RESUMEN
Giraffes--the tallest extant animals on Earth--are renowned for their high central arterial blood pressure, which is necessary to secure brain perfusion. Arterial pressure may exceed 300â mmHg and has historically been attributed to an exceptionally large heart. Recently, this has been refuted by several studies demonstrating that the mass of giraffe heart is similar to that of other mammals when expressed relative to body mass. It thus remains unexplained how the normal-sized giraffe heart generates such massive arterial pressures. We hypothesized that giraffe hearts have a small intraventricular cavity and a relatively thick ventricular wall, allowing for generation of high arterial pressures at normal left ventricular wall tension. In nine anaesthetized giraffes (495±38â kg), we determined in vivo ventricular dimensions using echocardiography along with intraventricular and aortic pressures to calculate left ventricular wall stress. Cardiac output was also determined by inert gas rebreathing to provide an additional and independent estimate of stroke volume. Echocardiography and inert gas-rebreathing yielded similar cardiac outputs of 16.1±2.5 and 16.4±1.4â lâ min(-1), respectively. End-diastolic and end-systolic volumes were 521±61â ml and 228±42â ml, respectively, yielding an ejection fraction of 56±4% and a stroke volume of 0.59â mlâ kg(-1). Left ventricular circumferential wall stress was 7.83±1.76â kPa. We conclude that, relative to body mass, a small left ventricular cavity and a low stroke volume characterizes the giraffe heart. The adaptations result in typical mammalian left ventricular wall tensions, but produce a lowered cardiac output.
Asunto(s)
Gasto Cardíaco , Jirafas/fisiología , Volumen Sistólico , Función Ventricular , Animales , Presión Sanguínea , Ecocardiografía/veterinaria , MasculinoRESUMEN
The high blood pressure in giraffe leg arteries renders giraffes vulnerable to edema. We investigated in 11 giraffes whether large and small arteries in the legs and the tight fascia protect leg capillaries. Ultrasound imaging of foreleg arteries in anesthetized giraffes and ex vivo examination revealed abrupt thickening of the arterial wall and a reduction of its internal diameter just below the elbow. At and distal to this narrowing, the artery constricted spontaneously and in response to norepinephrine and intravascular pressure recordings revealed a dynamic, viscous pressure drop along the artery. Histology of the isolated median artery confirmed dense sympathetic innervation at the narrowing. Structure and contractility of small arteries from muscular beds in the leg and neck were compared. The arteries from the legs demonstrated an increased media thickness-to-lumen diameter ratio, increased media volume, and increased numbers of smooth muscle cells per segment length and furthermore, they contracted more strongly than arteries from the neck (500 ± 49 vs. 318 ± 43 mmHg; n = 6 legs and neck, respectively). Finally, the transient increase in interstitial fluid pressure following injection of saline was 5.5 ± 1.7 times larger (n = 8) in the leg than in the neck. We conclude that 1) tissue compliance in the legs is low; 2) large arteries of the legs function as resistance arteries; and 3) structural adaptation of small muscle arteries allows them to develop an extraordinary tension. All three findings can contribute to protection of the capillaries in giraffe legs from a high arterial pressure.
