C3.9 - Mammalian Circulatory System

Introduction

• Blood arranged in 3 main cycles below:
• cardiac circulation: pathway of blood within heart
• pulmonary circulation: pathway of blood from heart to lungs
• systemic circulation: pathway of blood from heart to rest of body

Transport Vessels

• Arteries and arterioles (small arteries) carry blood away from heart
  • oxygenated blood
  • pulmonary arteries carry deoxygenated blood to lungs
• Veins and venules (small veins) carry blood toward heart
  • deoxygenated blood
  • pulmonary veins carry oxygenated blood to heart
• blood vessel: a tube for blood to circulate through
• Pulmonary cycle breaks oxygenated-deoxygenated trend
• Blood travels from arteries → capillaries → veins
• As blood travels through capillaries, pressure drops

Arteries

• arteries: thick, elastic, muscular transport vessels that carry blood away from heart
• Act as conduits for blood between heart and capillaries
• Act as a pressure reservoir to force blood into arterioles
• Dampens changes in blood pressure and flow caused by the heart
• Controls distribution of blood to different capillary networks by closing some arteries
• Walls of arteries made of epithelial tissue wrapped in layers of smooth muscle and connective tissue
• Blood pressure depends on…
  • the volume of blood in the arteries
  • properties of arterial walls

Veins

• veins: transport vessels that carry blood towards heart
• Large inside diamater
• Lack elasticity
• Large volume, low pressure systems
• Storage reservoir for blood
• Blood moves back to heart by gravity and skeletal muscle contractions
  • exercise is important
• Valves in veins prevent blood from flowing backwards
• Varicose veins: a disease where the valves in the veins do not function properly

Capillaries

Line 1 blood cell thick are capillaries

• capillaries: smallest (microscopic) blood vessels where nutrient and gas exchange between blood and body occurs
• Diameter just large enough for 1 blood cell to pass through
• High surface area, resembles network of tiny tubes
• Capillary wall regulates movement of fluids and nutrients into and out of blood
• Walls consist of very thin layer of epithelial tissue in moist membrane

Blood: The Transport Medium

• tissue: collection of cells that have specialized tasks
• blood: transport tissue of oxygen and nutrients
  • Body contains ~4-6 L of blood
  • also transports wastes out of system
• Composed of 2 phases
  • plasma: liquid part of blood (55% of blood)
    • ~90% water
    • ~10% of plasma: proteins, dissolved nutrients, wastes
  • solid cells (45% of blood)

Erythrocytes: Red Blood Cells (RBCs)

• red blood cells (RBCs): cells that carry oxygen from lungs to all tissues of body
  • 1 mm³ of blood (1 drop) ≈ 5 mil. RBCs
  • proper name: erythrocytes
• 44% of blood volume
• Bags of hemoglobin, made in bone marrow
• hemoglobin: protein that binds oxygen in the lungs and releases it throughout body
  • Each cell packed w/ 280M haemoglobin molecules
  • Haemoglobin takes up and releases oxygen
  • Oxygen bonds to Fe in hemoglobin, giving blood red colour
• Specialized for oxygen transport
• Produced in bone marrow
• Mature cells have no nuclei
• Lifespan: 3-4 mo.
• Factors that influence oxygen-haemoglobin binding
  • Low oxygen partial pressure: weakened bond, quick release
  • Increased blood acidity: weakened bond, quick release
  • Cooler temperature: strengthened bond, slow release
  • Carbon dioxide also binds w/ haemoglobin

Haemoglobin binding diagram

Leukocytes: White Blood Cells (WBCs)

• white blood cells (WBCs): cells that guard against infection, fight parasites, and attack bacteria
  • proper name: leukocytes
• 1% of total blood volume under normal, healthy conditions
• Protect body from infection
• Not constrained to blood vessels, can move through vessel walls as needed
• Keep their nuclei unlike RBCs
• 2 important types
  • macrophages
  • lymphocytes
• Macrophages may pass through capillary wall by amoeboid movements
  • to digest pathogens by phagocytosis
  • Part of innate immune response: general defence

Lymphocytes

• Lymphocytes part of acquired immune response
• Enable body to recognize and fend off pathogens
• 2 types
  • T cells (from thymus gland)
  • B cells (from bone marrow)
• Allow body to become immune to certain pathogens
• antibodies: Y-shaped proteins w/ a region that is variable in structure
• Variable region recognizes antigen carried on invading pathogens
• B cells remain in blood after an infection ready to trigger another immune response

Platelets

• platelets: cell fragments involved in clotting of blood
  • originate when cytoplasm of certain bone marrow cells divides
  • 1 mm₃ of blood ≈ 250k-500k platelets
• Platelets mainly produced in bone marrow
• After injury, broken blood vessels attract platelets to site of injury
• Platelets rupture and release substances that combine with other clotting factors in plasma
• Enzymatic reactions produce fibrin that forms a mesh preventing blood cells from escaping
• fibrin: strand-like protein involved in clotting
• Network builds up into a scab
  • Protects area while new tissue develops

Nutrient Exchange Between Blood and Cells

• diffusion: process where molecules move across a membrane from area of high concentration to low conc.
• diffusion gradient: gradual change in concentration of solutes in a solution as a func. of dist. through solution
  • i.e. if blood in capillary contains higher conc. of O₂ than fluid next to it…
  • … oxygen moves to fluid by diffusion (spont.)
• Capillaries no more farther than 10μm from body cell
• Nutrients, gases, and wastes diffuse between capillaries and body cells
• Body tissues are surrounded by fluid that acts as a medium for molecular exchange.
• Molecules in capillaries must first enter the surrounding fluid before entering cells
• Small molecules (e.g., oxygen and carbon dioxide) move by diffusion:
  • Oxygen diffuses from blood → fluid → cells
  • Carbon dioxide diffuses from cells → fluid → blood
• Larger molecules require other transport processes to cross membranes
• blood pressure: force exerted by blood on artery walls which drives:
  • Blood flow through arteries and capillaries
  • Movement of substances across capillary walls
• Nutrients and gases diffuse into cells from the fluid.
• Cellular wastes diffuse out of cells into the fluid and then into the blood.
• Blood transports wastes to organs for excretion from the body.