Ch.19 pt.2 Control of Blood Flow and Autoregulation in the Cardiovascular System

Control of Blood Flow

Tissue Perfusion

Tissue perfusion refers to the process of blood flow through body tissues, which is essential for maintaining cellular function and homeostasis.

• Delivery of O2 and nutrients: Blood transports oxygen and nutrients to tissue cells.

• Removal of wastes: Blood flow removes metabolic waste products from tissues.

• Gas exchange: Occurs in the lungs, where oxygen is absorbed and carbon dioxide is released.

• Absorption of nutrients: Takes place in the digestive tract.

• Urine formation: Occurs in the kidneys as blood is filtered.

• Precise regulation: The amount of blood flow is tightly regulated to meet the functional needs of each tissue or organ.

Mechanisms of Blood Flow Control

Blood flow is regulated by both extrinsic and intrinsic mechanisms to ensure proper tissue perfusion.

• Extrinsic control:

  • Involves the sympathetic nervous system and hormones that regulate blood flow throughout the body.

  • Acts on arteriolar smooth muscle to reduce flow to regions that do not require increased perfusion.

• Intrinsic control (Autoregulation):

  • Local control of blood flow, adjusted to meet the specific requirements of tissues.

  • Local arterioles feeding capillaries can modify their diameters.

  • Organs regulate their own blood flow by varying arteriolar resistance.

Comparison of Intrinsic and Extrinsic Controls

Intrinsic Controls	Extrinsic Controls
Control is entirely from within the tissue or organ	Control is from outside the tissue or organ
Uses paracrines or properties of muscle tissue	Uses nerves or hormones
Also known as autoregulation or local control	Systemic regulation

Example: Redistribution of Blood During Exercise

• At rest, skeletal muscles receive about 20% of total blood volume.

• During exercise, skeletal muscles can receive over 70% of blood volume.

• Intrinsic controls: Skeletal muscle arterioles dilate, increasing blood flow to muscles.

• Extrinsic controls: Decrease blood flow to other organs (e.g., kidneys, digestive organs).

• MAP (Mean Arterial Pressure): Maintained despite dilation of skeletal muscle arterioles.

Autoregulation: Intrinsic (Local) Regulation of Blood Flow

Autoregulation and Reactive Hyperemia

Autoregulation refers to local (intrinsic) conditions that regulate blood flow to a specific area. Reactive hyperemia is the increased blood flow to an area due to intrinsic factors following a period of reduced blood supply.

• Two main types of intrinsic mechanisms determine the final autoregulatory response:

  • Metabolic controls

  • Myogenic controls

Metabolic Controls

Metabolic controls adjust blood flow based on the metabolic activity of tissues.

• Increased tissue metabolic activity results in:

  • Declining levels of O2

  • Increasing levels of metabolic products (H+, K+, adenosine, prostaglandins)

• Effects of local chemical changes:

  • Direct relaxation of arterioles and precapillary sphincters

  • Release of nitric oxide (NO) by endothelium, a powerful vasodilator

  • Release of endothelins by endothelium, potent vasoconstrictors

  • Balance between NO and endothelins regulates vessel diameter; NO predominates when blood flow is inadequate, causing vasodilation

  • Inflammatory chemicals can also cause vasodilation

Myogenic Controls

Myogenic controls involve the response of vascular smooth muscle to changes in pressure to maintain constant perfusion and prevent tissue damage.

• Passive stretch: Increased MAP stretches vessel wall more than normal

  • Vascular smooth muscle responds by constricting, decreasing blood flow to tissue

• Reduced stretch: Decreased MAP causes less stretch than normal

  • Vascular smooth muscle responds by dilating, increasing blood flow to tissue

Key Terms

• Perfusion: The process of delivering blood to capillary beds in biological tissue.

• Autoregulation: The automatic adjustment of blood flow to each tissue in proportion to its requirements at any given point in time.

• Mean Arterial Pressure (MAP): The average pressure in a patient's arteries during one cardiac cycle. $MAP = rac{1}{3} imes (Systolic ext{ }Pressure - Diastolic ext{ }Pressure) + Diastolic ext{ }Pressure$

• Vasodilation: The widening of blood vessels resulting from relaxation of smooth muscle cells within the vessel walls.

• Vasoconstriction: The narrowing of blood vessels resulting from contraction of the muscular wall of the vessels.

Example: Exercise-Induced Blood Flow Changes

During vigorous exercise, local metabolic changes in skeletal muscle (e.g., increased CO2, decreased O2) cause arterioles to dilate, increasing blood flow to active muscles. Simultaneously, extrinsic controls reduce blood flow to less active organs, ensuring efficient oxygen and nutrient delivery where needed most.

Additional info: The balance between intrinsic and extrinsic controls is crucial for maintaining homeostasis during varying physiological states, such as rest and exercise.