Kristine Hovkjær Østergaard

Jugular venous pooling during lowering of the head affects blood pressure of the anesthetized giraffe

How blood flow and pressure to the giraffes brain are regulated when drinking remains debated. We measured simultaneous blood flow, pressure, and cross-sectional area in the carotid artery and jugular vein of five anesthetized and spontaneously breathing giraffes. The giraffes were suspended in the upright position so that we could lower the head. In the upright position, mean arterial pressure (MAP) was 193 +/- 11 mmHg (mean +/- SE), carotid flow was 0.7 +/- 0.2 l/min, and carotid cross-sectional area was 0.85 +/- 0.04 cm(2). Central venous pressure (CVP) was 4 +/- 2 mmHg, jugular flow was 0.7 +/- 0.2 l/min, and jugular cross-sectional area was 0.14 +/- 0.04 cm(2) (n = 4). Carotid arterial and jugular venous pressures at head level were 118 +/- 9 and -7 +/- 4 mmHg, respectively. When the head was lowered, MAP decreased to 131 +/- 13 mmHg, while carotid cross-sectional area and flow remained unchanged. Cardiac output was reduced by 30%, CVP decreased to -1 +/- 2 mmHg (P < 0.01), and jugular flow ceased as the jugular cross-sectional area increased to 3.2 +/- 0.6 cm(2) (P < 0.01), corresponding to accumulation of approximately 1.2 l of blood in the veins. When the head was raised, the jugular veins collapsed and blood was returned to the central circulation, and CVP and cardiac output were restored. The results demonstrate that in the upright-positioned, anesthetized giraffe cerebral blood flow is governed by arterial pressure without support of a siphon mechanism and that when the head is lowered, blood accumulates in the vein, affecting MAP.

Protection against high intravascular pressure in giraffe legs

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.

The thick left ventricular wall of the giraffe heart normalises wall tension, but limits stroke volume and cardiac output

ABSTRACT 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 300 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±38 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±1.4 l min−1, respectively. End-diastolic and end-systolic volumes were 521±61 ml and 228±42 ml, respectively, yielding an ejection fraction of 56±4% and a stroke volume of 0.59 ml kg−1. Left ventricular circumferential wall stress was 7.83±1.76 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. Summary: A left ventricular cavity and low stroke volume characterise the giraffe heart, resulting in typical mammalian left ventricular wall tensions but lowered cardiac output.

The giraffe kidney tolerates high arterial blood pressure by high renal interstitial pressure and low glomerular filtration rate

The tallest animal on earth, the giraffe (Giraffa camelopardalis) is endowed with a mean arterial blood pressure (MAP) twice that of other mammals. The kidneys reside at heart level and show no sign of hypertension‐related damage. We hypothesized that a species‐specific evolutionary adaption in the giraffe kidney allows normal for size renal haemodynamics and glomerular filtration rate (GFR) despite a MAP double that of other mammals.

Left Ventricular Morphology of the Giraffe Heart Examined by Stereological Methods

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 × 103 mm−3 in the adult and 504 × 103 mm−3 in the young LV. The total number (N) of myocyte nuclei was 1.3 × 1011 in the adult LV and 4.9 × 1010 in the young LV. In the adult LV the volume per myocyte was 39.5 × 103 µm3 and the number of nuclei per myocyte was 4.2. The numerical density of myocytes was 24.1 × 106 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. Anat Rec, 296:611–621, 2013.

Monodontella giraffae Infection in Wild-caught Southern Giraffes (Giraffa camelopardalis giraffa)

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.

Non-traditional Models: The Giraffe Kidney from a Comparative and Evolutionary Biology Perspective

Giraffes are the tallest living animals and endowed with the highest arterial blood pressure of any animal on Earth. Here we present novel data on kidney function in this extraordinary animal obtained over the course of two major expeditions in 2010 and 2012. As expected, the anaesthetised giraffes had very high mean arterial blood pressure ranging between 150 and 300 mmHg. However, despite the high filtration pressure, the rate of glomerular filtration (GFR) was only 0.7 ± 0.2 ml/min/kg, which is approximately 40 % below similar-sized mammals. The renal blood flow of 3.1 l/min accounts for approximately 20 % of cardiac output, and the calculated filtration fraction (GFR/ERPF) was approximately 0.3, and hence within typical mammalian values. The normal kidney function of the giraffes appears due to very high interstitial pressures within the kidney, a feature that is possible due to the very thick and strong capsule surrounding the kidney in combination with a vascular valve at the entrance of the renal vein into the abdominal cava. These relatively simple structural modifications normalize the Starling forces driving filtration over the Bowman capsula.