Fetal Pig - Cardiovascular

 
                                               
       
       
       
       
       
       
       
       
                                       
  Introduction
The function of the cardiovascular/circulatory system is the transport of materials to and from the cells. The organs of multicellular animals are too far removed from the external environment to enable them to exchange nutrients, oxygen and wastes directly by diffusion. Instead, the materials needed by cells must be brought to the cells by a circulatory system, and the cellular wastes must similarly be removed.

The cardiovascular/circulatory system includes the circulating medium of transport, the blood. The structures comprising the cardiovascular/circulatory system include the heart, arteries, veins and capillaries. 

Heart
The heart and the large blood vessels at the anterior end of the heart are enclosed by a fibrous, double-layered membranous sac, the pericardium.  The heart is located in the center of the thorax within the mediastinum.  It  is a muscular pump, conical in shape and slightly larger than a human fist consisting of two atria  and two ventricles.  The heart, responsible for the movement of blood, beats approximately 80,000 - 100,000 a day and pumps approximately 2,000 gallons of blood a day through the body.  To sustain this continuous beating, the heart is made of specialized muscle tissue.  The coronary arteries and veins on the surface of the heart supply the heart itself with blood.  The parts of the heart are:

Atria- the two top chambers of the heart.  The atria receive the blood that has returned to the heart from the body and lungs.  The atria contract, transporting the blood into the ventricles.  There are two atria:

Right atrium - receives blood returning from the body and releases it into the right ventricle via the tricuspid valve.

Left atrium - receives oxygenated blood from the pulmonary veins and releases it into the left ventricle via the mitral valve.

Ventricles - the two bottom chambers of the heart.  The ventricles contract to pump the blood out of the heart to the body and lungs.  There are two ventricles:

Right ventricle - receives blood from the right atrium via the tricuspid valve and releases the blood into the lungs to pick up oxygen via the pulmonary valve. 

Left ventricle - the largest and strongest chamber, receives blood from the left atrium and pumps blood to the aorta via the aortic valve. 

Septum -  The septum is a thick wall of muscle that separates the left and right side of the heart.  

Valves - A heart valve is a set of flaps called leaflets or cusps.  A difference in pressure across the valves causes them to open, releasing the blood in a forward direction then quickly closing to to keep the blood from flowing backward.  The four heart valves are:

Mitral - The mitral (bicuspid) valve has two flaps and releases the blood from the left atrium to the left ventricle.

Tricuspid - The tricuspid valve has three flaps and released the blood from the right atrium to the right ventricle.

Aortic - The aortic valve has three flaps and releases the blood from the left ventricle to the aorta.

Pulmonary - The pulmonary valve has three flaps and releases the blood from the the right ventricle to the pulmonary artery.

The fetal circulation has two adaptations for bypassing the inactive lungs. The first is the ductus arteriosus. It is a short vessel which connects the pulmonary artery to the aorta. The second is the foramen ovale, a small opening in the septum between the right and left atria which permits blood to enter the left atrium without first going to the lungs.

Next, the venous system of the fetal pig is dissected. Veins generally lie closer to the surface than arteries. In doubly injected specimens they are colored blue. Arteries, injected with red latex, are stud­ied last.

Veins - Anterior
We begin our study of the major blood vessels with the veins which carry blood to the heart from the head, forelimbs, shoulder, and thoracic region.

The anterior veins lie close to the surface of the body and are therefore studied before the arteries which lie deep to the veins. In doubly injected specimens veins will be blue, arteries will be red. This is due to the latex dye with which they were injected. This permits you to see these vessels more clearly and to differentiate between arteries and veins.

You have already exposed the heart and some of the major veins in your study of the thoracic cavity. Extend your incision into the neck area. If you have not yet done so, expose the trachea, larynx, and thyroid gland as in the photo. Remove the thymus gland carefully from the ventral surface of the heart and lower trachea.

