Mammalian Transport System Notes

Transport in Mammals

Closed Circulation

Mammals have a closed circulatory system with a powerful, muscular four-chambered heart.
Blood is pumped through arteries, veins, and capillaries under pressure.
The heart is divided into right and left sides.
Blood flows from the right side to the lungs and then to the left side, then to the rest of the body, and back to the right side.
This is a double circulation because blood passes through the heart twice in one complete circuit.

Advantages of Mammalian Circulation

Simultaneous high-pressure delivery of oxygenated blood to all body regions.
Oxygenated blood reaches respiring tissues without mixing with deoxygenated blood.

Comparison with Other Circulatory Systems

Open Circulation (Insects): Blood is pumped forward by a tubular heart into sinuses.
Single Circulation (Fish): Blood passes through the heart once per circulation, going to the gills first and then the body.
Mammals Double Circulation: Blood passes twice through the heart in each complete circulation, with pulmonary and systemic circuits.

The Transport Medium - Blood

Blood transports products of digestion, plays a role in defense against disease, and transports respiratory gases.
Blood consists of plasma (liquid medium) with suspended red blood cells (erythrocytes), white blood cells (leucocytes), and platelets.

Composition of Blood

Plasma (55%):
- Water (90%)
- Dissolved Substances (10%):
  - Proteins (7%)
  - Salts (1%)
  - Lipids (2%)
Cells (45%):
- Red Blood Cells (Erythrocytes): 5 million/mm³
- White Blood Cells (Leucocytes): 7000/mm³
  - Lymphocytes (form antibodies)
  - Monocytes
  - Neutrophils (phagocytes that engulf bacteria)
- Platelets: 250,000/mm³

Components of Blood and Their Roles (Table 6.7)

Plasma:
- Transports nutrients, excretory products (urea), hormones, dissolved proteins (including antibodies), and heat.
- Regulates osmotic concentration (water potential) of the blood.
Red Blood Cells (Erythrocytes):
- Transport oxygen from lungs to respiring cells.
- Transport carbon dioxide from respiring cells to lungs.
White Blood Cells (Leucocytes):
- Lymphocytes: Key role in the immune system, forming antibodies (discussed on pages 276-277).
- Phagocytes: Ingest bacteria or cell fragments.
Platelets:
- Role in blood clotting mechanism.

The Plumbing of the Circulation System

Types of Vessels

Arteries: Carry blood away from the heart.
Veins: Carry blood back to the heart.
Capillaries: Fine networks of tiny tubes linking arteries and veins.

Structure of Arteries and Veins

Arteries and veins have strong, elastic walls; artery walls are thicker and stronger due to collagen, elastic, and smooth muscle fibers.
Capillary walls consist only of endothelium.
Blood pressure is high in arteries leaving the heart, traveling in pulses, but much lower and without a pulse in capillaries.

Veins and Valves

Veins have valves to prevent backflow due to low pressure.
Sectioned veins may appear squashed, while arteries appear circular.

Structure of Blood Vessels - Tunics

Tunica Intima:
- Endothelium
- Subendothelial layer
- Internal elastic membrane
Tunica Media:
- Elastic fibers, collagen, involuntary (smooth) muscle fibers.
Tunica Externa:
- Elastic fibers and collagen.

Differences between Arteries, Veins, and Capillaries (Table 6.8)

Feature	Artery	Capillary	Vein
Outer Layer	Present (thick layer)	Absent	Present (thin layer)
Middle Layer	Present (thick layer)	Absent	Present (thin layer)
Endothelium	Present	Present	Present
Valves	Absent	Absent	Present

Variations in muscle and elastic fiber thickness exist throughout the circulation to maintain appropriate blood pressure.

Arrangement of Arteries and Veins

Pulmonary Circulation: Right side of the heart pumps deoxygenated blood to the lungs.
Systemic Circulation: Left side of the heart pumps oxygenated blood to the rest of the body.
Sequence: aorta → artery → arteriole → capillary → venule → vein → vena cava
Blood supply to organs named after them (e.g., hepatic artery to the liver).
The liver receives blood from the small intestine via the hepatic portal vein, carrying products of digestion.

