Chapter 21

5 types of blood vessels

arteries, arterioles, capillaries, venules, and veins

layers of a blood vessel

tunica interna, tunica media, tunica externa

tunica interna

• innermost layer, closest to the lumen,

• has direct contact with the blood and is continuous with the endocardium

• consists of an inner endothelium (simple squamous epithelium) and a basement membrane

tunica media

• Middle layer

• Smooth muscle and elastic fibers, their primary role is to regulate the diameter of the lumen (vasoconstriction and vasodilation)

tunica externa

• Outermost layer, adjacent to surrounding tissue consisting of elastic and collagen fibers

• Forms a protective layer

• Contains numerous nerves, tiny blood vessels that supply the tissue of the vessel wall called vasa vasorum vessels to the vessels

arteries

• carry blood away from the heart to tissues

• wall consists of tunica interna, tunica media (which maintains elasticity and contractility), and a tunic externa

functional properties of arteries

elasticity and contractibility

elasticity of arteries

• due to the elastic tissue in the tunica interna and media

• allows arteries to accept blood under great pressure from the contraction of the ventricles and to send it on through the system

contractibility of arteries

• due to the smooth muscle in the tunica media

• allows arteries to increase or decrease lumen size and to limit bleeding from wounds

types of arteries

elastic and muscular

elastic artery

• Conducting arteries because they conduct blood from the heart to medium sized muscular arteries

• Large diameter, thick tunica media dominated by elastic fibers

• More elastic fibers (elastic lamellae), less smooth muscle

• Are able to receive blood under pressure and propel it onward

• They function as a pressure reservoir (elastic fibers recoil and convert stored potential) energy in the vessels into kinetic energy of the blood and help propel blood

muscular arteries

• Distributing arteries

• Medium diameter

• More smooth muscle, fewer elastic fibers

• Distribute blood to various parts of the body

• They do not have the ability to recoil and help propel the blood

anastosomes

• The union of the branches of 2 or more arteries supplying the same region of the body

• This provides an alternate route for blood flow known as collateral circulation

• May also occur between the veins, arterioles and venules, (two veins or a vein and an artery)

end arteries

Arteries that do not form an anastomosis are called…

necrosis

If an end artery is blocked, blood cannot get to that particular region of the body and _________ can occur

arterioles

• Are very small, almost microscopic, arteries that regulate the flow of blood to capillaries

• There are approximately 400 million, have diameters that range in size 15μm to 300μm 

• It has all the tunicas

  • the tunica externa consists of areolar connective tissue containing abundant unmyelinated sympathetic nerves which regulate the rate of blood flow

resistance

• The opposition to blood flow from arteries into capillaries and altering arterial blood pressure

• Through vasoconstriction and vasodilation, arterioles assume a key role in regulating _________

metarteriole

The terminal end of the arteriole is called the…

capillaries

• 5-10μm

• Microscopic vessels that usually connect arterioles and venules 

• They are the smallest of blood vessels and approximately 20 billion in number

• Found near almost every cell in the body, but their distribution varies with the metabolic activity of the tissue

• The primary function of is to permit the exchange of nutrients and wastes between the blood and tissue cells through interstitial fluid

• Walls are composed of only a single layer of cells (endothelium) and a basement membrane, they lack tunica media and externa

• Branch to form an extensive network throughout the tissue

microcirculation

The flow of blood through the capillaries is called…

types of capillaries

continuous, fenestrated, and sinusoids

continuous capillaries

• Continuous tube interrupted by intercellular clefts, gaps between neighboring endothelial cells

• Found in CNS, lungs, muscle tissue, skin

Fenestrated capillaries

• Endothelial cells have many __________ or small holes(pores) raging from 70-100 nm

• Found in kidneys, villi of the small intestine, choroid plexuses of the ventricles in the brain, ciliary processes of the eyes, endocrine glands

Sinusoid capillaries

• Large fenestrations, incomplete or absent basement membrane

• Found in liver, spleen, anterior pituitary, parathyroid and adrenal glands 

Venules

• Are small vessels that are formed by the union of several capillaries

• Very porous-exchange of nutrients and wastes

• Drain blood from capillaries into veins

Veins

• Formed from the union of several venules

• Return blood to the heart

• Consist of the same three tunics as arteries but have a thinner tunica interna and media and a thicker tunica externa

