21. Physiology of the blood – functions. Composition and volume of the circulating blood – regulation of the volume. Blood plasma – composition and regulation. Haematocrit. Blood reservoirs FUNCTIONS OF BLOOD

physiologu of blood

Blood is a specialized liquid connective tissue with three essential functions:

A. Regulation

• Temperature: Maintains body temperature via heat absorption and distribution; adjusts blood flow to skin.

• pH: Regulates acid–base balance (buffers, proteins, ions).

• Fluid volume: Maintains osmotic pressure (mainly via albumin) and electrolyte balance.

B. Transport

• Gases: O₂ from lungs to tissues; CO₂ from tissues to lungs.

• Nutrients: From gastrointestinal tract to tissues.

• Hormones: From endocrine glands to target tissues.

• Waste products: To kidneys, liver, lungs for excretion.

• Heat: Distributed or dissipated as needed.

C. Protection

• Clotting factors: Prevent blood loss via coagulation.

• Immune defense: WBCs and plasma globulins (esp. gamma globulins like IgG, IgA) protect against pathogens and toxins.

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2. Composition and Volume of Circulating Blood

regulation of volume

A. Volume

• ~5–6 L in males, ~4–5 L in females

• Blood is ~6–8% of body weight

B. Composition

Component	Percentage
Plasma	~55%
Formed elements	~45% (RBCs), <1% (WBCs + platelets)

RAAS

3. Blood Plasma – Composition and Regulation

A. Plasma Composition

• ~90% water, ~10% solutes (proteins, electrolytes, gases, nutrients, hormones)

• Plasma Proteins (~60–83 g/L):

  • Albumin (35–55 g/L): Maintains oncotic pressure (Oncotic press = osmotic pressure specifically exerted by plasma proteins (especially albumin)), regulates water balance

  • Globulins:

    • α-globulins: Transport (e.g., caeruloplasmin for copper)

    • β-globulins: Transport (e.g., transferrin for iron)

    • γ-globulins: Immune defense (e.g., IgG, IgM, IgA, IgE)

  • Fibrinogen: Involved in clotting; regulated by glucocorticoids/cytokines

B. Electrolytes

Ion	Normal Range (mmol/L)
Na⁺	136–151
K⁺	3.5–5.6
Ca²⁺	2.12–2.62

C. Other Organic Components

• Glucose: 2.8–6.1 mmol/L (regulated by insulin/glucagon)

• Cholesterol: <5.2 mmol/L

  • HDL ("good"): Carries cholesterol to liver

  • LDL ("bad"): Deposits cholesterol in vessels

• Triglycerides: 0.6–1.7 mmol/L

D. Plasma Volume Recovery

• Loss of 500 mL plasma is compensated within ~25 hours

4. Regulation of Blood Volume

Blood volume directly affects arterial pressure and is tightly regulated by the renal and endocrine systems:

A. Renal System

• Increased blood volume → ↑ GFR (Glomerular Filtration Rate.)→ ↑ Na⁺/H₂O excretion → ↓ volume

• Decreased volume → activates compensatory mechanisms

B. Hormonal Regulation

1. Renin–Angiotensin–Aldosterone System (RAAS):

   • Renin → Angiotensin II → triggers aldosterone + ADH release + thirst

2. ADH (vasopressin):

   • From posterior pituitary; increases water reabsorption via aquaporins

3. Aldosterone:

   • From adrenal cortex; increases Na⁺ and water retention, K⁺ excretion

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• Glomerular Filtration Rate.= the rate at which the kidneys filter blood through the glomeruli

5. Haematocrit

• Definition: Proportion of blood volume occupied by red blood cells

• Measured as a fraction (L/L) or percentage (%)

Group	Normal Range
Men	0.40–0.54 L/L
Women	0.36–0.47 L/L

• Higher in men due to testosterone stimulating erythropoietin production.

critters = little insects: number of little insects occupying blood (little insects = RBC)

hematoCRITTERS

6. Blood Reservoirs

Certain tissues act as reservoirs, storing blood that can be mobilized during hemorrhage or stress:

Reservoir Site	Approx. Volume Released
Relaxed veins	Hold ~50–80% of total blood
Spleen	~100 mL
Liver sinusoids	Several hundred mL
Large abdominal veins	~300 mL
Venous plexuses in skin	Moderate amount

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