Cardiovascular system: Blood vessels, blood flow. Latha Rajendra Kumar, MD

Cardiovascular system: Blood vessels, blood flow Latha Rajendra Kumar, MD

Outline 1- Physical laws governing blood flow and blood pressure 2- Overview of vasculature 3- Arteries 4. Capillaries and venules 5. Veins 6. Lymphatic circulation 7. Mean arterial pressure and its regulation 8. Other cardiovascular regulatory processes

Physical laws governing blood flow and blood pressure Flow of blood through out body = pressure gradient within vessels X resistance to flow - Pressure gradient: aortic pressure central venous pressure - Resistance: -- vessel radius -- vessel length -- blood viscosity

Factors promoting total peripheral resistance (TPR) Total peripheral resistance = TPR -- combined resistance of all vessels -- vasodilation resistance decreases -- vasoconstriction resistance increases

Vasculature

Arteries and blood pressure Pressure reservoir Arterial walls are able to expand and recoil because of the pressure of elastic fibers in the arterial wall Systolic pressure: maximum pressure occurring during systole Diastolic pressure: pressure during diastole

Arterial blood pressure Figure 14.8

Blood pressure values: what do they mean? Pulse pressure: PP = SP-DP Mean arterial blood pressure = MABP MABP = SBP + (2XDBP) 3 CO = MABP = SV x HR TPR

Blood flow within each organ changes with body activities Reminder: The ANS controls blood flow to the various organs Figure 14.15

Capillaries Allow exchange of gases, nutrients and wastes between blood and tissues Overall large surface area and low blood flow Two main types: - continuous capillaries: narrow space between cells permeable to small or lipid soluble molecules - fenestrated capillaries: large pores between cells large molecules can pass

Local control of blood flow in capillaries Presence of precapillary sphincters on the arteriole and beginning of capillaries Metarteriole: no sphincter continuous blood flow controls the amount of blood going to neighboring vessels

Movement of materials across capillary walls Small molecules and lipid soluble molecules move by diffusion through the cell membrane Larger molecules, charged molecules must pass through membrane channels, exocytosis or in between 2 cells Water movement is controlled by the capillary hydrostatic and osmotic pressures

Forces controlling water movement Arterial side of the capillary: High capillary hydrostatic pressure (BHP), lower capillary osmotic pressure (BOP, due to proteins and other molecules in the blood) Net filtration pressure pushes fluid from the blood toward the tissue (but the proteins remain in the capillary Venous side of the capillary: - Lower hydrostatic pressure (due to resistance) and higher capillary osmotic pressure Net filtration pressure moves fluid back toward the capillary Interstitial fluid hydrostatic (IFHP) and osmotic pressures (IFOP) remain overall identical

Fluid movement in the capillary Arteriole side: fluid moves toward the tissues Venous side: fluid reenters the capillary Overall: for every 1 liter of fluid entering the tissues, only 0.85 l reenter the capillary The remaining 0.15 l is reabsorbed as lymph by lymphatic capillaries and eventually returned back to blood circulation When this system fails: Edema

Causes of edema Increased hydrostatic blood pressure - heart failure (left or right), - excess fluid in the blood Decreased blood osmotic pressure Liver, kidney diseases, malnutrition (kwashiorkor), burn injuries Increased interstitial hydrostatic pressure (lymphatic capillary blockage) - breast cancer surgery, elephantiasis Leaking capillary wall - histamine release during allergic reaction

Veins Veins are blood volume reservoir Due to thinness of vessel wall less resistance to stretch = more compliance

Factors influencing venous return 1- Skeletal muscle pump and valves 2- Respiratory pump 3- Blood volume 4- Venomotor tone

Lymphatic circulation Driven by factors similar to venous circulation: - muscle activity - valves - respiration Lymph = plasma-proteins Lymphatic circulation collects fluid not reabsorbed by the capillaries Lymph is filtered in nodes before return to blood circulation

Mean arterial pressure and its regulation Regulation of blood flow in arteries - Intrinsic control - Extrinsic control -- Neural control -- Hormonal control * Control of blood vessel radius * Control of blood volume