Your best dissection instrument at this point is the dissecting needle. Use it instead of a scalpel or scissors to expose and follow the path of a blood vessel. Scalpels and scissors, especially in the hands of the novice tend to destroy rather than preserve or expose. It is used for clearing a blood vessel of the connective tissue adhering to it, to tear into a thin muscle layer to follow a vessel, to separate one blood vessel from another, or from the nerves associated with it.

Proceed to expose the large veins surrounding the heart.

Push the heart gently to the left. Note a wide, blue blood vessel rising anteriorly from the diaphragm mid-dorsally. This is the posterior vena cava. In man, due to his upright position, this vessel is known as the inferior vena cava. It returns blood from the lower portion of the body and enters the heart at the right atrium. A similar, but shorter, blood vessel is seen anterior to the heart. This is the anterior vena cava, or superior vena cava, in man. It returns blood from the upper positions of the body, from the head and forelimbs. It also enters the heart at the right atrium.

Anterior (Superior) Vena Cava — Locate the trunk of this systemic vein above the heart. In the pig the term anterior vena cava is more correct and therefore preferred to superior vena cava. All anterior veins lead into the anterior vena cava.

Posterior (Inferior) Vena Cava — This is the major vein returning blood from the lower extremities and from the abdominal area. Again, this designation in the pig is preferred to inferior vena cava. It can clearly be seen rising from the diaphragm, which it has penetrated, to enter the heart at the right atrium together with the anterior vena cava. 

Innominate (Brachiocephalic) — Follow the anterior vena cava cranially from the heart. It divides into two equal branches, the “V” shaped innominate veins also known as the brachiocephalic veins. Each of these is formed by the union of the subclavian vein from the shoulder and the external jugular vein from the neck.

Jugular Veins — Two jugular veins lie along each side of the neck. The external jugular vein drains the head and neck, while the internal jugular vein drains the brain and spinal cord. They join to form the innominate vein.

Subclavian — This vein is a continuation of the axillary vein in the shoulder joint area. The axillary vein is a continuation of the brachial vein in the arm.

Internal Mammary (Sternal) Vein — On the ventral surface of the anterior vena cava about one inch above the heart the internal mammary or sternal vein arises. It results from the union of the paired internal mammary veins which drain the ventral thoracic body wall and the mammary glands of female pigs.

Subscapular — This vein drains the shoulder region and arises from the underside of the scapula. It enters the subclavian vein close to the base of the external jugular vein.

Cephalic — This vein lies upon the lateral surface of the foreleg and was seen when the pig was skinned. It is a long vein which continues up to the shoulder area and enters the external jugular vein at its base. The cephalic vein and the, deeper vein of the arm, the brachial vein, are joined in the inner elbow by the median cubital vein.

Long Thoracic — This vein arises along the inner surface of the pectoralis minor muscle then passes to the latissimus dorsi. It joins the axillary vein.

Thoracodorsal — This vein arises from under the scapula and similarly joins the axillary vein.

Hemiazygous — Push the left lung and the heart medially. On the dorsal body wall, to the left of the midline, and running parallel to the aorta, observe a vein which enters the anterior vena cava near its entrance into the right ventricle of the heart. This unpaired vessel is the hemiazygous vein. It drains the dorsal body wall and the dorsal musculature. Note that the intercostal veins between the ribs empty into the hemiazygous vein. Dissect the pairs of intercostal veins to observe this relationship. In man, the hemiazygous vein is replaced with the azygous vein which lies on the right side.

Veins - Posterior
The systemic veins below the heart empty into the posterior vena cava. The upper portion of this enlarged blood vessel was observed when we studied the thoracic veins. It passes through the diaphragm and lies along the mid-dorsal body wall of the thorax. It then enters the right atrium of the heart near the entrance of the anterior vena cava.

