**2. Vascular control of blood flow for regulating core body temperature**

Blood flow is necessary for effective thermoregulation in all mammals. Core body temperature is maintained and stabilized through constriction or dilation of vasculature to control blood flow in peripheral tissues for the regulation of heat dissipation from capillary beds in the skin (Chotani et al., 2000; Johnson & Kellogg, 2010). Excessive declines in core body temperature during cold ambient temperatures are controlled by vasoconstriction of blood flow that reduces heat transfer in the skin, whereas increases in core body temperature during warm ambient temperatures are mitigated through vasodilation to increase heat transfer (Chotani et al., 2000). During heat stress, there is decreased activity by sympathetic vasoconstrictor nerves and increased activity by a sympathetic cutaneous vasodialator nervous system (Johnson and Proppe, 1996). Expansion of the luminal area of blood vessels delivering blood flow to the skin during heat stress also provides the volume of water needed to drive sweating for evaporative cooling of the skin (Gagge and Gonzalez, 1996).

Blood flow rate is a function of cross-sectional luminal area of the vessel and flow velocity, with vessel luminal area having the greatest influence on flow rate. Based on Poiseuille's Law, volume flow rate (mL/min.) is directly proportionate to the fourth power of the radius of the cross-section of the vessel's lumen. Carter (2000) noted that a 10% decrease in radius of a tube under steady state conditions will reduce flow rate by approximately 35%. Constriction and dialation of arteries and veins are mediated in response to hot or cold

Reduced blood flows in cattle and sheep consuming endophyte-infected tall fescue was first reported by Rhodes et al. (1991) who used radioisotope labeled microspheres to estimate flow rates between those consuming endophyte-infected and endphyte-free tall fescue. Vascular dysfunction is also evidenced by in vitro findings of ergovaline induced constriction of bovine lateral saphenous veins (Klotz et al., 2006, 2007), uterine and umbilical arteries (Dyer, 1993), and rat tail and guinea pig iliac arteries (Schoning, et al., 2001). *Caviceps* spp. do not produce ergovaline, but can produce high quantities of ergotamine. Blaney et al. (2009) indentified ergotamine as the dominant ergopeptine produced by *Claviceps purpurea* sclerotia, which is the ergot that infects Australian rye. Blaney et al. (2011) reported severe hyperthermia in steers fed feedyard rations containing sorghum infected with Claviceps

Color Doppler ultrasonography has been used with humans as a noninvasive technique for real time diagnoses of aberrant blood flow (Someda et al, 1995; Whelan and Barry, 1992), stenosis (Hatle et al., 1980; Olin et al., 1995; Schmidt et al., 1997), and occlusions (Moneta et al., 1992; Müller et al., 1995). It also has been used to determine vasoconstrictive and blood flow responses in cattle (Aiken et al., 2007; 2009b) and sheep (Aiken et al., 2011) to ergot alkaloids, and was performed with steers to quantify artery lumen area and blood flow responses to heat and cold challenges (Kirch et al., 2008). Color doppler ultrasonography has potential use as a diagnostic or research tool in identifying and quantifying aberrant constriction caused by ergot alkaloids or other toxicants. This chapter will discuss: 1) blood flow aspects of thermoregulation in livestock, 2) effects of ergot alkaloids on blood flow and thermoregulation and, 3) procedures and sources of error in using Doppler ultrasonography as a research tool in evaluating vasoconstrictive responses in livestock exposed to ergot

**2. Vascular control of blood flow for regulating core body temperature** 

cooling of the skin (Gagge and Gonzalez, 1996).

Blood flow is necessary for effective thermoregulation in all mammals. Core body temperature is maintained and stabilized through constriction or dilation of vasculature to control blood flow in peripheral tissues for the regulation of heat dissipation from capillary beds in the skin (Chotani et al., 2000; Johnson & Kellogg, 2010). Excessive declines in core body temperature during cold ambient temperatures are controlled by vasoconstriction of blood flow that reduces heat transfer in the skin, whereas increases in core body temperature during warm ambient temperatures are mitigated through vasodilation to increase heat transfer (Chotani et al., 2000). During heat stress, there is decreased activity by sympathetic vasoconstrictor nerves and increased activity by a sympathetic cutaneous vasodialator nervous system (Johnson and Proppe, 1996). Expansion of the luminal area of blood vessels delivering blood flow to the skin during heat stress also provides the volume of water needed to drive sweating for evaporative

Blood flow rate is a function of cross-sectional luminal area of the vessel and flow velocity, with vessel luminal area having the greatest influence on flow rate. Based on Poiseuille's Law, volume flow rate (mL/min.) is directly proportionate to the fourth power of the radius of the cross-section of the vessel's lumen. Carter (2000) noted that a 10% decrease in radius of a tube under steady state conditions will reduce flow rate by approximately 35%. Constriction and dialation of arteries and veins are mediated in response to hot or cold

Africana, as compared to steers consuming non-infected sorghum.

alkaloids.

environments by endogenous biogenic amines: primarily serotonin, norepinephrine, and epinephrine (Johnson and Proppe, 1996; Strickland et al., 2011).
