Dissection and Comparative Anatomy: How the Three Systems Connect

Seeing the connections

Textbook diagrams show the respiratory, digestive and circulatory systems separately, in neat colours. Inside a real body, they are packed together and physically joined: blood vessels run through the membranes that hold the intestines, the liver sits right under the diaphragm that drives breathing, and the heart is wedged between the lungs. A dissection (real or virtual) or a detailed anatomical model shows how the three systems fit and work together, which is exactly what this lesson is about.

Why a fetal pig, and the choice not to cut

Pigs are mammals with organs arranged much like a human's. Fetal pigs used in schools are a by-product of the pork industry: they come from sows that were already being processed for food; they are not raised for dissection. Even so, many students and families have ethical, cultural, religious or personal reasons not to take part. Ontario students may be offered alternatives: virtual dissection software, 3-D models, videos or mounted anatomical models. These meet the same expectation, and the choice deserves respect. Respect for the animal matters either way: no unnecessary cuts, no playing, and proper disposal.

Safety (in a school lab): goggles, gloves and an apron; sharp scalpels and scissors cut away from you and others; preserved specimens may contain irritating chemicals, so work in a ventilated room and follow WHMIS labels; wash hands thoroughly; dispose of tissue as your teacher instructs.

Directions and the dissection plan

Anatomists use the animal's own directions: dorsal (back) and ventral (belly), anterior (head end) and posterior (tail end), and left and right are always the animal's left and right. The pig is placed on its back (ventral side up), and cuts are made through the ventral body wall, around the umbilical cord, with flaps folded open.

Umbilical cord: a cross-section shows two umbilical arteries (thick-walled; they carry blood from the fetus to the placenta) and one umbilical vein (larger, thinner-walled; it carries oxygen- and nutrient-rich blood to the fetus).

Thoracic (chest) cavity: the heart in its pericardium, between the lungs; the thymus (an immune organ, large in young mammals); the trachea with its cartilage rings and, dorsal to it, the esophagus.

Diaphragm: the dome of muscle separating the thoracic and abdominal cavities.

Abdominal cavity: the liver (the largest organ, just under the diaphragm), the gallbladder tucked beneath it, the stomach (on the animal's left), the spleen, the pancreas (a pale, grainy organ along the stomach and duodenum), the coiled small intestine held by the mesentery, and the large intestine (in pigs, partly coiled into a spiral).

Where the systems join

While dissecting, look for these physical links:

• Respiratory ↔ circulatory: the pulmonary arteries and veins connect the heart and lungs, which share the thoracic cavity.
• Digestive ↔ circulatory: the mesentery is full of blood vessels (and lymph vessels) branching to every loop of intestine. These drain into the hepatic portal vein, which carries absorbed nutrients to the liver.
• Digestive ↔ respiratory: the pharynx is shared by air and food; the esophagus runs right behind the trachea; the diaphragm, a breathing muscle, presses on the liver and stomach below it.
• All three: the aorta runs dorsally through both cavities, sending branches to the stomach, intestines, liver, spleen and kidneys.

Fetal circulation: a special case

Before birth, a mammal's lungs are filled with fluid and do no gas exchange. The placenta does that job, using the mother's blood. So the fetal circulation has shortcuts:

• The umbilical vein brings O₂-rich blood from the placenta into the fetus (much of it passes by the liver through a vessel called the ductus venosus).
• The foramen ovale, a hole between the right and left atria, lets most blood go straight from the right atrium to the left, skipping the lungs.
• The ductus arteriosus links the pulmonary trunk to the aorta, so most blood pumped by the right ventricle also skips the lungs.
• Umbilical arteries carry blood back to the placenta.

At birth, the first breaths inflate the lungs and pressures in the heart change. The foramen ovale is pushed shut and the ductus arteriosus constricts; within days to weeks both close for good. If one stays open, the heart and lungs can be overworked, and it may need treatment.

Comparing animals

Different animals solve the same three problems (gas exchange, digestion, transport) in different ways, matched to their size, activity and habitat.

Three adaptations worth a closer look

• Countercurrent exchange in fish gills. Water flows over the gill filaments in one direction and blood flows through them in the opposite direction. All along the gill, the water the blood meets has a little more oxygen than the blood itself, so oxygen keeps diffusing in. If they flowed the same way, the blood and water would quickly reach the same oxygen level and exchange would stop halfway.
• Bird air sacs. Birds have air sacs that act like bellows, so air flows one way through the lungs, and fresh air is present during both inhalation and exhalation. This helps supply the huge oxygen demand of flight, including for geese flying high on migration.
• Digestive tracts and diet. Plant material is hard to digest because of cellulose. Herbivores tend to have long guts and help from microbes. Ruminants (cows, sheep, deer, moose) have a four-chambered stomach: in the rumen and reticulum, microbes ferment cellulose; the animal brings up and re-chews its cud; the omasum absorbs water; and the abomasum is the "true" stomach with acid and enzymes. Rabbits and horses use a large cecum for fermentation. Carnivores, whose food is easier to digest, tend to have shorter intestines.