Posterior Vena Cava — This large blood vessel may be observed by displacing the abdominal viscera toward the left. It lies along the mid-dorsal abdominal body wall. Its major branches will be described in descending order, in a caudal direction.

Phrenic — Within the diaphragm the posterior vena cava receives these veins.

Hepatic — Below the diaphragm region the posterior vena cava passes through the sub­stance of the liver within which it receives the veins which drain the liver, the hepatic veins.

Ductus Venosus — Within the body of liver the inferior vena cava then receives the ductus venosus. This vessel is a continuation of the umbilical vein. The oxygen rich blood from the mother passes to the fetus through the umbilical vein within the umbilical cord. The umbilical vein leads from the cord to the liver where it forms the ductus venosus. In order to see the relationship between these veins it is necessary to remove some of the substance of the liver. Use your forceps or dissecting needles. You will discover a rich network of blood vessels running through the liver. The ductus venosus drains into the posterior vena cava posterior to the hepatic veins.

Renal — Find the renal vein exiting from the medial surface of each kidney and trace it to the posterior vena cava. In some specimens two renal veins drain each kidney.

Adrenal — The adrenal gland is an elongated oval structure lying medial to the anterior portion of the kidney. It is drained by the adrenal vein which empties the renal vein. In some specimens it enters the posterior vena cava separately.

Genital (Utero-Ovarian and Internal Spermatic) — The genital vein is known by different names in males and females. These are very narrow and difficult to locate and are most easily traced from the ovaries or the testes. The right genital vein enters the posterior vena cava next to the posterior end of the right kidney, while the left genital vein generally enters the renal vein.

Lumbar — Posterior to the kidneys seven pairs of veins drain the deep dorsal and lateral abdominal walls. The upper five to six pairs empty into the posterior vena cava, while the last pairs empty into the common iliac veins.

Common Iliac — The venous blood from the pelvic region and from the legs moves anteriorly toward the heart. The right and left common iliac veins serve as receiving vessels for all of this blood. They converge to form the posterior vena cava, forming the shape of an inverted “Y”. All veins below this point are branches of the common iliac veins.

External and Internal Iliac — The common iliac is a relatively short segment of blood vessel. Near its origin it may be seen as formed by two branches. The main, larger and thicker branch extends laterally. This is the external iliac. It extends out toward the legs. A shorter and narrower vessel, the internal iliac (hypogastric) vein extends posteriorly on either side of the midline. It drains the rectum, urinary bladder, and the genital organs.

Circumflex Iliac — This vein is a lateral branch of the external iliac vein. It drains the pelvic muscles.

Femoral— In the thigh the external iliac vein is known as the femoral vein. It receives several branches.

Deep Femoral — This vein drains the deeper muscles of the thigh.

Saphenous — This superficial vein may be seen lying on the medial surface of the thigh and lower leg. Above the knee it joins the femoral vein.

Popliteal — Posterior to the knee the femoral vein is joined by a deeper vein from the lower thigh and shank, the popliteal vein.

Middle Sacral — This, often paired vein, runs along the mid dorsal pelvic wall draining the musculature. It empties into the common iliac veins.

Caudal — This vein, extending into the tail, receives blood from the most posterior structures and the musculature of the tail. Anteriorly it joins the middle sacral vein.

Hepatic Portal System — The veins from most of the abdominal viscera do not join the posterior vena cava directly. Instead, blood from the stomach, pancreas, spleen, small intestine, and large intestine drain into a larger vein known as the hepatic portal vein which enters the liver and there breaks up into capillaries called sinusoids. The hepatic portal vein is rich in digested nutrients. Before these enter the general circulation the liver transforms them according to the needs of the body. Glucose may be stored as glycogen, amino acids deaminated, or fatty acids may be converted to carbohydrates.

In order to expose the hepatic portal vessels move the stomach, spleen, pancreas, small, and large intestines to the left. Frequently they are not injected with blue latex even in injected specimens, therefore, only the largest vessels can be readily identified.

