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USMLE step 1 Physiology Vascular system - 2023- Study guide - Study smarter not harder - your guide to pass the exam
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, Introduction
);;> Blood is the vital fluid tissue , circulates with CVS >-Total blood volume= 5600 ml (8% of body Wt.)
Functions of blood
0
~········
:
······························~············
For 0 2 , C0 2 ,
.........•........•••.
: By WBCs & antibodies : Stoppage of bleeding after : Keeping the
·······~
:
: glucose, hormones : against pathogenic injury by clotting : composition of internal :
: & waste products : microorganisms mechanism : environment constant :
~-················~·····················~···········································~
Composition of blood:
:·55% ·~i t~t~i .bl~~d· ~~~~-~~ ~
..........._..... ~PI~~~~· ~~ ·~ ·;,~ii~:;/~j;~·; ;i~id, RBCs (erythrocytes)
that clots on standing leaving WBCs (leucocytes)
the serum (plasma- clotting factors) Platelets (thrombocytes)
Composition of plasma:
(3) Organic substances
90% of 0.9% of plasma volume . 9.1% of plasma volume .
plasma • The main cation: (Na+) • Plasma proteins.
volume. (142 mEq I L.) • Plasma lipids: e.g. (cholesterol
• The main anion: (CI -) TG , F.A. & phospholipids}
(104 mEq I L.) & HC03- Present in combination with
• Small amounts of P0 4-3 plasma proteins ~ lipoproteins
& so4 , ca+2, Mg +2 & K+ • Others: as glucose, A.As, vitamins
The rate of albumin formation by the liver is high (200-400 mg/Kg/day)
This rapid production of plasma proteins by the liver is useful in cases
of massive protein loss (as in burns & nephrosis)
63
\
, Albumin I Globulin ratio (A/G ratio): N. (1.2- 1.6)
It decreases in:
1. Liver diseases: e.g. liver cirrhosis & hepatitis ~ H production of albumin.
2. Kidney diseases: e.g. nephrosis ~ ii loss of albumin in urine.
Functions of roteins
(1) Osmotic function
• Plasma proteins (mainly albumin) produce effective osmotic pressure across capillary walls
(colloidal osmotic pressure) oncotic pressure= 28 mmHg
• Albumin ~ 19 mmHg (the highest cone.& the lowest M.W.)
Cations ~ 9 mmHg (ma inly Na+ held in plasma by Donnan effect of plasma proteins)
• This osmotic pressure is important for regulation of blood volume & ISF volume.
>- The total osmotic pressure of plasma "' 5000 mmHg.
>- Most of this osmotic pressure is due to crystalloids (Na·, cr & HC03 -) but it is not effective
as the capillary membrane is permeable to them so they are equal in plasma & ISF.
>- Osmotic pressure caused by albumin isvery little, but is the most effective as capillary
membrane is impermeable to albumin ~ osmotic effect to draw tissue fluid to the blood.
Against microorganisms & their toxins by y globu lins (immunog lobu lins or antibodies)
(3) Capillary function
Plasma proteins maintain normal capillary permeability by closing pores in the capillary walls
(4) Blood clotting
Fibrinogen & prothrombin are essential for blood coagulation.
(5) Blood viscosity
• Blood is (3 times), plasma is (1.5 times) as viscous as water.
• Fibrinogen (h ighest M.W. & elongated shape) is responsible for most of plasma viscosity.
• Viscosity is important for production of the peripheral resistance & maintenance of ABP .
(6) Buffering action
• Plasma proteins are responsible for 15 % of the buffering power of blood.
• Amphoteric property of plasma proteins: they have free acidic (R-COOH) & basic (R-NH 2 )
groups which are easi ly dissociated ~ act as weak acid s or bases depending on pH of plasma.
• At normal pH (7.4), plasma proteins form the buffer (proteinic acid- Na proteinate).
(7) Transport & conservation of important substances
Albumin, a & ~ globulins transport important substances in blood (hormones , vitamins, lipids,
minerals "Ca ++, Fe ++ ")~ prevent their rapid loss in urine & provide a reservoir to be used when needed .
(8) Plasma proteins as a source of amino acids for tissues
• Plasma proteins function as labile protein storage.
• They act as a source of amino acids for tissue proteins replacement (when they are H ).
• Plasma proteins are engulfed by liver cells & macrophages (split into) am ino acids
~ transported back to blood ~ formation of tissue proteins.
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