Asunto(s)
Presión Arterial , Arteria Braquial/fisiopatología , Capilares/fisiopatología , Edema/prevención & control , Extremidad Inferior/irrigación sanguínea , Rumiantes , Arterias Tibiales/fisiopatología , Adaptación Fisiológica , Animales , Arteria Braquial/diagnóstico por imagen , Arteria Braquial/patología , Permeabilidad Capilar , Edema/fisiopatología , Presión Hidrostática , Masculino , Arterias Tibiales/diagnóstico por imagen , Arterias Tibiales/patología , Factores de Tiempo , Ultrasonografía , Resistencia Vascular , VasoconstricciónRESUMEN
The giraffe heart has a relative mass similar to other mammals, but generates twice the blood pressure to overcome the gravitational challenge of perfusing the cerebral circulation. To provide insight as to how the giraffe left ventricle (LV) is structurally adapted to tackle such a high afterload, we performed a quantitative structural study of the LV myocardium in young and adult giraffe hearts. Tissue samples were collected from young and adult giraffe LV. Design-based stereology was used to obtain unbiased estimates of numbers and sizes of cardiomyocytes, nuclei and capillaries. The numerical density of myocyte nuclei was 120 × 10(3) mm(-3) in the adult and 504 × 10(3) mm(-3) in the young LV. The total number (N) of myocyte nuclei was 1.3 × 10(11) in the adult LV and 4.9 × 10(10) in the young LV. In the adult LV the volume per myocyte was 39.5 × 10(3) µm(3) and the number of nuclei per myocyte was 4.2. The numerical density of myocytes was 24.1 × 10(6) cm(-3) and the capillary volume fraction of the adult giraffe ventricle was 0.054. The significantly higher total number of myocyte nuclei in the adult LV, the high density of myocyte nuclei in the LV, and the number of nuclei per myocyte (which was unusually high compared to other mammalian, including human data), all suggest the presence of myocyte proliferation during growth of the animal to increase wall thickness and normalize LV wall tension as the neck lengthens and the need for higher blood pressure ensues.
Asunto(s)
Artiodáctilos/anatomía & histología , Ventrículos Cardíacos/anatomía & histología , Adaptación Fisiológica , Factores de Edad , Animales , Artiodáctilos/fisiología , Capilares/anatomía & histología , Recuento de Células , Núcleo Celular , Proliferación Celular , Tamaño de la Célula , Circulación Cerebrovascular , Femenino , Ventrículos Cardíacos/citología , Hemodinámica , Masculino , Miocitos Cardíacos/citología , Función Ventricular IzquierdaRESUMEN
Giraffes are the tallest animals on earth and the effects of gravity on their cardiovascular system have puzzled physiologists for centuries. The authors measured arterial and venous pressure in the foreleg of anesthetized giraffes, suspended in upright standing position, and determined the ratio between tunica media and lumen areas along the length of the femoral/tibial arteries in the hindleg. Volume fraction of elastin, density of vasa vasorum and innervations was estimated by stereology. Immunohistological staining with S100 was used to examine the innervation. The pressure increase in the artery and vein along the foreleg was not significantly different from what was expected on basis of gravity. The area of the arterial lumen in the hindleg decreased towards the hoof from 11.2 ± 4.2 to 0.6 ± 0.5 mm(2) (n = 10, P = 0.001), but most of this narrowing occurred within 2-4 cm immediately below the knee. This abrupt narrowing was associated with a marked increase in media to lumen area ratio (from 1.2 ± 0.5 to 7.8 ± 2.5; P = 0.001), and a decrease in mean volume fraction of elastin from 38 ± 6% proximal to the narrowing to 5.8 ± 1.1% distally (P = 0.001). The narrowing had a six-fold higher innervation density than the immediate distal and proximal regions. The sudden narrowing was also observed in the hind legs of neonates, indicating that it does not develop as an adaptation to the high transmural pressure in the standing giraffe. More likely it represents a preadaptation to the high pressures experienced by adult giraffes.
Asunto(s)
Presión Sanguínea/fisiología , Miembro Anterior/irrigación sanguínea , Miembro Posterior/irrigación sanguínea , Rumiantes/fisiología , Animales , Arterias , Elastina/análisis , Femenino , Arteria Femoral/anatomía & histología , Arteria Femoral/fisiología , Miembro Posterior/inervación , Masculino , Arterias Tibiales/anatomía & histología , Arterias Tibiales/fisiología , Túnica Media/anatomía & histología , Túnica Media/químicaRESUMEN
Postmortem examination of seven wild-caught southern giraffes (Giraffa camelopardalis giraffa) from Namibia demonstrated focal discoloration, biliary thickening, and peribiliary fibrosis affecting mainly the left liver lobe. The giraffes were infected with Monodontella giraffae, previously associated with lethal infections in captive okapis (Okapia johnstoni) and giraffes. Contrary to this, all seven giraffes investigated in the present study were clinically healthy. Based on these findings, it is suggested that the nematode M. giraffae may not be an unusual parasite of the giraffe and that it does not necessarily cause detrimental liver disease.