Hepatic Portal Vein — This is the main vein of the hepatic portal system. All of the tributary vessels of the abdominal viscera merge with this single large vein. It enters the liver where it breaks up into smaller vessels then into capillaries. These are drained by the hepatic veins and posterior vena cava.  Some of the major veins joining the hepatic portal are:

Anterior Mesenteric — This vein is a union of branches from the many coils of the small intestines.

Gastroduodenal — This vein drains the pyloric region of the stomach and the duodenum before it joins the hepatic portal vein.

Gastrosplenic — As its name indicates, this vein originates at the stomach and spleen.

Arteries
Arteries are vessels which deliver oxygenated blood away from the heart to the capillaries. The heart pumps blood out through the aorta, which then divides and branches out into smaller arteries to each region of the body.  Consisting of three layers, an outer layer of tissue, a middle layer of muscle that is elastic and strong, expanding when the heart beats filling the artery with blood and contracting when the heart relaxes to push the blood along, and and inner layer of smooth epithelial cells allowing the blood to flow freely. 

The large pulmonary artery is seen leaving the heart from its ventral surface extending toward the left side. More distally the aortic arch leaves the heart and also extends toward the left side. In the fetal pig the pulmonary artery is joined directly to the aortic arch by means of a short vessel, known as the ductus arteriosus. It serves as a bypass to shunt the blood from the lungs into the systemic circulation. This connecting link persists till birth. It then shuts tightly, separating the two major blood vessels. It persists in the adult as only a small tendinous band.

Arteries - Anterior
Remove the major veins surrounding the heart. This will enable you to expose the arteries. Use your dissecting needle to clear arteries of connective tissue, to separate them, and to follow them.

Pulmonary Artery - On the ventral surface of the heart passing dorsally and to the left is the large pulmonary artery. It originates in the right ventricle. Try tracing its two branches to the lungs. The pulmonary is the only artery injected with blue latex to indicate that it carries deoxygenated blood. All other arteries will appear red or pink because they have been injected with red latex to indicate that the blood transported in them is oxygenated.

Aorta — Locate the aorta, the largest systemic artery of the body. It leaves the left ventricle, curves to the left, dorsal to the pulmonary artery, and continues dorsally in a posterior direction along the left side of the vertebral column. The proximal curved portion of aorta is called the aortic arch, while the next segment of the aorta within the thorax is known as the thoracic aorta.

Ductus Arteriosus — In the fetal pig, as well as in all mammalian fetuses, the pulmonary artery is joined directly to the aortic arch by means of a short vessel, the ductus arteriosus. It serves as a bypass to shunt the blood from the lungs toward the systemic circulation. This connecting link is about 1/4 inch long in the older fetal pigs.  The ductus arteriosus persists as a connecting link till birth. It then shuts tightly, separating the pulmonary artery from the aorta and the two major circulatory pathways, the pulmonary from the systemic. It persists in the adult as a narrow tendinous band.

Coronary — Near its origin within the left ventricle of the heart, the aorta gives off its first two branches, the right and left coronary arteries. Branches of these vessels may be seen upon the surface of the heart. These supply blood to the heart muscle (myocardium) directly.

The aortic arch gives rise to arteries that supply the neck, head, shoulders and forelimbs. Whereas in man three arterial trunks arise from the aortic arch, in the pig there are only two. These are the:

  1. Brachiocephalic (or Innominate) — This major artery is the first of the two trunks branching from the aortic arch. As its name indicates, it supplies blood to the forelimb and the head. At the level of the second rib it divides into the:

Right Subclavian — This artery leaves the thorax, gives off major branches to the musculature of the shoulder and thorax then continues as the:

Axillary — This artery is a continuation of the right subclavian in the region of the armpit and shoulder.

Brachial — This is the continuation of the axillary artery in the upper forelimb.

Radial and Ulnar — These are branches of the brachial artery within the lower forelimb.