• Have less elastic tissue and less smooth muscle than arteries

• Contain valves to prevent the backflow of blood 

64%

at rest the largest portion, _______ of the blood is in systemic veins and venules (blood reservoirs)

diffusion, transcytosis, and bulk flow

substances cross capillary walls by

diffusion

• Substances such as oxygen, carbon dioxide, glucose, amino acids, and some hormones cross capillary walls via…

• Plasma proteins and red blood cells cannot cross through capillary walls, because they are too large

transcytosis

• endocytosis

• mainly large, lipid-insoluble molecules cross by this route, also insulin crosses capillary walls in vesicles via…

bulk flow

• a passive process in which large numbers of ions, molecules, or particles in a fluid move together in the same direction

• occurs from an area of higher pressure to an area of lower pressure, and it continues as long as a pressure difference exists

• more important for regulation of the relative volumes of blood and interstitial fluid

hydrostatic pressure

due to the pressure that water in blood plasma exerts against blood vessel walls

osmotic pressure

due almost entirely to the presence of plasma proteins in blood

Blood hydrostatic pressure (BHP)

works against interstitial fluid hydrostatic pressure (IFHP)

Blood colloid osmotic pressure (BCOP)

works against interstitial fluid osmotic pressure (IFOP)

filtration

• a pressure-driven movement of fluid and solutes from blood capillaries into interstitial fluid 

• If the pressures that push fluid out of the capillaries exceed the pressures that pull fluid into capillaries, fluid will move from capillaries into interstitial spaces

• The pressures that push fluid out of interstitial spaces into capillaries exceed the pressure that pull fluid out of capillaries, then fluid will move from interstitial spaces into capillaries (reabsorption) 

• blood hydrostatic pressure (BHP) (~35 mmHg) and interstitial fluid osmotic pressure (IFOP) (~1mmHg) promote this

Reabsorption

• A pressure-driven movement of fluid and solutes from interstitial fluid into blood capillaries

• The pressures that push fluid out of interstitial spaces into capillaries exceed the pressure that pull fluid out of capillaries, then fluid will move from interstitial spaces into capillaries

• Interstitial fluid hydrostatic pressure (IFHP) (~0 mmHg) and blood colloid osmotic pressure (BCOP) (~26mmHg) promote…

Starling’s law of the capillaries

• Under normal conditions, the volume of fluid and solutes reabsorbed is almost as large as the volume filtered

• The near equilibrium is called…

net filtration pressure (NFP)

Balance of the pressure between the volume of fluid and solutes reabsorbed and the volume filtered

• (BHP + IFOP) – (BCOP+ IFHP)

  • ( 35 + 1 ) mmHg – ( 26 + 0 ) mmHg

blood flow

The volume of blood that flows through any tissue in a given time period (in mL/min)

cardiac output (CO)

• Total blood flow is….

• The volume of blood that circulates through systemic (or pulmonary) blood vessels each minute

• heart rate (HR) × stroke volume (SV)

• mean arterial pressure (MAP) ÷ resistance (R)

The distribution of cardiac output to various tissues depends on…

1. The pressure difference that drive the blood flow, 

2. The resistance to blood flow in specific blood factors

blood pressure (BP)

• The pressure exerted on the walls of a blood vessel

• the pressure exerted by blood on the wall of an artery when the left ventricle undergoes systole and then diastole.

• Measured by the use of a sphygmomanometer, usually in one of the brachial arteries

• In clinical use, ______ refers to pressure in arteries

• Contraction of the ventricles generates…

• Determined by CO, blood volume, and vascular resistance

• The higher the ______, the greater the blood flow

Systolic blood pressure

• the highest blood pressure attained in arteries during systole

• the force of blood recorded during ventricular contraction 

diastolic blood pressure

• the lowest arterial pressure during diastole

• the force of blood recorded during ventricular relaxation

Mean Arterial Pressure (MAP)

• The average blood pressure in arteries, is roughly 1/3^rd of the way between the diastolic and systolic pressures

• Equals diastolic BP + 1/3 (systolic BP-diastolic BP)

• Thus, in a person whose BP is 110/70 mmHg, this is about 83 mmHg [70 + 1/3(110 -70)]

Vascular Resistance (R)

• The opposition to blood flow due to friction between blood and the walls of blood vessels

  • The higher the ______, the smaller the blood flow

• Depends on:

  • Size of the blood vessel lumen: smaller the lumen of the blood vessel, the greater its resistance to blood flow

  • Blood viscosity: the higher the viscosity, higher the resistance. Viscosity depends on the ratio of red blood cells to plasma (fluid) volume. Dehydration increases viscosity, thus increases blood pressure 

  • Total blood vessel length: longer the blood vessel greater the resistance

size of the blood vessel lumen

• vascular resistance depends on this

• smaller the lumen of the blood vessel, the greater its resistance to blood flow

blood viscosity

• vascular resistance depends on this

• the higher the __________, the higher the resistance.