Regulation of blood flow in arteries It is important to adjust blood flow to organ needs Flow of blood to particular organ can be regulated by varying resistance to flow (or blood vessel diameter) Vasoconstriction of blood vessel smooth muscle is controlled both by the ANS and at the local level. Four factors control arterial flow at the organ level: - change in metabolic activity - changes in blood flow - stretch of arterial smooth muscle - local chemical messengers

Intrinsic control of local arterial blood flow Change in metabolic activity Usually linked to CO2 and O2 levels ( CO2 vasodilation blood flow) intrinsic control Changes in blood flow - decreased blood flow increased metabolic wastes vasodilation Stretch of arterial wall = myogenic response - Stretch of arterial wall due to increased pressure reflex constriction Locally secreted chemicals can promote vasoconstriction or most commonly vasodilation - inflammatory chemicals, (nitric oxide, CO2)

Extrinsic control of blood pressure Two ways to control BP: - Neural control - Hormonal control ** Use negative feedback

Control of blood pressure Importance: Blood pressure is a key factor for providing blood (thus oxygen and energy) to organs. SBP must be a minimum of 70 to sustain kidney filtration and adequate blood flow to the brain CO= HR X SV = MABP/TPR MABP= HRxSVxTPR heart rate, stroke volume and peripheral resistance affect MABP Main factors controlling BP: - Blood volume - Blood vessel radius

Neural control of BP - 1 Baroreceptors: carotid and aortic sinuses sense the blood pressure in the aortic arch and internal carotid send signal to the vasomotor center in the medulla oblongata Other information are sent from the hypothalamus, cortex

Neural control of BP - 2 The vasomotor center integrates all these information The vasomotor sends decision to the ANS center: - Both parasympathetic and sympathetic innervate the S/A node can accelerate or slow down the heart rate - The sympathetic NS innervates the myocardium and the smooth muscle of the arteries and veins promotes vasoconstriction

Hormonal control of BP Hormones can control blood vessel radius and blood volume, stroke volume and heart rate On a normal basis, blood vessel radius and blood volume are the main factors If there is a critical loss of pressure, then the effects on HR and SV will be noticeable (due to epinephrine kicking in) Control of blood vessel radius - Epinephrine - Angiotensin II - Vasopressin (?) Control of blood volume - Anti-diuretic hormone (vasopressin) - Aldosterone Control of heart rate and stroke volume - Epinephrine

Control of blood vessel radius Epinephrine: secreted by the adrenal medulla and ANS reflex increase HR, stroke volume and promotes vasoconstriction of most blood vessel smooth muscles. Angiotensin II promotes - vasoconstriction - - Angiotensin II secretion: Decreased flow of filtrate in kidney tubule is sensed by the Juxtaglomerular apparatus (a small organ located in the tubule) secretion of renin Renin activates angiotensinogen, a protein synthesized by the liver and circulating in the blood angiotensin I Angiotensin I is activated by a lung enzyme, Angiotensin-Activating Enzyme (ACE), angiotensin II Angiotensin II is a powerful vasoconstricted of blood vessel smooth muscles

Control of blood volume Anti-diuretic hormone = ADH - - Secreted by the posterior pituitary in response to blood osmolarity (often due to dehydration) Promote water reabsorption by the kidney tubules H2O moves back into the blood less urine formed

Control of blood volume Aldosterone: - Secretion by the adrenal cortex triggered by angiotensin II Promotes sodium reabsorption by the kidney tubules (Na+ moves back into the blood) H2O follows by osmosis Whereas ADH promotes H2O reabsorption only (in response to dehydration), aldosterone promotes reabsorption of both H2O and salt (in response to BP)

Clinical application: Shock Stage I: reversible, compensated shock Stage II: reversible, noncompensated shock Stage III: irreversible shock Death Stage I: Body reacts to maintain BP HR, vasoconstriction.. BP remains within normal range Stage II: Body reacts to maintain BP HR, vasoconstriction.. BP drops below adequate range (SBP 70). Can be reversed by medical treatment Stage III: Body is fighting to maintain adequate BP without success HR is very high not enough O2 for cardiac, brain cells to survive damages. Cannot be reversed by medical treatment