Bi-Carotid Trunk — This artery is the second branch of the brachiocephalic. It is only a short common pathway for blood to the head. It soon divides to form the:

Right and Left Common Carotids — These two arteries arise from the hi-carotid trunk, pass cranially, parallel to one another on either side of the trachea. Near the head each divides into an internal and external carotid artery.

  1. Left Subclavian — The second branch of the aortic arch is the left subclavian artery. It gives off branches to the musculature of the shoulder, then proceeds into the left forelimb as the left axillary, the left brachial, the left radial, and the left ulnar arteries. The subclavian artery gives off major branches to the musculature of the thorax and shoulder. Starting from its proximal end these include the:

Costo-Cervical Trunk — This is the first vessel to come off the subclavian artery. It originates from the dorsal surface of the subclavian at the level of the first rib. Some of its branches are the:

Vertebral — This is the most anterior branch of the costo-cervical trunk. The artery passes through the transverse foramina (channels) of the cervical vertebrae cranially toward the brain.

Deep Cervical — This artery sends branches to the anterior intercostal muscles.

Dorsal — This small artery is the third branch of the costo-cervical trunk. It supplies blood to lower neck and anterior part of the back regions.

Thyrocervical (or Inferior Cervical) — This comparatively large artery arises from the dorsal surface of the subclavian near the first rib. It supplies the thyroid gland, pectoral muscles, and parotid gland.

External Thoracic (Lateral Thoracic) — This arterial branch of the subclavian supplies blood to both the superficial and deep thoracic muscles.

Internal Mammary (Internal Thoracic) — This artery also originates from the subclavian opposite to the origin of the thyrocervical. It passes posteriorly to supply the pectoral muscles, the skin, the thymus gland, and the mammary gland. It is usually cut when exposing the thoracic cavity.

Subscapular — This artery originates just beyond the subclavian, in the axillary artery. It passes anteriorly to the shoulder region where it supplies the subscapular region with blood.

Thoraco-Dorsal — This artery is a branch of axillary artery. It originates opposite to the subscapular and passes posteriorly. In some specimens it is a branch of the subscapular artery.

We have traced the common carotid artery to the head. Just below the jaw it divides to form the internal carotid artery and the external carotid artery.

Internal Carotid — This artery runs anteriorly into the skull to supply the brain. Near its origin it gives off the occipital artery which passes dorsally to supply the occipital region.

External Carotid — This artery supplies the remainder of the head with blood. Some of its branches include the:

Lingual — This artery supplies the tongue.

External and Internal Maxillary — to the lower and upper jaws, the lips, and mouth

Posterior Auricular — to the ear

Superficial Temporal — found near the temple

Return to the aortic arch Trace its path to the left as it descends mid-dorsally within the thorax. It will be necessary to displace the left lung medially in order to observe this segment of the aorta. It is here known as the thoracic dorsal aorta. Upon closer examination, you will find paired arterial branches originating along the length of the thoracic aorta. These pass into the musculature between the ribs, and are known as the intercostal arteries. While most of them originate in this way directly from the aorta, the upper few originate from the costo-cervical trunk.

Arteries - Posterior

Follow the aortic arch dorsally and caudally through the thorax. Push the left lung toward the right and observe the thoracic aorta along the dorsal body wall to the left of the vertebral column.

Intercostal Arteries — Between each two ribs note the intercostal arteries. They are given off by the thoracic aorta in pairs, to the right and left sides, to supply the intercostal muscles.

Other branches of the thoracic aorta above the diaphragm include the:

Bronchial — These arteries supply the lung tissues with oxygenated blood.

Esophageal — This artery supplies the esophagus.

Phrenic — This artery supplies blood to the diaphragm.

