The Cardiovascular System — Veterinary Technology / Vet Assistant

Objective. Students trace the path of blood through the heart and pulmonary and systemic circulations, identify heart chambers and great vessels on a cardiac model, and locate auscultation and pulse-palpation sites on at least two species.

The Cardiovascular System: Structure and Function

The cardiovascular system is the body's primary transport network, delivering oxygen and nutrients to every tissue while removing carbon dioxide and metabolic waste. In veterinary practice, understanding this system is essential for monitoring patient vitals, recognizing signs of cardiovascular disease, and assisting the veterinarian with cardiac workups.

The Heart: Chambers and Valves

The mammalian heart is a four-chambered muscular pump divided into right and left sides by a wall called the interventricular septum. Each side has an upper receiving chamber (atrium) and a lower pumping chamber (ventricle).

Right atrium (RA): Receives oxygen-poor blood from the body via the cranial and caudal vena cava.
Right ventricle (RV): Pumps that blood to the lungs through the pulmonary artery.
Left atrium (LA): Receives oxygen-rich blood returning from the lungs through the pulmonary veins.
Left ventricle (LV): Pumps oxygenated blood to the entire body through the aorta. Its walls are the thickest because it generates the greatest pressure.

Four valves prevent backflow and keep blood moving in one direction:

| Valve | Location | Prevents backflow from… |

|---|---|---|

| Tricuspid (right AV) | Between RA and RV | RV into RA |

| Pulmonic | Between RV and pulmonary artery | Pulmonary artery into RV |

| Mitral (left AV, bicuspid) | Between LA and LV | LV into LA |

| Aortic | Between LV and aorta | Aorta into LV |

The tricuspid and mitral valves are atrioventricular (AV) valves held in place by chordae tendineae and papillary muscles. The pulmonic and aortic valves are semilunar valves.

The Two Circuits: Pulmonary and Systemic

Blood travels through two continuous loops:

Pulmonary circulation carries deoxygenated blood from the right side of the heart to the lungs and returns oxygenated blood to the left side. The path is: RA → tricuspid valve → RV → pulmonic valve → pulmonary artery → lung capillaries (gas exchange) → pulmonary veins → LA.

Systemic circulation carries oxygenated blood from the left side of the heart to every organ and tissue and returns deoxygenated blood to the right side. The path is: LA → mitral valve → LV → aortic valve → aorta → body capillaries (gas exchange) → cranial and caudal vena cava → RA.

A helpful memory rule: blue blood goes right, red blood goes left. This matches standard diagram coloring (blue = deoxygenated, red = oxygenated).

The Great Vessels

The aorta is the largest artery in the body, exiting the LV and arching (aortic arch) before descending through the thorax and abdomen. The pulmonary artery (pulmonary trunk) exits the RV and splits into left and right branches, one to each lung. The vena cava has two segments: the cranial vena cava (from the head, neck, and forelimbs) and the caudal vena cava (from the abdomen and hindlimbs). The pulmonary veins (four in dogs) carry oxygenated blood from lung lobes to the LA.

Clinical Connection: Mitral Valve Disease

Mitral valve disease (MVD) is the most common acquired cardiac condition in dogs, particularly prevalent in small breeds such as the Cavalier King Charles Spaniel. When the mitral valve degenerates, its leaflets do not close completely, allowing blood to leak backward from the LV into the LA (mitral regurgitation). This is heard as a left-sided heart murmur on auscultation. Over time, the increased volume load can lead to left-sided congestive heart failure and pulmonary edema. Recognizing the anatomy of the mitral valve directly explains why patients present with coughing and respiratory distress.

Auscultation and Pulse Sites

Cardiac auscultation in dogs is performed with the stethoscope placed against the chest wall at specific intercostal spaces (the spaces between ribs). The left cardiac auscultation field (approximately the 3rd–5th intercostal spaces, lower third of the thorax) allows best assessment of the mitral and aortic valves. The right cardiac auscultation field (approximately the 3rd–4th intercostal spaces) is optimal for tricuspid valve sounds. Always listen with the patient in a calm, quiet state and note rate, rhythm, and presence of murmurs.

Three important pulse sites used in veterinary patients:

Femoral pulse: Located on the medial thigh where the femoral artery runs; the most commonly used site in dogs and cats.
Dorsal pedal pulse: Located on the dorsal surface of the hind paw; useful when femoral access is difficult.
Lingual pulse: Located on the underside of the tongue; used during anesthesia when limb pulses may be hard to access.

Safety Points

When working with a live patient, always obtain instructor approval and use proper animal restraint before palpating pulses or applying a stethoscope. An improperly restrained animal can injure itself or students.
Never apply excessive pressure when palpating a pulse; moderate, gentle finger pressure is sufficient.
Stethoscopes should be cleaned between patients according to your facility's infection-control protocol to prevent disease transmission.
On a patient simulator, treat it with the same care as a live animal to build correct habits before clinical placement.
Do not attempt to perform cardiac assessment beyond your scope as a veterinary assistant or technician student; relay all findings promptly to the supervising veterinarian.

Key terms9

Pulmonary circulationThe circuit that carries deoxygenated blood from the right ventricle to the lungs for gas exchange, then returns oxygenated blood to the left atrium via the pulmonary veins.

Systemic circulationThe circuit that carries oxygenated blood from the left ventricle through the aorta to all body tissues, then returns deoxygenated blood to the right atrium via the vena cava.

Mitral valve (bicuspid valve)The atrioventricular valve located between the left atrium and left ventricle; prevents backflow of blood from the ventricle into the atrium during contraction.

AortaThe largest artery in the body; exits the left ventricle and distributes oxygenated blood to the systemic circulation.

Vena cavaThe large veins (cranial and caudal) that return deoxygenated blood from the body to the right atrium.

Semilunar valvesThe pulmonic and aortic valves, located at the exits of the right and left ventricles respectively; their half-moon-shaped cusps prevent backflow from the great vessels into the ventricles.

Femoral pulseA peripheral pulse palpated on the medial thigh over the femoral artery; the primary pulse-check site in dogs and cats.

Intercostal spaceThe space between two adjacent ribs; cardiac auscultation points are described by their intercostal space location on the left or right thorax.

Chordae tendineaeFibrous cords that anchor the leaflets of the atrioventricular valves to the papillary muscles of the ventricular wall, preventing valve inversion during contraction.

In class: Color-coded flow diagram exercise: students use red and blue markers to draw the complete circuit of blood from the right atrium through pulmonary and systemic loops back to the left ventricle, labeling all four chambers, four heart valves, aorta, pulmonary artery, vena cava, and pulmonary veins. · Cardiac model lab: students open a canine heart model, identify each labeled structure, and describe in one sentence what happens if the mitral valve is diseased, connecting anatomy to clinical importance. · Pulse and auscultation site practice on a live animal (canine patient simulator or supervised dog): students locate the femoral pulse, lingual pulse, and dorsal pedal pulse, and place a stethoscope at the correct intercostal spaces for the left and right cardiac auscultation fields, guided by an instructor checklist.
Materials: Canine cardiac model (dissectible), color-coded flow diagram worksheet, stethoscopes (one per student), canine patient simulator or live patient under direct supervision, auscultation site anatomical chart, Colville & Bassert textbook
Check: Skills station: student traces blood flow verbally while pointing to structures on the model (all major structures named correctly) and successfully locates femoral and dorsal pedal pulses on a simulator; evaluator checklist, pass requires all structures named and both pulses located within 30 seconds each.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.