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Chapter 20: The Circulatory System: Blood Vessels and Circulation

1 Structure and function of blood vessels

There are five main types of blood vessels: arteries, arterioles, capillaries, venules and veins. Arteries carry blood away from the heart to other organs. The smallest arteries are called arterioles and as these vessels enter tissues they become capillaries. The thin walls of capillaries allow exchange of substances between blood and tissue. Capillaries reunite to form venules, which in turn become larger veins. Veins return blood to the heart.

1 Arteries

1 Three tunics

The wall of an artery has three layers called tunics.

1 Tunica interna

The innermost layer. Has three layers. It is closest to the hollow center of the artery also called the lumen.

1 Endothelium

The lining of the artery composed of simple squamous epithelium. It is a continuous layer that lines the entire cardiovascular system. It is the only tissue that normally makes contact with the blood.

2 Basement membrane

3 Internal elastic lamina

2 Tunica media

This is usually the thickest layer of the artery. It consists of elastic fibers and smooth muscle fibers that encircle the artery.

1 External elastic lamina

Composed of elastic tissue, just outside of the smooth muscle fibers. The high proportion of elastic fibers in arteries give them high compliance, which means that they will stretch without breaking.

3 Tunica externa

The outermost layer of an artery. It is composed mainly of elastic and collagen fibers.

2 Control of artery diameter

The smooth muscle of the tunica media is innervated with nerve fibers of the sympathetic division (“fight-or-flight”). Stimulation by the sympathetic division causes the smooth muscles to contract, decreasing the diameter of the vessels. A decrease in sympathetic stimulation results in relaxing of these muscle fibers and an increase in the lumen diameter.

1 Vasoconstriction

A decrease in the diameter of a blood vessel.

2 Vasodilation

An increase in blood vessel diameter.

3 Elastic arteries

Elastic arteries are the largest at over 1 cm diameter. The tunica media of the large arteries contain a high proportion of elastic fibers. These arteries help to propel blood forward when the heart is in diastole (when the ventricles are relaxing). They are also called conducting arteries. Examples include the aorta, brachiocephalic, common carotid, pulmonary and common iliac arteries.

4 Muscular arteries

These are medium sized arteries, with relatively more smooth muscle and less elastic fiber than the elastic arteries. These vessels are capable of greater vasoconstriction and vasodilation to adjust blood flow rates.

2 Arterioles

Very small arteries. They play a great role in regulating blood flow by changing their resistance to blood flow. Contraction of the smooth muscle in these vessels narrows their diameter, resulting in more friction and lower flow rates.

3 Capillaries

Microscopic vessels that connect arterioles to venules. Capillaries are found near to nearly every cell in the body. Tissues with high metabolic activities have more capillaries than less metabolically active tissues.

Their prime function is to exchange materials between the blood and tissues. The walls of capillaries are composed of a single layer of endothelial cells and a basement membrane.

1 Capillary bed

A network of between 10 and 100 capillaries that perfuses a specific region, is called a capillary bed.

2 Thoroughfare channel

This is a vessel that bypasses the capillary bed. By diverting blood away from the capillary bed, the degree of perfusion of a specific tissue can be changed.

3 Precapillary sphincters

Smooth muscle rings around the arterial side of a capillary bed, which can control the extend of perfusion.

4 Three different types of capillaries

The three capillary types vary in the degree of leakiness between the endothelial cells lining the vessels.

1 Continuous capillaries

The plasma membrane of the endothelial cells lining these capillaries form a continuous tube, interrupted only by intercellular clefts.

Continuous capillaries are found in skeletal and smooth muscle, connective tissue and the lungs.

1 Intercellular clefts

These are gaps between endothelial cells that occur periodically in continuous capillaries.

2 Fenestrated capillaries

The cell membranes of the endothelial cells in these capillaries have many, small pores called fenestrations. These pore-filled capillaries are found in the kidneys, villi of the small intestine, some endocrine glands.

1 Fenestrations

Fenestrations are pores in the plasma membrane.

3 Sinusoids

Sinusoids are capillaries with extremely large fenestrations, absent basement membranes and very large intercellular clefts. These capillaries are found in the liver, red bone marrow, spleen and some endocrine glands.

4 Venules

Capillaries unite to form venules and deliver blood to veins.

5 Veins

Veins have the same three coats as arteries but the relative thickness of the layers are different: all generally thinner with little smooth muscle. They can distend enough to change their volume under different conditions, but they are not as resistant to high pressure as arteries.

