Blood Vessels

EMRG1230 Week 2 Day 1

Vessel Wall Makeup

1. Tunica Adventitia (outside)

2. Tunica Media (middle)

3. Tunica Intima (interior)

Decrease in vessel diameter → Decrease in thickness of walls

Capillaries only have one layer making them the only exception

Tunic Adventitia (Externa) (Outer Layer)

Made of strong, flexible, connective tissues

Holds vessel open and prevents tearing during body movements

In Veins- thickest of all 3 layers

In Arteries- 2nd thickest next to the middle layer

Tunica Media (Middle Layer)

Made of smooth muscle tissue sandwiched with layers of elastic connective tissue

Muscle Layers allow for changes in blood vessel diameter

Innervated by autonomic nerves to control diameter

Arteries have thicker Tunica Media then veins

Tunica Intima (inner layer)

Made up of endothelial cells (extremely thin)

In veins- these cells make up the semilunar valves

In Capillaries- only this is present.

The thinness is required for efficient exchange of materials between the blood plasma and interstitial fluid

Which 4 structures are always constant regardless of size?

1. Lining Endothelial Cells

2. Collagen Fibers

3. Elastic Fibers

4. Smooth Muscle Fibers

Lining Endothelial Cells

Lines entire vessel and creates a smooth luminal surface by inhibiting intravascular coagulation.

pores allow for diffusion and movement of substances into the blood.

Capable of reproduction. they provide new cells to increase blood vessel size or repair damaged cells

Collagen Fibers

Reinforced strands woven together (similar to walls of a garden hose)

Minimal stretch (2-3%)

Function to keep the lumen of the vessel open and strengthen the walls

Elastic Fibers

Made of elastin. A rubber-like network which is highly elastic and capable of stretching more then 100%.

Allow for recoil after distention

Maintains passive tension- maintains normal blood pressure

Smooth Muscle Fibers

Found in the wall of all segments of vascular system except capillaries.

Most numerous in elastic and muscular arteries.

Exert active tension when vessels contracted

Arteries

Thick walled muscular vessels that carry blood away from the heart

All carry oxygenated blood except for the pulmonary arteries

Highly sensitive to stimulation from Autonomic Nervous System

Causes change in diameter as they relax and contract. Role is to regulate Blood pressure

Elastic Arteries

Largest in body including Aorta and its major branches

Can stretch without injury to accommodate the surge of blood forced into them as the heart contracts

They recoil meaning when the ventricles relax, they accommodate

Muscular Arteries (Distributing arteries)

Carry blood farther away from heart to specific organs

Smaller in diameter but the walls are thicker then elastic arteries

Ex/ Brachial artery, Gastric Artery, Mesenteric Artery

Arterioles (Resistance Vessels)

Smallest Arteries not named individually but as a group

Main function is to regulate blood flow through the body and determines quantity of blood that enters an organ

Increased Contraction = Increased resistance to blood flow, regulating BP and vice versa

Veins

Operate on the low pressure side of the system and has thinner walls

Less capacity to decrease their diameter

Thinner walls make veins more likely to distend when exposed to small increases in backpressure

Veins become larger as they get closer to the heart

Their ability to stretch allows them to accommodate varying amounts of blood with almost no chance in BP

Vein System

Blood passes through arteries into capillaries and eventually the venules

Venules- first venous structures to receive blood after it leaves capillaries

Blood exits venules and goes into veins

Veins- Vessel that carries blood towards the heart

Capillaries

Microscopic Blood Vessels that carry blood from the arteries to the venules

Walls are extremely thin being only 1 cell thick

Transfer of nutrients and other viral substances between blood and tissue cells. Over 1 billion in the body not evenly distributed

Precapillary Sphincter

Regulate volume of inflow of blood through the capillary

Band of smooth muscle encircling the capillary

Open → Blood flows in

Closed/Partially Closed → Decreased flow

Peripheral Resistance

Resistance to blood flow imposed by the force of friction between blood and vessel walls

