Chapter 42 (3)

Blood vessels contain a central lumen lined ith __, a single layer of flattened epithelial cells. The smooth surface of the __ minimizes resistance to the flow of blood.
Surrounding the __ are layyers of tissue that differ among capillaries, arteries, and veins, reflecting the specialized functions of these vessels.

__ are the smallest blood vessels, having a diameter only slightly greater than that of a red blood cell. They also have very thin walls, which consist of just the __ and its __. This structural organization facilitates the exchange of substances between teh blood in capillaries and the interstitial fluid.

The walls of __ and __ have a more complex organization than those of __.
Both __ and __ have two layers of tissue surrounding the __:
1. an outer layer of connective tissue containing elastic fibers, which allow the vessel to stretch and recoil
2. a middle layer containing smooth muscle and more elastic fibers.

However arteries and veins differ in important ways. For a given blood vessel diameter, an __ has a wall about three tiems as thick as that of a vein. The thicker walls of __ are very strong, accomodating blood pumped at high pressure by the heart, and their elastic recoil helps maintain blood pressure when the heart relaxes between contractions.

Signals from the __ and __ circulating in the blood act on the smooth muscles in arteries, controlling blood flow to different parts of the body. The thinner-walled __ convey blood back to the heart at a lower velocity and pressure. Valves in the veins maintain a unidirectional flow of blood in these vessels.

Blood __ as it moves from arteries to arterioles to capillaries. Why?
- The reason is that the number of capillaries is enormous. Each artery conveys blood to so many capillary beds than in the arteries or any other part of the circulatory system. The result is a dramatic decrease in velocity from the arteries to the capillaries. Blood travels 500 times slower i the capillaries that in hte __.

TThe reduced velocity of blood flow in the __ is critical to the function of the circulatory system. Capillaries are the only vessels with walls thin enough to permit transfer of substances between the blood and interstitial fluid. THe slower flow of blood through these tiny vessels allows time for exchange to occur. After passing through the capillaries, the bloood speeds as it enters the __ and __, which have smaller total cross- sectional areas.

__ , like all fluids, flows from higher pressure to lower pressure. Contraction of a heart ventricle generates blood pressure, which exerts a force in all directions. The force directed lengthwise in an artery causes the blood to flow away from the __, the site of highest pressure.

The forces exerted against the elastic wall of an artery stretches the wall, and the recoil of arterial walls plays a critical role in maintaining blood pressure, and hence blood flow, throughout the __, Once blood enters the millions of tiny arterioles and capillaries, the narrow diameter of these vessels generates substantial resistance to flow. This resistance dissipates much of the pressure generated by teh pumping heart by the time the blood enters the veins.

Arterial blood pressure is highest when the heart contracts during __. The pressure at this time is called __. The spikes in bllod pressure caused by teh powerful contractions of the ventricles stretch the arteries. By placing your fingers on your wrist, you can feel a __- the rhythmic bulging of the artery walls with each heartbeat.

The surge of pressure is partly due to the narrow openings of arterioles impedintg the exit of blood from the arteries. Thus, when the heart contracts, blood enters the arteries faster than it can leave, and the vessels stretch from the rise in pressre. During __, the elastic walls of the arteries snap bacck. As a consequence, there is a loewr but still substantial bood pressure when the ventricles are relaxed (__). Before enought blood has flowed into the arterioles to completey relieve pressure int he arteries, the heart contracts again. Because the arteries remain pressurized throughout the __, blood continuously flows into arterioles and capillaries.

Blood pressure fluctuates ver two different time scales. The first is the __ in arterial blood pressure during each cardiac cycle. Blood pressure also fluctuates on a longer time scale in response to signals that change the state of smooth muscles in arteriole walls.

Physical or emotional stress can trigger nervous and hormonal responses that cause smooth muscles in arterioule walls to contract, called __. When that happens the arterioles narrow, therby increasing blood rpessure upstream in the arteries.

When the smooth muscles relax, the arterioles undergo __, an increase in diameter that causes blood pressure in the arteries to fall.

__ and __ are often coupled to chnges in cardiac output that also affect blood pressure. This coordination of regulatory mechanisms maintains adequate blood flow as the body's demands on the circulatory system change.

