Blood Vessels

There are several types of blood vessels: ateries (carry blood away from the heart), arterioles (little arteries), capillaries (where exchange happens), venules (little veins), and viens (carry blood back to the heart.

Arteries

Each artery has 3 layers: inner, middle, and outer.

• Inner Layer-Comprised of simple squamous epithelium, basement membrane, and an elastic lamina.
• Middle Layer-Comprised of smooth muscle and elastic tissues. Stimulation of the sympathetic NS causes vasoconstriction, and lack of stimulus causes vasodilation.
• Outer Layer-Comprised of elastic and collagen fibers.

Veins

Veins also have 3 layers, but are structured a little bit differently.

• The inner and middle layers are thinner than in arteries.
• The outer layer is thicker than in arteries.
• The lumen (space that blood flows through) is larger than in the corresponding artery.

Veins have valves that help control blood flow and prevent backflow.

• Vericose veins are caused by valves that have been weakened due to overloading.

More Specifics on Blood Vessel Types

1. Elastic Arteries-largest arteries (pulmonary artery, aorta & its branches). They help propel blood when the ventricles contract. They have MANY elastic fibers, which allow for ease of stretching and recoiling.
2. Muscular Arteries-medium-sized arteries with more smooth muscle. They do vasoconstriction to push blood through the body and play a major role in adjusting blood flow and B/P.
3. Arterioles-almost microscopic, help control B/P from arteries to capillaries, and have less smooth muscle.
4. Capillaries-exchange vessels. The need for exchange determines the amount of capillaries in a given area of the body.
5. Venules-same as arterioles, but are between capillaries and veins.
6. Veins-discussed above.

Capillary Exchange-this happens in a couple of ways.

1) Diffusion-movement from higher [ ] to lower [ ].

• i.e Oxygen and glucose in, carbon dioxide and waste out.

2) Bulk Flow-movement due to pressure differences.

• Happens when B/P in arteries is greater than B/P in veins.
• Ions and other molecules move out (filtration)
• Venous side gets bulk flow in (reabsorption).

Vasodilators cause the capillaries to expand and increase blood flow.

Vasoconstrictors cause the capillaries to shrink and decrease blood flow.

Tissues can regulate their own blood flow. This is called autoregulation.

Venous Return

1. Skeletal Muscle Pump
2. Ventricle Contraction
3. Respiration (Respiratory Pump): During inhalation the diaphragm contracts, which increases abdominal pressure and blood moves through the veins. During exhalation the diaphragm relaxes, and the venous valves prevent backflow.

Blood Circulation: How do we measure it?

1. Pulse-the number of heart contractions, recorded in beats per minute (bpm). An average pulse for an adult is 70-75 bpm. When the pulse is over 100, the person is said to have tachycardia. When the pulse is less than 60, the person is said to have bradycardia.
2. Blood Pressure (B/P)-normally taken from the brachial artery of the left arm. Both systolic (ventricular systole) and diastolic (relaxation) values are recorded: systolic/diastolic. Normal B/P values are 120/80 for men, and 6-8 mmHg less for women. The instrument used to measure B/P is called a sphygmomanometer (that thing the nurse wraps around your arm, cuts off your circulation, and gives it back again slowly with).

• Factors that affect pulse and B/P include, but are definitely not limited to age and exercise.