Develops partially because of Viscosity (higher proportion of RBC and protein molecules in blood)

Partly from diameter of arterioles and capillaries

Vasomotor Mechanism

Consists of vasoconstriction and vasodilation

Controlled in the medulla (vasomotor center or vasoconstriction center)

When stimulated- initiates impulse outflow by sympathetic fibers that end in the smooth muscles of the vessel walls causing constriction

Secondary to this, there are reservoirs throughout the body

Vasoconstriction

Reduction in blood vessel diameter caused by an increased contraction of the muscular wall

Increases resistance to blood flow thereby decreasing blood flow to the tissues

Vasodilation

Increases vessel diameter by relaxation of the muscular wall

Causes an increase in blood flow to the tissues

Reservoirs

Body houses blood reservoirs in the venous plexus (located in the skin and abdominal organs)

Serve as a slow moving stockpile or reserve of blood

Blood can move from reservoirs to arteries that supply heart and other organs when increased activity demands

Vasomotor Pressoreflexes

Changes in arterial blood oxygen or carbon dioxide content sets a chemical vasomotor control mechanism into operation

This changes the arterial BP - Initiates the Vasomotor pressoreflexes

One of two things can happen depending on the body’s needs:

Increase in Arterial BP

OR

Decrease in Arterial BP

Increase in Arterial BP

Simulation of the aortic and carotid baroreceptors (sense pressure changes)

Stimulates the Cardiac Control Center to lower the HR

Inhibits vasoconstriction center

More impulse per second goes out over parasympathetic fibers to slow HR and dilate the venues of blood reservoirs

Strives to bring back the BP to normal (Homeostasis)

Decrease In Arterial BP

Baroreceptors are stimulated to sense pressure changes

This stimulates the cardiac control center to elevate the HR

Sends more impulses to the medulla to stimulate vasoconstriction

Stimulates the SNS, increasing HR and vasoconstriction, therefore raising BP to normal levels (homeosotasis)

Vasomotor Chemoreflexes

Located in the Aortic and Carotid bodies

Sensitive to excess blood CO2 levels (Hypercapnia)

Less sensitive to low levels of O2 (Hypoxia)

When either occur, impulses are sent via chemoreceptors to the medulla’s vasoconstriction center. Vasoconstriction soon follows and HR increases

Emergency system when high CO2 or low O2 endangers the stability of the internal environment

Venous Return to the Heart

Refers to the amount of blood returned to the heart by veins

Done by several mechanisms:

1. Venous Reservoirs- BP drops, vein walls adjust and blood flows in to maintain optimal blood return

2. Elastic Nature Of Veins- BP rises, Vein walls expand allowing them to adapt to higher pressure

Both of these are referred to as “stress relaxation effect”

Gravity

Blood pooling in lower extremities can be combated by venous pumps by maintaining pressure gradients to keep blood moving into the central veins and back to the heart

There are 2 types” Respiratory and Skeletal

Respiratory Venous Pump

Caused by increasing pressure gradient between peripheral veins and vena cava

Inspiration- diaphragm contracts and thoracic cavity becomes larger and the abdominal smaller

As a result- Pressure in thoracic cavity, vena cava, and atria decrease and the abdominal veins increase

Expiration- Opposite

Change in pressure between inspiration and expiration acts as a respiratory pump that moves blood along venous route

Skeletal Muscles

Serve as booster pumps

As each skeletal muscle contracts, it squeezes the veins inside basically ‘milking’ the blood upward towards the heart

Semilunar Valves in veins then close and prevent blood from flowing back down as muscle relaxes

Total Blood Volume

Return of blood to heart can be influenced by factors that change total blood volume

Most quickly and effectively done by water moving into the plasma (Increasing blood volume) or out of the plasma (Decreasing blood volume)

