CVS 4 VENOUS RETURN & REGULATION OF CARDIAC OUTPUT.ppt
- 1. 1 VENOUS RETURN AND REGULATION OF CARDIAC OUTPUT VENOUS RETURNS: Is the rate at which blood RETURNS to the heart from the peripheral vascular beds, and thus the is the rate at which the blood enters the central venous system (i.e great veins in the thorax and Rt atrium. lung RIGHT HEART PUMP 100% LEFT HEART PUMP 100% HEART MUSCULAR 3% BRAIN 14% SKELETAL MUSCLES 15% BONES 5% VR GIT,SPELEEN 21% LIVER 6% KIDNEY 22% SKIN 6% OTHER 8%
- 2. 2 VENOUS RETURN CONT. An increase in peripheral venous tone will lead →↑VR→↑ central venous pressure and this will lead to an ↑ Cardiac output (C.O). ie Venous Return determine how much blood will be pumped out by the heart and this is what determine the C.O. At normal situation VR is equal to the C.O. However there can be temporary differences between VR and C.O. Cardiac output (C.O): Is the amount or quantity of blood pumped by the heart in the pulmonary artery and aorta per minute
- 3. 3 THE CARDIAC OUTPUT CONT. The main determinant of cardiac output are Stroke Volume (S.V.) and Heart Rate (HR) Stroke Volume is the amount of blood pumped out of the heart per beat ie C.O.= SV X HR HR = 75 beats /min SV = 70mls/beat CO = 75x 70 = 5250mls/min or nearly 5L/min Stroke Volume (SV) is determine by: - Preload - Contractility - Afterload
- 4. 4 CARDIAC OUTPUT CONT. • In the healthy young man the CO is therefore 5L/min in women the CO is 10-20%less Since CO changes with body size, we normally use the cardiac index (CI) CI = C.O body surface area.
- 5. 5 CARDIAC OUTPUT CONT. In a 70kg man has a body surface area of about 1.7sq m. Cardiac Index CI = 5L divide by 1.7= 3L/min per sq m Frank –Starling law: States that within the physiological limit or range of the heart , will pump whatever amount of blood that enters the Rt atrium. VR under normal condition is equal to C.O.
- 6. 6 CARDIAC OUTPUT CONT. • The heart plays a permissive role. • The heart can permit an increase in CO • Peripheral venous blood or pool has a pressure of 7mmHg and this is a MEAN SYSTEMIC OR CIRCULATORY FILLING PRESSURE. • It is an expression of the degree of filling of the CIRCULATORY SYSTEM of blood
- 7. 7 CARDIAC OUTPUT CONT. • The CO can reach or permit up to 13 to 15 l/min without stimulation of autonomic nervous system. • 7mmHg is the average effective pressure of the blood in the peripheral circulation that tend to push the blood toward heart.
- 8. 8 VENOUS RETURN AND CENTRAL VENOUS PRESSURE • NB: Venous Return increases as central venous pressure decreases ie. The central venous pressure influences the V.R. if other factors remain constant. • Factors that influence V.R. are: 1. peripheral venous pressure 2. central venous pressure ie an increase in peripheral venous pressure → an increase in V.R or a decrease in central venous pressure
- 9. 9 VENOUS RETURN CONT. Since Veins are capacitance vessels alteration of their blood volume (BV)→ alteration in venous pressure e.g. 1. Haemorrhage /vomiting/diarrhoea →a decrease in BV and this leads to a decrease in VR 2. Over transfusion →↑BV→↑CHP →↑VR 3. ↑Sympathetic discharge →↑venous tone→↑VR 4. ↑Parasympathetic discharge →↓venous tone →↓VR.