Chapter 20: The Circulatory System: Blood Vessels and Circulation

Arteries do what?

Carry blood away from the heart

Veins do what?

Carry blood back to the heart

Capillaries?

Connect smallest arteries to the smallest veins

Allow for exchange

Walls of arteries and veins have three layers (tunics)?

Tunica intima

Tunica media

Tunica externa

Tunica Interna?

Lines the blood vessel and is in direct contact with blood

Endothelium: simple squamous epithelium overlying a basement membrane and small layer of loose connective tissue

Acts as a selectively permeable → barrier

Secretes chemicals that stimulate dilation or constriction of the vessel

Normally repels blood cells and platelets that may adhere to it and form a clot

When tissue around a vessel is inflamed, the endothelial cells produce cell-adhesion molecules that cause leukocytes to adhere to the surface

Allows leukocytes to move to tissues when they are needed (infection)

Tunica Media?

Middle layer

Consists of smooth muscle, collagen, and elastic tissue

Strengthens vessels and prevents blood pressure from rupturing them

Regulates the diameter of the blood vessel

Chemical released by the endothelium with cause the smooth muscle of this layer to either go through vasoconstriction or vasodilation

Tunica Externa?

Outermost layer

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

Anchors the vessel and provides a passage for small nerves and lymphatic vessels

Vasa vasorum: small vessels that supply blood to outer part of the larger vessels

Arteries?

Are sometimes called resistance vessels

Strong, resilient tissue structure that can handle pressure the heart pumping creates

Can carry oxygenated (systemic) or deoxygenated blood (pulmonary)

Arterioles?

Smallest arteries

Control the amount of blood going into various organs

Thicker tunica media in proportion to their lumen than large arteries and very little tunica externa

Capillaries?

Exchange vessels: site where gasses, nutrients, wastes, and hormones pass

between the blood and → tissue fluid

Composed of endothelium and basal lamina

Absent or rare in tendons, ligaments, epithelia, cornea, and lens of the eye

Continuous capillaries?

Found in most tissues

Endothelial cells have tight junctions forming a continuous tube with intercellular clefts

Allow passage of solutes like glucose

Pericytes wrap around the capillaries and contain the same contractile protein as muscle

Contract and regulate → blood flow

Fenestrated capillaries?

Found in the kidneys and small intestine

Organs that perform rapid → absorption or filtration

Endothelial cells riddled with holes called filtration pores (fenestrations)

Spanned by very thin glycoprotein layer

Allow passage of only small molecules

Sinusoids (discontinuous capillaries)?

Found in the liver, bone marrow, and spleen

Irregular blood-filled spaces with large fenestrations

Allow proteins (albumin), clotting factors, and new blood cells to enter the circulation (large molecules)

Capillary beds?

Are networks of 10-100 capillaries

Usually supplied by a single arteriole or metarteriole

At distal end, capillaries transition to venules or drain into a thoroughfare channel

At any given time, 3/4th of body’s capillaries are → shut

Most control of flow involves constriction of arterioles that are upstream from the capillaries

Within the capillary bed, precapillary sphincters control flow

When sphincters are relaxed, capillaries have → blood

When sphincters contract, they constrict the entry to the capillary and blood bypasses the capillary

Veins?

Have a greater capacity for blood containment compared to arteries

At rest veins contain 64% of systemic blood (only 15% in arteries)

Thinner walls, flaccid, less muscular and elastic tissue

Collapse when empty, but expand easily

Have steady blood flow

Subjected to relatively low blood pressure

Averages 10 mm Hg with little change

Postcapillary venules?

Are the smallest veins

Even more porous than capillaries

Exchange fluid with surrounding tissues

Most leukocytes leave the blood and enter tissues from the bloodstream through venule walls

Tunica interna forms what?

Venous valves - prevents backflow of blood

Skeletal muscle pump propels venous blood back toward the heart

Whats the simplest and most common route for blood?

Artery → Arterioles → Capillaries(exchange) → Venules → Vein

Passes through only one network of capillaries from the time it leaves the heart until the time it returns

Portal system?

Blood flows through two consecutive capillary networks before returning to heart

Between hypothalamus and anterior pituitary

In the kidneys

Between intestines to liver

Blood pressure (BP)?

The force that blood exerts against a vessel wall

Measured at brachial artery of arm using a sphygmomanometer

Wrap cuff around arm superior to elbow

Increase pressure in cuff until it exceeds systolic pressure in the brachial artery

Pressure is released slowly, and examiner listens for sounds of Korotkoff with a stethoscope

Systolic pressure?