Key in maintaining constancy of blood flow

Diffusion

Oxygen and carbon dioxide pass through capillary walls from higher to lower concentration

Fluid movement across the wall is determined by a combination of hydrostatic and osmotic pressure

Osmotic Pressure

Movement of water into and out of the cell from high concentration to low concentration

Filtration

Forcing some water and dissolved substances through capillary walls by blood pressure

Filtration and Osmotic Pressure

Blood enters Capillary bed on arteriole end → Blood pressure in capillary vessel is greater than osmotic pressure of the blood vessel → Fluid moves from the vessel to the body tissue

Gas Exchange

Middle of capillary bed, BP in the vessel equals osmotic pressure of blood in vessel. Net result is that fluid passes equally between capillary vessel and body tissue. Gasses, nutrients, and wastes are also exchanged here

Venule end of capillary bed, BP in vessel is less than osmotic rpessure of the blood in vessel. Net result is that fluid, C02 and wastes are draw from the body tissue into the capillary vessel

Blood Pressure

High pressure in arteries must be maintained to keep blood flowing through system

Chief determinate is the blood volume

Blood volume and blood pressure are directly proportional

Pulse Pressure

Pulse Pressure=Difference between Systolic and diastolic pressure

Pulse is expansion and contraction of arterial wall during these phases of contraction and relaxation

Cardiac output and Peripheral Resistance are the 2 most important factors affecting BP (4 affecting factors total)

Factors Affecting BP

1. Cardiac Output

2. Blood Volume

3. Peripheral Resistance

4. Blood Viscosity

An increase in any one relates to an increase in BP

Cardiac Output

CO=SV (amt/beat) x HR (beat/min)

Affects blood entering arteries

If CO increases, amount of blood entering arteries increases, and tends to increase volume of blood in arteries

This causes increase in arterial blood volume=Increase in arterial BP

Blood Volume

Reduced by severe hemorrhage, vomiting, diarrhea, reduced water intake

When volume is replaced, BP returns to normal

Too much fluid = Increase in BP and blood volume

Peripheral Resistance

Friction of blood against vessel walls. Affects blood leaving arteries

If PR increases, decreases amount of blood leaving arteries which increases amount of blood left in them leading to an increase in arterial blood volume = increase BP

Antidiuretic Hormone (ADH)

Hormonal control of BP

Increases water reabsorption in the kidneys, thus increasing blood volume

(more water in blood → Greater plasma volume becomes)

Used for decreased blood volume and BP

Works as vasoconstrictor as well as to raise BP

Aldosterone

Secreted by Adrenal Gland

If decreased BP, it works to increase blood volume by increasing reabsorption of sodium ions and water (sweat, urine, GI system)

Increases osmolarity- Pushes fluid back into system

This increases the BP as a result of the increased blood volume

Renin-Angiotensin-Aldosterone System (RAAS)

In the kidneys and responsible for long term BP adjustments

Uses: Renin, Angiotensin I, Angiotensin II, Angiotensin Converting Enzyme (ACE)

How it works:

• Kidneys detect low BP (by decreased renal blood flow) Renin releases

• Renin activates Angiotensin I (Vasoconstrictor) simulating minor BP changes

• Then simulates Angiotensin II (Powerful Vasoconstrictor)

These all function to raise BP long term

Histamine

Increases blood flow and released with tissues are injured

Released by mast cells

Causes vasculature to dilate and increase permeability

Allows plasma and WBC to leave cell at injury site and encourage healing

When SNS stimulation is withdrawn, histamine is released, vasodilation occurs

This lowers BP

Kinins

Produce relaxation in smooth muscles of arteries

Increases capillary permeability

Vasoconstricts Venules

Lowers Bp

ACE inhibitors fall into this category with Kallidins and Bradykinin

Prostaglandins

When tissues are damaged, WBC flood to site to minimize tissue destruction

Prostaglandins are produced as a result

Different groups, some vasoconstrict, others vasodilate