Anatomy and Physiology Lecture Notes

Unit 7 Circulatory System - The Blood Vessels

 

Circles of Blood

The ancient Greeks believed that blood moved through the body like an ocean tide, first moving out of the heart and then ebbing back to it in the same vessels. It was not until the seventeenth century that William Harvey, an English physician, proved that blood did, in fact, move in circles through the body.

The walls of arteries are usually much thicker than the walls of veins. Their tunica media, in particular, tends to be much heavier.

This structural difference is related to a difference in function between arteries and veins.

Arteries are much closer to the pumping action of the heart. Their walls must be strong enough to take the continuous changes in pressure. On the other hand, veins are far from the heart in the circulatory pathway, and the pressure in them tends to be low all the time. Their walls do not have to resist pressures.

 

Because of the low pressure in the veins, and the fact that much of the blood in them flows against gravity, veins are modified to ensure that the amount of blood returning to the heart (venous return) equals the amount being pumped out of the heart (cardiac output) at any time. The lumens of veins tend to be much larger than those of corresponding arteries, and the larger veins have valves that prevent backflow of blood.

Skeletal muscle activity, the muscular pump, enhances venous return. As the muscles surrounding the veins contract and relax, the blood is "milked" through the veins toward the heart. Finally, when we inhale, the drop in pressure that occurs in the thorax causes the large veins near the heart to expand and fill. Thus, the respiratory pump also helps return blood to the heart.

venous valves and muscle "pumping"

 

The largest artery is the Aorta.

Blood leaves the heart in large arteries, moving into successively smaller and smaller arteries and then into the arterioles, which feed the capillary beds in the tissues. Capillary beds are drained by venules, which in turn empty into veins that finally empty into the great veins entering the heart.

The largest vein is the Vena cava.

Capillary Exchange:

Capillaries form an intricate network among the body's cells such that no substance has to diffuse very far to enter or leave a cell. The substances exchanged first diffuse through an intervening space filled with interstitial fluid.

Substances tend to move to and from body cells according to their concentration gradients. Basically, substances entering or leaving the bloodstream may take one of four routes across the plasma membranes of the singel layer of endothelial cells forming the capillary wall.

Diffusion of substances by the other two routes depends on the specific structural characteristics of the capillary.

Only substances unable to pass by one of these routes are prevented from leaving or entering the capillaries. These include protein molecules and blood cells.

There are also active forces operating in the capillary beds. Blood pressure tends to force fluids, and solutes, outward, while osmotic pressure tends to pull fluid back into the bloodstream. Whether fluid moves out of or into the capillary depends on the difference between these two pressures. As a rule, blood pressure is higher at the arterial end of the capillary bed, and osmotic pressure is higher at the venous end. For this reason, fluid moves out of the capillaries at the beginning of the bed and is reclaimed at the opposite end.

Not quite all of the fluid forced out of the bloodstream is reclaimed at the venule end. Returning that lost fluid to the blood is the chore of the lymphatic system - covered next week.

To make learning the arteries easier, be aware that that in many cases the name of the artery tells you the body regin or organs served (renal artery, brachial artery, and coronary artery) or the bone followed (femoral artery and ulnar artery).

The aorta curves upward from the left ventricle of the heart as the ascending aorta, arches to the left as the aortic arch, then drops downward following the spine as the thoracic aorta to finally pass through the diaphragm to become the abdominal aorta.

The branches of the parts of the aorta are listed below in their sequence from the heart and the organs served.

Branches of the Ascending Aorta:

Branches of the Aortic Arch:

The subclavian artery becomes the axillary artery, then continues into the arm as the brachial artery which supplies the arm. At the elbow, the brachial artery splits

Branches of the Thoracic Aorta:

Branches of the Abdominal Aorta:

Although arteries are generally located in deep, well-protected body areas, many veins are more superficial and some are easily seen and palpated on the body surface. Most deep veins follow the course of the major arteries, and with a few exceptions, the naming of these veins is identical to that of their companion arteries.

While major systemic arteries branch off the aorta, the veins converge on the vena cava.

Blood returns to the right atrium of the heart through the vena cava. Veins draining the head and arms empty into the superior vena cava and those draining the lower body empty into the inferior vena cava.