Trace the dorsal aorta as it continues caudally and passes through the diaphragm into the abdominal cavity. It is here known as the abdominal aorta. It gives rise to a number of arteries supplying the organs of the abdominal region. These include the:

Celiac — Immediately posterior to the diaphragm the abdominal aorta gives off ventrally a short unpaired blood vessel, the celiac artery, which branches extensively to supply blood to the organs of the upper abdomen. It divides into two main branches:

  1. Gastro-Splenic — This artery supplies blood to the stomach and to the spleen.

Gastric - This portion of the gastro-splenic artery passes to the underside of the stomach.

Splenic — The second portion of the gastro-splenic passes medially along the length of the spleen to supply blood to that organ.

  1. Gastro-Hepatic - This branch supplies blood to other portions of the stomach and to the liver. It passes through the pancreas to supply blood to that organ. Then it divides to form the:

Hepatic - This artery supplies the liver.

Gastro-Duodenal - This artery supplies the duodenum and stomach.

Anterior Mesenteric — This is the next arterial branch of the abdominal aorta below the celiac artery. It supplies the small intestine and the anterior portion of the large intestine.

Phrenico-Abdominal — This artery comes off the aorta near the anterior end of the kidney. It supplies the diaphragm and the lateral abdominal body wall with blood.

Renal — Each kidney is supplied with blood by the renal artery.

Adrenal — The adrenal gland is supplied by the adrenal arteries which may originate directly from the abdominal aorta or from the renal artery.

Genital — These arteries supply the male and female glands with blood. They are called:

Spermatic — These arteries pass posteriorly to become part of the spermatic cord to the testes within the scrotal sac in males.

Utero-Ovarian — The corresponding artery in females leads to the uterus and ovaries.

Posterior Mesenteric — This unpaired vessel originates from the aorta below the genital artery. It divides to supply blood to the anterior as well as the posterior portions of the large intestines.

Lumbar — Follow the abdominal aorta in the mid-dorsal region of the abdomen. Note six pairs of arteries passing laterally to the dorsal muscle wall. Those are the lumbar arteries.

At the level of the hip the aorta divides into two major vessels, one to each hind leg.

External Iliac — This artery gives off many branches to the organs of the pelvis, the genital and structures and to the hind legs.

 Note: Unlike the venous system, there is no common iliac artery. The external and internal iliac arteries arise as separate branches of the aorta.

Circumflex Iliac — This lateral branch of the external iliac artery supplies blood to the abdominal wall.

Internal Iliac — As already indicated, this artery arises from the aorta at the same level as the external iliac. It is, however, more medially located, thus serving the organs of the pelvis.

Umbilical — The narrow internal iliac arteries soon enlarge as the umbilical arteries. These major prominent features of the fetal circulation carry deoxygenated blood from the developing fetus to its mother. The arteries lie on either side of the elongated urinary bladder before exiting the body. They were already observed when the ventral abdominal wall was removed. After birth these arteries atrophy. Beyond the umbilical arteries, the internal iliacs continue posteriorly within the pelvis.

External Iliac (continued) — Return to the external iliac artery.

Femoral — The segment of the external iliac within the thigh is known as the femoral artery. It supplies the hind leg with blood.

Deep Femoral — This artery is a branch of the femoral artery. It supplies the medial upper region of the hind leg.

Saphenous — Just above the knee joint the femoral artery gives off the saphenous artery which passes medially down the leg.

Popliteal — This segment of the femoral artery originates at the back of the knee cap and passes to the lower portion of the hind leg.

Capillaries
Capillaries, the narrowest blood vessels, are a web-like network connecting oxygen-rich arteries to veins so the blood can be transported back to the lungs and heart. Capillaries are fragile and very thin enabling them to exchange oxygen and carbon dioxide through the thin capillary wall. Red blood cells inside the capillary release their oxygen which passes through the wall and into the surrounding tissue, the tissue then releases waste products such as carbon dioxide, which passes through the wall and into the red blood cells to be transported back to the heart.   Capillaries also aid in the release of the body's heat.