The lumen of a vein will be much larger than the lumen of a comparable artery.

Veins often appear collapsed when sectioned because of their relatively weak structure.

1 Valves

Many veins feature valves with the same function as those in the heart: keeps blood flowing in one direction toward the heart.

6 Anastomoses

The union of branches of two or more arteries is called an anastomosis. Anastomoses provide alternative sources of blood for tissues.

7 Blood distribution

At rest, most blood – about 64% - is in the systemic veins and venules. Systemic arteries and arterioles hold about 13% of the blood and capillaries hold an additional 7%. Pulmonary vessels hold about 9% and the heart holds about 7%.

1 Blood reservoirs

The veins and venules are said to be a blood reservoir from which blood can be diverted to other destinations. Constriction of veins results in a decreased volume occupied by the blood and movement to other areas.

2 Capillary exchange

Capillary exchange is the movement of substance between the blood and interstitial fluid. There are three basic mechanisms of capillary exchange.

1 Three mechanisms of capillary exchange

1 Diffusion

The most important mechanism of capillary exchange is by simple diffusion: movement of substances down their concentration gradient i.e. from high concentration to low concentration.

Water soluble substances like glucose, and amino acids pass through the intercellular clefts or fenestrations. Lipid soluble substances such as oxygen, carbon dioxide, and steroid hormones pass across capillary membranes directly.

Large molecules and red blood cells are generally kept within capillaries except in the sinusoids.

2 Transcytosis

Transcytosis is the movement of material across endothelial cells in membrane-bound vesicles. Important mainly for large, lipid-insoluble molecules that cannot freely cross the membrane. For example, the hormone insulin enters the bloodstream via Transcytosis.

3 Bulk flow: Filtration and reabsorption

The process where large amounts of ions, molecules or particles in fluid move together in the same direction. Movement of materials is faster than would proceed by diffusion alone.

1 Filtration

Filtration is pressure-driven movement of fluid or solutes from blood capillaries into interstitial fluids

1 Blood hydrostatic pressure

The pressure generated by the pumping heart promotes filtration.

2 Interstitial fluid osmotic pressure

This is the tendency for water to move from the blood into the interstitial fluids due to the amount of dissolved material in interstitial fluid. It opposes blood hydrostatic pressure and blood colloid osmotic pressure.

2 Reabsorption

Pressure-driven movement of substances from interstitial fluid into blood.

1 Blood colloid osmotic pressure

This is the main pressure promoting reabsorption. It is due to the large amount of soluble proteins in blood. The presence of large amounts of solutes in the blood itself means that water will have a tendency to move from the interstitial fluids into the blood (a tendency to balance the amount of soluble material in the two spaces).

3 Net filtration pressure

Whether or not fluids enter or leave tissues from the blood results in the balance of the above pressures: increase the blood pressure and you can force material into the interstitial fluid; increase the amount of dissolved material in blood relative to interstitial fluid and water will tend to move into the blood.

3 Hemodynamics: Factors affecting blood flow

1 Blood pressure

As the blood leaves the aorta and flows through system circulation, its pressure falls progressively as distance from the left ventricle increases. The blood pressure drops in the capillary beds, and then continues to drop in the venous side. As blood flows into the right ventricle it reaches zero mmHg.

1 Systolic blood pressure

The highest pressure attained in arteries during systole.

2 Diastolic blood pressure

The lowest arterial pressure during diastole.

2 Resistance

Vascular resistance is the opposition to blood flow due to friction between blood and the walls of the blood vessels.

1 Systemic vascular resistance

This term applies to all of the causes of resistance combined. The causes are listed below.

1 Size of the lumen

The smaller the vessel, the greater the resistance. This is because there is a relatively larger surface area of endothelium per lumen volume in smaller vessels.

2 Blood viscosity

Any condition that increases the blood's viscosity (thickness) increases the resistance in the vessels. Dehydration or overproduction of erythrocytes will result in higher viscosity.

3 Total blood vessel length

The longer the blood vessel, the greater the resistance.

3 Venous return

1 Venous return

How does the blood return to the heart via the venous system. It is due only in part to the blood pressure resulting from the pumping heart: blood pressure decreases with the distance from the heart and results in only about 16 mm Hg in the venous side.

There are two other methods for returning blood to the heart, both rely on functional valves in veins.

