Where the organs sit (lateral view)
Thorax: the heart sits between thoracic vertebrae 3 and 8, with the middle of its base at the level of the carina (intercostal space 5-6) and its apex on the sternum; on the lateral view it spans 2.5-3.5 intercostal spaces. The left lung has a cranial lobe (in two parts) and a caudal lobe, the right has four lobes; the thymus, bilobed, sits in front of the heart and shrinks with age.
Abdomen: the liver sits entirely within the costal arch, against the diaphragm; the stomach comes immediately caudal to it, on the left, with its axis perpendicular to the spine and the pylorus slightly to the right; the spleen follows the greater curvature along the left wall. The right kidney sits at roughly T13-L2, the left at L1-L3, retroperitoneally; the adrenal glands cranial and medial to them. The descending duodenum, the cecum and the ascending colon are on the right; the descending colon runs down the left side to the rectum.
Pelvis: the urinary bladder sits in the caudal abdomen, toward the pelvic inlet, pulled cranially by the weight of the urine; in the male, the bilobed prostate surrounds the urethra immediately after the neck of the bladder; in the female, the ovaries sit caudal to the kidneys, and the uterine horns run down toward the cervix, dorsal to the bladder.
Digestive
It begins with the mouth and includes the esophagus, stomach, liver, pancreas, intestines, rectum and anus. The teeth are its visible part; see the page on teeth.
Respiratory
The large and small airways and the lungs; gas exchange takes place in the alveoli. The upper airways, the nose first of all, also serve smell and cooling by panting.
Cardiovascular
The heart and the blood vessels (arteries and veins). The heart has four chambers; the right side pumps blood to the lungs, the left side to the rest of the body.
Nervous
The brain, the spinal cord and the nerves, plus the sense organs. It coordinates the whole body and allows the dog to perceive and respond to stimuli.
Urinary
The kidneys, ureters, urinary bladder and urethra: they filter the blood and eliminate waste as urine.
Reproductive
In the male: the testes and the genital tract. In the female: the ovaries, the oviducts (uterine tubes) and the uterus. These are two distinct anatomies, not sets belonging to the same animal; both are hormonally controlled by the pituitary and the hypothalamus. Puberty begins between 6 and 24 months, earlier in small breeds.
Endocrine
Tissues that release hormones into the blood, for example the thyroid and the pituitary; hormones regulate metabolism, growth and reproductive function.
Immune and lymphatic
The thymus and the bone marrow produce white blood cells; the spleen and the lymph nodes trap microorganisms. The skin and coat are the first barrier.