The veins listed below begin distally and move proximally to the heart.

Veins Draining into the Superior Vena Cava:

Veins Draining into the Inferior Vena Cava:

The inferior vena cava, which is much longer than the superior vena cava,
returns blood to the heart from all body regions below the diaphragm.

The alternating expansion and recoil of an artery that occurs with each beat of the left ventricle creates a pressure wave - pulse - that travels through the entire arterial system. Normally the pulse rate (pressure surges per minute) equals the heart rate (beats per minute). The pulse averages 70 - 76 beats per minute in a normal resting person.

A pulse can be felt in any artery lying close to the body surface by compressing the artery against firm tissue. Because it is so accessible, the point where the radial artery surfaces at the wrist is routinely used to take a pulse (the radial pulse).

To find your own radial pulse, rest your right arm in the palm of your left hand. Curl the fingers of your left hand up around the thumb side of your right wrist. Place several fingers of your left hand along and just to the outside (thumb side) of the tendon that runs along your wrist. With gentle pressure, you should be able to feel your pulse.

Several other clinically important arterial pulse points shown here. Because these same points are compressed to stop blood flow into distal tissues during hemoreage, they are also called pressure points.

Blood pressure is the pressure the blood exerts against the inner walls of the blood vessels, and it is the force that keeps the blood circulating continuously, even between heartbeats. The pressure is highest in the aorta and continues to drop throughout the system, reaching zero or negative pressure at the venae cavae.

Blood flows continually along a pressure gradient (from high to low pressure). Notice that if venous return depended entirely on high blood pressure throughout the system, blood would probably never be able to complete its curcuit back to the heart. This is why the valves in the larger veins, the milking actions of the skeletal muscles, and pressure changes in the thorax are so important.

Continual blood flow absolutely depends on the stretchiness of the larger arteries and their ability to recoil and keep the pressure on the blood as it flows in circulation. The importance of the elasticity of the arteries is best appreciated when it is lost, as happens in arteriosclerosis. This condition is commonly called "hardening of the arteries".

Because the heart alternately contracts and relaxes, the pressure in the arteries rises and falls with each beat. Two pressure measurements are made:

Measuring Blood Pressure with a Sphygmomanometer

Blood pressure is reported in millimeters of mercury (mm Hg),
with the systolic pressure written first.

Step 1

The artery used to determine BP is the brachial artery, which runs down the upper arm, splitting into the radial and ulnar arteries near the elbow.

A cuff is inflated around the arm - stopping the flow of blood through the artery.

Listening to blood flow below the cuff, the sound will stop when the ventricles are not producing enough pressure to force blood past the pressure of the cuff.

Step 2

Air pressure in the cuff is now slowly released. The first sounds of blood passing through the artery means that the ventricles have pumped with just enough force to overcome the pressure exerted by the cuff.

This measurement is the systolic pressure - the pressure of the blood when the ventricles contract.

Normal systolic pressure is about 120 mm Hg for males, AND 110 mm Hg for females.

Step 3

Air pressure is continued to be released from the cuff, listening for the disappearance of sound. This will happen when there is a steady flow of blood.

This measurement is the diastolic pressure - the pressure of the blood when the ventricles relax.

Normal diastolic pressure is about 80 mm Hg for males 70 mm Hg for females.

The pressure measured
in this example is 120/80.

Arterial blood pressure is directly related to cardiac output and peripheral resistance. Peripheral resistance is the amount of friction encountered by the blood as it flows through the blood vessels. Any factor that increases either the cardiac output or peripheral resistance causes an almost immediate reflex rise in blood pressure.

When arterial blood pressure is low, certain kidney cells release the enzyme renin into the blood. Renin triggers a series of chemical reactions that result in the formation of angiotensin II, a potent vasoconstrictor chemical.

1.      Hypotension, or low blood pressure, is generally considered to be a systolic blood pressure below 100 mm Hg. What does the term orthostatic hypotension refer to?

2.      A brief elevation in blood pressure is a normal response to fever, physical exertion, and emotional upset. Persistent hypertension, or high blood pressure, is pathological, and defined as a sustained elevated arterial pressure of 140/90 or higher. What damage is done to the body by persistent hypertension?