• depends on the ratio of red blood cells to plasma (fluid) volume.

• dehydration increases ______, thus increases blood pressure 

total blood volume length

• vascular resistance depends on this

• the longer the blood vessel the greater the resistance

Systemic vascular resistance (also known as total peripheral resistance)

refers to all of the vascular resistances offered by systemic blood vessels; most resistance is in arterioles, capillaries, and venules due to their small diameters

venus return

• the volume of blood flowing back to the heart through the systemic veins, occurs due to the pressure generated by contractions of the heart’s left ventricle

• occurs because of the pressure gradient between the venules and the right atrium

• Assisted by:

  • Valves

  • Respiratory pump

  • Skeletal muscle pump

blood flow

• the volume of blood that flows through a tissue in a given period of time

• inversely related to the cross-sectional area of blood vessels blood flows most slowly where cross-sectional area is greatest 

• decreases from the aorta to arteries to capillaries and increases as it returns to the heart

Syncope or fainting

Refers to a sudden, temporary loss of consciousness followed by spontaneous recovery. It is most commonly due to cerebral ischemia but it may occur for several other reasons.

control of BP

• Neural Regulation of blood Pressure

  • Baroreceptor reflex

  • Chemoreceptor reflex

• Hormonal regulation of blood pressure

• Autoregulation of Blood Flow

medulla oblongata

contains a cardiovascular center, which is a group of neurons that regulate heart rate, contractility, and blood vessel diameter.

Cardiovascular Center (CV)

• Receives input from higher brain regions and sensory receptors proprioceptors, baroreceptors and chemoreceptors.

• Output flows along sympathetic and parasympathetic fibers

increase

Sympathetic impulses along cardio accelerator nerves _________ heart rate and contractility

decrease

Parasympathetic impulses along vagus nerves ________ heart rate

vasomotor tone

The sympathetic division also continually sends impulses to smooth muscle in blood vessel walls via vasomotor nerves. The result is a moderate state of tonic contraction or vasoconstriction, called…

propriocepters

monitor joint movements

baroreceptors

• monitor BP

• are important pressure-sensitive sensory receptors that monitor stretching of the walls of blood vessels and the atria

• located in the aorta, internal carotid arteries and other large arteries in the neck and chest

• the two most important ________ reflexes are the: carotid sinus reflex and the aortic reflex

carotid sinus reflux

• concerned with maintaining normal blood pressure in the brain and is initiated by baroreceptors in the wall of the carotid sinus

• nerve impulses propagate from the _______________ over sensory axons in the glossopharyngeal (IX) nerves

aortic reflex

Concerned with general systemic blood pressure and is initiated by baroreceptors in the wall of the arch of the aorta or attached to the arch via axons of the vagus (X) nerves.

chemoreceptors

• monitor blood acidity (H+, CO2, and O2)

• sensitive to chemicals

• are located close to the baroreceptors of the carotid sinus and arch of the aorta

• they monitor blood levels of oxygen, carbon dioxide, and H⁺

Factors influencing BP: Cardiac Output (increased heart rate and contractibility)

• norepinephrine and epinephrine

• increases BP

Factors influencing BP: Vasoconstriction

• Angiotensin II, antidiuretic hormone, norepinephrine, and epinephrine

• increases BP

Factors influencing BP: vasodilation

• atrial natriuretic peptide, epinephrine, nitric oxide

• decrease BP

Factors influencing BP: blood volume increases

• aldosterone and antidiuretic hormone

• increases BP

Factors influencing BP: blood volume decrease

• atrial natriuretic peptide

• decreases BP

autoregulation

The ability of a tissue to automatically adjust its own blood flow to match its metabolic demand for delivery of oxygen and nutrients and removal of wastes.