Vessels role

Arteries: Vessels that convey blood from the heart (ventricles) to the tissues of the body at a high pressure.
Veins: collect blood at low pressure from the tissues of the body and return it to the atria of the heart.

The Heart as a Pump

The heart is the size of a clenched fist, located in the thorax between the lungs, and is protected by the pericardium.
The heart wall is supplied with oxygenated blood from coronary arteries.
Cardiac muscle consists of cylindrical branching columns of fibers, allowing for three-dimensional contraction and is of myogenic origin.
Intercalated discs transmit impulses for simultaneous contraction.

Chambers and Valves of the Heart

The heart has four chambers: two thin-walled atria (upper) and two thick-walled ventricles (lower).
The left ventricle wall is thicker than the right.
Valves prevent backflow of blood.
- Atrioventricular valves (tricuspid on the right, bicuspid/mitral on the left) prevent backflow from ventricles to atria; edges supported by tendons.
- Semilunar valves prevent backflow from the pulmonary artery and aorta into the ventricles.

Cardiac Cycle

The cardiac cycle is the sequence of events in a heartbeat.
The heart beats about 75 times per minute (each cycle is about 0.8s long).
Systole: Heart muscle contracts.
Diastole: Heart muscle relaxes.
Contraction decreases chamber volume, increasing pressure and forcing blood to lower-pressure regions.
Valves ensure one-way flow.

Stages of the Cardiac Cycle

Atrial Systole (0.1 s):
- Atrial walls contract, pushing blood into ventricles.
- Semilunar valves prevent backflow from the aorta.
- Contraction of atrial walls seals off vena cavae and pulmonary veins to prevent backflow.
Atrial Diastole (0.7 s):
- Atrium relaxes.
Ventricular Systole (0.5 s):
- Ventricles contract, closing atrioventricular valves and opening semilunar valves.
- Blood is forced into the aorta, generating a pulse.
Ventricular Diastole:
- Ventricles relax.
Pressure changes in the atria, ventricles, pulmonary artery, and aorta control valve opening and closing.

Heart Rate and the Pulse

Contraction of ventricle walls forces blood into the aorta and pulmonary arteries under pressure (stroke volume).
This stretches elastic fibers in artery walls, creating a pulse.
Artery walls distend and recoil, maintaining blood pressure and flow.

Blood Vessels - Changing Structure in Relation to Function (Table 6.9)

Component/Role	Structure in Relation to Function
Arteries (Aorta)	Thickest and strongest walls; tunica media is thickest. Walls stretch to accommodate blood surge. Elastic and collagen fibers prevent rupture.
Main Arteries	Distribute blood under high pressure. Become wider, lowering pressure. High proportion of elastic fibers stretch and recoil, keeping blood flowing.
Arterioles	Deliver blood to tissues under lower pressure (about $35 \text{ mm Hg}$ . High proportion of smooth muscle fibers regulate blood flow from arteries into capillaries.
Capillaries	Narrow tubes (diameter of a single red blood cell - about $7 \mu \text{m}$ ), reduce flow rate for exchange. Thin walls (single layer of endothelial cells) with gaps for blood components to escape into tissue fluid.
Veins (Venules)	Thin walls; tunica externa is thickest. Walls consist of endothelium and a thin tunica media with a few smooth muscle fibers. Tunica externa present, of elastic and collagen fibers.
Veins	Receive blood from tissues under low pressure (about $5 \text{ mm Hg}$ ). Become wider, lowering pressure and increasing flow rate. Tunica media contains a few elastic fibers and muscle fibers. Valves prevent backflow of blood.

Measuring Heart Rate

Heart rate can be measured in the carotid artery or at the wrist.
Each contraction generates a pulse.
Cardiac output = stroke volume × pulse rate.
At rest, cardiac output is about $5 \text{ dm}^3$ of blood per minute.