Heart and blood vessels

• Arteries carry oxygenated blood away from the heart to the body.
• Arteries transport blood away from the heart
• Arterioles are smaller than arteries and lead to capillaries
• Capillaries are the smallest vessels that allow the exchange of substances with body cells
• Veins transport blood to the heart
• Venules are smaller than veins and lead away from capillaries
• Arterioles require a thick layer of smooth muscle to regulate blood flow in vasoconstriction/dilation
• Arteries have a thick layer of smooth muscle to withstand high blood pressure and elastic fibers for elastic recoil to maintain high blood pressure
• The outer coat of arteries has collagen fibers to resist overstretching
• Arterioles are able to adjust their diameter to vary the blood supply to a capillary bed
• Veins have wider diameter lumens and thinner walls than arteries due to lower pressure and flow inside
• The thin muscle layer in vein walls can be compressed easily, allowing contracting skeletal muscle to push blood along
• Pocket valves in veins ensure that blood flows in one direction towards the heart
• Veins also have valves to prevent backflow of blood
• Contraction of skeletal muscles helps return blood to the heart
• Capillary walls consist only of endothelium and are one cell thick
• Capillaries are permeable to water and dissolved substances such as glucose, amino acids, urea, oxygen, and carbon dioxide
• Capillaries have a narrow lumen to restrict blood flow and allow more time for exchange
• Capillaries function to take blood as close as possible to all cells, allowing exchange of materials between the blood and tissues
• Capillary walls are made of endothelium, one cell thick, and porous, making them permeable to water and dissolved substances
• The narrow lumen of capillaries slows down the flow of blood to allow more time for exchange
• Arteries have a smaller lumen, while veins have a larger lumen to reduce resistance to flow
• Capillaries have a very small lumen where blood cells must distort to pass through
• Arteries do not have valves (except in the heart), veins have many valves to prevent backflow, and capillaries have no valves
• Arteries have high blood pressure, veins have low pressure that falls as the velocity of blood decreases
• Arteries usually carry oxygenated blood (except pulmonary arteries), veins usually carry deoxygenated blood (except pulmonary veins)
• Blood changes from oxygenated to deoxygenated in capillaries except in the lungs
• Arteries carry blood away from the heart, contain a low concentration of oxygen, carry oxygenated blood, and are named aorta
• Veins carry blood away from the heart, contain a high concentration of oxygen, carry deoxygenated blood, and are named vena cava
• Capillaries are adapted for the exchange between blood and body cells by being:

  • Thin – one cell thick, with permeable walls
  • Having a large surface area for the exchange of materials
  • Allowing blood to flow slowly through them, which permits time for the exchange of materials
• At any capillary bed, fluid escapes through gaps between cells in the capillary walls, forming tissue fluid
• Tissue fluid bathes the cells and allows the exchange of molecules:

  • Molecules required by cells like glucose, amino acids, and oxygen diffuse into cells from tissue fluids
  • Waste substances like urea and CO2 diffuse out of cells into tissue fluid
  • Newly formed tissue fluid is plasma minus plasma proteins
• Net filtration pressure at the arterial end of capillaries:

  • Fluid tends to flow out of the capillaries at the arterial end
  • Hydrostatic pressure is 33 mm Hg, and osmotic pressure is -20 mm Hg
• At the arterial end of capillaries, blood under pressure due to the heart's pumping action creates a high hydrostatic pressure that forces fluid through capillary walls. The outward flow is opposed by osmotic pressure, but as hydrostatic pressure is greater, there is a net flow of fluid out of the blood. Molecules dissolved in tissue fluid then diffuse into cells
• Net filtration pressure at the venous end of capillaries:

  • Fluid tends to flow from tissue fluid into the capillaries at the venous end
  • Hydrostatic pressure is 13 mm Hg, and osmotic pressure is -20 mm Hg
• At the venous end of capillaries, tissue fluid contains waste substances that have diffused out of cells. These can diffuse back into the blood in the capillaries down a concentration gradient. There is a lower hydrostatic pressure due to friction and lower volume of fluid in the capillaries. Osmotic pressure is now greater than hydrostatic pressure, causing a net movement of water back into the blood in the capillaries
• Approximately 90% of tissue fluid flows back into capillaries, while the excess 10% drains into the lymphatic system to form lymph, which eventually returns to the venous system via the thoracic duct
• The lymphatic system is involved in:

  • The absorption of lipids in the small intestine
  • The formation of lymphocytes and the prevention of disease
• The heart is located in the chest between the lungs behind the sternum and above the diaphragm
• The size of the heart is about that of a fist, and its weight is about 250-300 g