Two types of stimuli cause autoregulatory changes in blood flow

1. Physical changes

2. Chemical stimuli

physical changes

warming causes vasodilation, cooling causes vasoconstriction

chemical stimuli

• can lead to autoregulation

• vasodilating chemicals K⁺, H⁺, lactic acid, adenosine, NO, tissue trauma and inflammation causes release of kinins and histamine

• Vasoconstrictors: TXA₂, superoxide radicals, serotonin from platelets and endothelins

pulse

the alternate expansion and elastic recoil of an artery wall creating a traveling pressure wave with each heartbeat

resting heart rate

between 70 to 80 beats per minute

Tachycardia

a rapid resting heart or pulse rate (> 100 beats/min)

Bradycardia

indicates a slow resting heart or pulse rate (< 60 beats/min)

Korotkoff sounds

The various sounds that are heard while taking blood pressure are called…

normal BP

• Young adult male: 120/80 mm Hg

• 8-10 mm Hg less in a young adult female

• The range of average values varies with many factors

pulse pressure

The difference between systolic and diastolic pressure. It normally is about 40 mm Hg and provides information about the condition of the arteries.

shock

• an inadequate CO that results in failure of the CV system to meet the metabolic demands of body cells

• Cell membranes dysfunction, cell metabolism is abnormal, and cell death may occur

• Types:

  • Hypovolemic

  • Cardiogenic

  • Vascular

  • Obstructive

hypovolemic shock

due to decreased blood volume (loss of body fluids)

Cardiogenic shock

due to poor heart function (myocardial infarction)

Vascular shock

due to inappropriate vasodilation (anaphylactic shock-release of histamine and drop of bp)

Obstructive shock

due to obstruction of blood flow (pulmonary embolism)

Homeostatic Responses to Shock

• Activation of the renin-angiotensin-aldosterone system

• Secretion of anti-diuretic hormone

• Activation of the sympathetic division of the autonomic nervous system

• Release of local vasodilators

s/s of shock

• Clammy, cool, pale skin

• Tachycardia

• Weak, rapid pulse

• Sweating

• Hypotension (SBP <90 mmHg)

• Altered mental status (due to ↓ oxygen in brain)

• Decreased urinary output

• Thirst

• Acidosis

circulatory routes

The blood vessels are organized into routes that deliver blood throughout the body

• Systemic circulation is the largest circulatory route

• Pulmonary circulation

• Hepatic portal circulation

• Fetal circulation

systemic circulation

Takes oxygenated blood from the left ventricle through the aorta to all parts of the body, including some lung tissue (but does not supply the air sacs of the lungs) and returns the deoxygenated blood to the right atrium

parts of the aorta

the ascending aorta, arch of the aorta, and the descending aorta

systemic circulation of the heart

• ascending aorta → right coronary artery→ posterior interventricular branch or marginal branch

• ascending aorta → left coronary artery→ anterior interventricular branch or circumflex branch

systemic circulation of the abdominal aorta

• abdominal aorta → parietal branches

  • inferior phrenic

  • lumbar

  • median sacral

• abdominal aorta → visceral branches

  • celiac trunk

  • superior mesenteric

  • suprarenal

  • renal

  • gondal

  • inferior mesenteric

scheme of drainage: veins of the abdomen and pelvis

right/left, external or internal iliac veins → right/left common iliac vein → inferior vena cava → lumbar → gonadal → renal →suprarenal → hepatic/hepatic portal → inferior phrenic → heart

hepatic portal system

Collects blood from the veins of the pancreas, spleen, stomach, intestines, and gallbladder and directs it into the hepatic portal vein of the liver before it returns to the heart.

portal system

• Carries blood between two capillary networks, in this case from capillaries of the gastrointestinal tract to sinusoids of the liver

• This circulation enables nutrient utilization and blood detoxification by the liver

pulmonary circulation

right ventricle → pulmonary trunk → right or left pulmonary artery → air sacs of right or left lung → right or left pulmonary veins → left atrium

fetal circulation

• involves the exchange of materials between fetus and mother

• The fetus derives its oxygen and nutrients and eliminates its carbon dioxide and wastes through placenta

• Blood passes from the fetus to the placenta via two umbilical arteries and returns from the placenta via a single umbilical vein

• At birth, when pulmonary, digestive, and liver functions are established, the special structures of fetal circulation are no longer needed

development of blood vessels

• Develop from isolated masses of mesenchyme in the mesoderm called blood islands

• Spaces soon appear in the islands and become the lumens of the blood vessels

• Some of the mesenchymal cells immediately around the spaces give rise to the endothelial lining of the blood vessels

Effects of aging in the cardiovascular system

• Loss of compliance of the aorta

• Reduction in cardiac muscle fiber size

• Progressive loss of cardiac muscular strength

• Decline in maximum heart rate

• Increased systolic blood pressure