2 Skeletal muscle pump

Contraction of a skeletal muscle, especially in the lower limb, compresses veins and blood moves away from the center of compression. Because valves block the flow of blood away from the heart, the blood is forced in one direction to the heart.

3 Respiratory pump

The respiratory pump relies on the repeated contraction and relaxation of the muscles of respiration. Contraction of the diaphragm flattens it and pushes the internal organs agains the abdominal veins. Valves cause the blood to flow only in the direction of the heart.

4 Control of blood pressure and blood flow

Sections D. E. and F. are for your information only.

1 Role of the cardiovascular center

2 Neural regulation of blood pressure

1 Baroreceptor reflexes

2 Chemoreceptor reflexes

3 Hormonal regulation of blood pressure

4 Autoregulation of blood pressure

5 Checking circulation

1 Measuring blood pressure

6 Shock and homeostasis

1 Types of shock

2 Homeostatic responses to shock

7 Circulatory routes

1 The systemic circulation

1 The aorta and its branches

1 Ascending aorta

Emerges from the left ventricle. The beginning of the aorta contains the aortic valve. Two arteries branch near to the ventricle and supply the myocardium.

1 Right coronary artery

2 Left coronary artery

2 Arch of the aorta

The ascending aorta turns left and forms the arch. There are three branches from the arch.

1 Brachiocephalic trunk

The brachiocephalic trunk is the first major branch, which in turn branches into the right common and right subclavian arteries.

1 Right common carotid artery

Supplies the right side of the head and neck. Description below applies to the left side as well.

1 Right internal carotid artery

Supplies the structures external to the skull.

2 Right external carotid artery

Supplies structures internal to the skull.

2 Right subclavian artery

Supplies the right upper limb.

1 Axillary artery

The continuation of the right subclavian artery into the axilla (armpit). It supplies the shoulder, thoracic and scapular muscles and the humerus bone.

1 Brachial artery

The continuation of the axillary artery into the arm.

2 Radial artery

The smaller branch of the antebrachium that continues from the brachial artery. Passes along the radial aspect of the forearm.

3 Ulnar artery

The larger artery of the antebrachium passes along the ulnar aspect of the forearm. It supplies the wrist and hand.

2 Left common carotid artery

The left common carotid is the next major branch (after the brachiocephalic) along the arch.

It supplies the left side of the head and neck.

3 Left subclavian artery

The next branch is the left subclavian artery, which supplies the left upper arm. Note that the distribution of arteries in the left arm is the same as described below for the right arm.

3 Thoracic aorta

The section of the aorta that descends between the arch and the level of the diaphragm is called the thoracic aorta.

4 Abdominal aorta

The abdominal aorta between the level of the diaphragm and the division into the common iliac arteries is the abdominal artery.

1 Celiac trunk

The first visceral branch off of the abdominal aorta inferior to the diaphragm.

1 Splenic artery

Supplies the pancreas, stomach, greater omentum.

2 Common hepatic artery

Supplies the liver, gall bladder, stomach, small intestine, pancreas and greater omentum.

2 Renal arteries

Lower on the abdominal aorta, the renal arteries supply the kidneys.

3 Common iliac artery

The division of the abdominal aorta. There is a left and a right common iliac. Each common iliac divides into the internal and external iliac. The description below applies to both the left and right sets of arteries of the lower limbs.

1 Internal iliac artery

These are the primary arteries of the pelvis. Supplies the uterus, prostate, muscles of the buttocks, and urinary bladder

2 External iliac artery

Larger than the internal iliac. They begin at the division of the common iliac arteries.

1 Femoral artery

Continuation of the external iliac along the thigh.

1 Popliteal artery

Continuation of the femoral arteries they pass through the popliteal fossa which is the space behind the knee joint.

2 Veins of the systemic circulation

1 Veins returning blood to the heart

1 Superior vena cava

Empties blood into the superior part of the right atrium. It drains the head, neck, chest, and upper limbs.

1 Brachiocephalic veins

Formed by the union of the subclavian and internal jugular veins. Drain blood from the head, neck, upper limbs, mammary glands and superior thorax.

1 Subclavian veins

Deep veins, which are continuations of the axillary veins. They unite with the internal jugular veins to form the brachiocephalic veins. They drain the arms, neck, and thoracic wall.

2 Inferior vena cava

Enters the inferior part of the right atrium. Drains the abdomen, pelvis and lower limbs.