Peak arterial BP taken during ventricular contraction (ventricular systole)

Diastolic pressure?

Minimum arterial BP taken during ventricular relaxation (diastole) between heart beats

Blood pressure falls the further is travels away from the left ventricle

Pulse pressure?

Difference between systolic and diastolic pressure

Important measure of driving force on circulation and of stress put on small arteries by pressure surges generated by the heart

120-75 = 45mmHg

Mean arterial pressure (MAP)?

The mean pressure

Diastolic pressure + (one-third of pulse pressure)

Tells you if the patient's pressure is strong enough to get blood to the → organs

Average blood pressure that most influences risk level for edema, fainting (syncope), atherosclerosis, kidney failure, and aneurysm

Peripheral resistance?

The opposition to flow that blood encounters in vessels away from the heart

Resistance is based on three variables?

Blood → viscosity

Vessel → length

Vessel → diameter

Blood viscosity (“thickness”)?

The thicker the blood the more resistance

Vessel length?

The farther liquid travels through a tube, the more friction it encounters

Pressure and flow decline with distance

Vessel diameter (radius)?

Most powerful influence over flow

Vasoreflexes?

Changes in vessel diameter

Vasoconstriction?

When smooth muscle of tunica media contracts the lumen narrows

Vasodilation?

Relaxation of the smooth muscle, allowing blood pressure to expand vessel

From aorta to capillaries, blood velocity (speed) does what?

Decreases

Arterioles are most significant point of control over peripheral resistance and flow

From capillaries to vena cava, velocity increases again

Autoregulation?

The ability of tissues to regulate their → blood supply

Metabolic theory of autoregulation: if tissue is inadequately perfused, wastes accumulate it will stimulate vasodilation, which increases perfusion

Bloodstream delivers oxygen and removes metabolites

When wastes are removed and vessels constrict

The central and autonomic nervous systems also control blood vessel what?

Size

Vasomotor center of medulla has sympathetic control over blood vessels throughout the body

Stimulates most vessels to constrict, but dilates vessels in cardiac muscle to meet demands of exercise

Baroreflex?

Automatic, negative feedback response to change in blood pressure

Increases in BP detected by → baroreceptors

Glossopharyngeal nerve sends signals to the brainstem:

1) inhibition of sympathetic cardiac and vasomotor neurons

2) excitation of vagal fibers that slow heart rate and thus reduce BP

Decreases in BP have the opposite effect

Baroreflexes cause what?

Short-term regulation of BP

Adjustments for rapid changes in → posture

Not helpful in correcting chronic hypertension

After 2 days or less they adjust their set point

Chemoreflex?

An automatic response to changes in blood → chemistry

Especially pH, and concentrations of → O2 and CO2

Chemoreceptors called aortic bodies and carotid bodies

Located in aortic arch, subclavian arteries, external carotid arteries

Primary role: adjust respiration to changes in blood chemistry

Secondary role: vasomotion

Hypoxemia (low O2), hypercapnia (high CO2), and acidosis (low pH) stimulate chemoreceptors, acting through vasomotor center

Widespread vasoconstriction, increasing BP, increasing lung perfusion, and gas exchange

Also stimulates → breathing

Medullary ischemic reflex?

Automatic response to a drop in perfusion to the brain

Medulla oblongata monitors its own blood supply

Activates corrective reflexes when it senses ischemia (insufficient perfusion)

Cardiac and vasomotor centers send sympathetic signals to the heart and blood vessels

Increases heart rate and contraction force

Causes widespread vasoconstriction

Raises BP and restores normal perfusion to the brain

Other brain centers can affect vasomotor center

Stress, anger, arousal can also increase BP

What type of exchanges occur through capillary walls between the blood and surrounding tissues?

Capillary Exchange

Capillary exchange?

Two-way movement of fluid across capillary walls

Water, oxygen, glucose, amino acids, lipids, minerals, antibodies, hormones, wastes, carbon dioxide, ammonia

Chemicals pass through the capillary wall by three routes?

Through endothelial cell → cytoplasm

Intercellular clefts between endothelial cells

Filtration pores (fenestrations) of the fenestrated capillaries

Mechanisms involved

Diffusion, transcytosis, filtration, and reabsorption

Diffusion?

Is the most important form of capillary exchange

Glucose and oxygen, have a higher concentrated in blood, diffuse out of the blood

Carbon dioxide and other waste, have a lower concentrated in tissue fluid, diffuse into the blood

Capillary diffusion can only occur if?

