Sarah Lopes
A&P II – Dr. Jayne Jarger
The Digestive and Circulatory Systems
While all bodily systems are vital to our survival, the digestive and circulatory systems
are among the most prominent. Several times a day, we put the digestive system into action by
consuming food for energy, and we can physically feel our heart beating at all times. These two
systems are closely intertwined, and one could not operate without the other.
The digestive system begins with the mouth, where food is mechanically broken down
with the teeth and salivary amylase begins to chemically break down carbohydrates. The
muscular tongue maneuvers the food around the mouth during chewing and then aids in
deglutition of the bolus of food. The bolus travels down the esophagus as muscles contract
behind it to push it along. This process is known as peristalsis, and it occurs throughout the
alimentary canal. When the bolus of food reaches the stomach, it must pass through the lower
esophageal sphincter, which is also known as the cardiac sphincter, because the receiving end of
the stomach is referred to as the cardia. The stomach has other named portions, such as the
fundus, which is the most superior point, the greater and lesser curvatures on the lateral and
medial aspects, and the pyloric region on the exiting end of the stomach. The mucosa of the
stomach is covered in folds called rugae and also contains gastric pits, which become gastric
glands as they move deeper into the epithelium. This is where gastric juice is produced to
chemically digest the ingested food. The stomach is where protein digestion is initiated. The
mixture of gastric juices and ingested food is called chyme, and it is released in small amounts
through the pyloric sphincter into the small intestine. The small intestine is where lipid digestion
is initiated, and where the majority of digestion occurs. There are 3 sections: the duodenum on
the proximal end, joined to the pylorus of the stomach, the jejunum, and the ileum on the distal
end, joined to the large intestine. The liver and pancreas are considered to be digestive
accessory organs, and are connected to the duodenum via ducts which release bile and
pancreatic juice, respectively. Bile is composed of several compounds, including bile salts, which
emulsify fats. Pancreatic juice contains a variety of enzymes that break down the food
components of the chyme, as well as bicarbonate ions that neutralize the hydrochloric acid
released by the stomach. The lining of the small intestine has ridges known as circular folds,
which are covered in finger-like villi. The cells of the villi have their own projections called
microvilli, which make up the brush border. There structures serve to provide maximum surface
area for absorption of nutrients. As the chyme passes through the small intestine, nutrients are
absorbed through the mucosa and enter the bloodstream, with exception of the lipids, which
enter the lacteals of the lymphatic system and later enter the blood through lymphatic ducts.
Indigestible matter eventually reaches the cecum of the large intestine after useful nutrients
have been extracted, and this leftover material is formed into feces. Under normal
circumstances, water is reabsorbed at the large intestine (colon). After passing through the
ascending, transverse, and descending colon, waste matter arrives in the rectum and exits
through the anus.
The cardiovascular system includes the heart, arteries which carry blood away from the
heart, and veins which carry blood back to the heart. In systemic circulation, arteries carry
oxygen-rich blood to tissues, and veins carry oxygen-poor blood away from the tissues. On the
contrary, pulmonary arteries leading to the lungs carry oxygen-poor blood, and once
oxygenated, pulmonary veins carry the blood back to the heart so that it can be distributed
around the body. The heart has 4 chambers: the right and left atria and right and left ventricles.
Deoxygenated blood enters the right atrium from the superior and interior venae cavae, passes
through the tricuspid valve into the right ventricle, and then out toward the lungs through the
pulmonary trunk. Returning from the lungs, oxygenated blood flows into the left atrium,
through the biscuspid (mitral) valve into the left ventricle, and out through the aorta to all
tissues of the body. The familiar heartbeat sound is created by the closing of the atrioventricular
valves just before systole, followed by the closing of the semilunar valves just before diastole.
The beating of the heart is controlled by the cardiac conduction system, made up of cells that
have the unique ability to contract without nervous or hormonal input.
The arterial and venous systems are actually one continuous loop that carries blood to
tissues and back again. Similar to the alimentary canal, they are surrounded by layers of muscle
that constrict to propel their contents along. As these vessels move more distally from the
heart, they become more and more narrow until they reach such a small diameter, only one
RBC can pass through at a time. The RBCs carry oxygen via hemoglobin molecules, which
contain iron absorbed through the digestive system. As previously mentioned, all other
nutrients enter the bloodstream as well, and are carried to appropriate bodily tissues. At the
same time, mesenteric arteries carry oxygenated blood to the digestive organ tissues and
mesenteric veins carry deoxygenated blood away. Also, bile salts from the liver are recycled via
the portal vein, carried back from the duodenum to the liver several times during the digestion
of each meal. Lastly, water absorbed through the digestive system is needed for normal blood
plasma volume.