Page 142 - 84 human physiolofy part-1

Chapter 8

specimens (venipuncture) for testing purposes, and enabling the infusion of fluid, electrolytes,
nutrition, and medications (intravenous delivery).

Venules

A venule is a small vein that allows deoxygenated blood to return from the capillary beds to the
larger blood veins. Venules have three layers; they have the same makeup as arteries with less smooth
muscle, making them thinner.

The Cardiovascular Pathways

The double circulatory system of blood flow refers to the separate systems of pulmonary
circulation and the systemic circulation in amphibians, birds and mammals (including humans.) In
contrast, fishes have a single circulation system. For instance, the adult human heart consists of two
separated pumps, the right side with the right atrium and ventricle (which pumps deoxygenated blood
into the pulmonary circulation), and the left side with the left atrium and ventricle (which pumps
oxygenated blood into the systemic circulation). Blood in one circuit has to go through the heart to
enter the other circuit. Blood circulates through the body two to three times every minute. In one day,
the blood travels a total of 19,000 km (12,000 miles), or four times the distance across the U.S. from
coast to coast.

The Pulmonary Circuit

In the pulmonary circuit, blood is pumped to the lungs from the right ventricle of the heart. It is
carried to the lungs via pulmonary arteries. At lungs, oxygen in the alveolae diffuses to the capillaries
surrounding the alveolae and carbon dioxide inside the blood diffuses to the alveolae. As a result, blood
is oxygenated which is then carried to the heart's left half -to the left atrium via pulmonary veins.
Oxygen rich blood is prepared for the whole organs and tissues of the body. This is important because
mitochondria inside the cells should use oxygen to produce energy from the organic compounds.

The Systemic Circuit

The systemic circuit supplies oxygenated blood to the organ system. Oxygenated blood from the
lungs is returned to the left atrium, then the ventricle contracts and pumps blood into the aorta.
Systemic arteries split from the aorta and direct blood into the capillaries. Cells consume the oxygen
and nutrients and add carbon dioxide, wastes, enzymes and hormones. The veins drain the
deoxygenated blood from the capillaries and return the blood to the right atrium.

Aorta

The aorta is the largest of the arteries in the systemic circuit. The blood is pumped from the left
ventricle into the aorta and from there it branches to all parts of the body. The aorta is an elastic artery,
and as such is able to distend. When the left ventricle contracts to force blood into the aorta, the aorta
expands. This stretching gives the potential energy that will help maintain blood pressure during

142 | Human Physiology