1 Renal veins

Drain the kidneys directly into the inferior vena cava.

2 Hepatic vein

The hepatic vein drains the liver and this blood enters the inferior vena cava.

2 Veins of the head and neck

1 Internal jugular veins

The internal jugular veins unite with the subclavian veins. The internal jugular veins drain structures inside of the skull.

2 External jugular veins

The external jugular veins empty into the subclavian veins. They drain the superficial structures of the head.

3 Veins of the upper limbs

1 Axillary veins

Become the subclavian veins. Receive blood from the arms, axillae and parts of the chest walls.

1 Cephalic veins

A vein of the arm that drains lateral (thumb side) superficial structures. Originates in the hand. The cephalic veins join the axillary veins, just inferior to the clavicle.

2 Brachial veins

Deep veins that accompany the brachial arteries. They drain the forearms, elbow joints, arms and humerus. They join the basilic veins to form the axillary veins.

3 Basilic veins

Another superficial vein of the arm draining the hand on the medial aspect (pinky side) of the arm.

1 Radial veins

Deep veins of the lateral aspect of the forearm.

2 Ulnar veins

Deep veins of the medial aspect of the forearm.

4 Veins of the thorax

5 Veins of the abdomen and the pelvis

1 Common iliac veins

Formed by the union of the internal and external iliac veins. The common iliac vein is the distal continuation of the inferior vena cava. The common iliac veins drain the pelvis, external genitals and lower limbs.

1 Internal iliac veins

Drains the thighs, buttocks, external genitalia and pelvis. Located medial to the external iliac veins.

2 External iliac veins

Drain the lower limbs and abdominal wall. Lateral to the internal iliac vein.

6 Veins of the lower limbs

1 Femoral veins

Accompany the femoral arteries. Continuation of the popliteal veins just superior to the knees. The femoral vein is deep.

1 Popliteal veins

Drain the knee joints, the skin muscles and bones around the knee.

2 Great saphenous veins

The great saphenous veins are superficial. Ascend from the foot to the groin.

2 The hepatic portal circulation

1 Hepatic portal vein

The hepatic portal vein empties into the liver from numerous internal organs. With the exception of the kidneys, the blood from most organs pass through a capillary bed in the liver before entering the inferior vena cava.

1 Splenic vein

Drains the stomach, pancreas, and large intestine into the hepatic portal system.

3 The pulmonary circulation

1 Pulmonary trunk

Emerges from the right ventricle and carries deoxygenated blood to the lungs. Splits into the right and left pulmonary arteries.

1 Right pulmonary artery

Carries deoxygenated blood to the right lung.

2 Left pulmonary artery

Carries deoxygenated blood to the left lung.

2 Pulmonary veins

Returns oxygenated blood to the left atrium. There are two left and two right pulmonary veins.

4 The fetal circulation

This section is for your interest only.

1 The umbilical-placental circuit

The internal iliac arteries of the fetus give rise to the umbilical arteries. The blood in these arteries is low in oxygen and high in carbon dioxide and other fetal wastes. The umbilical arteries discharges waste in the placenta and acquire nutrients and oxygen. Blood returns to the fetus via the umbilical vein.

2 Three circulatory shunts

1 Ductus venosus

Most of the venous blood (which is high in oxygen and nutrients) bypasses the fetal liver via a shunt called the ductus venosus, which leads directly into the inferior vena cava.

2 Foramen ovale

Blood flowing via the fetal inferior vena cava flows into the right atrium. After birth, the right side of the heart pumps blood to the lungs for gas exchange. In the fetus, the lungs are bypassed via a hole between the right and left atria called the foramen ovale. As a result, blood flows from the inferior vena cava directly to the left side of the heart, which pumps it throughout the fetal tissues.

3 Ductus arteriosus

Some of the blood that enters the right atrium is pumped into the pulmonary artery toward the lungs, however it is shunted directly to the aorta via a short passage called the ductus arteriosus.

3 Neonate

After the umbilical is cut, the umbilical arteries and vein collapse and form scar tissue (become fibrotic). Parts of the umbilical arteries within the neonate retain some function, but the ductus venosus turns into ligament. Also, the foramen ovale seals over to become the fossa ovalis. Finally, the ductus ateriosis collapses and becomes a ligament.

8 Development of blood vessels and blood

9 Aging and the cardiovascular system

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