The solute can pass the plasma membranes of the endothelial cell

Also, find passages large enough to pass through

Filtration pores and intracellular clefts

Lipid-soluble substances?

Steroid hormones, O2, and CO2 diffuse easily through plasma membranes

Water-soluble substances?

Glucose and electrolytes must pass through filtration pores and intercellular → clefts

Large particles such as proteins are → held back

Transcytosis?

Endothelial cells pick up material on one side of their membrane by pinocytosis or receptor-mediated endocytosis, transport vesicles across cell, and discharge material on other side by → exocytosis

Important for fatty acids, albumin, and some hormones (insulin)

Fluid filters ___ of the arterial end of the capillary and osmotically reenters at the venous end

out

Delivers materials to the cell and removes metabolic wastes

Filtration (arterial end)?

Movement of fluid and solutes out of the capillary driven by pressure from capillaries into interstitial fluid

Reabsorption (venous end)?

Movement from interstitial fluid into capillaries

Hydrostatic pressure (HP)?

Force exerted by fluid pressing against wall

Capillary hydrostatic pressure (HPc)

Interstitial fluid hydrostatic pressure (HPif)

Capillary hydrostatic pressure (HPc)?

Capillary blood pressure that tends to force fluids through capillary walls

Interstitial fluid hydrostatic pressure (HPif)?

Pressure pushing fluid back into vessel, usually assumed to be zero because lymphatic vessels drain interstitial fluid

Colloid osmotic pressure (OP)?

Pulling "sucking" force through the capillary wall

Capillary colloid osmotic pressure (OPc)?

Sucking” pressure created by nondiffusible plasma proteins pulling water back into capillary

Interstitial fluid colloid osmotic pressure (OPif)?

Pressure is inconsequential because interstitial fluid has very low protein content

Venous return?

The flow of blood back to the heart

Relies on: pressure gradient, gravity, skeletal muscle pump, thoracic pump, and cardiac suction

Pressure gradient?

Blood pressure is the most important force in venous return

7 to 13 mm Hg venous pressure toward the heart

Venules (12 to 18 mm Hg) to central venous pressure: point where the venae cavae enter the heart (~5 mm Hg)

Gravity?

Drains blood from the → head and neck

Skeletal muscle pump?

In the → limbs

Contracting muscle squeezes blood out of the compressed part of the vein

Thoracic (respiratory) pump?

Inhalation - thoracic cavity expands and thoracic pressure decreases, abdominal pressure increases, forcing blood → upward

Cardiac suction?

Of expanding atria

Sucks blood from the vena cavae into the → atria

Hepatic portal system?

Receives blood from abdominal digestive organs: stomach, small and large intestines

Also the pancreas, gallbladder, and spleen

Capillaries of the abdominal digestive organs are _______ to capillaries of the liver before returning to the heart

connected

Intestinal blood is what?

Is nutrient rich after we eat

The liver accesses these nutrients first

Liver cleanses the blood of bacteria and toxins before it is circulated by the heart

The superior and inferior mesenteric, splenic, and pancreatic veins _______ to the hepatic portal vein, which also meets up with the cystic vein and gastric veins

converge

Blood _____ into the hepatic vein and into the inferior vena cava to the right atrium

empties

Fetal circulation?

Found only in the fetus and has special structures that allow the fetus to exchange nutrients with → its mother

Lungs, kidneys, and GI organs do not start to function until after birth

Mother and fetus exchange occurs through the?

placenta

Attaches to the fetus through the → umbilical cord

Fetal blood passes to the placenta through the umbilical artery in the cord

Removes CO2 and wastes

Umbilical vein?

Brings oxygenated and nutrient rich blood from the placenta to the → fetus

Material and fetal blood do not mix because of the placenta (blood surrounds chorionic villi and exchange occurs)

Blood entering through the umbilical vein enters what?

Enters the ductus venosus – drains into the inferior vena cava

This blood enters the → right atrium

Fetal heart circulation differs?

Blood does not pass from the right ventricle to the lungs, instead it goes through the foramen ovale – a hole in the septum between the right and left atria

Instead of going to the lungs, most blood goes through the ductus arteriosus - connects the pulmonary trunk with the → aorta

Aorta carries blood into systemic circulation

After birth?

The lungs, kidneys, and digestive system begin to function and changes occur

Ductus venosus collapses but remains part of the ligamentum venosum on the liver

Foreman ovale closes shortly after birth and becomes the fossa ovalis – depression in the interatrial septum, initiated by the baby’s first breath

Ductus arteriosus closes by vasoconstriction immediately after birth and becomes the ligamentum arteriosum