Blood has a COP of about 28 mm Hg, due mainly to albumin

Oncotic pressure

-       The difference between COP of blood and tissue fluid.

-       Tends to draw water into the capillaries by osmosis, opposing hydrostatic pressure

Net filtration pressure (NFP)

-Net hydrostatic pressure – oncotic pressure =NFP

Net reabsorption

-Causes the capillary to reabsorb fluid at the venous end because osmotic pressure overrides filtration pressure.

Venous return

-flow of blood back to the heart

Five mechanisms of Venous Return

1.The pressure gradient

2.Gravity

3.The skeletal muscle pump

4.The thoracic pump

5.Cardiac suction

Pressure Gradient

-pressure generated by the heart is the most important force in venous flow

-this and venous return increase when blood volume increases.

Gravity

-when sitting or standing, blood from your head and neck returns to the heart simply by flowing "down hill" through the large veins above the heart.

-the larger veins are normally collasped or nearly and their venous pressure is close to zero.

The skeletal muscle pump

-in the limbs, the veins are surrounded and massaged by the muscles. Contracting muscles squeeze the blood out of the compressed part of a vein, and the calved ensure that htis blood can go only towards the heart.

The thoracic (respiratory) pump

-Aids the flow of venous blood from the abdominal to the thoracic cavity.

-If adominal pressure on the Inferior vena cava (IVC) rises while thoracic pressure on it drops, then blood is squeezed upward toward the heart.

Cardiac suction

-during ventricular systole, tendinous cords pull the AV valve cusps downward, slightly expanding the atrial space. This creates a slight suction that draws blood into the atria from the venae cavae and pulmonary veins.

Types of Circulatory Shock:

1.Hypovolemic shock

2.Obstructed venous return shock

3. Venous pooling (vascular) shock

Hypovolemic Shock

-most common form

-produced by a loss of blood volume as a result of hemorrhage, traumna, bleeding ulcers, burns, or dehydration

Obstructed venous return shock

-occurrs when any object, such as growing tumor or aneurysm, compresses a vein and impedes its blood flow

Venous pooling (vascular shock)

-occurs when the body has a normal total blood volume, but too much of it accumulates in the lower body.

- can result from long periods of standing or sitting or from widespread vasodilation

-Neurogenic shock is a form of this

Most common cardiovascular disease

Hypertension- the major cause of heart failure, stroke, and kidney failure

Hypertension

"Silent Killer"

-increases the afterload on the heart, which makes the ventricles work harder to expel blood

-enlarges myocardium- becomes excessively stretched and less efficent

-strains BV and tears the endothelium

read pg. 802

Importance of the Lymphatic system in the circulatory system

As blood moves through the arteries and veins, 10% of the fluid filtered by the capillaries, along with vital proteins, becomes trapped in the tissues of the body.

-it collects this fluid and returns it to the circulatory system.

primary function of capillaries

-deliver nutrients and remove waste between the blood and tissue cells of the body

-they are part of the vascular system

Capillary Fluid Dynamics

1. Hydrostatic pressure

2. Osmotic pressure

3. Tissue Hydrostatic pressure

4. Tissue osmotic pressure

http://www.wisc-online.com/object/ViewObject.aspx?ID=NUR7808

Hydrostatic

- pushing, builds inside capillaries,it causes filtration, forcing solutes out through the capillary walls into tissues delivering nutrients arteriole (red) end

Osmotic pressure

- pulling, inward force: albumin within the capillary is pulling water from the tissues. This is happening throughout the capillary but exerts greater pressure at the venule (blue) end

Tissue Hydrostatic pressure

-pushing- an inward force: waste fluids are being pushed by the tissues into the capillary. This is happening at the venule (blue) end

Physiology Venous Return

Serves as:

1. a generalized raising or lowering of blood pressure throughout the body

2. selectively modifying the perfusion of a particular organ and rerouting blood from one region of the body to another

-fluid recovery

-immunity

-lipid absorption