 Dear students, in this topic, we shall discuss the effect of gravity on pressure and flow of blood in the circulatory system. The gravity affects column of blood in large arteries. So, the pressure in limbs and head which are supplied with larger arteries varies with the position of the body with respect to gravity. In the lying down position, when we are lying down, the heart is at the same level as the limbs and head with respect to gravity. So, blood pressure in arteries of head and limbs is equal. When we are standing, the relation between the head, heart and limbs changes with respect to gravity. Heart is quite down the head while heart is about a meter above the limbs. So, arterial pressure in lower limbs increases while it decreases in the head region. Dear students, now we shall discuss the effect of gravity on venous flow. Due to elasticity of vascular system, increase in absolute pressure expands the blood vessels. As the veins are more compliant, so they expand more with increased blood pressure. This results in pooling of blood in the veins in different regions of the body. When blood pressure increases, veins expand and the blood is collected in the veins. This pooling causes difficulty in maintaining capillary flow. This kind of pooling may have serious physiological consequences. For example, if this pooling of blood becomes in the kidneys, it changes the kidney filtration rates. This causes changes in the flow of excretive waste in the blood vessels. The pooling problems are particularly acute in sepices which have long necks and long limbs. For example, giraffe, camels and it was also present in the extinct dinosaurs. Dear students, now we shall discuss the regulation of blood pressure in the distant parts or regions of the body. The pressure generated by the heart is regulated with the position of long neck. When the neck of giraffe or camel is raised, heart pumps blood as at much higher pressure through the aorta. That is at about 195 to 300 mmHg. This high pressure maintains arterial pressure in the brain at the level of about 98 mmHg. The normal blood pressure required for the brain is maintained when a high pressure is generated in the aorta. Because of the distant position of the head due to long neck. However, when such a long neck is lowered to the ground, when the giraffe lowers its neck, the aortic pressure is reduced. Because if the high pressure is generated at that time, the brain will get a lot of high pressure due to which the blood vessels of the brain will go to a dangerous position. The variation in the aortic pressure with the head position leads to extensive pulling of blood when head is raised and decreased flow when head is lowered. In the arterioles, this happens in the arterioles of regions between the head and chest. The region between the head and chest where a lot of high pressure is generated and where a lot of pressure is generated, there is pulling of blood in this region. Pulling is prevented in these regions by vasoconstriction of peripheral vessels when head is raised. When head is lowered, there occurs extensive vasodilation. When head is raised, vasoconstriction is done. When head is lowered, vasodilation is done. This maintains flow of blood despite of lower aortic pressures during lowering of the neck. Dear students, the arterial pressure in the limbs of giraffe becomes higher than the aortic pressures due to the effect of gravity. It also causes pulling. To deal with this pulling, giraffe has large quantities of connective tissue surrounding the blood vessels of the limbs.