1. Blood Vessels
Anatomy Ch. 11 Part 1

2. Blood circulates inside the blood vessels.
The blood vessels form a closed system sometimes called the vascular system.
The idea that blood circulates is only about 300 years old.
Before this time it was believed that blood moved like the tide.
In the 17th century a physician named William Harvey proved that blood moved in circles.

3. As blood leaves the heart it moves first into the arteries.
The arteries that blood moves through become smaller and smaller.
After leaving the arteries the blood enters the arterioles.
The arterioles feed the capillary beds where gas and nutrient exchange takes place.

4. The capillary beds are drained by venules which empty into veins.
The veins merge into the superior and inferior vena cava which enter the heart.
The arteries and veins are simply conducting vessels.
It is only through the walls of the capillaries that exchanges between the tissue cells and the blood can occur.

6. In most cases in the body the arteries carry oxygen rich blood and the veins carry oxygen poor blood.
One location where the opposite is true is in the pulmonary (lung) circuit.

7. Tunics of Blood Vessels
Except for capillaries the walls of blood vessels have 3 coats or tunics.
Tunica intima
lines the lumen or interior of the blood vessel
composed of a thin layer of squamous epithelium that rests on a basement membrane
Cells fit closely together and form a slick surface that decreases friction of blood as it flows.

8. Tunica media Bulky middle coat Mostly smooth muscle and elastic fibers
The smooth muscle which is controlled by the sympathetic nervous system is active in changing the diameter of vessels.
As the blood vessels constrict, blood pressure increases. As the blood vessels dilate, blood pressure decreases.

9. Tunica externa Outer most tunic Composed of fibrous connective tissue
Functions to support and protect the vessel

10. Structural Differences in Arteries, Veins, and Capillaries
The walls of arteries are much thicker than those of veins.
The walls of arteries are much thicker because they are closer to the heart and must be able to expand as blood is forced into them.
The walls of arteries must be strong and stretchy enough to take the changes in pressure.

11. Veins in contrast are far from the heart and the pressure in them tends to be low all the time.
Veins have thinner walls than arteries.
Because of the low blood pressure and gravity, veins are modified to ensure that the amount of blood returning to the heart equals the amount of blood being pumped out of the heart.

12. To ensure that this takes place the lumens of the veins are larger and are equipped with valves to prevent backflow of blood.
The skeletal muscles also assist with venous return.
As the muscles around the veins contract the blood is squeezed through the veins toward the heart.

13. Another mechanism that assists the veins in bringing blood back to the heart is called the respiratory pump.
When we inhale there is a drop in pressure in the thorax which causes the large veins near the heart to expand and fill.

14. Arteries are generally located in deep well protected body areas.
Veins are more superficial and more easily seen.

16. The walls of the capillaries are only one cell layer thick (tunica intima).
Because the walls of the capillaries are so thin, exchanges are easily made between the blood and the tissue cells by diffusion.
Capillaries form networks called capillary beds.
The flow of blood through a capillary bed is called microcirculation.

17. A capillary bed consists of two types of vessels
Vascular shunts directly connect the capillaries to the arterioles and venules at opposite ends of the bed
True capillaries are the actual exchange vessels

19. Physiology of Circulation
A good indication of the efficiency of the circulatory system can be obtained by taking 1) arterial pulse and 2) blood pressure measurements.
These measurements, along with ) respiratory rate and 4) body temperature are referred to collectively as vital signs.

20. Arterial Pulse
The alternating expansion and recoil of an artery that occurs with each beat of the heart creates a pulse that travels through the entire arterial system.
Pulse rate equals heart rate which averages 70 to 76 beats per minute
Pulse rate is influenced by activity, posture changes, and emotions.

21. Your pulse can be felt in any artery that is close to the body surface.
The carotid (neck) and radial (wrist) are common locations.
Because these same points are compressed to stop blood flow, they are also called pressure points.
When checking pulse use fingers, do not use the thumb because it has its own pulse.

22. Blood Pressure
Blood pressure is the pressure the blood exerts against the inner walls of the blood vessels (arteries) and it is the force that keeps blood circulating continuously.
Blood pressure is highest in the large arteries near the heart and continues to drop throughout circulatory pathways until it reaches zero in the vena cava that enter the heart.
The blood pressure gradient is from high to low pressure.

23. Because blood pressure is so low in the veins, 1) valves in the veins and 2) skeletal muscles that surround the veins are necessary for the blood to complete its circuit back to the heart.
The ability of the arteries to stretch and recoil allow for continuous blood flow throughout circulation.

24. There are two arterial blood pressure measurements:
Systolic pressure: pressure in the arteries when the heart ventricles contract
Diastolic pressure: pressure in the arteries when the heart ventricles relax
Blood pressures are reported in millimeters of mercury (mm Hg).
Systolic pressure is measured first.
The auscultatory method is commonly used where pressure is measured in the brachial artery of the arm.

26. Increased blood volume or thickness can also raise resistance.
BP = CO x PR
BP – blood pressure
CO – cardiac output (amount of blood pumped out of the left ventricle of the heart in one minute)
PR – peripheral resistance
Peripheral resistance is the amount of friction the blood encounters as it flows through blood vessels.
There are several factors that increase resistance but the most important is the narrowing or constriction of blood vessels.
Increased blood volume or thickness can also raise resistance.

27. Factors that can alter blood pressure
Neural factors: vasoconstriction caused by the sympathetic division of the ANS
The kidneys: play a major role by altering blood volume
Temperature: cold has a vasoconstricting effect and heat has a vasodilating effect

28. Chemicals:
Epinepherine increases both heart rate and blood pressure
Nicotine and alcohol increases blood pressure by causing vasoconstriction
Histamine cause vasodilation and decrease blood pressure
Diet: a diet low in salt, saturated fats, and cholesterol helps to prevent hypertension or high blood pressure

29. Variations in blood pressure
Normal systolic pressure varies between 110 and 140
Normal diastolic pressure varies between 70 and 80
Blood pressure tends to peak in the morning
Hypotension is low blood pressure
Hypertension is high blood pressure

30. Capillary exchange of gases and nutrients
Substances move according to concentration gradient (high to low)
Respiratory gases ( O2 and CO2 ) pass across the plasma membrane by direct diffusion.
Fluids and small solutes move by limited diffusion through clefts (gaps) in the membrane.

31. Very free diffusion of solutes and fluids can occur through fenestrated capillaries (large pore). These are only found in areas where absorption (intestines and endocrine glands) or filtration (kidneys) occurs.
Lipid insoluble substances such as hormones are transport through vesicles
Proteins and blood cells are unable to pass through the capillaries

32. Because of openings in capillaries, excess fluids can leave the capillaries.
Blood pressure causes fluid to move out and osmotic pressure draws fluid in.
Blood pressure is higher than osmotic pressure at the arterial end of a capillary so fluid moves out.
Osmotic pressure is higher than blood pressure at the venule end so fluid is reclaimed.
However not all fluid is reclaimed at the venule end. The lymphatic system is responsible for returning lost fluid to the blood.