__ is a ptent inducer of vasoconstriction and the gas nitric oxide serves as a major inducer of vasodilation.

__ has a significant effect on blood pressure.
The challenge of pumping blood against gravity is particularly great for animals with very long necks.
__ is also a consideration for blood flow in veins, especially those in the legs.

Although blood pressure in veins is relatively low, several mechanisms assist the return of venous blood to the heart.
1- Rhythmic contractions of smooth muscles in the walls of venules and veins aid in the movement of hte blood.
2- he ocntraction of __ muscles during exercise squeezes blood throught eh venins toward the heart.
3- The change in pressure within the __ cavity during __ causes the __ and other large veins near the heart ot expand and fill with blood.

TRUE OR FALSE:
At any given time, only about 5-10% of the body's capillaries have blood flowing through them. However, each tissue has many capillaries, so every part of hte body is supplied with blood at all times. Capillaries in the brain, heart, kidneys, and liver, are usually filled to capacity, but at many other sites the blood supply varies over time as blood is diverted from one diestination to another.
True False

Given trhat capillaries lack smooth muscles, how is blood flow in capillary beds altered?
Tehre aretwo mechanisms, both of which rely on signals that regulate flow into capillaries.
1- involves contraction of the smooth muscles in the walls of an __, which reduces the vessels diameter adnd decreases blood flow to the adjoining __. When the smooth musc les relaxes, teh arterioles dilate, allowing blood to entre the capillaries.
2- involves the action of __, rings of smooh muscle located at the entrance to capillary beds. THe signals that regulate blood flow include nerve impulses, hormones traveling throughout the bloodstream and chemicals produced locally.

The critical exchange of substance between the blood and itnerstitial fluid takes place across the thin __ walls of the capillaries. Some substances are carried across the __ inv esicles that form on one side by __ and release their contents on the opposite sde by __. Small molecules (O2 and CO2) simply diffuse across the __ cells or throught he openigns within and between adjoining cells. These openings also provide the route for transport of small solutes such as sugars, slats and urea, as well as for bulk flow of fluid into tissues driven by blood rpessure within the capillary.

While blood pressure tends to drive fluid out of the capillaries, the presence of blood preoteins tends to pull fluid back into the capillaries. Many blood proteins and all blood cells are too large to pass readily throught eh __, adn they remian in the capillaries. THe proteins, especially __, creates an osmotic pressure difference between the capillary interior and the interstitial fluid. In places where the blood pressure is greater than the osmotic pressure difference, there is a net loss of fluid from the capillaries. In contrast, where the osmotic pressure difference exceeds the blood pressure, there is a net movement of fluid from the tissues into the capillaries.

Throughout the body, only about __ of the fluid that leaves the capillaries because of blood pressure reneters them as a result of __ pressure. Each day, this imbalance results in a loss of about 4 L of fluid from capilarries to the surrounding tissues. There is also some leakage of blood proteins, even though the capillary wall is not very permeable to large molecuels.

The lost fluid and proteins return to the blood via the __, which includes a network of tiny vessels intermingled among capillaries of hte cardiovascular system. After entering the __ by diffusion, the fluid is caled __; its composition is about the same as that of interstitial fluid. The __ drains into large veins of the circulatory systema t the base of the neck.
- This joining of the __ and circulatory systems functions int he transfer o f lipids fromt eh small intestine to the blood.

The movement of __ from peripheral tissues to the heart relies on much the same mechanisms that assist blood flow in veins. Lymph vessels, like __ , have valves that prevent hte backflow of fluid. Thythmic contractions of the vessel walls help draw fluid into the small lymphatic vessels. In addition, skeletal muschle contractions play a role in moving __.

Disorders that interfere with the __ highlight its role in maintaining proper fluid distribution in the body. Disruptions in the movement of lymph often cause __,eswelling resulting formt he excessive accumulation f fluid in tissues. Sever blockage of lymph vessels, results in extremely swollen limbs or other body partts, a condition called __.

Along a lymph vessel are organs called __. By filtering the lymph and by houseing cells that attack viruse and bacteria, __ play an important role in the body's defense. Inside each __ is a honeycomb of connective tissue with spaces filled by white blood cells. When the ody is fighting an infection, these cells multiply rapidly, adn the __ become swollen and tender. Because __ have filtering and surveillance functions, docs may exampine the __ of cancer patients to detect the spread of diseased cells.