Lecture 29 Blood Vessels

Question 1

Contrast the structure and function of veins, capillaries, and arteries.

Answer 1

Blood vessels - Tunica Interna (intima) – composed of simple squamous epithelium (endothelium) forms the smooth lining tissue of the lumen.
Tunica Media (muscularis) – composed of two layers. One of circular smooth muscle, and one containing both collagen and elastic fibers (external elastic lamina). Maintains the pulse in the arteries and provides some strength and support in the veins.
Tunica Externa (adventitia) – composed of dense irregular connective tissue.  Also adds strength to the walls and is used to attach the blood vessel to surrounding tissues. In large vessels, it is vascularized.

The circulation is a closed system. Blood is always retained in a vessel. Beginning at the heart the blood flows:
Blood first enters into the arteries from the heart, which carry the blood under pressure (about 120 mm Hg systolic pressure).
Arterioles are smaller than arteries and the walls are thinner so BP decreases (about 60 mm Hg)
Capillaries have walls only one cell thick (endothelium) allowing free exchange of gases, nutrients and wastes (about 20 mm Hg)
Post-capillary venules receive blood from the capillaries. Blood pressure is about 7-10 mm Hg. Depends on valves and muscle contraction.
Veins are the largest vessels, returning the blood to the heart. The largest, the vena cava, has no valves and depends on negative pressure from the heart for flow (about -5 mm Hg)

Question 2

Compare systematic, coronary, pulmonary, portal, and fetal circulation.

Answer 2

Unlike the Systemic Circulation, the Pulmonary arteries carry the least oxygenated blood, since this is blood returning from the Systemic Circulation.
The left and right Pulmonary artery enter the lungs and immediately branch into lobar arteries (3 on the right and 2 on the left).
The arteries end in capillaries surrounding the alveoli (air sacs containing breathed air) and carbon dioxide is exchanged for oxygen.
The Pulmonary Veins return to the heart, carrying the most oxygenated blood in the body.

In the fetus neither the lungs nor the alimentary canal play any useful role. Gas, nutrient and waste exchange occurs through the umbilical circulation and the placenta.

The placenta is a large, highly vascularized organ created by the fetus to take advantage of the oxygen and nutrients in the mother’s blood, and to release its waste products into that blood.
At birth, fetal circulation must suddenly shift from the placenta to the lungs and intestines. This takes place only a few minutes after birth.

Systemic Circulation – Blood exiting the left ventricle passes into the aorta , which supplies blood to all of the parts of the body including the lungs

Pulmonary Circulation – Blood exiting the right ventricle passes into the pulmonary trunk, which supplies blood to the left and right lungs, but is restricted to the alveoli for gas exchange. The purpose of the pulmonary circulation is to re-oxygenate the blood.

Question 3

Describe the importance and route of blood involved in a portal circulation.

Answer 3

There are two separate shunts that bypass much of the pulmonary blood flow (about 85% reduction) that must be closed. These are the foramen ovale, which allows blood to flow directly from the right to the left atrium and the ductus arteriosus, which shunts blood from the pulmonary trunk to the aorta.
There are also the umbilical arteries that come off the internal iliac arteries (L and R), which must be closed and the umbilical vein and ductus venosus shunt that bypasses the intestines and liver

Question 4

Explain the change in fetal circulatory structures after the establishment of post-natal circulation.

Answer 4

In the fetus neither the lungs nor the alimentary canal play any useful role. Gas, nutrient and waste exchange occurs through the umbilical circulation and the placenta.
The placenta is a large, highly vascularized organ created by the fetus to take advantage of the oxygen and nutrients in the mother’s blood, and to release its waste products into that blood.
At birth, fetal circulation must suddenly shift from the placenta to the lungs and intestines. This takes place only a few minutes after birth.